Galaxy AP, minimum FL required?

[Andy01]

Hi folks,
Toying with getting into capturing LRGB galaxies.
I'm experienced with big nebs and NB, but looking for a change.
Obviously will need a longer FL than my two short refractors, (336mm & 660mm)

Maybe I can trade one or both up for a 1000mm newt?
I have a QSI wsg8 with built in OAG and a hypertuned EQ6

Affordable recommendations?

Cheers
Andy

[Atmos]

It's more to do with image scale than focal length. For your camera something around the 1000mm range would be good. So a 8" F/5 or 10" F/4 or 12" F/3 ;)

http://www.astrobin.com/full/287438/D/
This was shot at 677mm on a night of awful seeing.

[alocky]

Hi Andy, as Colin says it's all about image scale, so the other question is what's the seeing like at your location? If you only get 2" seeing, then 1"/pixel is probably as good as you need. That would be anywhere between 1000 and 1500mm for that camera.
Cheers
Andrew.

[multiweb]

Have you considered using a x2 barlow and give it a go with your refractor first, test the waters with guiding and the mount.

[alocky]

That is quite possibly the best bit of advice I've seen in here! Where's the 'like' button?!
Cheers
Andrew.

[Camelopardalis]

Andy, another means of skinning the cat (image scale) is to decrease the size of your pixels...newer CMOS sensors may be more economical than changing your scope. I've been experimenting with the ASI178 and its 2.4 micron pixels show promise...but it clearly becomes seeing limited, even with my 550mm F/L scope.

[Andy01]

Hi Guys, sorry for the delayed reply, some personal life events took precedence.
I borrowed a 1000 mm f5 newt from a mate to play with last night.
Crikey, not easy for this refractoholic though!
Couldn't get it to focus for visual with my diagonal & eyepiece, so gave up and attached my camera instead.
Trouble is my QSI is a heavy beast, way too heavy for the crayford focusser anyway - I could see it slipping as I watched!
..And the tube is so long that it will easily hit my tripod's legs.

Long story short, I abandoned it and put my small frac back n and was imaging within 15 mins.

Think I'll look into the barlow suggestion, (thanks Marc) and also try using the original flattener that was supplied with my WO - as it's not a reducer, which gives me a FL of 770mm @ f7.0

Stay tuned :)

[Merlin66]

Andy,
as you've found Newtonians are not designed for use with diagonals....

I think the barlow solution is well worth trialing at least you can then determine the best fl for imaging with your camera.

Check out the CCDCalc page - it allows you to see what FOV you'll get with various fl and cameras - there's a good selection of astronomical objects to allow easy comparison.
http://www.newastro.com/book_new/camera_app.html

[gregbradley]

A barlow on a small APO will take forever to get some signal.
Not enough aperture for galaxy imaging.

You can do it but expect to put in massive hours.

Greg.

[alocky]

Hi Greg - my take on that piece of advice was that since Andy already owns the camera (v. expensive) and mount (also expensive) he should use the Barlow (cheap) to see if (1) his mount is up to guiding at long focal length, and (2) what the image scale and local seeing delivers at around 1.2m focal length before buying a new imaging ota (v. expensive) that may require another mount (v expensive) or camera.
I don't think anyone was suggesting it as an alternative final solution!
Cheers
Andrew.

[Merlin66]

I agree 100% with Andrew....

[glend]

I will throw in another suggestion, the Skywatcher MN190 Mak-Newt. Its 1000mm fl and f5,3 so good enough and fast enough for DSOs. It does not need a coma corrector as it has a great front corrector, so its flat field and there are no diffraction spikes as the secondary is mounted on the corrector. Fully baffled, images compare to larger APOs, and it produces true colour rendition. I have had one for years now and will not part with it. The stock focuser is just adequate for imaging with a DSLR but with your camera (and mine), the Moonlight upgrade is recommended (Ron makes a special adaptor kit for the MN190). Its more expensive than a GSO imaging newt of the same size, and heavier (due to the corrector) but well balanced.
Current price is around $1699 from Andrews.
You would need a NEQ6 as a minimum for imaging with it, but with your camera i would suggest a heavier payload mount, i use a CGX these days.
Just my 2 cents. Note i have APOs and Newts, but the MN190 is the easiest to use as you don't need to worrying about reducer correctors, flatteners, or coma correctors - just stick the camera in the focuser and your in business. :thumbsup:

[Andy01]

Hey Andrew, Thanks for clearing that up - I was browsing the Bintel website looking at 2x Barlows/Powermates today, but I'm uncertain which to get.

Televue? Powermate Other? Presumably they have screw threads at each end for affixing between the scope/flattener/camera

I'm assuming a barlow/powermate will slow the system down by a stop at least, so I'm guessing my 770mm f7.0 refractor becomes f8 or 9 @ 1540mm FL? That seems comparable to some of the affordable RC type scopes, and it's only a few hunderd bucks.



Thanks Glen, that's a very positive rap for a beast I know next to nothing about! Not sure I want to spend big $ upgrading my mount, OTA on this project yet, but good to know what's out there.

[Merlin66]

Andy,
Depending on spacing (for Barlows) the magnification will be as advertised ie x2, x2.5, x3 etc.
This means your f7.5 scope would end up f15 (1200mm fl), f18.75 (1500mm fl), f22.5 (1800mm fl) - much more than just a "stop" (That's one reason I went for the TMB x1.8 Barlow on my f6 system)
Again most Barlows are 1.25" or 2" fitment and may or may not have a T thread for camera connection.
My Powermates are 1.25" and 2" fitment but do have a T thread adaptor.

If the Barlow is (or can be) positioned closer to the camera than design distance, the magnification will be reduced. NB This doesn't apply to Powermates which have the same magnification at any/all distances.

Hope that helps

[Slawomir]

I think that astro-physics Barlow can also be positioned at various distances to change magnification from 1.8x up.

[Merlin66]

OT...
I'm currently testing the Baader Hyperion x2.25 Barlow with various refractors/ filters for solar imaging.
So far, it looks very promising.

[Peter Ward]

I disagree with the sampling argument as it ignores the intrinsic resolution of the instrument. If were stuck with average seeing limits all the time, then most high res planetary imagery would be impossible.

I frankly find images taken at around 1100mm a bit of a yawn. Rarely is there any filigree detail.

Ive also found galaxies to be a mix of deep sky and planetary imaging. Image scale AND brightness helps enormously IMHO. I'd suggest something at the FL 2500mm mark would be about right.

Assuming you dont want to be at F10, then that also means a fair sized aperture.

Now we are talking big scope....and big mount....assuming you don't want it wobbling all about the place.

So. Careful what you wish for...it could lead to aperture fever. ;)

[Andy01]

Yikes! Hmmm - well that's not going to work is it! :shrug:



Aaaaahgghhh - thanks anyway Peter - it's going to hard to get tthat past SWMBO though :lol:

[Merlin66]

10" or 11" SCT???
I use the C11 at f10 for spectroscopy on a NEQ6 mount. No issues, no drama

[Slawomir]

Drizzle...a poor man's solution to undersampling :thumbsup:

[strongmanmike]

Don't worry Andy... around 1200mm FL with 10"-12" aperture, on a modest and importantly affordable mount, coupled with a lower cost small pixel but sensitive camera, may well cause the more financially secure to feel tired...but the proof is out there and easy to find, that shows such a combo does pretty damn well on small faint extended objects, especially if it is also fast ;)...just look at Ray's (Shiraz) images :thumbsup:

Mike

[rustigsmed]

why not a 12-16" goto dob with a asi 174 or 1600 you can then also do some visual if you like.

seems to go alright.
eg
http://www.astrokraai.nl/dump/20160505_M51_2000x1s_AutoStakkert_A SI1600MM_Emil_Kraaikamp.jpg

[Peter Ward]

For sure that is innovative stuff :thumbsup:

(still a big scope BTW....so I see vindication in my perspective here)

But I can only look in absolute awe at the results of Chart32 guys

BIG scope, superb mount, pristine skies...sigh....
(eg NGC 1398 ) (http://chart32.de/images/phocagallery/galleries/galaxies/thumbs/phoca_thumb_l_ngc1398_80.jpg)

and marvel at their work. :bowdown:

[billdan]

2000 x 1 sec subs

Holy Dooley - If you ever wanted any proof that sub length is irrelevant and its total exposure time that counts, this is it. And its still only 33 mins total.

