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: