Picking the right camera for a focal length

[Gem (Grant)]

Hi all

I have been reading about pixel sizes, etc... it seems to me as some one new to imaging that certain cameras suit different focal lengths.
So... if I have a 2350mm focal length and a camera that is a bit better than an entry level (e.g. Meade DS I, etc..) for deep sky imaging (globs, galaxies especially)... what sort of cameras should I be considering and what pixel size is best for me?
Then I can start planning on saving...

[graham.hobart (Graham stevens)]

there is an equation you can use with your cameras pixel size and the focal length (including a constant value ) which will give your ideal sampling size i.e camera pixel for your seeing conditions.
I can't remember the name or the values of it but it's worth (when someone remembers it) calcualting the values for your scope else you may be over or under sampling
Vague I know but I am work and can't find it at the moment.
here it is
sample size (arc seconds) = 206.265/focal length mm x pixel size camera in microns
gives sample size for your camera to compare to your seeing conditions (arc seconds)

[Gem (Grant)]

Thanks.
I would need to work backwards though, since I want to find out what pixel size is best. How do you find out seeing conditions in arc seconds? What is typical for seeing conditions?

[graham.hobart (Graham stevens)]

really good seeing conditions from urban are I think usually 3-4 arc seconds
max 2 arc seconds with exceptionally dark skies
for example I have an LX 90 8" which is f/10 Focal length 2000mm, I also have a mono atik 314L which has pixels of 6.45microns, my sampling would be
206/2000X6.45= 0.66 arc seconds
which is way way too much sampling for my skies so I have to either use a focal reducer or bin the pixels 2x2,3x3 or even 4x4, this will decrease the sample size.
you can rearrange the equation to swap the values around if you assume your seeing is either 2 or 3 etc.

[Hagar (Doug)]

This probably the best explanation I have seen. It should help.

http://www.ccd.com/ccd113.html

[Gem (Grant)]

Thanks!
I am starting to get the picture!

[Gem (Grant)]

Originally Posted by graham.hobart

I also have a mono atik 314L

How is the Atik 314L? That is one of the options I was considering if it "fitted" the focal length.

[mithrandir (Andrew)]

Originally Posted by Gem

How is the Atik 314L? That is one of the options I was considering if it "fitted" the focal length.

With a C9.25 - fl 2350:

Atik 314L+ 6.45u pixel, 0.57 ASP (arcsec/pixel),FOV 9.8x13.1 arcmin

Comparisons:

Atik 383L+ 5.4u pixel, 0.47 ASP, FOV 19.8x26.5 arcmin

Atik 4000 7.4u pixel, 0.65 ASP, FOV 22.1x22.1 arcmin

Atik 11000 9u pixel, 0.79 ASP, FOV 35.1x52.7 arcmin

With binning or a focal reducer the ASP will change.

[renormalised (Carl)]

It also depends on what targets you wanted to concentrate on. If you want to do widefield shots mainly...nebs, large galaxies, many globs etc, you're going to need plenty of chip real estate and short focal lengths (which will mean a reducer in the optics), so the Atik383L+ is your best bet there. However, most targets will fit comfortably in the FoV of the 314L+ (most PN's and galaxies) and will do alright with the larger targets at native FL. But with the reducer, you'll do better for the larger targets as the FoV will increase.

Grab yourself this... CCDCalc, find the specs for the cameras you're looking at and then stick the specs into the program and see what you get.

[Bassnut (Fred)]

Originally Posted by renormalised

Grab yourself this... CCDCalc, find the specs for the cameras you're looking at and then stick the specs into the program and see what you get.

Yes, very usefull app indeed. Binning allows a variety of image scales, with other bonuses too.

[Gem (Grant)]

Originally Posted by renormalised

Grab yourself this... CCDCalc, find the specs for the cameras you're looking at and then stick the specs into the program and see what you get.

Thanks! Useful tool!