Gain and Offset settings for long exposure - Forums

Topic starter

Sep 19, 2016

Hi,
I just got my ASI1600MM C.
I tested it twice, and the result is nice. But i took Unity Gain and left the offset on 0. Ok, i found an discussion on the inet, where the offset should be set, that the lowest value in an bias shot is between 100 and 1000. In my case ist 16. so it seems i should raise the offset a little bit.
But what is about the gain for exposure >300s. Do you check that the maximum value is lower 65000 or do you use always the same settings?

regards
Thilo

Jason Guenzel

Sep 22, 2016

Hello Thilo,
There are default settings in the ASCOM driver that help with offset values. For gain/offset these are, 0/10, 139/21, and 300/50. If you draw a curve connecting these points on an X/Y scatter plot, it should give you an indication where you should specify offset at a given gain.

In general, the camera works very well at shorter sub exposures. The gain is obviously very configurable and that makes for a lot of decisions for the user. What equipment are you using with the camera?

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Thilo Frey

Topic starter

Oct 4, 2016

Hi Jason,
sorry i didn't got a message that an answer is here. I just saw it now.
Thanks, i will check this with the ascom driver. Always run it at unity until now.
I used the cam until now only with my 80/480 Apo, but want to use it also for widefields with my canon lenses later.

Carlos Uriarte

Oct 4, 2016

Hello Friend! I'm a similar situation about my new asi1600mmc. i have some subsexp with my William Optics Zenithstar80 FD. With orion nautilus filterweel and baader filters 1,25".
Well, my situation, its what is the best setup of gain and offset? For example: my subs are 200gain/0ofset. Its correct? or not?…
Thanks!

Ron Bokleman

Oct 4, 2016 · Edited: Oct 6, 2016

Jon Rista has done some significant testing of these values. The latest is 75/12 to 75/21, 139/21 to 139/35, 200/50 to 200/70. Obviously the higher the gain the shorter the exposures you will need to be to keep your star values below 65.504. From my experience this is what you'll see in PixInsight once you load a sample shot and clip/saturate a star. It will show up as 65.504.

Jon has also done several high gain images: http://www.astrobin.com/users/jrista/

NOTE: The new ZWO driver v1.0.2.16 appears to only allow an offset of 50 now.

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Carlos Uriarte

Oct 5, 2016

Ron, I'm surprised with Jon photos. Great! photos!!!

Thilo Frey

Topic starter

Oct 5, 2016

Yes i am also surprised. I have it only since short time and handled it like my CCD. But it looks i will needs to play a little with lower exposure time and higher gain.
I will try this in one of my next sessions

Thilo Frey

Topic starter

Oct 5, 2016

Steve

Oct 5, 2016

Interesting..I have zwo 178..I will have to try some of the above mentioned setting to see if that will help with star spikes..
I have been unable to find how to get camera back to defaults.

Jason Guenzel

Oct 5, 2016

You can use the camera "traditionally" with low gain and long exposures or with high gain and very short exposures. It's really up to the user to decide what the best fit is. Unlike a CCD, you can configure the gain how you like.

Honestly, fantastic results can come with either method as long as you carefully plan your exposure level within the dynamic range of the sensor. Just remember that there is no replacement for total integration time to pull in the faint bits.

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Steve

Oct 6, 2016

Ok,,thanks for the info..

Jon Rista

Oct 6, 2016

The camera is very flexible. It can be used in a variety of ways. It can be used at low gain much like a normal CCD, albeit with a lesser bit depth. It can be used at moderate gain settings for a better balance of dynamic range and low read noise. It can be used at higher gain to optimize for resolution and/or very high resolution scopes at very low read noise.

It just depends a lot on what you are interested in, what you are doing, what you are used to, and whether you are willing to explore some new possibilities that this camera can offer. For example, high gain, short exposure narrow band imaging such as I've been exploring recently. I've been amazed at what this camera can achieve with 90-120 second NB subs at high gain. I also think high gain can be a big bonus for high f-ratio imagers, as you can get the read noise down to around 1.1e- and still get moderately long exposures without clipping (something that is harder to do at low f-ratios, especially when you have a larger aperture).

Anyway, here are the gain settings I use, with a 150mm aperture at f/4:

Optimal SNR: Gain 75/Offset 15, 480-600s
Balanced SNR/Resolution: Gain 139/Offset 30, 210s
High Detail/Resolution: Gain 200/Offset 60, 90s

The exact exposure lengths you may need (or be able) to use at each gain setting will depend on your f-ratio and possibly your aperture (larger apertures can concentrate a lot more starlight on the same few pixels, which can make clipping a bigger issue).

I've mostly been using the High Detail option. It does not deliver the best SNR, so you might need to get some extra subs to balance out both resolution and SNR. One of the benefits of using short high gain narrow band exposures is, unlike with 1200, 1800, 3600 second exposures like you might use with a higher noise CCD camera, you have the freedom to discard subs that don't meet your resolution requirements. So it is kind of par for the course to acquire MORE subs than you actually plan to integrate, with the express intent of tossing the ones that are below your chosen quality threshold. The idea with high gain imaging is not necessarily to get maximum SNR, it's more of a means of getting the best detail. With modern NR tools like you can find in PI, stacking a couple hundred high gain subs can still produce very clean, low noise results, even if they are not quite as pristine as Gain 75 subs might be.

One of the main reasons I use Gain 75 is to get better SNR (I am finding that high gain is plenty good for Ha and sometimes even OIII, but SII really suffers). I've started getting all my SII at Gain 75 to maximize the SNR. I am still working out how to deal with the star FWHM differences...they are definitely larger with 600s subs.

One other thing I am still exploring is the best offsets to use at each gain. I was using an offset of 12 at gain 75, I upped that to 15. I upped the default offset of 21 at unity to 30. And I am going to try 60 maybe 70 at gain 200. Just to see if it will help me preserve more of the low level noisy pixels, which at higher gain settings still sometimes clip.

Hope that helps! ;)

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Carlos Uriarte

Oct 7, 2016 · Edited: Oct 7, 2016

Jon,
Thank you very much for your information so detailed. In my case I just 1sub: 600s 200gain / 0 offset.Halpha 6 nm.
http://foro.astrotorroja.es/download/file.php?id=5915&mode=view
but I have really wanted to try. Unfortunately we have some skies full of clouds. And I want to try with settings that you mentioned. Thank you very much!

Steve

Oct 7, 2016

Thank you Jon..
I tried some of those setting last night..Using a 102mm here so I expected to have to tweak the numbers a bit. But you gave me a basis on what to try and for that Thankyou Sir.

Jon Rista

Oct 7, 2016

Carlos Uriarte:
. In my case I just 1sub: 600s 200gain / 0 offset.

The sub looks pretty good! This is a very sensitive camera.

One thing though. You don't ever want to use an offset of 0. That basically guarantees data will clip unless you have a ton of LP. You want some kind of offset at every setting, even gain 0. At higher gain settings, a larger offset becomes more necessary. At gain 200, you will want an offset of at least 30, but in my experience that isnt' quite enough, so 50. I'm still seeing a little bit of semi-random banding with an offset of 50 at gain 200, which is the reason I plan to bump it up to 60 or 70 once I start imaging some new objects.

Anyway...keep offset above 0 at all times, and it should really be proportional to the noise you get on average at a given gain setting.

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Jon Rista

Oct 7, 2016

Steve:
Thank you Jon..I tried some of those setting last night..Using a 102mm here so I expected to have to tweak the numbers a bit. But you gave me a basis on what to try and for that Thankyou Sir.

For a given f-ratio things will largely stay the same. Smaller apertures will usually result in less intense stars, which is actually useful for higher gain settings as you can use longer exposures before you get too much clipping.

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Steve

Oct 7, 2016

OK,Thanks Jon..

I am rather new to the longer exp and tracking thing..Lots to learn..I spent a couple yrs shooting using a small cheap 80mm with my Nikon D-5100 at 1.6 seconds…Yes..1.6 lol…getting 2-10 mins now is amazing.
link to M16 from last night after collimating my 120mm as it was off very bad right from the factory..Just the way my luck goes..
hope link works for ya.
http://www.astrobin.com/267178/?nc=user

Carlos Uriarte

Oct 9, 2016

Jon Rista:
Anyway…keep offset above 0 at all times, and it should really be proportional to the noise you get on average at a given gain setting.

Certanly, was the first photo and I didnt know about gain/offset better setups.

Thomas Hellwing

Oct 11, 2016

Could someone please give me a hint or description what the exact function of the offset setting is? Thank you in advance! KR Thomas

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Chris

Oct 11, 2016 · Edited: Oct 11, 2016

I received my 1600 MM Cool yesterday and hope to get my 3nm NB filters very soon. Meanwhile, while I wait I will be trying out SG Pro with the camera hoping all will go smoothly including dithering. I assume the driver disc supplied with the camera will be out of date, so I will download the latest drivers online. Thanks to everyone for all the advice on these threads, especially the information shared by Jon

I'm really looking forward to trying out shorter subs for Narrow band, than those I'm used to using with my Mono cooled DSLR.
Chris

Jon Rista

Oct 15, 2016 · Edited: Oct 15, 2016

Could someone please give me a hint or description what the exact function of the offset setting is? Thank you in advance! KR Thomas

It adjusts the bias offset. In CCD cameras, the bias offset is usually fixed for you (in the case of QHY cameras, though, i think it is still configurable). DSLRs tend to use a fixed offset that is high enough for the highest ISO setting.

The ASI1600 has variable gain, and at higher gain settings, you usually need to use a larger offset to avoid clipping data to black due to noise. So the offset is variable as well. Here are the settings I use at various gains:

Gain 0 => Offset 10
Gain 75 => Offset 15
Gain 139 => Offset 21 (Unity)
Gain 200 => Offset 50
Gain 300 => Offset 65

For the most part, the optimal gain settings for this camera are 75, 139, and 200, which give you:

* Unity gain (139) which is sort of your "balanced" setting and is a good all rounder for the average user looking to keep things simple and easy.
* A high SNR/high DR gain (75) which gives you better SNR with longer exposures, but which might result in more star bloat if your tracking isn't great, and which might require longer narrow band exposures (600-900 seconds).
* A high gain/low noise gain (200) which can give you very low read noise (almost as low as it gets) without sacrificing too much on the SNR/DR front, and which would allow you to use very short subs to get the best resolution possible with just about any setup, even for narrow band (60-90 seconds!)

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David Dvali

Oct 15, 2016 · Edited: Oct 15, 2016

Jon Rista:
* A high SNR/high DR gain (75) which gives you better SNR with longer exposures, but which might result in more star bloat if your tracking isn't great, and which might require longer narrow band exposures (600-900 seconds).

Jon, isn't the Gain 50 with FW capacity > 10k e and a RN 2.5e would give you more CCD like performance? Why did you choose 75?
Is the reason 12bit ADC?..

CS

Thomas Hellwing

Oct 16, 2016

Jon Rista:
Could someone please give me a hint or description what the exact function of the offset setting is? Thank you in advance! KR Thomas
It adjusts the bias offset. In CCD cameras, the bias offset is usually fixed for you (in the case of QHY cameras, though, i think it is still configurable). DSLRs tend to use a fixed offset that is high enough for the highest ISO setting.

The ASI1600 has variable gain, and at higher gain settings, you usually need to use a larger offset to avoid clipping data to black due to noise. So the offset is variable as well. Here are the settings I use at various gains:

Gain 0 => Offset 10
Gain 75 => Offset 15
Gain 139 => Offset 21 (Unity)
Gain 200 => Offset 50
Gain 300 => Offset 65

For the most part, the optimal gain settings for this camera are 75, 139, and 200, which give you:

* Unity gain (139) which is sort of your "balanced" setting and is a good all rounder for the average user looking to keep things simple and easy.
* A high SNR/high DR gain (75) which gives you better SNR with longer exposures, but which might result in more star bloat if your tracking isn't great, and which might require longer narrow band exposures (600-900 seconds).
* A high gain/low noise gain (200) which can give you very low read noise (almost as low as it gets) without sacrificing too much on the SNR/DR front, and which would allow you to use very short subs to get the best resolution possible with just about any setup, even for narrow band (60-90 seconds!)

Thanks a lot Jon for these excelent explaining!

Thomas Hellwing

Oct 16, 2016

Jon you are never using the setting 0/10 for long exposures? I can do my shooting under a 20.8 mag/arsec sky (Unhidron SQM) and I am only using the 0/10 setting for exposures from 300-600 sek. Should I use the 75/15 setting?

fielderda

Oct 24, 2016 · Edited: Oct 24, 2016

Thanks for the great discussions regarding the Gain/Offset for this camera. I've just taken my first shots with my 1600MM cooled and Celestron EDGE HD 8 with 0.7x reducer at Gain 0/Offset 15 and found I had bloated stars with 300 sec exposures (with DGM NBP filter). I will try Gain 75 /Offset 15 and see if I can reduce the exposure times and hopefully the star bloat. It's quite different from a traditional DSLR!

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