[strongmanmike]

Forget the cheap and nasty Aldi gear, get a $3/4 Million 0.8m scope and put it in a $1/2 Million observatory up in the Andes and pay $50K a year to have it there yaaay :thumbsup:...or hey, THIS (http://www.astrosysteme.com/shop/asa-az800-ritchey-chretien/) is a steal at $1/2 Million :2thumbs:

Hope that was useful advice Andy, good luck with your decision :thumbsup:

Mike :P

[Slawomir]

Even if one purchased such instrument and installed it on the top of the highest mountain, somewhere there will always be a better scope with better skies...: http://www.spaceflightinsider.com/wp-content/uploads/2016/11/NASA-James-Webb-Space-Telescope-in-space-image-credit-James-Vaughan-SpaceFlight-Insider.png

:help:

:lol:

[gregbradley]

Ah yes it would be a good test. I have done some APO galaxy imaging using a Tak 1.6Q extender on a TEC180. I imaged the Sombrero. It looked reasonably good but the subs were not very bright at all. In my case the mount held up well (Tak NJP mount) but that mount was maxed out by the refractor so results could be variable.

Greg.

Greg.

[strongmanmike]

Wow that's a great scene that Suavi huh?...can't wait till it launches :)

Sorry Andy....no more OT, now get yourself that $Miilion scope :thumbsup:

Mike

[multiweb]

:eyepop: :lol: Does it come with bob's knobs and mag wheels?

[Merlin66]

Re original OP....
I used a 320mm f5 (1560mm fl) - Canopus 320 for many years in the 80's to visually check many, many of Greg Thompson's initial draft "Supernovae search charts" down to 15.5 mag. Generally a 12.5mm UO Ortho eyepiece....
Would be interesting to see how this would perform with today's CCD's.

[Peter Ward]

Assuming the optics are well corrected, quite well I'd expect. Imagers such as Shriaz certainly have pushed into the outliers of what is possible at that FL.

A big budget is hardly germane to the discussion as Rolf (across the pond) has proven time and again.

The fact remains, galaxies typically have small angular sizes and are faint.

Many of us don't have the luxury of time or clear weather to gather the data hence using a big scope simply makes sense.

[Paul Haese]

Where's the popcorn? :D

Seriously though, I have just ventured along this path of large aperture, fast f ratio with small pixels. What prompted me along this path was the results Mike gets with his system. I found typically I was doing three times the amount of imaging time and a lot of the time not getting quite the same results. That was in dark, still skies with an AO.

However, this type of imaging is not without its own problems. Fast Newtonians require coma correctors and those can be very sensitive to tilt. Not to mention any other flexure that can occur in a fast Newtonian. Narrowing down the problems can be time consuming if you can't afford an expensive scope which already has most of these issues resolved. The system has to hold collimation well and there has to be no movement in the focuser. Even expensive focusers can have slight slop. It can be a bit of a mine field with cheaper gear but it can work if you persist.

Overall, I think the image scale debate has been settled and I am a late convert to the concept. Fast imaging speed with good guiding is the way to go.

[Andy01]

Thanks everyone for all your input so far.

Seems that I'll need at least f5.6 or less, and 1000mm at absolute minimum, with an ota being whatever the max load my EQ6 will hold, with the QSI WSG8 in place.

That reduces some of the options, affordability being the last one - can't see me spending more than $2k on this, and that will mean parting with my WO FLT 110 first.

Not sure about metal bodied Newts either, as the temperature/focus thing would do my head in,
So that means either a carbon fibre ota or maybe Glen's idea of a Mak Newt which also eliminates the need for coma correctors as Paul mentioned.

More research to do, all good fun hey :D

[Peter Ward]

Oh come on Andy... $2k is chump change. :D

You want to go galaxy hunting, but know the sad truth: one can't even buy a decent Canon L-Series lens for that ;)

Carpe noctem !!!

[rbronca]

Andy, your current thinking is similar to what I have.

Skywatcher 10" F4 CF quattro. 2.5" Moonlite, QSI683 wsg-8 with unmounted filters, MPCCIII and a Loadstar for company. Mine is on an AZ-EQ6.

Field of view is 1.03 x 0.77 degrees at 1.11"/pixel.

May I suggest you select the scope and camera in a field of view calculator, say http://www.skyatnightmagazine.com/astronomy-field-view-calculator and check out what you get for a number of objects.

You commented on my slightly cropped Lagoon image from a few weeks ago to give a feel of what this is solution is capable of.

This is getting close to the maximum load for this size mount. Your not going to squeeze into a 12" or a 10" RC without guiding suffering.

[strongmanmike]

Hey Andy, just out of interest, this is what a fast well tuned and collimated 10" F5 Newt, mounted on an EQ8, in good sky conditions with a small pixel, sensitive camera and in competent processing hands, can deliver ;)

NGC 253 (http://www.astrobin.com/full/275078/0/)

One simply does not need a $100K+ budget

Mike

[Andy01]

Beautiful :thumbsup: - I've always been a fan of Ray's work - his gear delivers good bang for the buck too!

Thanks for posting Mike :)

[strongmanmike]

Ya not wrong! That was not up in tbe Atacama either. Did you check out the full res :eyepop: why would you feel you need 2500mm + FL when 1200mm can produce this? :shrug:.... with the right camera, you just dont.

Mike

[gregbradley]

I am not totally convinced of these arguments. I have both types of scopes - the AP Honders 305 F3.8 1159mm focal length scope and a Planewave CDK
17inch 432mm aperture scope at 2936mm focal length.

The Honders is better at widefield. It can do galaxies well with small pixels (Sony 694 sensor) but I get a better result from the CDK. I have seen Peter Wards images with similar setups show the same results.

It may be more the aperture rather than the focal length but there is a point where large aperture is hard to physically handle without being in a more compact form like an RC or CDK etc. A 17 inch Newt would be very hard to handle and wind prone as well.

Perhaps its the old adage "aperture rules" at work more than the focal length but the 2 concepts are really intertwined.

Small pixel cameras come with their own baggage as well. Small wells, Sony
CCDs are more prone to fixed pattern noise too.

Greg.

[Paul Haese]

Let's look at this way. The RC12 with a 9 micron pixel sensor gives me an image scale of 0.76" per pixel. The Newt 12 with 5.4 micron pixels gives me an image scale of 0.92" per pixel. The Newt 12 image is smaller for any given target but so far it appears the resolution looks very similar to me. Both scopes are physically a similar size, in fact GSO used the same truss system for both scopes except they put a secondary cage on the top of the RC truss. So up to 12" I'd say the argument is null and void. Over that size then you have a point. I am not a fan of tubes in general and long tubes do get caught by the wind, so there lever arm and gravity to contend with. And; the point has been well made here, not many people can really afford a 30-50K OTA. If you have the money that is all well and good. I personally cannot afford an F8 CDK or RC over $10,000 so I needed to find a faster solution to the problem. I put a fast Newt on a great mount and I think once I have it firing it will produce similar results to both Ray and Mike. They both have the newer sensors but the results should be similar in resolution.

The long and short of it is, if you want to spend that sort of money on a scope, then go for your life. If you are happy with that sort of imaging speed, so be it. If you want faster you need to consider other options. Andy, I think if you look along the 10" newt path that should serve you well here. As Rob pointed out he has a 10 Newt and he gets good results. He has to do a bit of work to sort it out but now the results are coming forth.

