74 replies to this topic

[Jim Waters]

I want to double-check before I spend multiple nights imaging DSO.  What rules would you follow for selecting the Gain or would you always use a specific Gain and why (100)?  Are there any conditions where you would use one over the other?

 

What would you use for imaging nebulas (M42), galaxies (M81 / 82), fast vs. slow scopes, ..etc?  What about imaging from areas with heavy light pollution using filters vs. very dark sites?

Edited by Jim Waters, 29 April 2020 - 03:33 PM.

[bulrichl]

See https://www.cloudyni...ng-sense/page-3 , post #53.

 

Bernd

[Jim Waters]

Thanks Bernd.  I was involved in this post but I didn't see Jon's post.  It sounds like you should always use Gain 100.

[mewmartigan]

Jim, I am dealing with the same issue now that I have my 2600 in hand.

I kinda wanted to keep my 3 minute subs for broadband so I guess maybe gain 100 is best to ensure I am above read noise and the dynamic range is very close according to Jon's info. For my L-enhance I would probably keep 100 gain but maybe bump exposure time....although with my 294 I was able to continue using 3 minute subs.

[Jim Waters]

Thanks mewmartigan.

[Coconuts]

In general, I would prefer gain 0 for most cases, to avoid saturating stars with the higher full well capacity, and to limit the total number of subs to something reasonable from a processing time perspective.  If I was putting narrowband filters in front of the OSC sensor, then maybe gain 100.

 

All the best,

[Jim Waters]

I don't follow how shooting at Gain 0 would help limit the total number of subs.

[bobzeq25]

You need longer exposures at Gain 0, so, for the same total imaging time (the key concept), fewer of them.

 

With my C8 RASA (F2) in Bortle 7, I have little choice.  Gain zero means 10" exposures, hundreds of them.  Gain 100 would get even worse.

Edited by bobzeq25, 29 April 2020 - 04:27 PM.

[Jim Waters]

To Coconuts point above, would shooting at Gain 0 be best for images with bright start?

[Jim Waters]

With my C8 RASA (F2) in Bortle 7, I have little choice.  Gain zero means 10" exposures, hundreds of them.

So you use Gain 100?

 

I will be imaging from a Bortle 8 site with f/7 to 4 scopes & lenses until things get better - (COVID-19).

[airscottdenning]

There's no magic. Higher gain means BOTH more signal AND more noise in each sub. You can see more with fewer subs but a clean final image still takes just as many hours with Gain=100 as with Gain=0. 

 

My own preference is Gain=0 for "traditional" DSO imaging (many hours to many nights per final image), because I get better dynamic range in each sub.

 

OTOH, I juice up the gain as high as tolerable (200? 300?) for live video (EAA), so I can see a lovely color image quickly, and just put up with the extra noise.

Edited by airscottdenning, 29 April 2020 - 05:35 PM.

[Jim Waters]

 Higher gain means BOTH more signal AND more noise in each sub. 

No - Look at the ASI2600MC Pro plots from ZWO - Gain 100 noise.

 

https://astronomy-im...600mc-pro-color

[Jon Rista]

There's no magic. Higher gain means BOTH more signal AND more noise in each sub. You can see more with fewer subs but a clean final image still takes just as many hours with Gain=100 as with Gain=0. 
 
My own preference is Gain=0 for "traditional" DSO imaging (many hours to many nights per final image), because I get better dynamic range in each sub.
 
OTOH, I juice up the gain as high as tolerable (200? 300?) for live video (EAA), so I can see a lovely color image quickly, and just put up with the extra noise.

Higher gain means less noise per sub. You are making a common mistake by looking at the data "the wrong way", in ADU form rather than electron form. In electron terms, higher gains have less noise, and this can be demonstrated through testing. Here is some test data, represented in different ways, from an ASI183:
 

 
The top row here is what most people see if they simply stretch a raw dark or bias frame (or even a light frame) from a camera, and "see" that at higher gains, there is "more noise". Problem is, your eyes deceive you! There is NOT actually more noise...the higher gain means that each electron is represented by MORE ADU, which makes it SEEM like there is more noise, simply because AS REPRESENTED, there is a greater deviation around the mean. But this is a RELATIVE thing, not an absolute thing. 
 
The bottom row here is after the data was re-converted back to an electron representation. In this representation, a representation of the true analog signal, the drop in read noise becomes quite apparent...and the higher gains are exceptionally clean. Not just from a spatially random noise standpoint, but also from a pattern noise standpoint. 
 
This is the image form of the data. You can also take any slice of this data and render a 2D noise plot from it. Now, a slice through the middle of the top row results in something like this:
 

 
Again, it APPEARS as though higher gains have more noise. But again, this is simply because each electron (an absolute unit) is represented by more ADU (a RELATIVE unit) at higher gains than lower gains. In fact, this increased representation by ADU improves your ability to detect faint signals (small signals, few electrons), as with more ADU each electron, and all the noise within the signal, is more accurately represented. Note that while the lower gains with fewer ADU seem to have a smaller distribution...note the "stepped" or "blocky" nature of those plots...there are not all that many different possible points in the plot. This is quantization error.
 
This final example is a slice through the middle of the bottom row. The high quantization error at the lower gain is now quite apparent, while at higher gains the high precision and very low distribution is quite clear:
 

 

These plots demonstrate the true nature of what is really going on, before an arbitrary and relative unit, the ADU, obscures reality and misleads you into thinking high gain means more noise.

 

Now, you can "swamp" the read noise at any level, and effectively bury the differences between gains. This usually means long exposures at low gains, medium exposures at middle gains and short exposures at high gains (all on a relative scale of course...there is no absolute time here...short may be 10 minutes at high gain, if you are using 60 minutes at low gain...)

