Sequence Gain Setting (feature?)


Like many others I have pondering over my next camera and whether it will be CMOS. One of the things that I am very aware of is that, to overcome the lower ADC bit depth effect on read noise (among other things), it is recommended that you change the camera’s gain setting depending on the object type. For instance, ATIK recommends a low gain setting for stars and a high gain setting for imaging nebula.
For my targets, I always have narrowband events and I round up with RGB events to capture star color, combining both in the processing software.

Is there a way (or can it be) to have gain settings set by event within a single target (or at least have a gain setting per target, so I can run the target twice within a sequence for the different target content)?


You can set the gain per event. Click on the “Event Options” icon (the gear) in the first column of the sequencer panel, and select “Event Settings”.


Brilliant, thanks Don. I hadn’t noticed that. Now I just need to decide on a camera…


@buzz My typical workflow is just like yours and I use a QHY163M. Just a reminder: with most CMOS cameras, when you change the gain you should change the offset as well. As far as I understand, the ASCOM platform does not support changing the offset yet, so you won’t be able to do it through SGP.

In my case, when I need to combine events with different gains into the same sequence, I have to insert a pause between events, then change manually the gain AND the offset through the driver setup dialog. I plan to write a small app to change the offset automatically by running a custom Event Script in SGP (Pre-Event Options, Run script), but I’m afraid that it won’t be generic enough to be used by others.

As a general advice, while you’re looking for a new CMOS cameras you might want to check for models that handle automatically the offset value.


Thanks for that Alessio - I’m writing again, this time on a beginner’s book and I feel that I need to look more seriously at CMOS cameras, their price entry point makes them tempting for the first timer. My nagging doubt, triggered by the steady stream of forum issues, is the robustness of some the OEM’s hardware and software offerings. I’m leaning towards those companies, with a more established track record, even if they are slightly more expensive.


I also have a QHY163M and it has been rock solid for me. No issues with hardware or software. I’ve had it for about 1.5 years. At the time I bought it, I was a little turned off by ZWO constantly changing their hardware. If I had bought the first generation ASI1600 I would have been a little miffed when the second generation came out just a month or two later. Now of course there are a few more companies like Atik offering them. Still, I would buy the QHY163M again in a heartbeat.

Alessio is correct that currently ASCOM does not allow changing the offset so it cannot be done directly from SGP. However I tend to disagree that one “should” change the offset when changing gains. After carefully measuring noise profiles and dynamic range profiles of my camera, I have settled on gain 75 for LRGB and gain 200 for narrowband, but I stick with offset 35 for both. The offset isn’t critical. It just has to be set high enough so that the histogram is shifted to the right far enough to avoid clipping data. This means the background of my LRGB images is slightly brighter than my narrowband images, but there is still plenty of room on the left side of the histogram to adjust the black point for both images. Also, if you change the offset then you have to have matching darks as well. That’s a lot more darks in your library for something that doesn’t really need to be changed.


I would think you should redo your darks if you change the gain as well?


Yes, you need to have darks for each different gain (and offset) setting.


I know this is deviating from the original thread intent - but might this be accommodated by PixInsight’s calibration routine which scales the dark frame subtraction to reduce overall noise level?


I think we are basically saying the same thing. My mother tongue is Italian and I used the term “should” with the meaning “recommended for optimal results, but in no way mandatory”. I totally agree that offset isn’t critical at all and that is “has to be set high enough so that the histogram is shifted to the right far enough to avoid clipping data”, which is basically my recommendation (while others stick with formulas like offset= gain / 2). Full agreement on the QHY163M quality too.

I have a different experience regarding offset value at medium-high gain: I found that a narrow-band long exposure (10’ or so) at gain 200 requires a slightly higher offset (in the 50-60 range), but that could be attributed to variance among single instances of the camera/sensor. Regarding the dark library, as long as you stick with a single offset value for each gain value you’re going to use, you’ll have no issues. By the way, I think most users select just two or three gain levels, so you need only two or three dark frame sets for each exposure time and each temperature.

@buzz Unfortunately, this camera shows amp glow in long exposures and it also implements black point compensation. This means that you cannot scale dark frames, as they are not linearly related to exposure time, so PixInsight’s optimization is doomed to fail. By the way, for this camera I always recommend to calibrate manually (i.e. avoid using Batch Pre-Processing script) and to disable dark optimization. I wrote a paper/tutorial on this topic, so if you’re interested please let me know.


Thanks Alessio - I think I recall your paper. I just need to consider these sensors as the being equivalent to DSLRs- since the camera is doing its own dark subtraction using masked pixels and PI optimization fails.


I think you refer to my review about the QHY163M. The paper I was talking about (which I never really advertised in its English version) deals with dark current measurement and dark frame calibration, including a step-by-step tutorial about calibration of QHY163M frames using PixInsight. It’s available in here


Yeah @alessio.beltrame, we’re on the same page regarding offset. I just didn’t want Buzz or anyone to think that setting offset was mandatory when changing gains. You cleared that up nicely.

My offset of 35 for narrowband (gain 200) is admittedly perhaps a tiny bit too low. However I still have room on the dark end of the histogram so for my skies offset 35 works ok. Since ASCOM doesn’t have the ability to change offset I just know that I would constantly forget to change the offset myself. That’s another big reason why I keep the offset the same all the time.


Very good point. It happened to me more than once.


Thanks for the link Alessio. I was in NE Italy last week. Wonderful countryside, people and food. All I needed was clear nights!


Hi Chris,

I guess this might be a misunderstanding. Alessio is not referring to dark frame subtraction which is an option that you can choose in some DSLR cameras (e.g. Canon), but to a dark supression mechanism. This is quite different. A dark suppression mechanism is working in the electronic circuit before the AD converter and cannot be deactivated. The result of dark suppression is that bias frames and dark frames have about the same median intensity - the mean dark current is subtracted. The camera is subtracting a (for all pixels) constant offset that is determined from the optically black region of the sensor resulting in a black point correction. This has nothing to do with an automatic dark frame subtraction.



I’m probably using the wrong terminology. I’m not referring to DSLRs taking a second dark frame and subtracting but to the internal sensor mechanism, that uses the masked pixels on the chip to form its own subtraction… leading to bias frames being brighter than the dark frame of an extended exposure.


@buzz BTW, I live in NE Italy! 120 Km NE of Venice, 50 Km from Slovenia and 100 Km from Austria. Excellent wine too… but the latest months have been awful as regards weather.

Bernd’s remark is perfect. Therefore, you still need to do dark calibration, mainly to get rid of amp glow.


This sums up this forum - informed opinion, shared best practice and friendly help. Thanks to all and have a great weekend.