Gain on 533mc pro with fast lens?

[900SL]

Is there any benefit from using zero gain on the 533mc when using a Samyang 135 at f2.8 in a bortle 2 area? I know the full well capacity is significantly increased, and there's a minor bump in dynamic range, but offset against increased dark current.

The stock recommendation seems to be unity gain however the samyang is a fast lens and I don't want to saturate stars on widefield if I can help it.

[900SL]

Answering myself here.. based on Dr. Glovers youtube video about gain, it would seem that 0 gain is preferred as long as the exposures are not beyond the mount tracking capability, for full well depth and dynamic headroom

Using the 533MC exposure spreadsheet from Cloudy Nights, I'm getting an exposure from 1'30' to 2' depending on assumed transmission efficiency. I'll give it a go and see what happens   

 

 

[Pitch Black Skies]

But wouldn't your read noise be much higher at 0 Gain?

I would have thought unity gain is always best and then adjust exposure length until you are near the first third of the histogram.

I'm a beginner so take what I say with a pinch of salt. Hopefully some of the more experienced members will chime in. #@vlaiv

[SteveNickolls]

Not because it's unity gain but a gain of 100 or 101 will give you the benefit of lower read noise and higher dynamic range looking at the camera's data charts. Try it and see.

Cheers,

Steve

[vlaiv]

@900SL

Faster the optics - it is "more OK" to use lower gain settings.

If you want to use lower gain settings just to get "dynamic range" or to stop stars from being clipped - well - there is not much point in doing that as there will always be some stars that clip and dynamic range is really not important in single exposure - as stacking increases dynamic range with each added sub.

As far as stars clipping - there is universal way to solve that that works for every well depth and every camera - use shorter "filler" exposures that you'll use to get the data for clipped regions of long exposures.

There is really simple method of determining gain setting and exposure length. Lookup read noise for particular gain setting you want to use and measure what sort of light pollution you have (measure background ADU with that particular setup you have - from any of the images you've taken with that setup - just be sure to convert to electrons from ADU after you measure and that you measure on linear calibrated data).

Ratio of read noise to background noise should be at least 3 but preferably 5 or more.

According to ZWO website ASI533 has about 3.9e of read noise at gain 0:

This means that you need to make your exposure long enough so that background signal (LP level) reaches about 380e (3.9 x 5 all squared).

If you are comfortable exposing for that long (and that should not be a problem with short focal length and low working resolution) - then by all means, use gain 0.

[900SL]

On 07/03/2022 at 17:10, vlaiv said:

@900SL

Faster the optics - it is "more OK" to use lower gain settings.

If you want to use lower gain settings just to get "dynamic range" or to stop stars from being clipped - well - there is not much point in doing that as there will always be some stars that clip and dynamic range is really not important in single exposure - as stacking increases dynamic range with each added sub.

As far as stars clipping - there is universal way to solve that that works for every well depth and every camera - use shorter "filler" exposures that you'll use to get the data for clipped regions of long exposures.

There is really simple method of determining gain setting and exposure length. Lookup read noise for particular gain setting you want to use and measure what sort of light pollution you have (measure background ADU with that particular setup you have - from any of the images you've taken with that setup - just be sure to convert to electrons from ADU after you measure and that you measure on linear calibrated data).

Ratio of read noise to background noise should be at least 3 but preferably 5 or more.

According to ZWO website ASI533 has about 3.9e of read noise at gain 0:

This means that you need to make your exposure long enough so that background signal (LP level) reaches about 380e (3.9 x 5 all squared).

If you are comfortable exposing for that long (and that should not be a problem with short focal length and low working resolution) - then by all means, use gain 0.

Thanks Vlaiv

I used the 533MC spreadsheet from Steven Bellavia here:  https://drive.google.com/drive/folders/1EnRw69iA5_kTBxObnU9CKj5GilnZW8VJ

Adjusting for my local conditions I get a zero gain exposure of around 90" to swamp read noise by a factor of ten. I think this follows a similar process to the Sharpcap exposure calculation.

I'll give it a try end of March and report back, sandstorms permitting. I'll also check using the median ADU value for a bias frame to cross check the ADU value for my lights at that exposure

Edited March 8, 2022 by 900SL

[Adam J]

On 07/03/2022 at 14:10, vlaiv said:

@900SL

Faster the optics - it is "more OK" to use lower gain settings.

If you want to use lower gain settings just to get "dynamic range" or to stop stars from being clipped - well - there is not much point in doing that as there will always be some stars that clip and dynamic range is really not important in single exposure - as stacking increases dynamic range with each added sub.

As far as stars clipping - there is universal way to solve that that works for every well depth and every camera - use shorter "filler" exposures that you'll use to get the data for clipped regions of long exposures.

