I finally figured it out!
I was having the damnedest time over the last year trying to take proper flats with my f/5 Dobsonian and my f/4 imaging Newtonian. The biggest issue was my Nikon DSLR that I was using. It added an awful chromatic rotational gradient that was very difficult to get rid of in post processing, and it would never completely correct with flats. Updating the camera to a dedicated astronomy camera solved that and, overall, was a huge improvement to my images.
However, with the new camera, I would still have problems with my flats not correcting completely. They would either not correct the vignetting, or they would leave an awful ring or other artifacts in the center of the image. I reckon that the ring in the images, both in the lights and flats, is due to reflections from the inside of my coma corrector, about which I don’t dare do anything. With the mirror cell on the SkyWatcher Newtonians, there is potential for light leaks, as well. Here’s what I’ve tried:
- Daytime sky flats. These never worked remotely correctly for me. The light leaks left awful blobs and artifacts in the middle of the image.
- Daytime sky flats with a diffuser. I’d try T-shirts, plastic, foam, and all kinds of things to diffuse the light and also dim it, but it never worked. And the more I dimmed it, the worse the light leaks would affect my images.
- Twilight sky flats. These would sometimes be pretty good, and other times not. I think the light leaks would affect these a lot as well, depending on how bright the sky was when I was taking them. If the flats were longer than five seconds, the light leaks were always a problem.
- Night sky flats. I could never get the star trails to reject out completely, so these never worked in production.
- Studio light box. I bought a cheap light box that has a diffuser screen on the front. I tried this right on top of the aperture, both with and without varying diffusers and dimmers. I also tried having it a few feet away from the telescope. The flats would always be kind of close, but never perfect. Usually, the vignetting never corrected properly. Also, with the light right on the aperture, some of it could go directly into the end of the coma corrector, and this would make all kinds of weird blobs and artifacts that were a nightmare to correct.
After having a good think one day, I hypothesized that I needed to somehow combine the proper vignetting correction of twilight flats with something brighter that would limit the affect of the light leaks. As a test, I stretched some white fabric over a large cardboard box and pointed my studio lightbox at it. I waited until complete darkness and took flats against this. When preprocessed with a master bias, I finally had true flat images! I think the keys are:
- Having a good distance between the aperture and the flat field to approximate the collimation of light from the sky. A flat field right on the aperture can lead to all kinds of reflections in the telescope that would not occur when imaging at infinity. Light could also directly enter the drawtube from the flat field source. Both of these problems will results in a flat that does not match the lights.
- Using a bright light source so that exposures are quick enough to “drown out” the affects of any light leaks. During the night, light leaks should be minimal in the sky exposures, unless maybe you have an awful lot of moon or security light illuminating your yard. If the flats accentuate the light leaks by requiring long exposures in anything but true darkness, you will add unwanted artifacts to your processed images.
With all of this in mind, I used the following to piece together a system that takes perfect flats with my 12″ reflector telescope.
- A reflective projector screen. Anything white and reflective should work, just make sure it is large enough to cover the field of view of your telescope at whatever distance you mount it. If it’s not large enough, you will not properly model the vignetting.
- A studio lightbox and a bright bulb. I know next to nothing about lighting, but you want a bright, neutral, diffuse source lighting your flat panel.
Here is how I have this set up:
I just manually point the scope at the center of the projector screen. Once it is true night, I turn on the light and make sure it is oriented so that the illumination is even across the screen. I then let NINA take flats as it calculates is correct. Over the last few nights, this has given me perfectly corrected images.
If you use Siril for processing, check out my guide on calibrating.