Altair 26c smart histogram (par deux)

Discussion of features and issues relating to Altair Astro Cameras
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AndyM274
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Joined: Sun Jul 26, 2020 3:29 pm

Altair 26c smart histogram (par deux)

Post by AndyM274 »

Hi guys,
Ive changed my config a bit & re-ran the smart histogram, only this time my interest is in maximising the dynamic range rather than gain given that the ultra mode gain is 100.
The smart histogram recommended that I use a gain of 200 - which kind of defeats the object of running ultra mode. I spoke to Nick from Altair briefly earlier & he suggested I posted in here for advice. So my question boils down to whether I change the gain to what is suggested or do I keep it at 100 & expose for longer? Ideally, I'd like to keep the gain at 100, so open to suggestions.
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Many thanks
Andy
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Re: Altair 26c smart histogram (par deux)

Post by admin »

Hi Andy,

at gain 200, you have read noise ~1.3, full well depth ~17000, so SNR per frame is ~13000. You can take exposures of 37s.

Drop that to gain 190 and you have read noise ~3.0, full well depth ~27000, so SNR per frame is ~9000 and your exposures will need to be about 197s, so you will get five times less subs in the same imaging session as at 200 gain.

So you can see that the higher gain should in theory lead to a higher SNR for the final stack (higher ratio between the brightest non-saturated possible star and the faintest detail that is visible above the noise).

Now of course you can capture at gain 190 if you want (if your guiding is up to the longer exposures). Just click on the graph in the brain window at any point and it will read out the recommended exposure for a particular gain.

Note that there is currently a bug in SharpCap with the 26C - the actual camera gain at 200 gain is 3.0x (it should be 2.0x) and the cliff edge where the read noise drops should occur at 300 gain. Basically what should be gain values 200 to 299 are unavailable and higher values are 100 lower than they should be.

cheers,

Robin
AndyM274
Posts: 7
Joined: Sun Jul 26, 2020 3:29 pm

Re: Altair 26c smart histogram (par deux)

Post by AndyM274 »

Hi Robin,
thanks for the response that's exceptionally useful - wasn't aware that the graph in the brain would give me the exposure length by gain - very very useful.
200 gain it is then!
Cheers
Andy
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