There is really simple method of determining gain setting and exposure length. Lookup read noise for particular gain setting you want to use and measure what sort of light pollution you have (measure background ADU with that particular setup you have - from any of the images you've taken with that setup - just be sure to convert to electrons from ADU after you measure and that you measure on linear calibrated data).

Ratio of read noise to background noise should be at least 3 but preferably 5 or more.

According to ZWO website ASI533 has about 3.9e of read noise at gain 0:

This means that you need to make your exposure long enough so that background signal (LP level) reaches about 380e (3.9 x 5 all squared).

If you are comfortable exposing for that long (and that should not be a problem with short focal length and low working resolution) - then by all means, use gain 0.

I really don't think that there is any justification for using gain 0 on this camera when you get nearly the same dynamic range at gain 100 and only 1.5e read noise. 

Adam 

Edited March 8, 2022 by Adam J

[vlaiv]

3 minutes ago, Adam J said:

I really don't think that there is any justification for using gain 0 on this camera when you get nearly the same dynamic range at gain 100 and only 1.5e read noise. 

Adam 

And why is dynamic range important in AP?

[Adam J]

Just now, vlaiv said:

And why is dynamic range important in AP?

Dynamic range is simply the trade off with gain / read noise and so I mention it. But I am struggling to see any technical factor that would make me choose 0 over 100. I would always take shorter exposures required to bury read noise if I could. I would actually push it up to 200 as 12 stops is still more than required. The only advantage of longer exposures is if you are worried about storage space. 

Adam

[vlaiv]

29 minutes ago, Adam J said:

Dynamic range is simply the trade off with gain / read noise and so I mention it. But I am struggling to see any technical factor that would make me choose 0 over 100. I would always take shorter exposures required to bury read noise if I could. I would actually push it up to 200 as 12 stops is still more than required. The only advantage of longer exposures is if you are worried about storage space. 

Adam

I agree with you, and personally don't advocate use of gain 0, but OP asked for gain 0 and as long as read noise is treated appropriately with adequate exposure length there is no reason not to choose it either.

[900SL]

The way I see it is if I can take 120 X 1 minute subs at gain zero then why take 480 X 15s at gain 100, if the end result is the same? 

I'll check the spreadsheet but I thought I noticed a better SNR shooting at zero gain for the same total exposure.

I'm lucky in that I can shoot from bortle 2 - 4 locations here

 

Edited March 8, 2022 by 900SL

[Adam J]

2 hours ago, 900SL said:

The way I see it is if I can take 120 X 1 minute subs at gain zero then why take 480 X 15s at gain 100, if the end result is the same? 

I'll check the spreadsheet but I thought I noticed a better SNR shooting at zero gain for the same total exposure.

I'm lucky in that I can shoot from bortle 2 - 4 locations here

 

If you are hell bent on it just make sure that you don't get any weird effects at zero gain in Darks first. 

But in answer to your question, there is no disadvantage to shooting 100 gain with the same exposure you propose. 

Adam

 

Edited March 8, 2022 by Adam J

[CCD-Freak]

Gain =100 is all I use and I often image at F2.8.  I then try to use subs that are short enough to keep from "blowing out" (saturating the sensor) the stars thus loosing their color. 

The chart shows that the Dynamic range jumps back up to almost as much as gain=0 but with much less noise.   I use -5C for cooling since it gets quite warm here in the summer and the ASI-533MCp is quieter than many cameras are at -15C .   M45 image 150 x 30 seconds at F2.8

Edited March 12, 2022 by CCD-Freak

[BrendanC]

I know this is an old topic but I'm always on the lookout for interesting new ways to confuse myself about exposure times.

So, @vlaiv, I've tried following your method. Assume I'm using a 533MC Pro, at gain 200, and my latest shots have a mean ADU of around 5000.

1. Lookup read noise for particular gain setting you want to use: for gain 200, according to the ZWO charts, seems to be about 1.4e
2. Measure what sort of light pollution you have (measure background ADU with that particular setup you have - from any of the images you've taken with that setup): my most recent shots are around 5000 ADU average (it was a full Moon).
3. Convert to electrons from ADU after you measure and that you measure on linear calibrated data: not sure what I'm doing here, but there's a chart for the 533 that seems to imply that e-ADU at gain 200 is about 0.4. So, does that mean I multiply 5,000 by 0.4, to get electrons? In which case, this is 2,000. Also, why would I need to use calibrated data here? 
4. Ratio of read noise to background noise should be at least 3 but preferably 5 or more. This means that you need to make your exposure long enough so that background signal (LP level) reaches about 380e (3.9 x 5 all squared). This is where I'm really lost. I don't understand how 'the ratio should be at least 3 but preferably 5' equates to '3.9 x 5 all squared'. I understand that the 3.9 here is the read noise at gain 0, so I would substitute 1.4 here, but I still can't quite make the leap from step 3, to this one.

