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Old 25-06-2019, 02:19 PM
Imme (Jon)
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An old thread back to life I know......however this seems to be the 'goto' advice on the web.

Just moving in to NB and have done a few tests the last couple nights however my stars are being blown out significantly using far less than recommended exposure times.

As I said, new to NB, so a question - should I expect the vast majority of stars reading at 65K+adu when reviewing subs on SGP?

Using the table I ascertained I needed a 30'ish minute sub using an F6.7 with the asi1600 set to 200 gain/50 offset. (Sky is easily in the 21+ mag range and picture taken pre moon rise). To be honest 30 minutes scared me so I went 10 as a trial.......the result, about half my stars were saturated and I had a background ADU around 3000 (bias subtracted - value 180)

I cant think what a 30 minute sub would end up like!

I'm thinking tonight I will work out exposure times based purely on background ADU rather. If I recall correctly I did similar with the ASI1600 OSC I used and those pics came up well.
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  #22  
Old 26-06-2019, 05:37 AM
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Camelopardalis (Dunk)
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Yeah I’d go with using the “expected ADU over bias”. Even 10 minutes sounds too long at 3000 ADU for background sky
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  #23  
Old 27-06-2019, 03:49 PM
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Shiraz (Ray)
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Quote:
Originally Posted by Imme View Post
An old thread back to life I know......however this seems to be the 'goto' advice on the web.

Just moving in to NB and have done a few tests the last couple nights however my stars are being blown out significantly using far less than recommended exposure times.

As I said, new to NB, so a question - should I expect the vast majority of stars reading at 65K+adu when reviewing subs on SGP?

Using the table I ascertained I needed a 30'ish minute sub using an F6.7 with the asi1600 set to 200 gain/50 offset. (Sky is easily in the 21+ mag range and picture taken pre moon rise). To be honest 30 minutes scared me so I went 10 as a trial.......the result, about half my stars were saturated and I had a background ADU around 3000 (bias subtracted - value 180)

I cant think what a 30 minute sub would end up like!

I'm thinking tonight I will work out exposure times based purely on background ADU rather. If I recall correctly I did similar with the ASI1600 OSC I used and those pics came up well.

that is a bit of a worry Jon - the tables are not super accurate, but should not be that far out.

I just checked some old ha data and, at f4 under about m21 sky, I get about 400ADU above bias for gain 200 and 10 minutes - this is consistent with the table.

I cannot suggest why you might get 3000ADU under the same sort of sky and with a slower scope, particularly if you are getting excessive numbers of saturated stars. Maybe our filters are vastly different - what do you use?

cheers Ray

Last edited by Shiraz; 27-06-2019 at 04:01 PM.
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  #24  
Old 27-06-2019, 04:18 PM
Imme (Jon)
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Iím using Astrodon 3nmís. Iíll check back over everything tonite and make sure Iím not mistaken. Out of interest what was your bias value?

I did previously use these tables for an asi1600 OSC (75 gain) and found on this same scope, on same mount and in same obs that 3 minutes was a sweet spot which more or less aligns with the table.

Very strange
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  #25  
Old 27-06-2019, 05:22 PM
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Shiraz (Ray)
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with the default setting, the bias level from my cam is about 540 at gain 200 and -15C Cheers Ray
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