[strongmanmike]

To cover most deep sky objects and coupled with the right two cameras, I recon you have the perfect combination of scopes there really Greg :prey2::thumbsup: but aaaand hey, I could be wrong :P...I'm gathering Andy is also not in the position to acquire a 17"CDK and a 12" AP Honders with the two mounts and cameras required either...?

At the end of the day, people like Andy need to do the image comparisons for themselves and do them properly ie compare full res versions, allow for image display size, use of AO, type of camera/pixels used and site quality etc. and critically judge the level of difference and be able to differentiate whether these differences are perceived or indeed real and then do what they wish with the budget they have. After already doing this though, I know what I recon :)

Mike

[Camelopardalis]

The way I understand goes something like this....

A telescope is a photon funnel. The bigger the aperture of the funnel, the more photons end up hitting the sensor.

Traditionally, large photon buckets (pixels) were necessary to achieve acceptable SNR from the stream of photons hitting the sensor, largely because of high read noise and significant dark noise.

The only problem with tradition is that over time it gets outgunned by the technological revolution.

CMOS sensors are now starting to get competitive with their forebears with a combination of high QE and low noise, resulting in (commonly) smaller pixels that are able to achieve similarly acceptable SNR from fewer photons. That doesn't mean that chasing the faintest of fuzzies doesn't take a long time, but what it does it mean is that it isn't impossible with more humble kit.

Nothing is ever going to beat aperture...but what's happening with the newer technology is that there are more ways to achieve acceptable SNR that don't require the outlay of a family car. It doesn't diminish the amazing work done by the dedicated few with deeper pockets, but greater accessibility in this hobby can only be a good thing.

Some day I might buy a similar imaging newt that the OP is considering, hence my interest :D but in the meantime, I'm having as much fun as I can handle with a small scope.

[Peter Ward]

Beg to differ.

For sure, sensors have improved immensely.

But while I cherish my optics, I upgrade my camera bodies often, simply to take closer scrutiny of what the glass has to offer.

I've put together a small web page here (http://www.atscope.com.au/BRO/tutorials/LensesBigandSmall.html) to illustrate what I'm on about.

Sure, going from a 52mm diameter objective, to a 100mm is not the same as comparing a 300mm vs 400mm aperture telescope.

But I'd suggest we are simply talking shades of grey here.

To argue a 3" objective with say 1 micron pixels, can capture the same detail as a 30" and 10 micron pixels is a nonsense. You just need to decide where along the continuum you want to operate.

[Slawomir]

An interesting short article talking about aperture (and also benefits of adaptive optics) that I find relevant for our discussion: http://www.cfht.hawaii.edu/en/outreach/NHN/size.html

[RickS]

The main improvement has been a reduction in read noise (there are CCDs with high QE as well.) This allows the use of much shorter subs. It doesn't make a lot of difference to the total integration time required as you still have to overcome shot noise. Going large on aperture and pixels is still a winning combination for those who can afford it.

Cheers,
Rick.

[gregbradley]

Thanks Mike. I wasn't suggesting though that he should spend mega dollars it was more about the theory that you can get exactly the same results.
Its certainly close but that extra bit does seem to come from aperture. GSO has made large aperture a lot more affordable in just a few years.
Just putting in a different point of view.

Greg.

[strongmanmike]

Hey no problems Greg, all good fun musing :thumbsup: I was just trying to illustrate that a relatively inexpensive fast Newt around 1100-1200mm FL, coupled to a small pixel sensitive camera does pretty ok on small objects and (unlike the smartR's link to the CHART32 class outfit :rolleyes:) could perhaps fit in Andy's budget :lol:. Hey and then if one attaches a bigger chip you would also have a bigger field = win-win :)

Of course aperture is desirable but it comes at a $ cost and in typical Aussie conditions often fails to give the commensurate returns on the significant investment required in the OTA, suitably massive mount and necessary observatory.

I'd happily have your gear though :D

Mike

[Slawomir]

This is such an interesting thread :)

I have been thinking..(oh no!)... and came to a conclusion, that perhaps it wouldn't be entirely incorrect to assume that small pixels are more demanding on optics than large pixels, except for the size of corrected circle?

I feel it is not so easy to achieve the same level of off-axis optical correction on a smaller telescope with small pixels, as it would be with a longer focal length combined with large pixels, given both are giving exactly the same image scale. A small telescope will have more significant field curvature that needs to be corrected/flattened, and then of course diffraction becomes more significant with smaller apertures.

And since most amateurs are on a fairly tight budget, rare are setups that can take a full advantage of small pixels with quality small optics, good mount and excellent dark skies. I feel we more often see images taken with sensors with small pixels that are put at the end of a less than perfect (“mass” produced) optics, less than perfect mount and in less than perfect conditions - city backyards/balconies. It would be a rather rare sight to see an FLI 16803 attached to a budget but large telescope riding on EQ6/8 and set up on a balcony in a city.

So perhaps in many cases effects of diffraction on a small telescope and limitations of small pixels are actually not the main “issues” affecting quality of data?
What say you?

[Merlin66]

The title of the thread......Minimum fl.....
As the focal length increases so to does the size of the seeing disk - larger seeing disks can accommodate larger pixel sizes and still achieve good sampling.
It think it comes back to larger apertures = fainter targets, longer focal lengths = smaller FOV (unless we increase the camera frame size)
For 20 years the f10 SCT almost ruled supreme....add the x0.63 reduce and you have a long focal length solution.

[Atmos]

This is a reasonably complicated discussion as there are so many factors that have to be taken into account.
Mike uses a 12" F/3.8 with a ICX-694 which gives a sampling around 0.82"/pixel. This just so happens to be almost identical to a 12" F/8 with a 16803/11002/6306 which all have 9 micron pixels (I think the 6303 does anyway). Technically its closer to a F/7.5 than F/8 but as a standard RC is F/8 lets roll with that.

Both scopes need correctors of various kinds, both can be built both optically and structurally good, both have the same aperture and image scale (or near enough). Both of these should perform very similar to one another but one has about twice the focal length of the other. Both of these should perform similar to a 12" f/3 with an QSI690 as the image scale remains the same as does the aperture.

In terms of cost effectiveness I would say.
12" Newt + ICX-694
12" RC + 9 micron pixel (choose your poison)
RH300 + ICX-814

The difference between all of these systems them comes down to the quality of the optics and mechanics. GSOs are generally optically pretty good but mechanically leave a bit to be desired. A reasonable newtonian isn't overly expensive and even some of the premium ones are still less than what a comparable RC would cost. Throw in a smaller (cheaper CCD) and CONSIDERABLY cheaper filters, a good newtonian is difficult to beat from a price/performance aspect.

Suavi does bring up an important point, smaller pixels are most definitely more stressing on any optical system.... but likely not in this case. One of the benefits of higher resolution imaging is that your seeing conditions do help mask a lot of problem. Peter Ward a while back made the comment that he thought he had his RC nicely collimated until he had a night of very good seeing which then showed that it was a smidgen off.

A good example of this is with some of the Celestron Edge 14" scopes, some of them have an on-axis performance of something like 20 microns but because they're a 14" f/10, that equates to 1.16" resolution which when using a 11002 sensor still equates of a FWHM of 2 pixels under average seeing. If a small refractor had 20 micron on-axis spot sizes you'd send it back.

The cheapest option would be to get a reasonable 10" F/5 with a RCC Coma Corrector (think I got that right). Teleskop Service make a 10" F/5 ONTC with carbon fibre tube and various mods that they can do pre-shipment.

[peter_4059]

I thought Andy said his budget was $2k? Also worth remembering he has an EQ6. Can't believe people are discussing RH and CDK optics in this context. Perhaps his price was a typo and he meant $20k?

[Slawomir]

I think for under $2k it will have to be a Newtonian. And EQ6 will only carry so much, so I will second Colin - a 10" f/5 should be a one of the best options. I would personally go for 10" f/4 as it would be a bit lighter and easier to guide, but more challenging to collimate. Also, drizzling good quality data can always give that extra detail.

