Preprocessing with Siril

I’ve been having quite the time figuring out how to properly calibrate the images from my big ass Newtonian. I think I might be getting somewhere!

The first key was taking proper flats, which I will do a separate post on soon. Along with flats, you also need bias frames. With dithering, however, I haven’t really felt the need to take darks. Noise on my new ASI2600MC is much better than with my DSLR, and with some decent dithering, I don’t find anything except fine luminance noise in my finished integration. However, to get something usable, you need flats and biases.

Step one: Bias frames

You will definitely need bias frames. I finally realized this when, once I started taking good flats, they over-corrected. I use an offset of 50 on my camera, so we need to at least account for this during flat application. There may also be noise or other offsets in the camera that can noticeably affect how flats correct an image.

For my camera, I took maybe 50 images at 1/1000 of a second with a lens cover on. You definitely do not want any light leaking onto the sensor. Load the images into a sequence in Siril, no need to debayer them. Go straight to the Stacking tab, and combine them. Make sure not to use normalisation.

Your result will be a master bias, or “offset” as Siril calls it. You can reuse this frame, so you don’t need to retake the images and reprocess them each time. At least, I don’t, and it doesn’t seem to be a problem.

Step two: Flat frames

Acquire your flat frames however works best for you. Load them into a sequence in Siril. Then, go to the Pre-processing tab. You will want to use the master bias that you just created. Don’t debayer.

Once the flats are preprocessed, go to the Stacking tab. Here, make sure to use the Multiplicative normalisation.

You now have a master flat.

Step three: Calibrate

Finally, create a sequence with all of your light files. In Pre-processing, you will want to use the master bias and master flat that you created. Also, use the “Auto evaluate normalisation value” and also debayer, if applicable. You should now have some pretty flat fields!