To Gain or not to overGain

Discussion of using SharpCap for Deep Sky Imaging
AstroDude
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Re: To Gain or not to overGain

Post by AstroDude » Mon Oct 15, 2018 7:55 pm

Hey Guys , now that we have covered the section on OSC hypercam gain/ exposures...
we have a new set of issues to work out.!

NB_SHO imaging AA183MM

i ran the Brain 2 nights ago and it reported = failed to find any exposure in 300 sec using the Ha filters Gain at 400 to start and 40 offset.
the image was black under SQM = 20.08 ,
So now we need to address the NB SHO problems with the Hypercams 183MM .
sos we are right back where we started , in a way with :

1-HIgh Gain ( 500~1200) / short expos ( under 60 Sec)
2-then low gain (400 and under) and 300s ~ 600 sec subs to just reach 100 ADU on the histo gram. and a huge Star burst to boot ....( and amp glow)

All my friends with ASI 1600 MM shoot at Unity gain 139 (of 600) and offset 21 / with 300 sec subs and get an ADU of 1250 ish . at 16 bit.
now i understand that the ASI is only 56% QE and the AA183MM is 84% , but that should help it now make it worse...
SO now for some test the AA183MM to see how long of an exposure we need to reach ADU 100 so that DSS4.1 and Pixinsight 1.8 doesnt cry a river with
( NO STAR FOUND ) empty image ... :-(
So back to the drawing board on the over gain under exposures, to get the right dynamic range and be able to stack the subs....
3- if using 300 sec subs to get a target you will find yourself short on subs for noise reduction on the hypercam ( i usually shoot 200-600 subs at 30 sec )
but now in a NB SHO environment this wont work... or i will only get 20 -30 subs on each channel and this will make problem with noise reductions.
IF the star burst was not a problem then i would shoot LOW gain and long subs to get my ADU to 100 or over ( higher) for stacking reasons.

will keep testing and waiting on any of you guys remarks , and also on Robins take on this ....

so far all testing has been done with SGP and that is a problem SDK DRiver wise...
Attachments
Gain 800 300sec.jpg
Gain 800 300sec.jpg (92.28 KiB) Viewed 170 times

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turfpit
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Re: To Gain or not to overGain

Post by turfpit » Mon Oct 15, 2018 9:27 pm

Mitch

I ran up against this a couple of weeks ago see viewtopic.php?f=16&t=1148

This was my first exposure (no pun intended) to narrowband imaging. What I did notice when I came to take flats was my usual 150ms turned into 15s to get a decent histogram for a flat - a 100x increase!

In a discussion with Nick @ Altair, he suggested 20% for gain. So I started to do some research. I started with someone who knows what they are doing with narrowband - Sara Wager https://www.swagastro.com/ic434-b33.html

I will let you do your own research but here are some questions to consider:
  • What type of camera is being used?
  • How is the camera cooled?
  • What is the cost of the camera?
  • Pixel full well depth?
  • What is the exposure time for a frame?
  • What is the total integration time?
Then move on to other world class imagers like Terry Hancock and ask the questions again. Look round on Astrobin.

A pattern will emerge ;) Your findings will be interesting.

If you want to put your images + darks somewhere, I can have a go at stacking them using DSS (Dave's Secret Stacker).

I have an Atik 314E now and will be having a look at Ha again with this camera but I think I will need to get PHD2++ going first.

Also, is the Brain validated against narrowband filters?

Dave

AstroDude
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Re: To Gain or not to overGain

Post by AstroDude » Mon Oct 15, 2018 10:17 pm

well on the Brain validation i haven't heard anything from anyone, so i'm going to assume you should do a NB sensor analysis with full filters on!
then see if that is the way to go with NB SHO...
it probably wont work , but it worth a try...

otherwise its a trial and error and see and wait while changing Gain values and exposure times...
if you like and i gather subs ona target and let you go at it..

Mitch

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turfpit
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Re: To Gain or not to overGain

Post by turfpit » Mon Oct 15, 2018 11:20 pm

Mitch
assume
will make an ASS of U and ME ;)

Understanding will come from answering the questions listed using the link above. Off to bed now, look forward to reading the answers in the morning.

How full is your well? :lol:

Dave

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oopfan
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Re: To Gain or not to overGain

Post by oopfan » Tue Oct 16, 2018 3:22 am

Mitch,

I am having trouble parsing your post. I don't know what you are asking but it sounds like you are frustrated with the results.

First off, your image of the Fish Head Nebula, you don't explicitly say what we are looking at. Is that a single frame or a stack? I am guessing that it is a single frame without dark frame subtraction. Is that right? You've got a fair amount of amp glow but so do I using long exposures. Dark subtraction will take that out. Are you saying that DSS won't stack frames that look like that? I have a hard time believing that. You sound experienced with DSS. You do know that there is an "Advanced" tab on the "Register" dialog that has a slider control. The minimum value you can select is 2%. it detects the most number of stars at 2%. I adjust the slider so that the total number of stars is just under 100.

