Does my result look correct? (ngc281, SIRIL)

[ausa]

Hey there!

I have to calibrate CCD images (ngc281)  for an astronomy practicum at my university. I have three narrow band filters (H alpha, OIII and SI) and bias, darks and flats for each filter. I stacked each filter individually and put them in to the RGB and then edited the result (I'm working with SIRIL). I finally came to a more or less satisfying result, but when you zoom into the nebula the colour of the stars are somehow strange. Is that normal like this? If not, how can I change that? 

Thank you so much!

 

[vlaiv]

Hi and welcome to SGL.

I see two things - first is that stars are bluish and second is that channels are not fully aligned properly.

Given that this is false color image - made out of narrowband data - stars simply won't have proper color. You are taking recording of very specific wavelengths and assigning them to whole part of spectrum (when doing RGB compose).

If you look at iconic image - Pillars of creation, by Hubble - you'll see very strange star color as well:

Pink stars? That does not happen. Stars lie on Plankian locus and have one of following colors:

If you want to try to fix channel shift - I suggest that you register channels against one (say OIII and SII against Ha). That should align data between channels.

[rickwayne]

For artistic purposes, we often use software to pull the stars out of an image and process them separately -- for narrowband, white is about as good as you can get. A good trick is to take RGB data of the same target and composite those natural-color stars with the narrowband nebulosity data. Not sure what your goals are for the class, though. In this context, "calibration" usually refers to using catalog photometric data to adjust the image's overall color balance so that G2v stars (such as the Sun) come out white. And as Vlaiv points out, for narrowband data that just ain't gonna happen! But of course it can have other meanings.

Edited January 12, 2022 by rickwayne