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Old 09-07-2014, 12:00 PM
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Shiraz (Ray)
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Shiraz is offline
Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,785
Originally Posted by rally View Post

The A/D like many cameras may be 16 bits but the camera itself is really only capable of producing 11.6 bits of information because of the well depth and read noise.

Log2 (WellDepth e- / ReadNoise e-)

Although that is then artificially increased by the non linear antiblooming capability of the CCD circuit in terms of reproducing the dynamic range of the original target, the camera is still only giving you 11.6 bits of useable data.

Unfortunately some camera makers like to "oversell" to their consumers by quoting the A/D circuit capability ! I am being kind here !!

True 16 bits would be 64,000 e- well depth and 1 e- read noise or 256,000 e- and 4 e- read noise !

very well summarised Rally - not widely accepted though, which is a pity.

Originally Posted by rat156 View Post
Hi Ray,

Great explanation. How does this effect a non-saturated signal, say one at about 5,000? Would you still get the full 20,000 if you binned?

The reason I ask is that you guys turned me away from taking my RGB at 2x2 binned because you argued (quite convincingly) that I was better off getting RGB at 1x1 on a KAF8300. But if the argument is only for saturated signals, really it doesn't matter to me as these are usually just the stars, which I would tend to take shorter subs for and overlay the saturated ones in PS.

And, yes, cloudy days are great for expanding you knowledge rather than actually doing any astronomy.

Hi Stuart. with wells of 5,000 ADU, the binned signal would be 20,000 ADU, but that would then be scaled by about 0.5 (along with the noise). Net result is better SNR from binning. If you use HDR methods, the loss of dynamic range on stars is not a problem.

However, apart from the loss of dynamic range, I understand that binning with the 8300 is problematical for 2 reasons:
1. the read noise increases for binned operation (probably not big issue, but requires longer subs than expected)
2. the saturation in the registers somewhere along the line result in one sided blooming on brighter stars (I have no experience with this chip, but the effect is widely documented). Presumably there are parts of the chip that are not protected by ABG - they would not normally need to be, but are pushed into saturation by the higher net signal.

In summary, imaging at 1x1 with subs that are long enough to bury read noise allows full resolution for targets with small bright features that might benefit from high res. If you decide that you don't have enough colour data at 1x1, you could alternatively use software binning on this data to get the full binning advantages of 4x dynamic range, 2x SNR and no artefacts. So I guess that the basic question is why bother with on-chip binning.

Clouds?*++%. I have a new mount sitting here and an AO to try out - naaah not going to happen any time soon.

Last edited by Shiraz; 10-07-2014 at 11:20 PM.
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