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suggested sub exposure tables for ASI128MC and QHY 128C cooled cameras

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#1 Shiraz

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Posted 16 September 2018 - 10:00 PM

Please find attached suggested sub exposure tables for IMX128 based cooled cameras.

 

Published data for ZWO and QHY cameras are significantly different at very high gain. However, Buckeyestargazer published some measurements for the QHY camera that matched well with the ZWO data, so the tables are based on the ZWO data, cross matched by well depth to get the corresponding QHY result - best I could do.

 

To use the tables, first determine your sky brightness. If possible, measure with an SQM, but otherwise, find your zone from a sky brightness atlas or the Bortle scale. Maybe start at https://www.skyandte...e-your-skyglow/
 
When you know your sky brightness/zone, go to the appropriate table and find the column for your scope fNo. Then find the sub length for your chosen gain setting – lower gain for dynamic range, higher gain for shorter subs or narrowband. If the exact fNo for your scope is between the listed values, your exact sky brightness is not covered, or want to use a gain value that is not listed, look at the nearby data and estimate – that will be good enough.
 
The value from the table is the shortest sub length (for that scope, sky and gain) that will still be shot noise limited (5%RN criterion) - this is an efficient way to image. The suggestion is definitely not a hard and fast rule though - you can use longer subs, but more stars will be saturated. You can use shorter subs, but the total exposure time to get to a given image quality will be longer.

If you wish to refine the sub length, first take a sub at the recommended exposure and use the cursor of your acquisition software to look at the ADU values in a background sky region of the scene (no stars and no hot pixels, preferably away from vignetting). Move the cursor about slightly to estimate the average ADU background value in the clear region. From this background ADU value, subtract the bias value for your camera (again - use the cursor to estimate the average ADU signal in a bias frame taken at the same gain and offset as the light sub). Then compare your sub-minus-bias value with the “expected ADU above bias” in the final column of the table. If your sub-minus-bias is a lot lower than expected, increase the sub length to compensate – if you get a much higher value, consider reducing the sub length. However, don't sweat about getting an exact match – getting in the same ballpark will be close enough. (eg if the “expected” value is 500ADU, measured values from maybe 300 to 700 would be fine).

 

Have also included a graph of dynamic range vs gain for a typical imaging situation. Dynamic range is defined here as maximum possible signal divided by the total noise in a dark sky region - after the stacking process. The cameras have good dynamic range at most gain settings - to put this in perspective, a typical benchmark mono 8300 camera would have about 75dB dynamic range under the same conditions - and this is generally sufficient to produce good images.

 

I do not have access to either camera to validate these calculations, so would greatly appreciate any feedback - particularly if you find anything that is obviously wrong.

 

Thanks for looking. Cheers Ray

 

zwo128.jpg

 

zwo128dynamicrange.jpg


Edited by Shiraz, 17 September 2018 - 04:07 AM.

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#2 ehunnell

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Posted 17 September 2019 - 02:01 PM

Thanks for posting this. I have a ASI128MC Pro with a 11" RASA and have some questions. It appears from your comments that the goal is to not let most stars saturate. But what if you are imaging a very faint nebula (say the horsehead and flame nebula)? Most deep sky imaging is not targeting the stars but the dim nebula in between. Do you have any recommendations for that situation or are you saying that these numbers will work fine for that?  Also, do you have any comments on how to adjust these when using a filter (such as the L-enhance or a CLS)? Thanks for your insight.

 

Eddie Hunnell

Longmont, CO



#3 Shiraz

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Posted 17 September 2019 - 07:30 PM

Hi Eddie

 

the tables are based on the idea that getting good SNR in the sky between the bright bits is most important, so, yes, the numbers should work fine for faint nebulas. However, it is possible that exposing in this way will cause some bright parts of some targets to run into saturation - if that is bad enough to affect your images, reduce the exposure time until you fix the issue.

 

CLS etc should reduce the background sky brightness, so you will be able to use longer subs if you want - as a complete guess maybe double what is suggested. You can best adjust exposures through filters by trying to get somewhere near the expected ADU above bias in the last column.

 

Cheers Ray


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#4 ehunnell

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Posted 18 September 2019 - 09:00 AM

Thanks for the quick answer. I used a C9.25 and a C11 for 3.5 years before getting the RASA. These seem like such short exposures times, even at F2. Worried I will get almost nothing. But I will give it a try. Thanks

 

Eddie



#5 bobzeq25

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Posted 18 September 2019 - 09:36 AM

You've made a _great_ contribution here with these tables. 

 

One suggestion.  One doesn't need to buy a SQM, or use a quite possibly outdated and/or unreliable map.  This procedure is free and measures your actual sky value.  It's "good enough" to get someone on the right table.  Also good enough to measure the variability in your skies.

 

https://www.pbase.co.../image/37608572



#6 bobzeq25

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Posted 18 September 2019 - 09:44 AM

Thanks for the quick answer. I used a C9.25 and a C11 for 3.5 years before getting the RASA. These seem like such short exposures times, even at F2. Worried I will get almost nothing. But I will give it a try. Thanks

 

Eddie

These fast optics, with a low read noise CMOS camera, are a different ballgame.  Particularly in light polluted skies.

 

Here's my first light with a C8 RASA.  It's not a great image, but it makes the point.  183C camera.  Red Zone, Bortle 7, mag per arc sec squared low 18s.  It's 150X15 seconds, and those subs are overexposed, too many stars saturated.  Longer subs would have not made this any "deeper", the answer is more subs, not longer ones.  I'm now doing a couple of hundred, and going up.  That's gain 50, I need to try Gain 0 to try and get the subs just a bit longer, I'm gathering gigantic amounts of data, and having to process a lot of subs.

 

https://www.astrobin...3196/B/?nc=user

 

Minor point.  An individual sub will look like there's nothing very dim there.  Not to worry.  A few hundred of them...  <smile>  It can make framing a dim target challenging.


Edited by bobzeq25, 18 September 2019 - 09:52 AM.

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#7 Shiraz

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Posted 18 September 2019 - 11:08 PM

have fun Eddie. Looking forward to seeing some images.

 

Bob, that image is very impressive - the star shapes in particular look really good for such unforgiving optics. CMOS cameras sure can gereate lots of subs though - I got so snowed under by 1600 subs that it took a new Ryzen PC to dig my way out. Worth the effort though.

 

Cheers Ray 


Edited by Shiraz, 19 September 2019 - 04:01 PM.


#8 vdb

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Posted 19 September 2019 - 12:17 PM

So my QHY 128C at gain 1208 and 5 minute or 10 minute subs are way over board for SQM 21.5 skies for an FSQ 106 at f3.8?

 

/Yves



#9 Shiraz

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Posted 19 September 2019 - 04:01 PM

So my QHY 128C at gain 1208 and 5 minute or 10 minute subs are way over board for SQM 21.5 skies for an FSQ 106 at f3.8?

 

/Yves

Hi Yves. Your choice, but, depending on gain, you could probably use shorter subs and still get the same final SNR with fewer saturated stars. eg at gain 50, you could maybe use subs around 2 - 3 minutes.

 

Since the tables are based on general assumptions, suggest you tweak exposures to get the background sky minus the bias level to be around the "expected ADU above bias".

 

Cheers Ray


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