Various focal length scopes for deep space imaging

[iceman]

So i'm wondering what I should consider next for expanding the range of options for deep-space imaging with my 350D.

Currently:

- I have a Saxon ED80, focal length 600mm f/7.5. Good for fairly large objects or wider starfields but not so great for smaller objects, as they cover a very small area on the camera chip.
- I'm getting a WO 0.8x reducer, which will give me a focal length of 480mm and a focal ratio of f/6. Again, good for large, wide objects or wider starfields.

But what if the object is small and I want a closer view..

- I could get a 2" 2x barlow, giving me a focal length of 1200mm, but slowing the scope down to f/15.. meaning longer exposures are required.
- I could use a 2" 2x barlow in combination with the WO reducer to give me around 960mm focal length but still very slow at f/12.

So what about a newt then? A 6" or 8" newt would give me similar focal lengths (800-1000mm). The advantages include a larger aperture, and a faster focal ratio (f/5 or f/6 usually) which means I can use much shorter exposures. (and look at what Garyh and Ezystyles (Eric) do with their 6" and 10" newts!! :eyepop: )


Sounds all good?

I guess there are some disadvantages. I'd also need:
- An MPCC to reduce the coma,
- A set of rings/dovetail to hold it on the EQ6,
- Some sort of piggyback system for the ED80 or cheapo refractor to guide with (although I still need to do this with the ED80 on its own for guiding).

So, more cost. Probably more hassle. And do I really want to delve deeper into deep space imaging? I'm not sure yet :)

Anyway i'm just thinking out loud so your opinions and suggested are most welcomed.

Cheers

[Benno]

hahaha sounds like me Mike, although you have been in the business much longer then me. at the moment im not too fussed about longer exposures hence why im looking at the Meade ETX-125PE due to the goto capabilities (unless there is a better one out there for around the same price??) so i can learn the sky. but on the other hand, im also looking at the 10" Dob due to the aperature and focal length for the deeper space stuff.
personally id hit up a cheaper refractor piggybacked to a 10" newt and use that to guide it with (not sure if its possible, but would look cool nonetheless and it would be a decent finderscope haha).
anyways thats my two cents....

[h0ughy]

Now Mike, you seem to have answered most of your own questions, and you know what we have:lol::whistle:

[[1ponders]]

You know what you need to do. There is no turning back. So far you have only dabbled around the edges of the dark side. Welcome to "The Pit"!!!!

See PM.

[Alchemy]

What i find most important particularly with the 350d if your f ratio, the lower you go the deeper and more detail you capture. the 350d from my experience is good for about 6 mins of exposure after that color balance goes red and no noticeable improvement in depth

Dont go the barlow as it will be too slow exept for maybee really bright stuff M42 etc.

i cant see the eq6 holding the 12 inch plus your ed80 for guiding i use this on a g11 and i reckon its as heavy as i can go.

perhaps a fast 8 or 10 inch

a few colorful ideas anyway

[jase]

Like many do, don't lose sight of the arcsec/pixel combination Mike. Your telescope and camera are one. They should compliment each other, not work against. As the focal length changes, so will the arcsec/pixel value. By all means you can image DSOs at .25 arcsec/pixel, but will be wasting sensitivity with little gain to resolution (due to seeing limitations). A higher arcsec/pixel such as 2 or so will capture more light per pixel increasing sensitivity. Too high and you'll lose resolution. This is the reason why on the end of many long focal length instruments, you'll find cameras with 16u - 24u+ pixel size. They are a better match for the telescope focal length.

[iceman]

Thanks Jase - a good point I hadn't though of. Unfortunately, I don't understand how to interpret the image :)

also, how do I work out the arcsel/pixel combination for a given telescope with my 350D?

And what is the pixel size on the 350D?

[iceman]

Just found the pixel size of the 350D is approx 6.5 microns.

[ving]

sounds like you are off on another little adventure :)

best of luck mike. and may the force be with you :)

[jase]

Mike, to use the chart, simply draw a straight line from your right to left. i.e start with your pixel size (right) and draw a line through to the left for DSO arc/sec pixel (left). As the line intersects the middle virtual line - you will know the focal length in mm to achieve this desired combination. You can of course calculate from left to right - what ever way, the result is the same.

I should point out that this is only a reference. There isn't any hard and fast rules. For example, the Tak FSQ I use has a arcsec/pixel of 3.501 when coupled to the SBIG STL11k 9ux9u pixel size and makes an excellent all weather wide field instrument that is not troubled by seeing. Nyquist Theorem indicates you should sample at twice the frequency you want to resolve.

