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Old 10-03-2015, 04:45 PM
glend (Glen)
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Matching CCD Camera to Scope

I have been doing some reading on matching a CCD camera to scope, like this article:

http://www.astro-imaging.com/Tutorial/MatchingCCD.html

and Craig Stark's review:

http://www.stark-labs.com/craig/reso...-the-Bench.pdf

and even Jeff's thread here on exactly what I am thinking of doing:

http://www.iceinspace.com.au/forum/a.../t-131003.html

and tend to believe that something like the QHY10 would be ideal for my 10"(250mm) f5 (1250mm fl) imaging newtonian. Currently I am using a Canon 450d which has a pixel size of 5.2 um and this camera has served me well in the past (targets are mainly DSOs).

Is this a reasonable choice on my part (the QHY10)? I am looking to move to a cooled CCD camera in the next few months. I'd prefer the One Shot Colour based on my DSLR experience but is the Mono a better option for Ha, etc?

Last edited by glend; 10-03-2015 at 05:05 PM. Reason: Sale Pending
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Old 10-03-2015, 05:04 PM
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http://new-astronomy-ccdcalc.software.informer.com/1.5/

This is a free calculator that helps work these things out.

Basically .5 to 1 arc sec/pixel is good practice.

Your 5.2 nm Canon is at .86 arc seconds/pixel.

The KAF8300 seems to work with most scopes so is a pretty safe bet and that is 4.54 microns. It also works well with short focal length scopes like FSQ106 etc.

Undersampling is common with short focal length scopes and as your article says there are plenty of good FSQ106ED and 9 micron CCDs like SBig STL11 or Proline 16803 images.

If you over sample you lose some sensitivity and takes longer exposure time to achieve the same signal to noise. Not much of a problem.
Small pixels have smaller full well capacities but with Sony low read noise chips stacking shorter exposures means there is no disadvantage there.

So in short your 10/12mp one shot colour APSc sized ccd you mentioned
should be fine.

One shot colour is not so good for Ha - no. Because only one in 4 of the pixels is registering red which is where Ha is. Mono is quite a bit superior for that. One shot colour is handy but from my experience it means worse signal in the dim areas which means more noise. So its fine for bright objects but if you start imaging things that are dim or Ha O111 narrowband imaging they aren't so good.

Starlight Express MX25C has a good reputation and high sensitivity. Also the Sony ICX694 or 814 colour should have good sensitivity as the mono version is one of the most sensitive ccds to narrowband.

Greg.
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Old 10-03-2015, 05:48 PM
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codemonkey (Lee)
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Quote:
Originally Posted by gregbradley View Post
Basically .5 to 1 arc sec/pixel is good practice.
I've been reading lately (Craig Stark's great articles on SNR) that you should target half your seeing, as you can't leverage greater resolution (you can't record what's not there) and so you lose out on SNR and make life more difficult for yourself and your equipment in terms of guiding, tolerance for error etc by going beyond that.

Apparently 3" is considered typical seeing, which suggests that an ideal target would be in the area of 1.5"/px in such circumstances. Of course, even if 3" is typical seeing, typical means nothing if your particular site is atypical, so you should definitely measure this yourself.

Here's a link to one of the articles I'm referring to; there's an example of resolution vs FWHM roughly halfway down the page that illustrates what I'm talking about: http://www.cloudynights.com/page/art...sampling-r1970

Definitely made me wonder whether I'm doing myself any favours with the 1.8m focal length I have coupled with 6.45 micron pixels.
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Old 11-03-2015, 05:28 AM
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There's also an article by Stan Moore that suggests .5 to 1.
I've heard 1 before from Roland Christen of Astrophysics.

The best way to decide is to move away from theory as not everyone agrees and look at various astro images and see the size of the stars etc
and level of detail of people using similar scopes to what you plan to use.
What camera did they use?

From what I have seen .5 to 1 seems correct. 1.5 also seems fine.
I am not sure its that sensitive and perhaps has been overplayed as an issue in imaging.

I agree with Paul its better to be slightly undersampled than oversampled as all it costs you is a slight loss of sensitivity and you get round stars.

But then there are tons of great images with 9 micron cameras. I have mainly used 9 micron cameras and they performed the best overall but I think that was mainly sensor size as well.

Greg.
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Old 11-03-2015, 06:23 AM
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Quote:
Originally Posted by gregbradley View Post
The best way to decide is to move away from theory as not everyone agrees
Fully agree! :-)

As soon as I'm able I'll be testing this personally so I can see what works for me, with my equipment, in my skies.

In the meantime, that article I linked to shows a comparison of images taken at difference resolutions with different seeing conditions (simulated) which is definitely food for thought. Here's a direct link to the image.
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Old 11-03-2015, 07:03 AM
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One thing is the article kind of assumes DSLRs or Kodak CCDs. They have relatively high read noise. The Sony read noise is a fraction of the Kodaks and this allows higher res using smaller pixels to some degree.

Interesting point about showing up guide errors more with oversampling. I would not have thought of that as an issue. Not sure I have seen it myself though with different camera setups. Round stars are sometimes an illusion in the sense that the star has been moved around evenly during the course of the exposure rather than a regular drift in one direction causing eggy stars.

Greg.
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Old 11-03-2015, 07:54 AM
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I think the "general rule" is that if your guide errors are below your resolution you're ok, so as your focal length gets bigger you have less margin for error, which is why I sold my NEQ6 recently and bought an EQ8.

With the RC8 I'm capturing 0.74"/px (not the 0.82 I thought, because it turns out the older RC8s don't seem to be the same focal length as the new ones!) and I was never able to get the RMS error consistently below 1.2" This led to a lot of frustration, blurry subs, and wasted time.

If I'd just been using my ED80 with the Atik 314L+ where I'm significantly undersampled at 2.2"/px that 1.2" would have been totally fine.

The interesting thing is based on Craig's article I'm not sure there's any point in imaging at the full focal length of the RC8 where I am, as I don't believe I have better than average seeing. If the theory is correct, I'd probably be much better off getting a 0.8x reducer.
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