Here is a link to my recent shot of a section of the Soul Nebula using a 6nm Ha filter. I used Unity Gain at 200 seconds. When I stacked it in DSS I had to adjust the slider to 5% in order to get the star count just under 100.

viewtopic.php?f=16&t=1163

Brian

PS: Don't waste your time using short exposures in Ha. I tried to capture the Wizard Nebula with 50-second exposures. DSS barely found the minimum number of stars for alignment (9, if I remember correctly). The stack was a noisy mess even with 45 minutes total integration time. Avoid anything less than 100 seconds. I've had good results using 200 seconds.

AstroDude
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Re: To Gain or not to overGain

Post by AstroDude » Tue Oct 16, 2018 1:50 pm

Hey Brian ,
let me clarify all of the above in the shortest possible way.
1- with NB imaging gain and offset and exposures are not the same as OSC.
2- if my Gain /Offset/Expo are not long enough Pixinsight reports the above ( no stars) Not so in DSS.but still very low count ( 3-8)
3- the ADU is to low in SGP with low gain (nothing under 800) in NB imaging so i use 243 to 300 sec to get enough stars to show up in PI.
4- the fish head was 180 sec single frame, Gain 1000 if i recall and offset 120, so that some kind of nebula would show up and some stars would be detected in the registration in PI.
5- sometime the darks DO not remove the star burst at all and that's doing darks right after lights at the same night same temp.
6- now with my new setup i cant Autofocus in SGP in HA because not enough stars are detected in 10 sec expo bin 2x2 for the routine to work.( last night is an example of that ) (Gain at 400) offset 120)
------------------------------------
so not being a newb ( 40 years exp in astrophotography) it not about frustration , its about getting around the star burst glow and getting at the very least 120 ADU on each filter so that Pixinsight can register these frames. its just a lot of ranting looking for help from others who have a
hypercam AA183 MM-- NOT cooled.
-----------------
does that help?
------------------
if you want to see my OSC work click link below...
it shows that i have no problem with LOw gain short exposures and low black point / offset
but in SHO NB that's a whole different set of #s... So some experimenting required.
if you suggests Gain / offset / and exposure times , im more then willing to run them through on any given clear night and report back to you.
now the gear also tells tale ... i have the ALtair astro 115 MM F 5.8 EDT with the AA183M and the plamostar 0.79 + xagyl 5 pos filter wheel SHO.
----------------
If you need more info ( for you to help me out, let me know) .... trying out NB for the very first time is very strange but it shouldn't be. lol
-----------------
If your using an ATIK and im using a 183M hypercam then sharing gains wont work ( i think) plus maybe you have a C11 and i have a 4 inch refractor..

https://www.flickr.com/photos/rcfmitch/

Mitch

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oopfan
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Re: To Gain or not to overGain

Post by oopfan » Tue Oct 16, 2018 2:08 pm

Mitch,

Regarding:
5- sometime the darks DO not remove the star burst at all and that's doing darks right after lights at the same night same temp.
With uncooled cameras you will get more consistent results by taking darks before lights. Although you may think that the temperature is the same, if it is off by 0.5F then you've got problems, and more often than not ambient temperature goes down as the night wears on.

Regarding focusing in Ha, I am not yet fully automated so I have no experience with the SGP focusing assistant. I've tried Bahtinov mask but there is too little light. I've tried SC's FWHM tool but the results are inconsistent. I use the "eyeball method". I crank up the gain with 300% magnification, I tweek the fine focusing knob until I find the best looking star disc. Works well for me.

I know that Chuck of "Chuck's Astrophotography" does a lot of narrowband since he lives in Detroit. He's fully automated using SGP for focusing. You might want to contact him on AstroBin or YouTube for some insight. He images with a short refractor and 1600MM-C.

Brian

AstroDude
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Re: To Gain or not to overGain

Post by AstroDude » Tue Oct 16, 2018 2:27 pm

Hi Brian , i know chuck well , we talk all the time , his answer is " im using an ASI 1600 MM Pro TEC so my setting probably won't work with your hypercam""
ive done the eyeball focusing , it also is an option but, it defeats the 500$ electric focuser. lol

thanks ,


Mitch

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oopfan
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Re: To Gain or not to overGain

Post by oopfan » Tue Oct 16, 2018 2:34 pm

Mitch,
it defeats the 500$ electric focuser. lol
Yes, but at least you can tweak it from the comfort of your home! :D

Brian

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oopfan
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Re: To Gain or not to overGain

Post by oopfan » Tue Oct 16, 2018 2:44 pm

Mitch,

Regarding Chuck's advice, take what he gives you and tweak it a little here and there. There is really not much difference between his camera and yours especially as we move into colder nighttime temperatures. Chuck keeps his knowledge close to his chest. His reply to you is classic Chuck. Form your questions with pinpoint focus, something very specific that he can answer in a sentence or two. If you do that enough times you can get the broad picture.

Brian

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