Arcsec/pixel = pixel size in microns / (0.00485 x telescope focal length in mm).If your pixels aren't perfectly square you can simply square root the two - sqrt(W x H)= X microns). The most accurate way to determine the arcsec/pixel combination is to plate solve.

The downside of a DSLR is the lack of support for binning. Binning provides greater flexibility when trying to reach the optimal arcsec/pixel combination.

[Alchemy]

given the chart then a 350d for deep sky would be 600-1000mm range.

so whats wrong with 1500mm as it seems to work ok,( im not trying to be difficult)

[JohnG]

Following on from what Jase said, the various combinations for your ED80/350D are:

2x Barlow - 1200mm fl - 1.12 arcsecs/pixel

Prime focus - 600mm fl - 2.19 arcsecs/pixel

0.8 reducer - 480mm fl - 2.72 arcsecs/pixel

Given that typical seeing is around 2 arc seconds the use of the 2x barlow is limited by the seeing.

Cheers

[[1ponders]]

Mike, download the CCD Calculator (http://www.newastro.com/newastro/book_new/camera_app.asp)from The New Astronomy Press website. It is great for modifying FL, Pixel sizes, FR and gives results of pixel resolution, images size in arcmin. You can even input and save cameras and scope dimensions that are not included in the supplied list.

I find it fantastic and would be lost without it when trying to calculate the best options for camera/scope/object.

[davewaldo]

Hi Iceman!

I think the 8" Meade LXD75 schmidt-Newt would be a great scope for the 350D! Striker used to use one of these on an EQ6 and had very promising results. http://www.iceinspace.com.au/forum/showthread.php?t=13725

At only F4 its fast too! And at 800mm FL it would be a good match to your 350D.

The only downside is, they are quite expensive for the OTA. Better value to buy with mount but then you have a mount to sell.

This is the scope I would love to get when I get into imaging :)

Cheers,

Dave.

[jase]

There is nothing stopping you from using the 350D at 1500mm. How optimal is the question. If we take the example of a C9.25 @ F/6.3 - it delivers 1480mm. Coupled to the 350D pixel size you get an arcsec/pixel combination of .90 arcsec/pixel.

Now ask yourself, how many nights a year will the seeing get below an average FWHM of 1 to take advantage of this arcsec/pixel combination?

[Bassnut]

But Jase, OK .90 arcsecs/pixel doesnt make full use of the available res, but given the huge leap to an astrocam, its not a big disadvantage, surley. Better that than too far the otherway;-).

Im not sure about this, but wont a barlow give massive vignetting with a DSLR?.

Mike, the ED80 DSLR is a nifty combo, seems to me you could get some increadible images 1st before you move to a longer FL. A longer FL especially over 2000mm is fraught with pain to get the image quality I suspect you will eventually hope for, youd really need to spend big for a good mount 1st methinks.

[iceman]

Thanks guys.

Fred I have no intention of going over 1000mm focal length. You're right - I'd need a much more solid mount that my shaky EQ6 which has been driving a 12" newt around on it.

Plus, at longer focal lengths, perfect polar alignment is much more critical and I am too lazy to properly polar align.

[netwolf]

I am a bit confused by this. Is it not better to have a smaller arcsecond value per pixel?

[[1ponders]]

Only if the seeing will allow it Fahim. There is no point in having a 0.5 arcsec/pixel resolution when the seeing will only allow 2 aresec/pixel. You are sacrificing sensitivity for unachievable resolution. Better to reduce your focal length or increase your pixel size (binning) to increase sensitivity rather than attempt high resolution images.

This is applied to DSO imaging rather than planetary imaging. By using many images as is done with planetary imaging improved resolution can be achieved by interating bits of repeated detail from multiple images.

[netwolf]

I have spent many hours looking over photocollector and Ezystyles images and from what I can gather they are using sub to arcsecond setups. How do they get such great images? Paul is using a 12" F5 1500mm and Eric is using 10" F4.7 1175mm these would produce ~1 arcsecond. Whats the secret? I am considering a SN6 + ED80 setup (ideally i would like a SN8 hard to find), perhaps later a custom made Newtonian like Bird's. Right now I can go 8" F6.3 (with FR) on my LX90 and I have seen good results from this combo from 5ash who has produced some great images with this setup.

