Star bloat with blue filter

[mikeyj1]

Hi,

can anyone help me - I've attached some png's showing stacked channels on the pacman nebula.  Ha and Green are really sharp, but the red and blue are showing really bloated stars.  i didn't re-focus on filter change as I had left it running and don't have an autofocus on that scope.  Having said that, the small / faint stars in the images still seem nice and sharp...so is there another explanation, and how should i handle the processing?

thanks in anticipation

Mike

 

NGC281_Pacman_Nebula-Blue-session_1.fits

[vlaiv]

What scope was this?

If using refractor, you should really refocus on filter change.

There is exaggerated focus vs wavelength curve:

Look at dashed red line. Green is at 550nm, blue is at around 450nm while red is at 650nm - they all have different focus positions.

Another thing to be careful with is what RGB filters you are using. Sometimes filters can have UV or IR leak at far ends of spectrum and that can cause star bloat. In that case - use UV/IR cut filter together with particular color filter.

Btw, you can process above image in the following way:

make it LRGB image and use Ha as luminance - because it is both sharp and captures most of the nebulosity. Human eye is much less sensitive to noise and blur in color than it is in luminance.

[mikeyj1]

It’s an 102ed Altair refractor.  The fits file is also there for info, the graph you show is interesting, so you think its definitely a focus issue?

I focused on LUM, which i didn't even use...

i guess i need to do some experiments on focusing next clear night

thanks Vlaiv 

Edited August 1, 2020 by mikeyj1
added to my short phone written reply

[vlaiv]

5 minutes ago, mikeyj1 said:

It’s an 102ed Altair refractor.  The fits file is also there for info, the graph you show is interesting, thanks Vlaiv 

Ah ok, yes Altair 102ed has FPL-51 glass and is not corrected for color perfectly.  Unlike Altair 102ED-R which is newer model with FPL-53 glass and better color correction.

With this scope, you'll most definitively need to refocus between filter changes and when shooting luminance - maybe it would be best if you get Astronomik L3 as that one removes violet and deep red parts of spectrum to suppress star bloating in such scopes.