Thank you Colin for clarifying a few things, for me anyway. As for telescope options, would a 140 mm triplet at f/6.5 with ICX814 (0.84"pp) yield similar data to say RH300 or 12" fast Newton, or would diffraction get in the way with a 140mm aperture? I understand that a refractor would be slower with the same camera.

[Merlin66]

What about a s/h 10", 11" SCT with a x0.63 reducer???
Costs may be in the right ball park....

[peter_4059]

That's the path I went down but it isn't as simple as just buying a GSO newt. He will need:
1. a better focuser with a QSI683wsg8 as this is a heavy camera (I've got the 683ws8)
2. a decent coma corrector - at least $300 worth I suspect
3. if it's f4 he'll need good collimation tools (Catseye)

He might need to reinforce the tube where the focuser attaches.

[Peter Ward]

Is this a rhetorical question?

To quote from the book of The Castle: "Tell 'em their dreaming!"

With a 140mm vs 300mm objective you are virtually in the same territory as my previously uploaded link (http://www.atscope.com.au/BRO/tutorials/LensesBigandSmall.html).

The images were not fudged. If you can't see a difference in the roll-over
images......... might be time to see an ophthalmologist :)

[Slawomir]

Im not sure how daylight snapshots of an object a few hundred meters away really compares to long exposure astro photography through the entire atmosphere and on a moving mount :shrug:

I didn't see your comment on the information from the article I provided a few posts down - I would be really interested in reading your opinion on that :thumbsup:

[Peter Ward]

The houses were not a few hundred metres away....several kilometres in fact.

I thought they pretty clearly showed how aperture influences image resolution.
(sampling between the two systems by dumb luck was almost identical)

Deep sky imaging simply has a few more layers of image abberration by looking through a tad more atmosphere, keeping the shutter open for longer and having to pan the camera....none of which reduce the effect of your starting point.

The image simply degrades even further from there. :thumbsup:

[peter_4059]

Just read the article Suavi. Interesting.

[RickS]

Reviewing the Ray-Fan plots and spot diagrams in "Telescopes, Eyepieces Astrographs - Design, Analysis and Performance of Modern Astronomical Optics" I think the argument may not be so much about aperture as scope design. Fast refractors inherently have relatively lower resolution compared to slower, large aperture catadioptric systems (e.g R-C, corrected Dall-Kirkham, corrected Newtonian).

Tiny pixels only produce better resolution if the optics support it and even the amazing FSQ-106ED won't benefit from tiny pixels. Take a look at these spot diagrams which use a 100 micron scale: http://www.takahashi-europe.com/en/FSQ-106ED.optics.spots.htm Measuring generously, at the centre of field you're looking at 10 micron spot size. And that's a high-end refractor/astrograph...

Cheers,
Rick.

[Peter Ward]

I agree :thumbsup::thumbsup::thumbsup:

Leonard Cohen said it way better than me: Hallelujah

[Andy01]

Fyi, I'm following this with great interest and appreciation for all the thoughtful input from you all.
I'm learning heaps from your combined contributions. :thumbsup:
As you know, I'm more creative than technical, so I'm all ears to your input.

My experience and results with refractors appears to have been satisfyingly simple in hindsight, by comparison with say Paul H & Lee's experience with bedding down their fast newts and the issues they have had to patiently work through!

Probably worth mentioning that in my very hands on, manual workflow I use a mac, focus manually (once only, at the start of an imaging run, just set & forget, with parfocal Astrodons that don't seem to shift) spend ages framing my targets for aesthetic reasons, and shoot mainly NB from my suburban Melbourne backyard - with occasional trips to my mate's country property for LRGB stuff.

So I'm kinda hoping for a solution that just works out of the box!

My pimped out hypertuned, belt modded EQ6 was aquired from a fellow iis member who used it to do amazingly sharp 1hr subs with a homemade a 10" f4 CF newt riding on top.

So as I have a young family with 3 lots of school fees, I've no plans to upgrade the mount (or my camera) yet, and that's going to have a significant bearing on the max weight of any proposed ota.

I've also considered & been researching longer refractors, Mak-Newts, CF Newts, Truss Newts, SCT's, hyperstar etc - seems that there are a great many options out there.

So please keep the discussion going- I'm sure that not only myself but many others are benefiting from your combined wisdom :)

Cheers
Andy

[alocky]

The impact of aperture on resolution is pretty old news, the size of the airy disc is determined solely by the aperture of the telescope. This is because the hole in the end of the tube is a boxcar window filtering the wavefront coming towards us. Even if you don't know what a Bessel function is, the fact is, an infinitely wider aperture gives you a perfect spike response to a star, and a small aperture gives you a sort of ringing peak with sidelobes that would be a lot easier to draw than describe.
This leads to another of the frustrating myths - my 4" APO is better than a 25" dob because I can see the airy discs on a good night. The fact is, those discs are more than 6 times bigger in the 4"!
Now of course then you have the impact of central obstruction and all the other fun and games but these are secondary (excuse the pun).
Unfortunately, galaxies are small and faint.
And apart from Mike, we really aren't getting much out of the planetary nebulae yet! That's next challenge.
cheers,
Andrew.

[Merlin66]

Andy,
Yes, but....
""the size of the airy disc is determined solely by the aperture of the telescope""

you should say "the angular size...." the seeing disk still dominates.
If you have say, excellent seeing conditions of 2 arc sec.
The seeing disk (FWHM) would be:
400 fl = 3.9 micron
1250 fl = 12.1 micron
2500 fl = 24.2 micron

Lucky fast frame imaging (ie solar etc.) can effectively freeze the seeing and give much better results.

[Andy01]

Just doing more research, any thoughts on RC's like this?

From Andrew's website...



RC-10" f/8 Carbon fibre astrograph w/M-LRC F/L 2000mm
Newest model with Carbon fibre tube, Super-flat field Now A$2299


RC-10 10" carbon fibre has TWO Losmandy "D" style 3" wide dovetail
plates fitted, 40cm long each; one on top and one underneath.
Includes one 50mm and two 25mm focal extender rings!
Resolution of 0.46 arc-sec. Weighs about 16kg.

We recommend Skywatcher EQ6PRO, EQ6R, AZ-EQ6 or
Celestron CGEM, CGEM-DX & up computerised
equatorial mounts for use with C.F. RC-10".

[peter_4059]

That's a good choice. Definitely get the 10" though.

[Andy01]

Maybe so, but further research suggests that a decent focusser upgrade & reducer/flattener are required as well as a fancy Tak collimator, heaters & dewshield, which I'd guess brings it up well past $3k. Hmmm :question:

[Atmos]

You will definitely want both a flattener and a better focuser otherwise you won't be happy with the results.
The Tak collimating scope may help but you can get along without it.

[rustigsmed]

Perhaps a 10" cf newt instead, investigate using it with a Barlow and a focuser upgrade will be cheaper and probably easier than the rc

[peter_4059]

Maybe true but you avoid the-ho hum 1000mm focal length and once you get the collimation dialed in and CALIBRATED you will be set for any galaxy you like to point it at.

[RickS]

I was pretty happy with mine but I did put an Atlas focuser on it.

CALIBRATED CALIBRATED CALIBRATED CALIBRATED CALIBRATED CALIBRATED CALIBRATED CALIBRATED CALIBRATED CALIBRATED

[Andy01]

Out of curiosity, what's the advantage of the 10" vs. the 8" other than 2000mm vs 1600mm?

[RickS]

100/64 times the light gathering ability for a start. And big scopes are cool :)

[Andy01]

Ok, notwithstanding size does matter lol :lol: if they're both f8 how is it different? (or am I missing something basic here?)

[Octane]

Bigger mirror = more light. :)

H

[rustigsmed]

Very subtle Rick :)
:thumbsup:

[RickS]

The exchange rate on photons has been very favourable lately.



