Best way to take Flats

[IDM]

I have been using flats for several years but recently was wondering if maybe the flats I was taking might not be as good as they possibly could. My current method is to use a light panel with several sheets of white A4 on top to dim the light source. I then adjust both the gain and the exposure time so that I can achieve an exposure time in the region of 1.5 seconds and the histogram at or around 50%.

In order to see if I was doing anything wrong or if my results could be improved I had a look around the web and seemed to find some conflicting information. Some sites state that the camera gain should be the same as used to capture lights. The problem I have with this is that if I keep the gain the same I end up with very short exposure times if I am to keep the histogram at or around 50%. This then conflicts with other online advice that states that flats exposure time should be somewhere in the range of 1-3 seconds.

Given I have spent hours optimising and capturing data I don’t want potentially reduce the quality of my processed image through poor calibration. Therefore What do people consider to be the best way to capture flats?

Thanks,

Ian

[Xilman]

Curious. I very rarely find that an exposure of less than several seconds is usable with a light panel. The time required depends on the filter used, of course. Perhaps your panel is intrinsically brighter than mine.

FWIW, I would suggest you continue with your current practice. Do you have any evidence that the flats you take are unsatisfactory? That is, do you see any residual gradients, vignetting, etc after applying your flats to your lights?  If not, leave well alone - i.e. if it aint broke, don't fix it.

[Elp]

I've looked into this a lot, especially once I got a 294mc camera which is notorious for calibrating (it's in fact nothing to do with flats but the construction of the sensor glass). What I did was use around four sheets of perspex going from opal down to an almost black. Even on the dimmest setting if I'm using a UV/IR or lum filter the exposure ends up being around 1-2s which I'd still consider fast. Using a computer controller helps as they usually have an auto exposure mode that works it out automatically when it takes test exposures prior to the run. If I'm doing it manually I try to make sure the histogram peak is around a third across from the left, but this largely depends on the F ratio you're imaging at, at F2 you'll find the centre of the flat looks like it over saturates very quickly so you have to adjust accordingly.

Your camera settings should be the same as the lights (gain, temperature etc) the only thing you adjust is the exposure time to get the histogram as you want it. If it's too bright very quickly you need to diffuse the light even more.

[IDM]

16 minutes ago, Elp said:

Your camera settings should be the same as the lights (gain, temperature etc) the only thing you adjust is the exposure time to get the histogram as you want it. If it's too bright very quickly you need to diffuse the light even more.

That's very helpful, so don't touch the gain just dim the panel with more layers . I am thinking that this is likely to need tp be very dim and therefore even with the light panel on the end of the scope there will need tp be some sort of seal to  prevent stray light creeping round the edge, or take the flats in total darkness. Would that be right?

[Elp]

What scope it is?

[IDM]

51 minutes ago, Xilman said:

Do you have any evidence that the flats you take are unsatisfactory? That is, do you see any residual gradients, vignetting, etc after applying your flats to your lights?

Unfortunately, I am seeing vignetting, this doesn't seem to happen with my refractors it is something that has become worse when trying to take Flats with my C8 SCT. Currently Graxpert is doing a good job of sorting it out, but that clearly isn't the best approach!!

[IDM]

1 minute ago, Elp said:

What scope it is?

Problem occurs with my C8 SCT with f6.3 reducer

[Elp]

What dew shield?

[IDM]

Just now, Elp said:

What dew shield?

The standard Celestron Velcro one. Though, I have been wondering if that's part of the problem as I have taken the flats without the dew shield, is that wrong?

[Elp]

Yes, as the vignette profile will likely change. I've got the C6/C8 shield, think it's the same as yours but my scope is C6. I find prior to imaging I have to be absolutely sure that the shield is square in both axis to the central axis of the scope (x and y if you're looking right at the corrector plate face on) or near enough as you can make it, and make sure the cutout is butted up to the start of the dovetail and that the circumference of the shield is fairly snug. This serves two purposes, as I point my scope directly up to take flats and rest the panel on top of the dew shield, so the shield doesn't move. I have on occasion had it shift but luckily the flats worked fine once I shifted the shield back straight. It's not an ideal method, and with your C8 the panel will be heavier.

I've thought about getting the aluminium dew shield to mitigate this issue but it's not exactly cheap. In the meantime I've got ND film to wrap around my panel so it's lighter to put on top of the dew shield then with the perspex I'm using currently, this may sort my problem.

But yes, you need to keep the optical path the same as your imaging conditions. It doesn't matter if light leaks out onto the sides of the outside of the shield as long as the panel is flat to it, covers it, and the dew shield is wrapped sufficiently around the OTA body that you don't get light leaking into it from underneath.

[IDM]

3 minutes ago, Elp said:

Yes, as the vignette profile will likely change. I've got the C6/C8 shield, think it's the same as yours but my scope is C6. I find prior to imaging I have to be absolutely sure that the shield is square in both axis to the central axis of the scope (x and y if you're looking right at the corrector plate face on) or near enough as you can make it, and make sure the cutout is butted up to the start of the dovetail and that the circumference of the shield is fairly snug. This serves two purposes, as I point my scope directly up to take flats and rest the panel on top of the dew shield, so the shield doesn't move. I have on occasion had it shift but luckily the flats worked fine once I shifted the shield back straight. It's not an ideal method, and with your C8 the panel will be heavier.

I've thought about getting the aluminium dew shield to mitigate this issue but it's not exactly cheap. In the meantime I've got ND film to wrap around my panel so it's lighter to put on top of the dew shield then with the perspex I'm using currently, this may sort my problem.

But yes, you need to keep the optical path the same as your imaging conditions. It doesn't matter if light leaks out onto the sides of the outside of the shield as long as the panel is flat to it, covers it, and the dew shield is wrapped sufficiently around the OTA body that you don't get light leaking into it from underneath.

Superb advice. I will make the changes and try again

Many thanks,

Ian

[Clarkey]

Good advice above, but I would add that it is very dependent on your set up. I image with a 1600MM pro and a RisingCam IMX571 colour camera and I have no problem with a standard LED panel (tracing panel from Amazon) and very short exposures << 1sec. (Not recommended with a 294 sensor). This is particularly convenient when taking NB and LRGB flats at the same time as I do not need to alter the screen brightness. Similarly, with the RC8 I remove the dew shield for the flats and it is fine. Obviously, this may not be the case with an SCT due the effect of the corrector plate.