To Gain or not to overGain

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

Post by AstroDude » Tue Sep 11, 2018 5:29 pm

HI Robin, there has been so much talk with us guys and the AA183M/C hypercams about not getting enough stars to register in Pixinsight , and that has lead to hot discussions about the Gain values ... tool low ( 4) 400) not enough stars Ransac error in PIxinsight .
to hi a Gain 800~1250 (8-12) and now we are dealing with a big star burst. and a noise factor.

the thing is with the sensor analys which i like BTW, it would suggest Gain 350 offset 0... 68 sec subs on a 0.69 -e/Pix/sec
plug those # into SGP and your in big trouble, the ADU is about 78 out of 4098 and PI say " not gonna happen"...
after 6 mths of testing , we have had to get aggressive with Gain ( not so much with offset) to get any kind of image to stack.
then there the competition with there ASI1600MM Pro shooting Gain 200 ( above unity gain of 139) and 300~900 sec subs .with barely any noise
( I have the proof ) but who cares about that . LOL

SO the question is , what happens when we are shooting IC405 or IC1805 a faint target and we have to use the recommended Gain 400 or UNity
and also having to stay under 60 ~120 sec subs. the desired result do not match the expectations.

the other thing is Gain ( at unity ) 4/400 is fine if you want to shoot 300 subs per night and deal with the huge GIGAbyte ( 40-80)(bin1x1) total files in preprocessing. while the ASI1600MM boys only have to deal with 24-48 subs At 300 sec. in SHO.
SO again my Question is where is the happy medium ?
is it (Gain 350/offset 50 x 300 subs @ 60 sec) or (Gain 1250/offset 50 x 90 subs @ 45 sec)

So..low gain tons of subs or high gain fewer subs for the same dynamic range. or image brightness.

in simple terms LOw gain with 100s of subs.... or real high gain with very few subs. not with standing the offset ( blackpoint) always at 40 ish.
take all of this with a grain of salt, its just astro talk to find out why some do this and get great results and other try that and its left wanting.

According to NIck the ASI1600MM is a Panasonic chip and only is 56% QE as compared to our AA183M hypercam 86% @.

i suppose i need to push the Gaineven lower and try 4-8 minute subs and see if i can deal with the Starburst amp blow situation.

CHeers and keep up the great job via changes in SC 3.1 and beyond.

Astrodude / Mitch
https://www.youtube.com/watch?v=QgX9XDZlTfw&t=741s

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oopfan
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Re: To Gain or not to overGain

Post by oopfan » Tue Sep 11, 2018 7:22 pm

Mitch,

Just a thought: In File | Settings, General tab, do you have this checked: "Save 10/12/14 bit images in FITS files without stretching to 16 bit (Altair, QHY, ZWO cameras only)"? I requested this option several months ago to facilitate photometry. If you are doing AP or EAA and you are stacking off-line with DSS or PixInsight then you need to uncheck it. My camera is an Altair 290M. If I check this box then stars that saturate come in at 4095 not 65535. This upsets DSS I know for certain. Once I had to discard a entire night's worth of AP because I had this option checked.

Brian

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oopfan
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Re: To Gain or not to overGain

Post by oopfan » Tue Sep 11, 2018 7:58 pm

Mitch,

Actually, now that I think about it, I didn't have to throw out images. I used ImageJ to multiply each pixel by 16.

Brian

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markmac99
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Re: To Gain or not to overGain

Post by markmac99 » Tue Sep 11, 2018 9:50 pm

Brian - out of interest, how did you do that in ImageJ?
Mark M
---
Equipment:
Hypercam183C, modded EOS1100D, ASI120MM
Altair RC8, WO ZS70, PST
NEQ6 and EQ5, both on fixed piers

AstroDude
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Re: To Gain or not to overGain

Post by AstroDude » Tue Sep 11, 2018 10:07 pm

oopfan wrote:
Tue Sep 11, 2018 7:22 pm
Mitch,

Just a thought: In File | Settings, General tab, do you have this checked: "Save 10/12/14 bit images in FITS files without stretching to 16 bit (Altair, QHY, ZWO cameras only)"? I requested this option several months ago to facilitate photometry. If you are doing AP or EAA and you are stacking off-line with DSS or PixInsight then you need to uncheck it. My camera is an Altair 290M. If I check this box then stars that saturate come in at 4095 not 65535. This upsets DSS I know for certain. Once I had to discard a entire night's worth of AP because I had this option checked.

Brian
when you say "In File | Settings, General tab" are you talking about PIxinsight , i do not use DSS 4.1.1 and i should have been more specific as to the acquiring software which is SGP Pro. Gain at 4 and offset at 4 .
then Pix has a hard time finding any stars.. does that help.

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markmac99
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Re: To Gain or not to overGain

Post by markmac99 » Tue Sep 11, 2018 10:25 pm

Astrodude - the setting is in Sharpcap.

I had it on, and it probably explains why my images were unstackable!
Mark M
---
Equipment:
Hypercam183C, modded EOS1100D, ASI120MM
Altair RC8, WO ZS70, PST
NEQ6 and EQ5, both on fixed piers

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turfpit
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Re: To Gain or not to overGain

Post by turfpit » Tue Sep 11, 2018 11:19 pm

"Save 10/12/14 bit images in FITS files without stretching to 16 bit (Altair, QHY, ZWO cameras only)"
The default for this box is unchecked.

Robin - perhaps a note on the screen or a tip to say something along the lines of "only enable for use with photometry" otherwise we will all spend the rest of our imaging days having to deal with the unstackable frames issue?

Dave

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turfpit
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Re: To Gain or not to overGain

Post by turfpit » Tue Sep 11, 2018 11:27 pm

Mark

ImageJ - I got these instructions from Brian when I had the same problem (that will teach me to tinker with defaults).
  • Open an image file
  • From the menu, click PROCESS -> MATH -> MULTIPLY
  • In the dialog box, enter the value 16 (optionally check Preview)
  • From the menu, click FILE -> SAVE AS (choose FITS or TIFF or any other)
I found this document helpful with the batch processing:
http://www.sussex.ac.uk/gdsc/intranet/p ... essing.pdf

Dave

AstroDude
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Re: To Gain or not to overGain

Post by AstroDude » Tue Sep 11, 2018 11:28 pm

markmac99 wrote:
Tue Sep 11, 2018 10:25 pm
Astrodude - the setting is in Sharpcap.

I had it on, and it probably explains why my images were unstackable!
Done!!! is was checked mark ...

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turfpit
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Re: To Gain or not to overGain

Post by turfpit » Tue Sep 11, 2018 11:47 pm

Mitch

Back to your original post. How about this:
  • choose some object
  • capture using smart histogram suggested settings
  • capture using a variety of exposure, gain, offset values (user defined)
  • process captures ideally following the same process
  • report back with images and findings
I have not seen any systematic evaluation yet of outcomes from smart histogram settings versus user supplied settings.

No gain without pain ;)

Dave

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