Detail vs Long Focal Length

[Emuhead]

Hi all,

I'm interested in going deeper into space. I have been imaging at 800mm f/4 with my newt and very happy with the results so far, but long story short I'm looking to take the next step closer and try for something in the 1400 - 2400mm focal length range. Have an EQ6R Pro mount, 20kg payload and like to keep it under 13kg. Also whilst I'd love a CDK, or Dream Astrograph (or even an EQ8R mount with a 1200mm f4 newt) I'm not quite ready for that financial leap. So have been looking at GSO RC8 & Edge HD 8 & 9.25.

What's imaging at f7, f8, f10 like in practical terms. I know that f7 is twice as fast as f10 for example but how does that translate into the subs. Does an f7 sub appear twice as bright as an f10 sub?

My other question is which scope of those, and please suggest anything else I might have missed, would give the best image quality? I have been spending some time on Astrobin and looking at a selection of images from these scopes and they somehow seem less detailed & more blurry than id be happy with. To clarify it's almost like using digital zoom on a photo rather than optical zoom, it's like the target has increased in size but not increased in the same relative amount of detail as you might expect.

Is detail at that aperture and weight possible at long focal lengths or am I looking for something that is beyond what is technically achievable? As an amateur astrophotographer without limitless funds (*should have bought that Bitcoin), should I be realistic and just be content with detail at 1200mm or less?

Would appreciate people's thoughts on this.

[multiweb]

Aperture will give you more light gathering and resolution. Focal length only gives you magnification.

[alpal]

Yes you should.
The detail is decided by the aperture but
is in practical terms controlled by "the seeing".
Your 200mm aperture is giving you a Rayleigh Limit of .69 arc seconds.
https://astronomy.tools/calculators/telescope_capabilities
I know that in Melbourne the best seeing I ever measured -
by using the FWHM of stars was 2.8 arc seconds.
That is well short of the 0.69 arc seconds capability of your scope.
So - we are limited by seeing not by focal length.


cheers
Allan

[Emuhead]

Appreciate the responses.

The penny dropped finally on the seeing limit, that FWHM example really helped. So in Melbourne, even if i could get down to 2 arc seconds i would only need a fairly small aperture (70mm) to max out the detail level. Then it's down to f-ratio to determine the magnification & brightness of that image (given same sub exposure time).

So anything greater than about 70mm of aperture isn't providing more detail, it's simply allowing for a brighter & more magnified image potentially.

Or have i missed a few things? Just trying to find a scope that will give me highly detailed up-close images that'll suit my mount.

[Andy01]

For what my 2c is worth, I had an RC8 (FL 1500mm) and it was challenging to collimate, and fairly slow at f8. Paul (Discoduck) on the other hand gets consistently spectacular images from dark sites with his finely tuned RC8.

My former f4 10" newt on the other hand was a delightful light bucket that really sucked in those photons! With the paracoor the FL was 1148mm.
Contributors here on IIS like Lee Borsboom have done spectacular galaxy images with a beast such as this, but under darker skies.

Kevin (Kosborn) uses a 10" f5 Newt very well on an EQ6 too, his FL is a tad longer at 1270mm.

So yes, seeing is a big factor, and a road trip to LMDSS can help with that! :)

[alpal]

There is also the quality of the telescope to consider.
A Newt. with a small diagonal on its camera sensor
will work well but for a large frame camera - the small
spot size of say a Ritchey–Chrétien
will give smaller star sizes at the edges of the frame.
Refractors have a better contrast ratio but ones that have a high Strehl ratio
are very expensive and they don't have the light collecting power of a large Newt.


You also need to consider the application of Drizzle in stacking
and that will depend on your arc second per pixel count.
http://www.iceinspace.com.au/forum/showthread.php?t=158389


and here:
http://www.wilmslowastro.com/software/formulae.htm




cheers
Allan

[Startrek]

I use an 8” f5 newt with focal length of 1000mm
With the right camera , good conditions , and some creative processing you can image fairly “deep” without going to the next step with long focal lengths , expensive mounts , expensive scopes etc... plus a requirement to guide a lot tighter than you would normal be comfortable with
Here’s some “Zoomed in images” of the outer arm of Carina nebula , Rosette nebula , Centaurus A Galaxy and Sombrero Galaxy showing a fair bit of detail and resolution at 1000mm focal length ( images were cropped a bit to “zoom in” close )
My imaging camera ZWOASI2600MC
Subs no longer than 3 minute
Nebula imaged with L Extreme filter under full moon
Galaxies imaged with no filter during New moon ( dark )
Hope this demonstrates what is achievable at 1000mm FL
Cheers

[Emuhead]

Thanks all for the helpful & informative responses with calculators & examples too. Much appreciated. I've only now got more questions though. :)

Andy I checked out both of the astrophotographers noted and their work is very impressive indeed. I don't think I'll have the patience that Discoduck has though (50hrs), and I'm not game like Kosborne to go a 10" mirror on an eq6r pro, at least not yet while im still playing nice with the mount.

