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Old 21-06-2013, 06:22 PM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
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Originally Posted by alistairsam View Post
Hi Ray,

How do you calculate sensitivity to be 1/3rd?

Doug, sorry don't mean to hijack your thread, I've been thinking of a similar issue of matching pixel size with my 10inch F4. I currently use a QHY8 OSC with 7.8micr pixels, what would be best pixel size if I went with an OSC and which one's best for a mono.
again, seeing is not great.

what other factors do we need to consider with sensors, the high end ones have a well depth of 100k, others 20k, others 25k. how does this fit in with your pixel scale recommendation. does it influence it at all?
other factor would be read noise and i'm guessing sampling rate.

would the recommended 9um for Doug's setup change based on these factors from one chip to another?

with your permission Doug, re Alistair's questions, as I see it:

1. the sensitivity scales with the pixel area for a given scope - more area means more photons collected in each pixel - but at the expense of less resolution of course.
2. the only "crime" in choosing pixel size is to use ones that oversample, ie they are smaller than those required to extract all of the detail that the atmosphere gives you. then you just lose sensitivity with no extra resolution. You can of course choose to give up some resolution and have large pixels relative to the seeing spot for high sensitivity - for example Bert (Avondonk) does this very successfully with his f3 system - depends on what you want to do.
3. If your choice is for ultimate resolution with the 800mm scope in 2 arc sec seeing, you would want about 4 micron pixels. OSC requires slightly smaller pixels for the same resolution result as mono - Craig Stark did a good analysis on this - see ref
4. read noise is important by allowing you to use more and shorter subs - if the read noise is low you can do more reads for a given noise level. there are lots of system advantages in having short subs - tracking, flex, field rotation etc.
5. large well depth allows you to image longer before you run into saturation and the ABG takes over. However, limited well depth is not a big deal with low read noise chips - you can readily increase the headroom by combining multiple subs. For example, all else being equal, adding 4 x 1 minute subs taken with a chip with 20000 well depth and 5 electron read noise will give the same result (headroom and noise) as a single 4 minute sub from a chip with 80000 well depth and 10 electron read noise. So don't rely on well depth alone - dynamic range (welldepth/readnoise) tells the whole story and most chips on the market have somewhere around 70dB+, which seems plenty for most purposes.

Advice to Doug is: aim for 9 microns: ABG is useful: highest quantum efficiency you can get: lowest read noise. Another possibility is to image with a focal reducer if one is available for the scope. That would bring the chips with smaller pixels into consideration. Also be aware that some chips may require IR flooding to overcome residual bulk image and some older chip designs also have relatively high thermal noise that requires deep cooling - camera makers will generally take care of these issues though.

regards Ray


Last edited by Shiraz; 23-06-2013 at 10:03 PM.
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