:thanx:

[DJT]

Hi Andy

Currently chasing ARP galaxies with an RC8 and an ST2000 from Sydney close to CBD. Saw your image on AB and hope I can get somewhere close to that!

Yes, get a better focuser, yes, get the Tak collimator, you shouldn't need a dew shield and I rarely put my dew heaters on. You should get Bobs knobs and a tilt plate and also you would probably need counter weights as it's ar.! Heavy.

On point of principle I havnt put a reducer on, I would just lose the focal length I am after and I image " naked", ie, no flattener as for this specific small ccd sensor /scope combination, you dont need it. Even with a larger CCD I just Crop though I do have a flattener that does work well when I get round to setting it up. ( very amused with the constant underlining of the phrase super flat field on the ad.....)

Slow bugger though but a larger mirror like a 10 or 12 is very tempting and am not in a rush.

Even with the upgrades though you are getting good bang for buck over the more expensive RC's without over capitalising. Holds Collimation very well and barely needs refocusing during the night.

If you get hacked off with LRGB, do what SteveC does and chase NB PN's

Gawn, you know you want one ;)

[Andy01]

So to update this again, as much as I like the idea of an RC, f8 or F10 would be impractical for the occasional trips to dark skies. Probably wouldn't get sufficient data for a single image in one night at that speed.

Other considerations ...

Max ota weight under 15kgs to suit my EQ6.
1200mm Fl
F4/F5
Decent focusser for my QSI Wsg8 (2kgs)
Corrector/collimator
CF tube (I have no dome, although that means wind issues too)

Guess that adds up to a Newt (or Mak/Newt), as already advised above by many.:thumbsup:

So what's the difference between Asian manufactured units incl. GSO, Saxon/Skywatcher etc, vs Euro stuff like TS & Orion?

Seriously considering parting with my WO FLT110 so that will free up the budget (a little) as well, so maybe $3k- ish to spend.

[Atmos]

There is surprisingly very little difference.
Saxon/Skywatcher/Orion are all owned by Synta and therefore exactly the same except for their decals.

GSO is its own, has been around for quite some time although big in the western world, pretty sure they provided the optics and components for the Meade Lightbridge series way back when though. We've only started hearing about GSO quite recently but they've been around for a while.

TS is probably the oddball in all this mix. Their baseline reflectors are all GSO but they do allow for customisation on their ONTC series which can either use GSO mirrors or higher quality ones from another German company (used to be Orion Optics UK but their quality control has really gone down the gurgler).
TS get their carbon fibre tubes custom manufactured, not sure how they compare to the likes of GSO or Synta ones. They do use GSO mirror cells but have some custom secondary supports.

Just to clarify, there is Orion which is Synta and then there is Orion Optics UK which is different... KEEP AWAY! That's about all I can say. Although some good scopes do make it out of there, there are more negative than positive reports/reviews of their products. Customer support appears to be next to non-existent.

Also to clarify, the TS newtonian are largely just rebadged GSO but allow for greater customisation. Their customer support is one of the best in the business.

So to actually answer your question, there isn't much of a difference. When it comes to the non-truss newtonian, the TS ONTC line is possibly a bit better quality as I believe (although not entirely sure) it may have a stiffer CF tube but it has the benefit of having them customise it a bit. You can get them to put on a Moonlite or Feathertouch focuser in factory, you can pay a bit extra for a higher quality primary or get stronger secondary supports or a double vein if you like.

[troypiggo]

I think Mike is showing everyone in the deep sky images section of the forum what an Orion Optics UK focal length around 1100mm can do in the right hands.

[Atmos]

As I said, SOME made it out without issues :P
There are a LOT of threads over several forums about issues with Orion Optics UK telescopes that have had primaries and secondaries with really bad figures, mirror cells that cannot hold collimation over 25° movement, focusers missing parts, the list goes on.

When they get it right they can produce an excellent telescope but you are taking a very expensive gamble as to whether you'll have to send it back at your cost for repairs or it'll work out of the box.

[codemonkey]

I bought the TS ONTC because I get the impression that GSO and other cheap newts all require a significant amount of tweaking to overcome flex and various other issues. Whether the ONTC proves better remains to be seen, but I'll probably never know because I've never owned a cheaper newt.

Even within the ONTC line there's differences. Some of them come with GSO mirrors, some of them (more expensive ones) come with a OOUK primary.

The ONTC has a 5-6mm thick foam-core carbon fibre tube said to be much stiffer / better than the tubes found on cheaper newts.

Seems that OOUK used to have a good reputation but it turned to **** after a while, particular with the mechanics of their scopes. Supposedly they still produce good mirrors but QA is not so great. TS test the OOUK mirrors themselves to verify that you don't get a bad one. Mine came with a report detailing 0.99 Strehl, but the report came from OOUK and if you do some research you'll find that their reports seem to be largely worthless.

[Peter Ward]

I helped someone recently after he was struggling with an Orion UK telescope. To cut a long story short the optics were rubbish, and Orion blamed the customer! Would not touch them with a barge pole. Worst business morals I’ve ever experienced. GSO while not an A list product, at least will replace a dud at no expense to the customer and kudos to them for that.

[rustigsmed]

I'd go with the skywatcher 10" cf or TS carbon fibre. get a feathertouch or moonlite, good coma corrector and investigate barlows if you wanted the extra focal length.

as for the cheap options the build quality of my 12" skywatcher is way better than my 12" GSO (steel tube however).

didn't know that OOUK went downhill thats interesting.

[DJT]

I remembered this article from way back which has a great nomogram showing the relationship between pixel size, focal length and image scale.

Interesting article as well.

http://www.skyandtelescope.com/astronomy-resources/astrophotography-tips/of-pixel-size-and-focal-reducers/

[gregbradley]

I generally operate off of sample images I like and what was the gear used. A very objective empirical approach rather than theoretical.

Which images do you like that were taken by gear you can afford is the million dollar question?

In your example 10 inch carbon fibre RC. I have seen many excellent images from these scopes.

Generally, smaller sensors are more forgiving of the optics. Large sensors start testing the correction of the optics and the strength and lack of flex of the focuser and frame of the scope.

Avoid closed tubes due to thermal currents if you can. Or at least make sure that have fans.

My Honders has a 14 inch tube and the optics are 12 inch plus it has fans as well as removable back cooling plates. The point being great care has been taken to manage thermal tube currents. Keep that in mind when selecting an RC type scope.

Greg.

[SimmoW]

Yes this is a really useful thread, dare I say 'award winning! 😁

I've heard from one technical expert how bad Orion Optics can be, having to recoat a new expensive mirror! And another industry supplier saying pretty disappointing things about their tactics. Just rumour of course, but enough for me to be glad I've never used them.

At these longer focal lengths, once off focusing at start is not possible. Temp changes cause the focus point to vary too much

[Slawomir]

Earth's atmosphere is nearly 500 km thick ;) ( even if most of it is within less than 20 km of the surface. )

[HenryNZ]

This is a very interesting thread. Thank you.
May I ask for some suggestions please? I have been imaging with a 6" RC reduced to about 1000 mm with a reducer. With my atik 460 the pixel scale is about right for my seeing (0.9 arc sec per px). However I have recently acquired a refractor which has only slightly shorter focal length so I feel that it is now redundant and I would like to take this opportunity to upgrade it to something with longer FL so I can image galaxies that are even smaller. I am thinking about a 8" RC with a flattener will give 1600mm at 0.6 arc sec per px, or shall i jump straight to a 10" for 0.46 arc sec per px? Would that be way too oversampled? I guess with the 10" I have the option to use a reducer to bring it to 1600 mm focal length but at a faster speed, but the jump in cost is quite substantial esp as the 8" carbon tube is on special at the moment.

Ps. I don't like imaging with my edgehd because the few times I tried it it dewed up so quickly and mirror flop was a bit of a pain. It will cost me almost as much to upgrade the focuser to something decent than to buy a new 8" Rc OTA and move the moonlite from my 6" to 8". I will likely keep it for visual or planetary only or to sell it.