Where does that leave me in terms of going deeper and getting the most detail possible.. 8" f5, or maybe even the elongated 8" f6, which is where the next question comes in..

3 scopes:
8" f4, 8" f5, 8" f6.
Given aperture determines resolution.. do they all provide the same level of detail ultimately? That is assuming the sensors pixel scale stays between 1 & 2 arc sec/pixel via binning & drizzling where needed, and just imaging longer on the slow scope vs the fast scope to ensure the overall SNR remains consistent across all 3. If I did all that, once in post if i cropped all 3 images to the same FOV would they all basically look identical or would there be more fine details on the f6 vs f4 image?

(Last question: Or based on actual experience with scope types, do refractors arguably provide the sharpest detail available and should I head in that direction..?)

[Startrek]

Some more information for your perusal

My example images were 3 minute subs , maximum 2 to 2.5 hours of integration on each object
No darks needed
Never take flats or bias
No camera binning, only software binned in Startools at 50%
My mount is an EQ6-R pro
PHD2 Guiding around 0.70 to 0.80 total rms ( Dec 0.45 to 0.50 and Ra 0.60 to 0.65 )
My image scale is around 0.80 arc sec per pixel
Cheers

[alpal]

I think you need to read it again:

I am using a KAF8300 sensor.
It has 5.4 um pixels.

My arc-sec/pixel = 1.11
http://www.wilmslowastro.com/softwar...m#ARCSEC_PIXEL (http://www.wilmslowastro.com/software/formulae.htm#ARCSEC_PIXEL)


Let's round that off to 1 arc second per pixel.
If my best seeing is 3 arc seconds per pixel then

that 1/3rd of the best seeing is about right in terms of Nyquist sampling.
see here again:
http://www.iceinspace.com.au/forum/showthread.php?t=158389&page=2


Nyquist says minimum sampling is a factor 2 ( but 3 is better. )
https://astronomy.tools/calculators/ccd_suitability

OK so - 10" f4 or f5 or f6 won't make any real difference to the detail
give that the pixel size is matched correctly as per the above.
A 10" f4 will work better for imaging as it's a faster scope.
Speed is important otherwise dim areas will be very noisy
unless you have large numbers of stacked frames.
Stacking with 3 x drizzle will improve everything but
not many people bother doing it.
It takes longer and is harder to do.



cheers
Allan

[Atmos]

This really is a bit of a minefield. One thing worth mentioning is that there can be some incredible seeing in Melbourne, I'm talking top shelf stuff. Take this image of 47 Tuc.
Full Resolution (https://cdn.astrobin.com/images/14547/2020/fda832f3-fb19-4a4c-89e1-d2b5db24e3b4.jpg)

It has been taken with a Mewlon 250 at F/10 with an ASI094 which results in 0.4"/pixel. This is about 40 minutes of 120s exposures, the final stack has a FWHM of ~1.4" with the best single exposure being closer to 1.1" seeing. That's about 3-4 pixels at 0.4"/pixel. Now 0.4"/pixel is a bit over the top but the best single sub I've managed from Melbourne as been 0.96" so that's a fraction under 1" seeing from my suburban backyard.

Don't get me wrong, I've had terrible nights of 4" seeing but my average is around 1.5-2".

[alpal]

Hi Colin,
I've never heard of seeing that good in Melbourne.
I think the best I ever measured from any single frame
was 2.8 arc seconds FWHM in Ha.

I have never measured a final stack - only individual frames.
Are you sure you had the correct information loaded
into Maxim DL or whatever you were using to make the measurement?
On some nights the seeing was as bad as 6 arc seconds FWHM.

The best seeing I've ever heard of in Melbourne
before tonight was at Mt Macedon -
close to Melbourne -
FWHM of 2.2 arc seconds.


cheers
Allan

[DiscoDuck]

Very kind words, Andy. Just a note that I use a focal reducer on my RC8, so run it at 1200mm (f6). At that focal length, my camera gives me 0.65" per pixel. I'd suggest you want to ideally get at least 2 * sqrt(2) pixels across your seeing distance. So 2" of seeing would need 0.7".

