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what am i missing here


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scratching my head all the time these days. I have had to reset everything up the last few months , I am using the ZWO ASI 1600 mono, equinox ED80. been trying to find out the best gain/offset exposure times and theres a chart (on here i think)  basically gain 0= 400 ADU etc etc,  and as the gain increases so does the ADU, which always puzzles me as I always think of gain like ISO (i realise its not). Everytime i have tried to follow the guidlines my stacking has failed. Anyway last night I had a go in sharpcap and tried out the sensor analysys, the chart I am left with goes the opposite way?  can anyone enlighten me I will try putting up the two charts so you can see what i mean. just looking for a happy medium in unity gain ( I was doing gain 50 but going back to unity).

Cheers

zwo sensor analysis.JPG

zwo.JPG

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yes thats the chart looking at that  gain at 139 orange zone F6 refractor I,m looking at a 15sec exposure in LUM?, my rgb I have been doing about 3 or 2 mins , any lower and pixinsight fails to process them in batch processing, this is why I have been looking at it

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Just now, brrttpaul said:

I was doing gain 50 offset 15 I am not sure what the median was, I do have images  will it show the median in PI ?

 

Yes. The statistics process can give you the median value. You also need to take a bias frame 0.3s with the asi1600 with the same gain/bias and subtract its median from the lights median. 

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2 minutes ago, brrttpaul said:

I will have to take  bias frames then, I was told the 1600 does not like bias so stopped using them

It doesn't, hence the 0.3s exposure time. I don't think you need to use boas frames,  but you need to work out your median bias so you can subtract it from your light median. That gives you your value above bias.

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You don't need to sweat over offset. I just set it to 50 on the ASI1600 and leave it at that. The reduction of dynamic range at low gain is insignificant but leaving it constant saves mistakes. That is unless you are using the presets in the ASCOM driver which for some models also adjusts the offset. In that case its easier to use the defaults.

Also, the gain and read noise numbers provided by ZWO are near enough to what you measure so no need to re-measure them. Your graph is ok - what might be confusing you is that gain is given in e-/ADU by convention but it is more intuitive for it to be in ADU/e-

5 hours ago, brrttpaul said:

zwo.JPG

These numbers don't quite match what I'd expect from the Shiraz tables on CN. e.g. At Gain 0 Offset 10 he says 420 ADU above offset which would be 420+16x10 = 580. You've got 400 ADU with offset which is just 240 above offset so only about half the exposure. 

One of the problems with the tables is that it depends a bit on getting a reasonable estimate of your sky background. I prefer to calculate (or look up) the target ADU then take a sample sub to see if the median is near enough to that target. Its not a precise science - targets of 3xRN^2 to 10xRN^2 are ok. More than 10xRN^2 is also ok if you are not saturating.

I'm wondering if your stacking problems are more related to the SNR of the individual subs. I've found the recommended ADU values under strong LP, less than ideal seeing and faint stars  leads to very few alignment stars being found due to poor SNR per sub. In that case the only answer is to expose longer per sub than recommended.

 

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now that makes sense to me, when I image and ignore the chart and go by what my sample image looks like I can usually stack no problem, when i go to the chart I always get the "cant stack not enough stars". 

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