[Peter Ward]

Having spent the last 30 years operating airliners in the tropopause I’m fairly familiar with how density decreases with altitude ;)

(interestingly, if a turbofan could still produce the same thrust at altitude as it does as sea level, there would be enough thrust there to get to orbit)

I digress, adding extra image abberation in by pointing an optical system vertically doesn’t change the system’s intrinsic resolution. It all still goes south from the starting datum.....

[Merlin66]

I'm sorry.... I seem to be losing the plot here...
The resolution obtained in long exposure images is limited by the seeing conditions, not the theoretical optical resolution of the optical system.
Over sampling I think is generally better than undersampling..

[Peter Ward]

It’s not that simple.

Seeing affects smaller apertures differently to larger ones.

While the larger aperture typically presents an image that fuzzes in and out, it remains positionally stable on the sensor.

The smaller aperture images presents less perturbed airy disk, but it is positionally unstable.

Seeing clearly varies from night to night....but to assume 2 arc sec is as good as it gets is nonsense. Some nights fleetingly present sub arc second seeing.

Having the aperture to gather signal quickly in those conditions is simply another reason as to why aperture rules :)

[Merlin66]

Peter,
I'd be surprised that those " fleeting moments" can actually impact on a long exposure image....lucky fast frame imaging is a different issue.

[Slawomir]

For the record, I'm converted and saving for the biggest telescope my current mount can handle. One beautiful day, it will be either a fast 10" Newton or a 10" RC.

[Peter Ward]

A big decending high pressure cell (the kind that usually causes fog in the mornings) gives hours of marvellous seeing. I am surprised you do not seem to have experienced these conditions during a run of imaging sessions. :shrug:

[Merlin66]

My practical experience is in holding a target star image for subs of >10 min with a C11 at f10 on a slit gap of 20 micron - this is less than the average FWHM I experience.....

[Peter Ward]

What can I cay? You definitely need bigger+ faster scope :)

[Merlin66]

I agree with the larger aperture, but slit spectroscopes need f10. They don't work effectively at faster f ratios...

[Peter Ward]

Really? I used a SBIG self guiding spectrograph (18 micron slit) for about 12 months on a 14” F8 system. The spectrograph was somewhat immune to F ratio, (worked well at F6) but I found dispersion from the grating made aperture my friend.

TOTALLY off topic BTW.....

[Merlin66]

Just to finish the OT discussion...
SBiG no longer manufacture spectrographs...
The current commercial Littrow instruments (Shelyak) are optimized for f10.
The point was regards the FWHM size of the target star image in long focal length instrument.

[gregbradley]

My CDK17 has a carbon fibre truss. Once the mirrors have equalised in temperature its focus does not shift hardly at all. In fact the next night it will still be almost exactly in focus much like RCOS used to promote with their scopes. Still in focus the next night.

So focus shift may be true of some scopes but not the CDK or RCOS in my experience. I still check and refocus after a substantial temp shift and I can get a slight improvement but its a small gain. An FSQ shifts the most of any scope I have used. The AP Honders shifts a bit as well but not that much and it seems to be fairly predictable and temp compensation of Sky X seems to keep up with it.

Greg.

[Slawomir]

Greg's experience with telescopes confirms what theory predicts. From what I understand, it is easier to focus larger apertures and at slower focal ratios. The most difficult telescopes to focus precisely are fast telescopes with small apertures.

My 105mm CFF definitely requires refocusing every 1 degree Celsius at f/4.5, ideally perhaps every 0.5 degree to maintain perfect focus. So it is best to use temperature compensation at that focal ratio and with this telescope, instead of frequent refocusing.

At native f/6 the same telescope needs refocusing every 1.5 degree change in ambient temperature.

I'm yet to test whether larger aperture telescope also has a larger critical focus zone...

[Atmos]

The size of the CFZ is determined by the focal radio as it is the steepness of the light curve. The size of the pixels also has to be taken into account as this is the area in which the CFZ occupies.

When you have a larger slower telescope you typically have larger pixels and at higher resolution you are never diffraction limited. Take what Mike and Trish have, a 20” CDK with a 16803. When the seeing is less than perfect the CFZ aid basically made larger via atmospheric blurring.

4-5” F/5 refractors really struggle because their CFZ is the determining factor of perfect focus where as a CDK can largely be determined by the seeing conditions.

[Slawomir]

Yes, I forgot to mention the size of pixels also needs to be taken into account. Small pixels in a smaller sensor are not necessarily easier to handle...

[Camelopardalis]

I find my motor focuser plots a nice V curve - the beauty of a focuser with oodles of steps :D

More aperture would be great, but I’m not ready to give up the relative simplicity of a refractor...

[Merlin66]

As Colin says, The CFZ is directly related to the f ratio.
The Airy Disk diameter is:
2.44*lambda*F/D (micron) where F is focal length, D is aperture and lambda the wavelength of the light being studied.

A 200mm f8 telescope gives a diffraction disk of 11 micron, a 400mm f4 it would be 5.5 micron. To catch the "lucky seeing" the CCD pixel size needs to be 1/2 to 1/3 the diffraction disk.

To maintain the Raleigh limit (1/4 wave), the CFZ is = 4 *lambda*f ratio^2 micron. Based on green light 550nm examples:
f15 gives 495 micron
f10 gives 220 micron
f8 gives 141 micron
f5 gives 55 micron
(The tolerance at UV wavelengths is 1/2 that of Ha)
- the average human hair, for comparison is 70 micron!)

[ericwbenson]

There is IMO an improvement on the CFZ equation that takes into account the seeing component, see here:
http://www.goldastro.com/goldfocus/ncfz.php
I can vouch for the logic, it correlates with what I have observed with a C11 and a CDK20.

EB

[Merlin66]

For imaging, yes the seeing is King!
The original CFZ as is mentioned, was for visual and Airy Disks.
This new version just acknowledges the reality of seeing conditions.

[Shiraz]

another 5 cents worth Andy - most of which has already been said.

You don't need a big scope. The linked composite image shows M83 taken with scopes from 8.2mVLT aperture down to Codemonkey Lee's 0.12m Chinese refractor. The big scopes do better, but the VLT clearly does not produce 5000x more information than Lee' scope - the differences in detail can reasonably be explained by the seeing - 0.7arcsec on average for the VLT and maybe around 2 for Mt Lemmon and Lee's system. And of course, nothing comes remotely close to Hubble with no atmosphere. http://astrob.in/314502/0/ .

ie, the atmosphere is the primary resolution bottleneck for scopes above about 0.1m aperture (unobstructed) and going bigger may increase sensitivity, but does not get sharper results in typical Australian conditions. A central obstruction changes things a bit, so you could aim for something around 200mm as a minimum aperture.

The focal length should be long enough for the pixels to extract all of the detail left in the optical image, after it is blurred by the atmosphere - with your CCD, about 1.5m will do the job in 1.5-2 arcsec seeing. You could go longer, but would not get significantly better detail - and you would need to spend a longer time to get an image (eg going from 1.5m to 3m would increase the imaging time by a factor of 4x).

Given that you might want something with >100 mm clear aperture (~200mm+ if obstructed) and 1.5m fl, that will work on an EQ6 - you could consider:
- an 8 inch f8 RC with field flattener (will take a long time, but give best resolution)
- a vastly expensive APO refractor

You could consider a shorter scope and use drizzle to recover some of the detail lost in sampling. Then suitable scopes would include:
- an 8 inch f5 Newtonian with coma corrector
- a 10 inch f4 CF Newtonian with CC (on the weight limit for the EQ6)
- a 7 inch Mak Newt (also a bit heavy)

Whatever you get will need a good digital focuser - my CF f4 Newtonian needs refocusing (by SGP) about every 10 minutes in typical conditions. The Newtonians will need at least a 3 element coma corrector and the Skywatcher (a bit better mechanically at this size) or GSO tubes will require stiffening around the focuser to hold your heavy camera - some sections cut from a cheapo tube ring can be glued and screwed to the OTA to do this.

