I bought a cheap Chinese “UHC” filter on Amazon that only lets through light around the 500nm band, which is a cyan-ish color. It did work pretty good at making the Orion nebula pop in a big eyepiece. However, it seems to have some kind of reflection problem. Stars would appear thrice in my images. The advertisement for it also lied that it lets in light around the hydrogen-alpha and sulfur-ii bands, which are a dark red color. I think I’ll want to get a better filter.
No matter, I decided to try to photograph M42 again for fun. It’s basically the only nebula I can find so far, so you’re probably going to see it a lot before it goes over the horizon for the year.
I made some covers for my telescope to take dark and flat frames. I used some cheap fat quarters of fabric and stapled them to a wire wreath form that fits over the top of my telescope.
I took about 50 dark frames where the black cover was on the end of the telescope. These were taken at the same exposure and ISO that the pictures were taken at. Then, the white cover was placed over the end of the telescope, and I shined a white flashlight on it. I took 28 pictures like this at a shorter exposure of 1/25s. Later, I took 40 bias pictures at the same ISO and 1/4000s, with the lens cover on the camera.
The newest version of Siril, 0.9.11-alpha4, comes with a script to process the darks, flats, and biases into the picture. One button click, and lots of CPU cycles, later, a ton of the noise was removed from my relatively short 1/2s exposures. I stacked it using an average rejection scheme with “Windsorized clipping,” whatever that means!
I then used Gimp to increase the dark values to brighten up the nebula. Finally, I used several layers in subtract mode filled with gradients to remove most of the background and random noise.