So, what would be my next step after step 3 above, given the data I already have? 

 

Edited April 10, 2023 by BrendanC

[900SL]

It's complicated by 14 bit to 16 bit conversion, and offset.

Do you have your median ADU for any bias frames? Mine is around 2600 with an ASI Air Pro (default value)

[900SL]

12 hours ago, BrendanC said:

I know this is an old topic but I'm always on the lookout for interesting new ways to confuse myself about exposure times.

So, @vlaiv, I've tried following your method. Assume I'm using a 533MC Pro, at gain 200, and my latest shots have a mean ADU of around 5000.

1. Lookup read noise for particular gain setting you want to use: for gain 200, according to the ZWO charts, seems to be about 1.4e
2. Measure what sort of light pollution you have (measure background ADU with that particular setup you have - from any of the images you've taken with that setup): my most recent shots are around 5000 ADU average (it was a full Moon).
3. Convert to electrons from ADU after you measure and that you measure on linear calibrated data: not sure what I'm doing here, but there's a chart for the 533 that seems to imply that e-ADU at gain 200 is about 0.4. So, does that mean I multiply 5,000 by 0.4, to get electrons? In which case, this is 2,000. Also, why would I need to use calibrated data here? 
4. Ratio of read noise to background noise should be at least 3 but preferably 5 or more. This means that you need to make your exposure long enough so that background signal (LP level) reaches about 380e (3.9 x 5 all squared). This is where I'm really lost. I don't understand how 'the ratio should be at least 3 but preferably 5' equates to '3.9 x 5 all squared'. I understand that the 3.9 here is the read noise at gain 0, so I would substitute 1.4 here, but I still can't quite make the leap from step 3, to this one.

So, what would be my next step after step 3 above, given the data I already have? 

 

I found the answers to this thread pretty useful for the 533MC  

https://theskysearchers.com/viewtopic.php?t=13708

[900SL]

 

Also this thread.

 

https://www.cloudynights.com/topic/772214-zwo-533-offset-multiplier/

 

If using a 533mc pro with an ASI Air Pro the default offset bias is 70, which ASI Air Pro scales by a factor of 40x (14 to 16 bit conversion plus some other factor?), so typical bias frame median ADU will be shown as around 2800 in AAP histogram

 

 

Edited April 11, 2023 by 900SL

[BrendanC]

Thanks, but I've given up on all the online calculators. I just don't understand them. I've even created my own from the Robin Glover presentation... but I don't really understand how or why it works.

I just want to know how to continue with the 'simple' method outlined in this thread - and it doesn't surprise me in the slightest that it's not as simple as I thought! I really really really wish it was as simple as with a DSLR: exposure to 1/3 to 1/2 way across the histogram.

To answer your questions:

• Median for a bias is around 2,000
• Offset is 50

So, how would I progress from step 3 above to an actual figure for my exposures?

[Elp]

After much testing I haven't really seen much of a difference for DSO work no matter what the setting of gain, much more important to spend time gathering the images in the first place via total imaging time in semi decent conditions. So I've settled on a high gain of around 200 for my 183, and usually take 60-120s subs. That's it.

You could look down the avenue of utilising full well depth and bit depth at low gains, but not at the cost of high read noise. If you also compare FWD graphs across many cmos cameras you'll see most of them drop very very quickly after just a little gain increase which leads me to suspect it's just marketing fluff with no real practical use. As long as your histogram doesn't look clipped you should be okay.

The only camera I've used where gain setting is critical is with a 294.

Edited April 11, 2023 by Elp

[BrendanC]

Thanks, but I really really really really really want to know how to continue the method above, from step 3 onwards. I also shoot gain 200 with 300s for duoband, gain 101 with 60/120s for broadband... but I want to get a deeper insight as to how and why this works.

Edited April 11, 2023 by BrendanC

[900SL]

Im quoting from the link in the earlier post here:

'Assuming an offset of 50, the desired 16bit sky background you would want for 10 times RN squared would be about 300 at unity gain (the 300 being the 16 bit value). Read noise at unity is 1.5e- so [10 * (1.5)^2 + 50]*4 = 290'

HOWEVER where I get confused is the way the Asi Air Pro reports ADU. The bias frame has mean ADU at 2800 ( at 70 offset) so the offset is factored by 40.

Does this mean the target ADU should be around 3700? ie ((10×rn^2)+70) x 40?

 

[BrendanC]

I've no idea. I just want someone to help me with step 3 onwards!

Having said which I totally appreciate I may have hijacked your thread here. I just figured it was an old one and it would be OK to revive it.

Edited April 11, 2023 by BrendanC

[vlaiv]

22 hours ago, BrendanC said:

3. Convert to electrons from ADU after you measure and that you measure on linear calibrated data: not sure what I'm doing here, but there's a chart for the 533 that seems to imply that e-ADU at gain 200 is about 0.4. So, does that mean I multiply 5,000 by 0.4, to get electrons? In which case, this is 2,000. Also, why would I need to use calibrated data here? 