I reckon that a 200mm Skywatcher steel tube f5 Newtonian (drizzled) with the Skywatcher CC and Moonlite stepper focuser should do the job and be close to your budget. The Skywatcher optics should be quite good enough out of the box. Sounds like a fun project.

Cheers Ray

[SimmoW]

wow that's great useful advice Ray, thanks for posting.

[Andy01]

+1 above - thanks Ray, that's an excellent summary - appreciate the time you took to post this (I'm a big fan of your galaxy work!) :thanx:



Yep, was heading down that path, but missed out on one in the classifieds :sadeyes: still, they're not too expensive new at around $1k, and 1200mm is great, but f8 is VERY slow, maybe even too slow to get sufficient data in one night at a dark site (I don't often get to one, so I would want to make the trip worthwhile)



I really do like refractors, but a quality 120/130mm +/- 900mm FL, well - yes big $$$



Yes, probably the most sensible, logical solution :) Although cumbersome and prone to wind issues



I'm very curious about these. :question: Around $2.2k for the MN190 or there's a Russian one for sale under $2k in the classifieds, f6 too - hmmm no diffraction spikes either ...



MMM, this is new territory for me - I typically set & forget focus at the start of an imaging run with my two refractors. Motor focus will require imaging with a PC (I currently use a mac for everything, I've never used a PC :rolleyes:)

But you certainly have given yet more food for though, many thanks indeed :D

[codemonkey]

Probably a lot variables in that, mechanical and environmental. One guy who uses a TS ONTC newt says he hardly even needs to refocus during the night.

IMO you can get some decent results using a moderate sized refractor with small pixels... it's certainly been my approach and, not to blow my own trumpet, but I think my images hold up well to much larger, much harder to manage and potentially much more expensive scopes, even if it is well within the "yawn" zone as some might put it.

I've been eyeing off the ASi 178mm-Cool as well. Very small sensor, but only 2.4 micron pixels that I think will do well with smaller galaxies on moderate sized 'fracs; it's relatively cheap too and because of the small sensor size it'd be less demanding on optics.

[strongmanmike]

And They do!...sad really, the elitist hubris is quite laughable..?.. :lol:

I agree, your images are great and I certainly dont yawn when looking at them....

Mike

[Slawomir]

Thank you for sharing your experience Lee - your images certainly are great and inspire me and many others :thumbsup:

Galaxy imaging is something I would like to do in the future, so this discussion is of great interest to me.

I put together a simple excel spreadsheet in an attempt to roughly estimate which system would be optimal for galaxy imaging with the seeing we have in coastal Australia.

It looks like my camera's small pixels are always limited by any smallish optics slower than f/6 until we get to 140 mm aperture and above, when seeing will become limiting factor. So for my current camera, either a 250 mm f/4ish Newton or 140 mm f/6ish refractor would do for galaxy imaging, with Newton being significantly faster but would require collimating.

Those two might be my options since I feel that in the future we will see more new sensors with smaller pixels that will most likely will be more affordable than sensors with large pixels.

[DJT]

Just on this, I have been chasing ARP14, a tiny little galaxy, from Sydney with an RC8 (1628mm, not 1200 per your earlier post) and an older ST2000XM with a 7.2 pixel size. Its doable but needs patience. This was after 6 hours and I need to get more colour data so definitely a WIP, noisy little devil.

ARP14 (NGC7314) A Seyfert Galaxy in Aquarius (http://astrob.in/full/314150/0/)

No Crop here, it is as presented.

But this would be a struggle if the plan is to do single nights at a dark site. It may work because you get all those dark sky benefits but given that the usual rules apply (alignment issues, chasing collimation in the field, etc,) it might be a struggle, unless you have your setup process nailed completely.

Theres comments about refocussing but these things can hold focus and collimation all night so that's a plus.

Good luck with the search. I have to say, hunting down the unusual galaxies can be addictive ;)

[peter_4059]

Just wondering if there is any trade off in small pixel size? Does it change anything other than arc seconds per pixel?

[HenryNZ]

Longer focal length instruments suitable for imaging get expensive quickly. Wouldn't it be nice if we can just have one OTA in the medium focal length range and just change the cameras to suit the targets. I guess with smaller pixels the area covered by each pixel becomes smaller rendering each pixel less sensitive. My question is, what is the current sweet spot in terms of smallest pixel size that will still have sufficient sensitivity? I am really tempted by the QHY183 (or the ASI equivalent) as it has 2.4 micron pixels and reasonable FOV at 6MP.

[Slawomir]

Shallower wells but also usually less read noise. However, in spite of having lower read noise, dynamic range is generally lower with smaller pixels, so more shorter exposures are needed if we want to control saturation of stars. The biggest advantage of small pixels on a small chip for me is substantially lower cost of the entire imaging apparatus.

As I understand, 2.4 micron pixels are in most cases best matched with fast telescopes (f/5 and faster).

[HenryNZ]

Can the problem with lower dynamic range be mitigated by HDR techniques?

[peter_4059]

That's what I suspected. I guess the trade off then is in image processing time?

Is it possible to get the same depth in an image with many short exposures vs fewer longer ones?

[peter_4059]

Suavi,

Can you explain the rational for your spreadsheet? Are the numbers arc seconds per pixel? What are your criteria for optimal sampling and seeing limited?

I get 0.97 aspp with my QSI683 (5.4 micron pixels)/Paracorr (1.15 focal length multiplier)/SN10 (10" f4) combination.

[Camelopardalis]

I’d agree with a seeing-limited approach to pixel/scope choice, but with newer technology the smaller pixels aren’t always at a substantial advantage.

For example, the well depth of the IMX178 is about 15000 whereas the ICX694 is around 19000. Not only does the 178 have lower read noise, but with the use of BSI it has a reportedly higher QE too. With a 14-bit ADC the 178 isn’t at any significant disadvantage either.

[Camelopardalis]

1) processing time - depends on the capabilities of your computer. Registration and Integration is multithreaded in PixInsight now, so a modest quad core and a solid state drive works wonders

2) depth - so long as your images are sky limited, stacking many short images should still amount to a similar numerical result. If a photon arrives from a faint part of an object every 10 minutes and you take 10 minute subs, over the course of an hour you collect 6 photons. With 1 minute subs you still collect 6 photons in the same time...

[Peter Ward]

Wouldn't many of the one minute exposures have no signal at all? Hence you collecting noise rather than signal.

Also, if the camera is downloading, that lone photon might be missed entirely....as opposed to the camera with its shutter being open most of the time.

[Camelopardalis]

1) Many of the exposures would indeed record no photon, only background sky. However, since the SNR increases with the number of subs, the statistics quickly discerns between noise and faint signal

2) Given the random nature of photon arrival, over the course of the 10 minutes the window of opportunity to miss the photon is longer with many shorter subs but over the course of the hour it is improbable (although not impossible) that the photon arrives while the camera is downloading on each occasion, and thus the stack still records a statistically significant photon

[Peter Ward]

So I'll play devil's advocate here. Draw this to an extreme...say millisecond exposures. Still think we'll be collecting signal..if any...as quickly as the open shutter case?

[Merlin66]

Peter,
The statistics of the "Shot noise" probability says yes... some of those very short exposures will record a photon.
Over an extended period (ie leaving the shutter open) the probability of collecting the same number of photons is the same.

[peter_4059]

If the shutter is open for 1 min and the download time is 8 seconds then the percent of time capturing is 88%. If the shutter is open for 10 mins and the download time is still 8 seconds then the percent of time capturing increases to 99%. I'd have thought the probability of collecting the elusive photon must be higher if the shutter is open a larger percentage of the time?

[Camelopardalis]

Actually this is where it gets a little more interesting...the “traditional” long exposure paradigm = software gets in the way here, as it doesn’t make any sense to take millisecond exposures with conventional software, working through an ASCOM driver which takes an exposure, downloads, then gives the OK signal for the software to initiate the next sub.

