MSG Script- Rejection in galaxy and nebula.

Roger Smith

I asked John Murphy about making big rejection areas covering a galaxy within NSG script. He wrote back to me on his script portion of PI Forum:

"Provided the scale factor (determined from the stellar photometry) is accurate, and a single scale factor is valid for the whole image, then it is actually desirable to have the sample squares cover the nebula or galaxy. Remember, we are not trying to find the background. We are only trying to measure the difference between the reference and target image. Hence samples over the nebula or galaxy help contribute to the relative gradient model."

So I will not manually restrict galaxies or nebulous regions. His comments are on P12. Previously you and I both blocked the galaxy, but with little possible detriment since the galaxy is small, and gradient small. Probably significant in a large nebula though.
Roger

BlockHead

Right! You do not need to restrict samples of nebulous regions. If I made a statement to the contrary I need to fix that. If there is a major difference in the gradient graph (which is a graph of differences)- then I might consider excluding. Either way, it is likely not a big deal...Certainly if the nebulosity fills the field... there shouldn't be an issue. The issue is usually scattered light or other local differences (perhaps due to flat fielding... who knows).

-the Blockhead

Roger Smith

After my re-checking, in your processing of Cosmic Canvas NG3614, 3rd video at 16:30 you excluded the galaxy. (You never mentioned nebulosity.) 

I looked at the gradient graph during my processing with and without the galaxy excluded. The gradient line difference was almost imperceptible. So no affect (effect?). Probably because the galaxy was small compared to the rest of the image (and few data points show up). A big nebula area would not be this case.

IMO the script should have automatically excluded that bright star to the left you had to manually exclude.

My guess is that in Josep's M51 data, where the galaxy is pretty big, the galaxy should not excluded in the NSG script. I want to go back and use the NSG script on it this time. On my long To Do list.

I am doing M31 (first time ever with my color CMOS) using the new separate channels debayering, then NSG script on each. I will not exclude the galaxy. NWeights on each channel will be a plus. Then MureDenoise on the individual channels, another plus. 

I would be almost at a loss without your videos telling me what is behind each step as you do it. 

Roger