The RC8 was a serious challenge to collimate. I used a combination of a few techniques, but none of them I think guaranteed to converge! So some luck involved still!:)



Re the patience, LOL yeah that is needed for that. But that is for imaging from the suburbs - for which I need at least 5x more subs in my light polluted skies. From dark sky sites, 10-15 hours or so on an image seems to provide a good set of data - depending on the object of course.

[Emuhead]

Labouring this point.. so an 8" mirror can only ever give the same resolution, regardless of whether it's an f4 newt, or an f8 RC, or an f10 SCT, or any of the above with reducers or paracorrs?

The magnified result (determined by the f ratio) will only just increase the size (much like an eyepiece does) without providing any more or any less detail, so it wouldn't benefit me to look at any other 8" scope & expect more detail (unless I've been incorrectly sampling but with 3.8 micron pixels (around 1 arcsec/pixel) i think there's not much to be gained by going with smaller pixels) so i could consider the case closed. :)

Thanks all

[alpal]

3.8 um pixels will be slightly oversampling but that's OK.
It all boils down to what kind of seeing you'll get in Melbourne -
it's normally 3 to 6 arc seconds -

or consider some other place if you try imaging there.
Given that Mount Macedon can sometimes give
2.2 arc seconds of seeing then the longer focal length scopes would
give you some more detail or resolution.


Using a 10" f4 Newt. -

some of my examples from Melbourne are here:
https://www.flickr.com/photos/24719437@N03/31782635104/in/photostream


and a crop using 3 x drizzle
https://www.flickr.com/photos/24719437@N03/32759728116/in/photostream




cheers
Allan

[Atmos]

I make my decision as to whether I continue imaging early on in the night depending on how the auto focus is coming along but also how MaximDL deems the FWHM. When I do my proper measurements though it's all done in PI and done manually where I select at least a dozen stars. I know my pixel scale (0.4") and if a bunch of stars measure 3 pixels then I know I have 1.2" seeing.

Here are three crops. NGC 288 measures at 1.48" first off. Next is NGC 6752, a forgotten globular near NGC 253, it measures down to 1.42". Lastly is a crop of Eta Carina, actually cannot remember exactly what it was but it wasn't as low as those globulars; might have been around 1.8-1.9".

[alpal]

Fair enough Colin.
As a standard candle.

Your Finger of God in Eta Carinae is amazing.
That certainly looks better than 2.8 arc seconds
which is all I ever got on that.


Your make my pics look blurry now - LOL


cheers
Allan

[Imme]

I’m just over the next range (Cobaw Range) from Mt Macedon (approx 15km) and can confirm we get some cracking seeing here. 2.2 wouldn’t be deemed the best setting we get, definitely gets better than that..... although as others have said, some nights it’s best to stay inside.

[alpal]

Wow - that's all news to me.
Looking at planets I've seen very clear images
from Melbourne but only fleeting for 1 second at a time.


cheers
Allan

[Slawomir]

A bit late to the discussion. Everything said so far matches either my understanding or experience.

However, I think there could be some difference in data quality between 8" f/4 and say 8" f/6. I'm not talking about quite obvious differences in SNR, but spot size. This is to do with the size of the secondary and also possibly some difference in field curvatures and the degree of coma across the frame (coma corrector reduces coma but I do not think it does eliminate it entirely, particularly further away from the optical axis). Yet these potential optical advantages could be offset by a more mechanically demanding accurate guiding of a longer tube, so in the real world it could be actually detrimental to data quality to use a slower Newtonian.

I feel that it is common to chase higher SNR with ever faster f-ratios, but having settled on the aperture size, perhaps we could be sacrificing a bit of detail with the faster optics. Or perhaps there is a potential for greater gains in resolution in images from a more precise guiding, be it a better mount or adaptive optics :question:

[strongmanmike]

You don't need big long focal lengths to get decent detail in your galaxy and PN (or any other object) images, just match the focal length with an appropriate size pixel, buy the largest aperture you can afford, then be lucky enough to have some reasonable seeing, easy :thumbsup: :D

To add to the other good examples already posted too ilustrate what's possible (I second looking at Lee Borsbooms images!), the following extreme close up crops, were all done at the native prime focus, 1120mm FL of a fast 12inch F3.8 Newt, with a small pixel camera, providing 0.84"/pix image scale (no lucky imaging used, just straight 5-15min exposures, stacked), from just outside Canberra, so reasonable seeing conditions generally.

As you can see, plenty of detail can be discerned

NGC 6872 (https://pbase.com/strongmanmike2002/image/161119808/original)

Shapely 1 (https://pbase.com/strongmanmike2002/image/165589596/original)

The Southern Crab (https://pbase.com/strongmanmike2002/image/165378489/original)

NGC 1566 (https://pbase.com/strongmanmike2002/image/158452106/original)

Good luck with your (confusing :lol:) quest :thumbsup:

Mike

[alpal]

Hi Mike,
your pics bear testimony to the fact that you don't need
long focal length to take great pictures.
Of course you correctly mention lucky imaging as being an exception -
we only need to see Anthony Wesley's images with Barlow lenses added to see that.
http://www.acquerra.com.au/astro/


cheers
Allan

[strongmanmike]

Aaah yes, if only there were more deep sky objects as bright as planets :thumbsup:

Mike

[alpal]

Well - there is the Homunculus Nebula. :)

[kosborn]

I'm very late to the conversation but thought I'd add my recent experience.

I started with an Esprit 100ED about 3 years ago and wanted to add more focal length. I thought carefully about adding an 8" RC but in the end decided to go for a high end Newtonian. I was nervous about the size and weight of the Newt (and it really does look top heavy in the attached pic) but it actually works well on the EQ6-R as long as there isn't too much breeze. I was equally nervous about the collimation and the big f number of an RC. A Newtonian is infinitely easier to collimate than a Ritchey-Chretien!

I dither every sub and lose a lot of time waiting for the scope to settle, but I live in a Bortle 4-5 suburb of Canberra and don't have to throw away too many subs. The Newt is a 254mm f/5 carbon fibre tube from Sidereal Trading and I've added OAG to reduce weight as much as possible. It weighs in at 17kg with imaging train which is a lot for an EQ6-R but it all seems to work. With a Baader MPC Mk III coma corrector it gives me a great flat field edge to edge and the focal length of 1270mm together with an ASI1600mm and ASI2600MC (and with the Esprit with or without a Starizona 0.65 reducer) gives me a great range of focal length and framing options.

The bottom line is don't underestimate what you can get with a good quality Newtonian!

[Startrek]

Couldn’t agree more , except I use basic entry level newts and have done so since I started this hobby just over 4 years ago
6” f6 Bintel newt on an EQ6-R mount in Sydney for AP
8” f5 Bintel newt on an EQ6-R mount in my NexDome on the South Coast NSW for AP
12” f5 Skywatcher Goto dob for visual on the South Coast NSW
My next scope for AP at some stage in the future will definitely be a 10”f5 newt
Never really looked at refractors when I first got into the hobby. As a novice , I thought they were just too expensive compared to newts , so I just stuck with newts and have done ever since.

[Emuhead]

I bought the 8" Quattro f4 800mm and haven't taken it off. Absolutely love the thing and its speed, and i have a 550mm refractor that I've not yet used because i like the longer focal length, and I cant fault the newt. Grown to really like diffraction spikes too.

So a 10" mirror on an eq6r (f4 or f5) is not too heavy after all, I'm hearing more & more from people who have done it successfully. Just might consider that.. 1000mm f4 or 1200mm f5 is the question, guess this thread has shown it makes no difference to detail/resolution as long as the pixel scale still falls in the sweet spot, it'll be the exact same image if cropped to match, so may go the f4 route for the speed & to keep the windsail size down.

[kosborn]

I've done everything I can to save weight on the Newtonian including carbon fibre tube and OAG. Wind is still the killer though. If you go f/4 keep in mind what you'll use as a coma corrector. I think Andy had a Sidereal 1000mm f/4 with a Paracorr which actually has a 15% Barlow effect. I think that means he was actually imaging at 1150mm and f/4.6, not much different to my 1270mm and f/5. I use a Baader MPCC III which can be used at f/5 and above and leaves the focal length and f ratio unaltered. I don't know if there are coma corrector options for Newtonians faster than f/5 that leave the focal length unchanged... I think in general an f/5 Newtonian is cheaper than an equivalent f/4 (and the Baader is cheaper than the Paracorr). Obviously a 1000mm Newt has less length than a 1270mm Newt when it comes to windsail but I don't know how much difference that extra 270mm actually makes in real life.


Kevin