That is right.

You need to use calibrated data because you want only background signal. You don't want offset / dark current signal when you measure average background level as that will give you wrong reading.

22 hours ago, BrendanC said:

4. Ratio of read noise to background noise should be at least 3 but preferably 5 or more. This means that you need to make your exposure long enough so that background signal (LP level) reaches about 380e (3.9 x 5 all squared). This is where I'm really lost. I don't understand how 'the ratio should be at least 3 but preferably 5' equates to '3.9 x 5 all squared'. I understand that the 3.9 here is the read noise at gain 0, so I would substitute 1.4 here, but I still can't quite make the leap from step 3, to this one.

Ok - here it is a bit better explained.

Say that you have 1.4e of read noise at gain 200. Say that we set our ratio to 5 (you can use 3 here if you want, but let's go with 5).

This means that background noise should be at least 1.4e * 5 = 7e. This is background noise and in order to get background signal - you need to square this value because noise is equal to square root of signal it relates to.

So you need to have background value expressed in electrons to be at least 49 electrons.

You've got too high value of 2000e for your background signal in step 3 - and there could be two reasons for that:

1. you haven't calibrated your sub properly (haven't removed offset / dark current)

2. you've been using wrong e/ADU value

you say that you think e/ADU is 0.4 at gain 200?

ASI has gain expressed in 0.1 dB values. Unity gain is at 100, so gain 200 is actually 10 dB higher than gain 100.

10 dB is ~3.162 times difference, so actual e/ADU value is 1 / 3.162 = 0.3162 e/ADU

5000 * 0.3162 = 1581 electrons (but again - have you removed offset?)

 

[BrendanC]

I wasn't using calibrated subs, but what I can say is that my bias ADU is 2,000, so could that help here? So that if my sub's ADU is 5,000, and my bias is 2,000, does that mean I could assume 3,000 for a calibrated sub? The reason I ask is that I want something I can understand and use in the field, ie take a sub, evaluate it (using APT's Pixel Aid if possible), and act on that. Calibrating the sub wouldn't really be part of this process if at all possible.

I have literally no idea whether e/ADU is 0.4. I was reading one of the ZWO charts that seemed to be relevant.

OK, so this simple method is turning out not to be so simple, as is often the case.

What I will never understand is why this was so very simple with a DSLR (expose until 1/3 to 1/2 across the histogram), and so very opaque with a CMOS camera.

Since moving to a CMOS camera over a year ago, I have never found a simple, understandable, step-by-step method for determining an optimal exposure time. Like I say, I have a calculator of my own based on Robin Glover's presentation with mysterious C factor values and suchlike, but I don't really understand the logic of what I'm doing, or how to vary things depending on shooting conditions. All I know is that given different gain values I can have different noise values, but it all seems very arbitrary to me.

So, thanks anyway, but I think I'll just never understand this.

Edited April 11, 2023 by BrendanC

[vlaiv]

6 minutes ago, BrendanC said:

I wasn't using calibrated subs, but what I can say is that my bias ADU is 2,000, so could that help here? So that if my sub's ADU is 5,000, and my bias is 2,000, does that mean I could assume 3,000 for a calibrated sub? The reason I ask is that I want something I can understand and use in the field, ie take a sub, evaluate it (using APT's Pixel Aid if possible), and act on that. Calibrating the sub wouldn't really be part of this process if at all possible.

Yes, you can do that - just remove bias (and if you you have darks - maybe better to do average dark ADU value for that exposure).

5000-2000 = 3000 ADU, 3000 * 0.3162 = ~ 948.6 = ~950e

Given that your exposure requires only 50e of background signal, you are exposing for x19 longer with your setup than you need to. I'm guessing that you have fast optics and heavy LP?

8 minutes ago, BrendanC said:

I have literally no idea whether e/ADU is 0.4. I was reading one of the ZWO charts that seemed to be relevant.

Probably just a quick and imprecise glance at ZWO chart?

It is hard to precisely determine e/ADU value from graph like that. It is a bit closer to 0.25 than it is to 0.5 (and it should be since the value is ~0.3162).

You should be able to access this data in fits header of your subs, so check values stored there.

10 minutes ago, BrendanC said:

What I will never understand is why this was so very simple with a DSLR, and so very opaque with a CMOS camera.

Thanks anyway, but I think I'll just never understand this.

I find this method much more complicated with DSLR as there is no e/ADU value for given ISO. How do you convert to electrons with DSLR?

Not sure what you find complicated - you take a calibrated sub (you should really calibrate your subs for stacking, so there is no reason not to have calibrated value) - you measure median value on patch of background in the image - multiply with a number and compare to reference value you calculated from read noise.