And I have read somewhere b4 that binning is possible with DSLR images in photoshop, is this right?


Regards
Fahim

[Alchemy]

just had a thought...would stacking say 20 pictures then using things like unsharp give you the extra detail similar to webcam imaging. only for deep sky.

So consider this... jases resolution chart..... the bottom end is for deep sky ie 1 photo, the top end is for planetary images 1000 photos ..........
could you then have a scale for no of images to join the two , like a log based scale. Which then you could increase your focal length and still get good results ? :)or:(

[[1ponders]]

Fahim I think much of the secret to Erics, Pauls and Scotts images are their wonderful processing skills. Yes there equipment may be imaging at 1 arcsec resolutions, but that doesn't mean they are actually achieving 1 arcsec resolution as their results.

While I'm generally only using the ED80, either native or with the WO 0.8 FR, I have a 0.5X NextGen FR and Flattener coming for my 8" SCT which I am really looking forward to trying out. This will give me an 1.52 arcsec/pixel resolution which I think will be about as low as I can go with my atmospheric conditions on good nights.

I'm not sure how to go about binning a DSLR in PS.



I don't believe so. If the detail/resolution isn't there in the first place no amount of unsharp masking will bring it out. You may achieve better results using a deconvolution algorithm like Lucy-Richardson or Maximum Entropy to improve resolution

[davidpretorius]

great thread

[Alchemy]

looks like:(

[jase]

The software binning you’re referring to follows a similar concept, but different to in camera binning. In camera binning (either 2x2, 3x3 or what ever your camera supports) electronically combines pixels. When the camera is placed into a 2x2 mode, it groups four pixels to make them act as one. This increasing camera sensitivity, but at the same time potentially reduces resolution depending on your arcsec/pixel scale.

In comparison, software binning does not increase sensitivity. It follows a similar principle of combining pixels, but does this by taking the average of each pixel to make the final pixel output. i.e. with a software 3x3 bin, results in one large pixel containing the averaged value of the nine pixels. You’re probably thinking “Why would I want to do software binning if it doesn’t improve sensitivity?”. Well… software binning from an image processing perspective is one method of smoothing data to reduce noise. As the pixels are averaged, it is makes it easy to remove outlier pixels. First upscale your data, then active the software bin mode of choice. There is of course many other noise reduction methods which I think are more efficient than software binning, but it does work well for RGB data which is later to be combined with a 1x1 luminance.

It is also important to note that with any form of binning, image size is reduce. This makes sense considering you are combining pixels. So if your camera pixel array is 1024x1024 in a 1x1 configuration, binning 2x2 results in half the array size i.e 512x512. This is the reason why I mentioned to upscale your data first so when the pixels are averaged you finish with the original sized image (before binning).

[netwolf]

jase, thanks for clearing that up. But can you explain further how you upscale data?

Paul, Is that the Optec FR your getting? How about the Optec 3.3 FR would that be any good for DSLR with the 8"?

Regards
Fahim

[Bassnut]

Jase, one of the reasons for in cam binning rather than sofware binning is the read out noise is proportionally less after binning in cam.

[[1ponders]]

Yes it is Fahim. I'm looking forward to giving it a go. The 3.3 isn't suitable for the DSLR, the Optec site specifies (http://www.optecinc.com/optec_026.htm)a max chip diagonal of 9mm, way too small for DSLR. Even the 0.5 will be pushing it as the 300D diagonal is 27.3mm and the 0.5 is suitable for 17.5 mm. That is a lot of lost FOV but it will considerably reduce imaging time and flatten the field for what is available.

[iceman]

So just an update on this, been thinking about it again lately..

I was hoping to get a 6" newt, thinking they'd be around 1000mm focal length, but from looking this morning, they're mostly 1200mm focal length, giving f/8 focal ratio.

It looks like Skywatcher have an 8" newt (SW600) with a focal length of 1000mm, giving f/5 focal ratio for around $600 - perfect for what I'd use it for (deep space imaging).

Anyone else got any suggestions for a ~1000mm focal length, fast focal ratio newt with a 6" or 8" mirror?

[davewaldo]

GSO have an F4 and F5 8" Newtonians. Andrews sells them with mounts but may be available OTA only. May be cheaper than the skywatchers.

:)

[[1ponders]]

Or you could go for a 10" MakNewt @ f4 (?). Mind you anything at these f ratios will probably neeed a coma corrector as well.

[iceman]

Yes you're right - any of the f/4 or f/5 newts, i'd also get the baader MPCC.

