Hi
I’ve been taking flats using the t-shirt over lens and pointing at the sky method, however the exposure varies quite strangely between shots. Most of the time I get images with the max and median values down in the low thousands and minimum down in the hundreds but sometimes I get ones where the values seem to be maxed out pretty much at 16384 (14 bit camera). I’m taking the images at 0.001 secs exposure with a sub second download time so I don’t think the sky brightness is changing that rapidly to cause this.

I also seemed to have similar problems determining a suitable exposure to actually take them at with a seemingly random correlation between exposure time and the statistics in the fits viewer.

This is using a sony a7rii camera connected to an astroberry running the latest kstars.

Has anyone noticed this with their setup?

Regards

Nigel

Nigel,

The trouble with taking sky flats is the light is always changing, which makes it a problem if you want to subtract darks, although with the speed you are using subtracting bias may be more appropriate.

I have even found this with a light source, but it is generally more consistent.

My suggestion is to move from sky flats to a flat light source.

In addition, that is a very fast exposure time and if the camera has a shutter it may be interfering.

I would aim for a less bright light source to allow an exposure time of around a second.

Paul

Hi Paul

yes it is rather faster than I was expecting. I’ve tried this on both blue sky and cloudy sky days with similar problems. Did wonder if it was shifts in brightness due to clouds but it just seems so random, rapid and extreme. Most shots are showing similar stats but every now and then there’s these jammed up at the limit.

Currently on the initial part of the learning curve about these things but can see the impact of flats so want to try and get these right. Biases I can vaguely understand but I’m not yet at the stage where I notice much change to my images, darks just seem a pain. Given the changes from people using ccd to cmos and the comments people make I do wonder which are worth the effort though definitely flats are.

I’m going to look for a light panel, tried holding an ipad in place but it’s just to awkward as I’m using that to control things.

Regards

Nigel

ok I’ve now got an led flat panel by the magic of Amazon.
I tried the auto flats option in ekos which came up with an exposure around 0.6 sec and also manually using aperture priority which gave 0.07 secs. Both came out with pretty similar distributions of pixel levels.

I still had first exposure out of the 10 or so I did hit 16384, the rest seemed fine.

The interesting/learning thing was that I used AstroImageJ to inspect the images using a 3d plot and found that my old flats where over exposed in the middle. You could see the dent of it hitting the limit in the middle of the dome of the vignetting. The new ones were a nice smooth dome which didn’t get flattened by hitting the limit. Expect that will make a difference to the final images.