Flats calibration wizard results are not good


first off, i’m using SGP
I’m experiencing issues with the Flats Calibration Wizard.
The problem is that the wizard does what it’s supposed to do, but when I apply the wizard’s results in a sequence, all my flats are overexposed.
Does this have anything to do with gain?
I’ve always found it strange that you can’t set gain in the wizard.
I use the ZWO ASI 2600MM camera on gain 100 and offset 25.

What did you use as your target ADU? If it’s too bright for you, I would suggest one of 2 things:

  • Sometimes this happens because the EL panel “warms up” and becomes brighter during integration leading to overexposed flats. Allow the EL panel to remain at the same level for around 3 - 5 minutes before starting the run.
  • Or… simply reduce the target ADU

Gain is not a property of flats, it’s a property of some cameras and, by extension, some sequences and equipment profiles. We realize it’s not almost the most intuitive process, but the idea of the flats wizard is to produce flats metrics compatible with an equipment profile (and sequence) and not for a unique set of parameters entered just before the process starts. If we allow users to uniquely apply gain and offset during this process we increase the likelihood that these flats metrics are not going to be compatible when you use them with your sequence or profile.

Well yes Ken, I agree that gain is not a property of flats. A properly exposed flat does not care what the gain setting was. This means a sequence flat does not have to be at the same gain as the corresponding sequence light to properly flat field calibrate the light frame.

However, if you calibrate your flat exposure time at one gain, and then take your sequence flat at a different gain you will not reach your desired ADU. If you do not set a gain (“gain not set” value) for a sequence event (flat or light), the gain used will be whatever the camera was set for in the previous target/event. The point is if you don’t set a gain, you really don’t know what it is (unless you take extraordinary measures such as opening the ASCOM profile for the camera and see what it is currently set at - or manually set it in the SGP settings for the camera).

When you do a flat exposure calibration, you must know what the gain currently is. I usually use unity gain for flat calibrations. Then when you program your sequence flat event you must set the same gain. For example, say you calibrate your flat exposure at unity gain. Then you run a sequence with a series of lights - the last light event before your flat events is an event with a NB filter with a gain of 2X unity gain. If you do not set unity gain for each each subsequent flat event (and leave the setting “not set”), you would overexpose your flats because they would run at 2X unity gain - the last gain setting command sent to the camera.

I am wondering if this is Miguel’s issue (or some variant of it).

Jim T.

Hi Ken,

i use 30K ADU, but i’ve had the issue with other camera’s too.
However, if you say gain is not a property of flats, does this mean that if i create a sequense for flats, i shouldn’t set a gain ?
If so, this might be the reason i’m overexposed ?

Thanks !

Hi Jim,

i believe the gain setting might be the issue here, i’ll test this next time i’m shooting flats.

I think maybe I expressed this poorly. For obvious reasons, if your camera supports it, gain and offset are used when taking flats or any other type of frame. I did not mean to imply anything else. Your camera doesn’t have the ability to “not use gain”, it will use it for every single frame.

The flats calibration wizard is designed to capture measurements that can, at a later time, be used to make flat frames as part of a sequence. Before you start the calibration run, you are required to select an equipment profile to which the results of the calibration run will be applied when it is done. You can either select the current sequence or any of your existing equipment profiles. Gain and offset, within the bounds of SGPro are currently properties of a sequence and are not currently properties of a frame type. A sequence or profile can set gain and offset in two different ways (per-binning and per-event). When you load a sequence or profile before you start the calibration run, the gain and offsets associated with that sequence or profile will be used. Later, when you create a flats event, the data captured during the calibration run will be used, but also, the same gain and offset will be used because they are properties of the sequence / profile that was also used for the flats calibration run.

All this is not to say it can never be… I am only speaking in terms of the current set of functionality.

I have always used a “profile” rather than a “sequence” to calibrate exposure time for a flat. But if I were to use a sequence that had per event gain settings - say a lower gain for RGB filters and a higher gain for NB filters, would the flats calibration wizard set gain differently for NB’s and RGBs as it went through the exposure time calibration? But is it true that all it would record in the filters list section of the sequence would be exposure time - right - and not the gains?

When I use the “flats wizard” to populate a sequence with flats, the flat events always come up with “gain not set” in the flat events. Intended gain setting for RGB vs NB flats seems arbitrarily dependent on the order of the flats and lights. In my experience - I need to set flat gains accordingly for RGB and NB Flats the same as what gains the the exposure time was calibrated to. I am using latest version 3.2 SGP. Maybe things have changed in version 4.1?

I think you should indeed manually set a gain to calibrate exposure time in the flats calibration wizard. If you don’t it will simply use the last gain the camera was programmed to. Further, I think you have to set a gain for each flat event in your sequence (see above hypothetical sequence with RGB and NB filters at different gains).

I use Unity Gain for my flat exposure time calibrations. I have to set this manually with camera settings before I run the flats calibration wizard - a one time exercise for each profile I create. When I populate a sequence (that uses the intended profile) with flat events, I set each flat event for unity gain. This way I always know that the actual flat gain is set the same as that used for the exposure time calibration for the respective filter flat. My flats are always properly exposed. If you do not set a gain explicitly (for calibration and for flat events), how will you know your flats are consistently using the intended gain (as calibrated).

I suppose if you ALWAYS use the same gain regardless of filter or bin and all of your lights use this same gain, you are probably OK as long as you are sure your flats calibration is using the same gain.

Currently, flats added to a sequence will use the per-binning gain and offset (same as the calibration wizard). I guess we never assumed that flats would have the need for a special or different gain or offset, but, all this stuff was written when CMOS wasn’t a thing. A lot of these assumptions are no longer valid and if folks find that there is benefit to using separate values just for flats, we can try to work that into the feature set at some point.

Ken, That’s good to know about the setting in “per-binning” gain and offset applying to the generated flats. Thanks for that explanation. I have not used this feature. Does a specific event gain setting override a “per-binning” gain setting? Or the other way around?

Yes, I use CMOS cameras and I get my best lights calibration happens when I take flats, light darks, and flat darks for each session. That is easy to do with an observatory and it doesn’t really cost time (my target has sunk too low andI’m usually sleeping during the darks). It just costs a bit more of disk memory. My CMOS cameras don’t respond well to dark scaling so I don’t use masters.

I typically set event gains for each light event. I may start using the per-binning gain setting, but that still means I usually want to set specific gains for lights (and thus light darks) depending on the target and filter. I would not want my per-binning gain setting to override the individual event gain settings for these lights and light darks.

It does. The order of priority for gain and offset is:

  • per-event
  • per-binning
  • manufacturer default (usually found in ASCOM settings)

Thanks Ken,

i’ll look into the fact of selecting the right equipment profile.
But i think this should be OK since i have set the profile where i use my 2600MM as default.

I still think something is wrong with the wizard, but i can’t put my finger on it, i’'ll investigate it further.

Well, what are the ADU values of a flat frame? How far are they off from your target 30000 ADU?