I'd preferably want something 6" or 8", rather than 10".. just to keep the size/weight down.

[h0ughy]

vixen R200SS

[[1ponders]]

I've been very tempted with getting an 8" MakNewt for a while now for attaching the 300D to. I've got a NextGen 0.5 FR coming for the SCT for use with the SBIG, but there is just something about using an 8" scope (not refractor :eyepop: ....in my dreams :lol: ). They are a good size in aperture, not too difficult to manage weight wise on a mount and you still get a decent focal length for the f ratio.

Though a 10" is really tempting :evil:

[iceman]

I'm sure I mentioned my other requirement..

On a budget.. :)

[Lee]

I've been having similar thoughts..... I love refractors, but as we know to get a good, fast refractor with a longish focal length, ie big aperture, is crippling, cost wise and mount wise!
I was thinking long the lines of a Schmidt-Newt or something too..... I figure if it is 8" f/4, you can barlow it, and only end up at f/8 - not too bad, considering the ED80 is f/7.5 anyway.....

[gbeal]

Great thread, as DP said.
I have just been down this road.
I owned a Tak refractor, a 4" f8, so about 800mm focal length. Drop dead gorgeous, and about US$2500 or so. Took great images, with both my 20D and also the ST2000XCM.
In a fit of lunacy I sold it, and bought a set of GSO 8" f4 optics (see the similarity here folks). Same focal length, so same image scale, and 4 times faster, if I am not mistaken. Biggest gain though was that it cost a fraction of the Tak price.
OK, I bought the optics, built the tube, and all, and then fitted a very expensive focuser and focus motor to it, but it is still about 250% of the price of the refractor. Images as good? I think so, and so does my wallet.
If you are not up to building one, then buy the likes of the GSO8" f4, and the Baader MPCC. AND a decent focuser.
That way Mike, you can have a lighter purpose built DSO imaging scope, plus the ED80 on the side for guiding. Later when you get to a dedicated CCD, with an in-built guide chip, you can dispense with the ED80.

[netwolf]

Interesting turn this topic has taken, this is the reason i was earlier investigating a SN8 OTA the FL is just right for what we need. But I did contact Andrewscomm about the GSO 8" f4 a while back and is well priced for the pocket. I think it was 399 for the OTA.
Maybe we can put in a bulk order Mike delivered to the NOC ;)

Regards
Fahim

[davewaldo]

I've been thinking about a similar scope...

I just spoke to andrews... GS-600 8" F4 newt OTA. $499. :)

I assume this comes with rings and dovetail although I'm not sure.

As Gary has mentioned, you will want to replace the focuser, but it seems like a good buy. Much cheaper than the Meade 8" SN (but one of those would be great too :D)

[davewaldo]

you beat me to it Fahim :)

hmm.... I wonder what the best price they could do it for. I might be interested in going in on a group purchase!

[netwolf]

But Dave did you ask him about shipping to Brisbane.
I would be sure your local stores would match the price.

Regards
Fahim

[davewaldo]

:shrug: I don't think any local stores sell GSO scopes.....

Sirius optics no longer deal wth them. :(

[seeker372011]

why not consider an Intes Mak newt or an Intes Micro Mak ?
The Intes Micro is what I have in mind (budget being an important consideration for me). Sub 1000mm- but still much longer than the ED80

[iceman]

Aren't they expensive?

[dannat]

this is the Aus distributor for intes
http://members.ozemail.com.au/~aecclaudio/stock.htm
I think they are in the medium price range.

[gbeal]

I still reckon the humble newt is THE way to go. A few years ago I built up a 10" f5 Maksutov newton, from an optical set. It was lovely, expensive but lovely. I used it for images of Jupiter etc and was chuffed. The major snag was that at the same time, I still had the humble "had it for years" 10" f5 newt. Side by side there was precious all difference in what I saw, and or imaged, and when you factor in the cost of the MN optics at about US$2500, and the cost of the newt optics at about US$250 it was a no brainer.
I love the Maksutov design, and for an all rounder, the MN is hard to beat but it costs, and the appreciation that I have for my newts is based on this.
So back to the start of all this posting, I still think the 6 or 8" newt, possibly even a 10" is where you should headed Mike. It will fit OK on the EQ6, and give you the around 1000mm focal length you seek. Plus it will deflate the wallet the least.

[Starkler]

Excellent thread that deserves a sticky :thumbsup: