Pairing the right CCD to a telescope

[nmoushon]

I bought my first CCD (a second hand SBIG 2000XM) about a year ago with the intent of getting over the learning curve by pairing it up with my little ED80 and by the time I save up enough money to make the jump into long FL imaging (C11HD on a AP900) I'll have it down so that I can focus on the long FL learning curve. I had originally bought the SBIG because it has the self-guiding chip, so no need for an OAG and guide cam. I also bought it because I thought that through my "extensive research" that I came to the conclusion that the chip size was a good match to the scope and in my budget. But after using it for the past year, though it really wasnt that much due to weather, I've started doubting that I made a good choice.

The first problem I came across was that with the self-guider it is hard to find a guide star and have the object framed correctly. Some objects I had no problem getting a star and the frame to line up how I wanted them but others I had to switch to my finderguider because I couldnt find a star and have the object fit in the frame. Now I dont think I'll have as much of this problem with the C11 because my primary targets are going to be galaxies and I would think they would be small enough that framing wouldn't be as big an issue as it was with my ED80 and imaging large nebulas. Have other long FL imagers had this problem with a similar setup or found other problem that I havent run into yet with a self-guiding setup? Should I not worry about this issue too much or should I just focus on an OAG?

The next thing I started worring about is my pixel scale/resolution. With the pair, at native FL, I'm getting 0.55"/pixel. With the reducer I get .78"/p. With my moderate LP and my local sky/weather conditions I thought this was pretty decent. But after working with this for the last year and learning more as I've gone along I worried I might not have paired it up correctly. One thing i picked up after I did my first "extensive research" and bought the camera was about under/oversampling. This is one thing I'm still not very sure on and don't really understand and wouldn't mind if someone explained a little on this.

With these two things eating away at me and making me double guess my selection choice I would really like some more expert opinions here. If I did make the wrong choice I would like some suggestions on what I need to be looking for in a new camera. Thanks in advance for any advice.

[The_Sarge]

Hi nmoushon

I have a C11 and planning on a SBIG STT-8300m with the self guiding filter wheel.

I have been doing research myself and the ideal pixel scale should be around 2-3 arsecsonds/pixel for average seeing conditions.

If you use binning which is to create an effectively larger pixel from an array of nxn pixels i.e 3x3 binning would give you 2.34"/pixel

I believe this also makes the camera more ”sensitive” and allows shorter exposure times which would be better for long FL scopes like the C11.

Hopefully got that right am sure the experts will correct me if I am wrong.

Paul

[steppenwolf]

One thing i picked up after I did my first "extensive research" and bought the camera was about under/oversampling. This is one thing I'm still not very sure on and don't really understand and wouldn't mind if someone explained a little on this.

You may find this article on my website of some use.

[nmoushon]

Hi nmoushon

I have a C11 and planning on a SBIG STT-8300m with the self guiding filter wheel.

I have been doing research myself and the ideal pixel scale should be around 2-3 arsecsonds/pixel for average seeing conditions.

If you use binning which is to create an effectively larger pixel from an array of nxn pixels i.e 3x3 binning would give you 2.34"/pixel

I believe this also makes the camera more ”sensitive” and allows shorter exposure times which would be better for long FL scopes like the C11.

Hopefully got that right am sure the experts will correct me if I am wrong.

Paul

Since I'm intending to image at F/10 I'm not too concerned about time...plus not living in the UK I have a bit for cloud free nights .  I know binning does increase the per pixel ratio but doesnt that also increase the FoV and thus reduce the image scale and resolution? Not sure if i understand that or not. I usually image at 1x1 binning for my LRGB currently...mainly because I dont want to bother with trying to match different binned subs.

You may find this article on my website of some use.

That was a great write up and explained it very well thank you.

[The_Sarge]

It won't change the FOV as that is fixed by the focal length and the size of the imaging sensor.

It does reduce the image scale (resolution) but you can't capture anymore detail than the seeing will allow.

As for combining different binned subs it isn't an issue. Some of the guides to LRGB imaging I have read will do the luminance run at nxn to match the resolution to the seeing to capture the fine detail of the image in the luminance layer. The RGB runs are done at 2nx2n to collect the colour information for the image with a similar length exposure rather than increased length.

I will try and find one of the guides and post a link.

If you have such great skies you can afford the time to experiment with different sampling rates and see what gives you the best results

[The_Sarge]

http://www.jtwastronomy.com/tutorials/lrgb_primer.html

Here is a good link