These devilish CMOS sensors come equipped with video capabilities, since many of them were designed for use in security cameras and driver aids in cars requiring a constant video stream.

So using the appropriate tool such as SharpCap you capture the entire video stream, no shutter involved...

[Peter Ward]

Yep, I get that bit...it's all the other camera noise sources going to zero (thermal, read, bulk layer , etc.etc. ) and not swamping the signal I am having trouble getting my head around.

[Camelopardalis]

Correct, but as the nature of the photon arrival is random, on average you’re only disadvantaged 1/8 of the time...

But also see shutterless software use above. More CMOS chips are rolling shutter than global shutter, so unless the photon strikes the pixel at the instant it is being read, then there is no loss taking place.

[Atmos]

I’m afraid everyone seems to be missing the point, we’re discussing integration time, not time under the stars. One hour worth of subs is going to capture one hours worth of photons. Whether that hour is taken in single 10 minute subs over five nights or a single hour in one night makes no statistical difference as photon capture is a random event.

[Slawomir]

Yes, arcseconds per pixel.

Criteria? Pretty much got them out of thin air :rofl:, but yes, I believe, perhaps naively, that sampling for galaxy imaging between 1" pp and above 0.6" pp should in a costal location yield comparable and optimally sampled data, as long as aperture will not limit the resolution and guiding will be spot on. Going below 0.6-0.7"pp for deep sky imaging in a typical coastal location in AU is a bit pointless, I presume :question:

[peter_4059]

I'm discussing the pros and cons of small pixels and long vs short subs...not sure what you are discussing?

[Atmos]

Below

[Shiraz]

Hi again Andy.

apart from resolution, sensitivity also varies with aperture. The attached graphs show how resolution and relative sensitivity vary with aperture for practical systems around 0.75arcsec sampling.

resolution flattens out pretty well by 200mm (within a few % of best possible), but the sensitivity increases with aperture. ie, set your focal length to 1.5m to match your pixels and then get the fastest scope that your mount will carry.

Although small scopes with small pixels will get similarly resolved results to larger scopes, they will take a lot longer to get there. Chips with small pixels do not make large scopes obsolete - they do however bring high resolution imaging within the reach of small scope/mount owners.

cheers Ray

[Andy01]

Yes your images are inspirational Lee, amazing results :thumbsup:



+1^ :prey2:



Yes, good arguments for the CF RC8 ;)



Thanks again Ray - as mentioned above I'm leaning towards a carbon fibre RC8 to suit my EQ6/5.4m pixels combo.
1625mmFL, 7Kgs, short OTA (less wind issues than a newt at my exposed location), good bang for the buck but will need collimation & upgraded focusser.



Agreed, a bit slow at f8 but the short OTA, longer FL & best resolution are attractive.
Maybe more so than a shorter FL, longer OTA 8" f4 CF Newt, and anything 10" is too heavy, as is the MN190 MakNewt.

I'd like to one day get decent A4 size printable images to enter in print competitions, so this is looking like a good entry point to start working towards that goal. :D

Thanks again to all for your input, glad several other IIS'ers have got something out of this thread as well.:thumbsup:

[rally]

The OP's question was about the minimum focal length required and the discussion has wandered around this and its been interesting, but I cant help but think that the FL he needs is more likely constrained by the FL that the rest of his system can support.

Anyone who has imaged at long FL has discovered how much more exponentially worse all the problems become.

Things that werent even considerations become significant problems and obstacles.

Talking about 1 arc sec seeing or sub arc sec image scale becomes a moot point if the rest of the noise in your system is being measured in many 10's of arc secs or even arc minutes !

eg your mount has more Periodic error than a few arc secs and there is no PEC, if mount backlash exists and is a problem, if mount capacity is at its limit, mount's ability to hold stability with light breeze etc, if polar alignment is not perfect - eg under say 10 arc secs, if tracking is limited or if there is no tracking and pointing correction for example, if your guiding errors are more than an arc sec, if you can't hold focus across an imaging session, if guiding flexure or mirror flop is occurring . . . its a long list.

You simply cannot capture the theoretical image detail in your subs if the mechanical noise in your system exceeds the telecope spot size, image scale or the seeing.
If that noise greatly exceeds the theoretical limits then its a failed exercise.

So IMO the answer needs to at least consider as much about what the OP's existing system is capable of yielding, since the budget is only $2000 it doesnt allow for any other upgrades other than OTA.

So for example
What is the existing PE and backlash ?
What is a typical PA for this system and the methods used
What is the tracking and guiding providing now at low FL and how will that be affected by increasing FL etc.
The choice of camera and sub length is relevant too - if the minimum sky noise and read noise limited sub length is sufficiently long but the mounts ability to track accurately or remain focussed for that length of time is compromised then its going to be difficult to obtain good results.

Imaging at longer FL is a whole new ball game, the status quo is not the same as imaging at a forgiving 770mm on a small light weight scope at an image scale of around 1.5 as/p

Its all doable and encouragement given, but knowing what you are up against is an important consideration.
I've seen EQ6's (and I see this one is belt modded) that had well over 1 arc minute of mechanical error - backlash and PE
Even after hypertuning, relubing, swarf removal and bearing upgrades the errors were still in the 10's of arc secs.

My 2c worth

Cheers
Rally

[Merlin66]

Rally,
Back, as you say to the original question....
I'm glad to see that seeing conditions are mentioned.
I semi-regularly (weather limited) use the C11 on the NEQ6pro at f10 (2800mm fl) with a reflective slit plate guiding (similar to an on axis OAG) and Lodestar.
My target is to keep the FWHM star image central on a 20 micron slit for at least subs of 10 mins. At "good" seeing of 2 arc sec the FWHM is 27 micron.
Using PHD2 (or the AA6 guiding) this seems doable.
Just my 2c

[gregbradley]

A large CDK scope can be very difficult to focus in poor seeing. So much so that its really a waste of time trying to image with that sort of long focal length scope in poor seeing.

I have had it where it seems in focus you take another focus shot and the seeing pushes it out of focus, you adjust and all you are doing is chasing the seeing and its all over the place and unpredictable.

Temperature compensation is a good tool but its somewhat imperfect because it assumes a straight line response between temperature and focus and I don't know that it is. Its not a bad approximation but I imagine its not as accurate as continuous autofocus or refocusing at some regular interval which matches your scopes focus shift with temperature.

For example I have noticed my Honders will shift focus noticeably after an hour of thermal equalizing and then the differences become much less after its settled.

The carbon fibre scopes so far have been very forgiving of temperature focus shift.

Some scopes have built in adjusters to match the focus shift. Roland Christen has said aluminium tubes focus shift matches reasonably closely the shift in the lens focal point so it tends to cancel out.

Greg.

[luka]

Andy, just wondering if you made a decision yet? Also, not trying to advertise anybody in particular but I saw that Andrews Communications has 10% off sale right now which may be of interest.

Wonderful and very informative thread!!!

[Andy01]

Hi Luka,
Yes- after all the amazing, informative advice offered here so freely and generously I’ve ordered a GSO CF RC8 from Andrews.
It ticked most of the boxes for me.
ie: Budget, FL, Weight, OTA length, my camera pixel size etc. maybe a tad slow but my SFFR70APO flattener/reducer mates with it so that will help speed it up & reduce the FL a bit.
(Another Astrobin user has this same combo working well too)
Hoping it arrives soon so I start get it collimated/shakedown/sorted before the star party at VicSouth next weekend!
Then as Rally rightly mentioned, I’ll need to review pec & pa procedure to get the most from my mount at this FL.... and when Santa comes, upgrade the focusser to a Feathertouch.
Once again, thanks to all who have contributed to this thread, much appreciated!
Cheers :thumbsup:
Andy

[gregbradley]

Good choice Andy.

Looking forward to your images with this new rig. A whole new world should open up for you with this setup.

Greg.