Sky Flats. (Yes Again, Again)


How about:


but would need to make your own frame …



If the camera doesn’t have a shutter than you can take flats in bright daylight in a uniform section of blue sky - in which case the exposure isn’t changing much at all and the flats are captured fairly quickly. That is how I normally operate with the asi1600.

Even if the exposure changes a bit it doesn’t really matter because the flats are usually normalized before stacking. You just don’t want them too bright to introduce nonlinearities, or too faint to reduce the total photon count in each pixel.

In my case I don’t adjust exposure at all while capturing the flats in a given filter. But I do have to tune in the exposure for each filter in each session since the sky brightness varies from session to session. This would be much simpler if it were automated. A more sophisticated routine could look at each flat exposure as it comes in and slightly adjust the exposure to keep them exactly on target - but I don’t think that added complexity is needed.

For ccd’s with shutters you need longer exposures so you can’t do daylight and you need to do twilight flats. And since the exposures are longer and the sky brightness is changing - that is a much more challenging situation - and it would benefit from automatically adjusting exposure after each flat. But for shutterless cameras and daytime flats this isn’t really needed.



There is also a way to do it during the day (sky flats) by using film similar to solar filter but not as dark… you can also throw a T-shirt over it if you need to for LRGB flats.

I can’t think of what it is called or where to buy it but I think it might be a good idea.
Or use some dark tinted acrylic plastic sheets famed in a box that slides down over the front of the OTA to dim the sky.

I have not done any of this. I used to use CCDAP to do it but the download speed of my camera was much faster than it is now. These days it would be midnight before I had my flats done :slight_smile:


Resurrecting this thread as I have also been looking at ways to use SGP to take flats.

For now I have just been doing the following:

  • Create a second “flats” target in each sequence
  • This target is time delayed to start sometime just before dawn
  • Points at whatever part of the sky works best
  • Cycles through each filter, taking the flats I need.

I guess I’m curious what additional features a “sky flat” feature would add in SGP, above and beyond what I have described above?




i guess that we need automation for points 2 and 3 of your list. “whatever part of the sky works best” was defined in a paper by neil degrasse tyson (believe it or not) and is a moving target in time and position in the sky, both as the sun goes down/comes up and as the earth goes around the sun. so it needs to be computed every night/morning.

furthermore as the sky brightens or darkens, the exposure has to be adjusted to keep the flats with in an acceptable range of brightness, to make sure the exposure is in the linear range of the sensor. this varies of course with what filter you are using as well. so after every flat, SGP would have to evaluate the brightness, figure out how much time elapsed since the previous flat finished and the next would start, and lengthen or shorten the next exposure appropriately. again, the speed that the sky changes brightness is dependent on the time of year and your latitude. so there’s a lot that has to be automated to make the flat acquisition truly automatic.


If the sequencer calculated the ADU of the flat, and checked it against the desired ADU, it then could adjust the exposure time to hit it. This wouldn’t be too bad for LRGB flats, but Narrowband can be a challenge, since they tend to be fairly long, thus you could end up in a cycle where the ADU never hits the right target, or you run out of time to take flats since it is continually exposing, checking, adjusting, etc.


it works. what i have described is what ACP has been doing for years and years. once flats are in progress, the next exposure length is calculated and the flat is taken - there’s no checking before taking a flat. if it’s too dim or bright, it is what it is.


I built a light box that runs off my deep cycle battery.

Using an ATIK383L+ one of the problems was getting an exposure with a long enough duration that you do not get shutter shadow. At least 2-3 seconds. This requires a pretty dim light source and sky flats will not work at all.

The light box is adjustable. For the Lum Green and Red filters I use one setting, when I get to the blue and narrow band filters I pause the sequence, turn the flat box pretty much all the way then run those. If I did not do that the lum flats would be about 5 seconds and the Nb filters would be out to about a minute at least. Way too long :wink:

I do not run flats each and every session. In fact I only shoot a new set when something on the imaging train changes be it the OTA a different focal reducer or something like that.

I carefully index all ‘moving’ parts with painters tape and a sharpie so that when I do remove the camera it can be placed back very close, close enough for the previous set of flats to be used. Repeatbility is key there.

I have a few threads and images on other forums to illustrate what I am talking about but don’t want to cross post here. Just look for my user name on those other forums :slight_smile:


Ekos will check each flat, and if it goes outside of the tolerance range you provide it will change the exposure time for you.


yes, so does ACP. you described a degenerate case where the software just keeps checking exposures and never completes a flat, and i was trying to explain that it takes the flat, then adjusts the exposure afterwards.


Seems to be a lot of talk by users for this feature but little from the devs? Sky flats feature is a good addition to SGP and needed. Been to long without it. Any chance getting this happening?


Ditto - I’ve had to give up on my light box (DIY project) as I too get artifacts with my ASI1600mm. I’ve been doing sky flats, but the time it takes to sit there at the computer changing the exposures to make sure I’ve got reasonable sets of flats is annoying. Please consider this as a welcome enhancement to an already wonderful tool!


+1 Yes I 'd like this feature


+1 from me

I really like the idea of automated flats without the need to use flat boxes.
Normally I use my gear in the garden and have no choice to use wall-panels so this would be the most convenient way to make the flats.

BTW, if I‘d use some frosted acrylic panel wouldn‘t this be enough to eliminate any minor gradients or persistent stars in the sky???

Cheers Chris


Stars wont matter once the master flat is made. Leave tracking off when doing flats.


+1 for sky flats. Prism has it. ACP has it. CCD Autopilot has it. CCD Commander has it.

Come on SGP :slight_smile: !



Hello! I would like features for twilight flats as well.

FWIW, I find a median normalized sky flat from images of constant exposure times but varying ADU values to be my easier than trying to adjust image exposure times during the run.

Of note, I do not vary my exposures. After 2 or 3 runs of twilight flats, I have a good idea of when 1-second exposures go out of the linear zone of my CCD. I use about 50,000 to 10,000 ADU. I cycle through this range, starting a before and ending after each cut off. I get about 20 or so images in each of my photometric filters - B,V, and I.

I tried longer exposures after or before the main sequence depending on evening or morning twilight, but stars started to appear, and 20+ images from the 1-second exposures were sufficient for flats.

The constant 1-second exposure makes things easy for dark subtraction.

A median normalized combine then makes the final flat and normalizes the different image ADUs for the final image.

Depending on the sky conditions, I may switch to Light box flats or combine Light box flats and twilight flats. Best regards.