 

 

In this plot you can see how increasing the shot noise in each sub ultimately results in the signal, its noise and its distribution being totally dominated by what you get from the sky. Now, inherently, for a given exposure length, the amount of signal in a sub is the same...doesn't matter what gain. The signal you get from the sky is fixed for a given TIME. However, you can CHOOSE how long you expose for at any gain. You may clip at some gains, you may not swamp the read noise at others, but how much signal is in each sub is dependent on TIME, not gain. More time with the shutter open, the more signal you get. Increasing gain does NOT increase the amount of signal...it only reduces the amount of noise. So higher gains does not mean both more signal and more noise. On the contrary, usually, higher gains, since you also lose dynamic range with most cameras at higher gain, means less signal...and also less noise since read noise drops as well. Used properly (same total integrated time), you can get the same signal at a high gain as at a low gain. The differences is not necessarily how much signal, but how well the faintest signals are represented...and that is where higher gains can often do a better job with more reasonable exposure times (2, 5, 10 minutes, rather than 30, 40, 60, 90 minutes, per sub). 

Edited by Jon Rista, 29 April 2020 - 07:15 PM.

[mewmartigan]

Any thoughts on offset? NINA defaulted to an offset of 1 whether I am at 0 or 100 gain.

[Gert]

Hi Marcus,

 

Any thoughts on offset? NINA defaulted to an offset of 1 whether I am at 0 or 100 gain.

 

I've shot 0.1sec darks (in lieu of bias) with the ASI2600 @0C 100gain using NINA. Inspecting the FITS in 'ccdstack' I see an avg of 500 which is probably the offset. (there will not be dark current in 0.1sec)

 

 

Was wondering if one should spend any energy in thinking about changing that?

 

Clear Skies,

Gert

[bobzeq25]

To Coconuts point above, would shooting at Gain 0 be best for images with bright start?

There can be a number of considerations.  But lower gain, like lower ISO with a DSLR, generally means more dynamic range.  Considering that factor alone, you're correct.

 

Some cameras do have a "high dynamic range" mode that kicks in at something like gain 100.  Scroll down to the dynamic range graph here.

 

https://astronomy-im...600mc-pro-color

 

But note that gain zero still has a bit better dynamic range, though hardly enouugh to matter.

 

So you use Gain 100?

 

I will be imaging from a Bortle 8 site with f/7 to 4 scopes & lenses until things get better - (COVID-19).

Sorry, that was ambiguous.  I use gain zero to avoid having even shorter subs than 10", and having to do many more to get the same total imaging time.

 

Unless I'm doing narrowband.  <smile>

 

It's never simple.
 

Edited by bobzeq25, 29 April 2020 - 08:37 PM.

[Jim Waters]

It's never simple.

It never is.  Thx

[ZL4PLM]

my experience so far on my imaging with the 2600 is that the gain 0 vs gain 100 change isn't huge but there is def improved DR at gain 0

 

Gain 100 did not give a massive improvement in noise reduction and I have settled on gain 0 right now

 

Still working through my first uses of this camera but so far gain 0 seems to be favourable in terms of low noise / best DR

[elmiko]

I don't have that camera, but I use Unity gain with my ASI 071McPro. I figure as long as you collect enough data, the magic of pixinsight will pretty much fix most anomalies in your image.  

[Gert]

Hi Folks,

 

I've done the below from my suburban apartment with gain 100 and offset 500. Ca. 80x2min with 130mm F6.5 APO and flattener. Cooling at 0C. Image acquisition NINA. PI processing.

 

Clear Skies,

Gert

 

[Jim Waters]

Hi Folks,

 

I've done the below from my suburban apartment with gain 100 and offset 500. Ca. 80x2min with 130mm F6.5 APO and flattener. Cooling at 0C. Image acquisition NINA. PI processing.

 

Clear Skies,

Gert

 

Gert - These are really nice images...!  Great processing too.

 

You are using an Offset of 500?  I thought Offset for the 2600 was best at 50?  Am I doing things wrong?

[Gert]

Hi Jim,

 

Gert - These are really nice images...!  Great processing too.

 

You are using an Offset of 500?  I thought Offset for the 2600 was best at 50?  Am I doing things wrong?

Hm, maybe my wording was wrong. I was seeing that my FITS have 500 counts pedestal and I was assuming this was the same as offset. (See post #15) I don't have my laptop and camera handy. Once I have it fired up I'll look through the software settings and try to see what the offset says.

 

Just my thoughts about this. I would not understand why offset would be too high as at some point I am cutting down the dynamic range. from 500 to 65536 is less than from 50 to 65536. Alas it's splitting hairs at that level.

Clear Skies,

Gert

[buckeyestargazer]

In the ASCOM driver at least, the offset only goes to 240 which is ridiculously high.  Set at 50 and forget about it.

[PilotAstronomy]

I'm currently processing two different images at two different gain settings. Because it's two different targets it won't be a perfect comparison. But I'll show you what comes out of the stack when I'm done today or tomorrow.

B150 at 0 Gain 180 seconds and NGC6946 at 100 Gain 60 seconds. Both gave me roughly the same histogram peak in ADU. I can't say which image has better DR vs Gain yet (working through the process...it's a lot of files) but I can tell you without question that the 0 Gain, 180 seconds B150 was much easier to get through the initial process with due to being 1/3rd as many of these huge files coming out of this camera.

[alanrock]

I have the same camera, and after having done all this same research--Gain 100, Offset 50. :-)

 

Highly recommended reading: https://daleghent.co...g-camera-offset

Edited by alanrock, 26 August 2020 - 04:10 PM.