R,G, B filters focus green using Blue focus?

[Adam J]

I have been using my ASKAR FMA180 quite allot recently in my current situation its a easy way to image and the image scale makes things easy. 

I note that when using it with my ASI1600mm pro I still get slightly larger stars in red and Blue in comparison to G, this is down to the speed of the scope and budget ED glass. Focusing between R G and B filters as I cycle them. 

I have been wondering if I would be better off focusing on blue when imaging with green. My reasoning being that this seems to lead to the star sizes being better matched between all three channels but at the expense of losing the max sharpness of the Green channel. 

The underlying cause of this is spherochromatism aberration as opposed to classic Chromatic aberration, hence the focus is nearly identical but the scope is just sharper in green in comparison to Red and Blue. 

Anyone ever deliberately de-focused the green channel like this?

 

Adam

 

[vlaiv]

That will affect image more than defocus in blue and red.

We are most sensitive in brightness and green part of spectrum carries the most of brightness information. You can do small experiment.

Take any image, separate in R, G and B channels - then blur each channel and recombine image. Create one image with blurred red, one with blurred green and one with blurred blue channel and see which one looks the sharpest.

[ollypenrice]

I wouldn't run G out of focus. Rather, just increase the saturation in R and B a little to compensate.

Olly

[vlaiv]

I just now realized what was original question.

Don't do it - image with green being in focus.

You can always blur green channel a bit if you decide to "equalize" star FWHM. That + split bin would be my choice.

Alternative would be to use Green as luminance (works only on some targets), or to process stars separately and use Green as luminance for stars only. Later would be good on say Ha regions - since Ha is red signal mostly - it will be a bit blurry anyway so no point in correcting that but tight good looking stars can be had if you starnet++ image and extract stars only and then blur green to match R and B for color but use Green that is sharp for luminance.

[PadrePeace]

I’d strip the stars out using Starnet or Straton and then reduce the red and blue equally in PS. 
 

skinning cats here I think.

Edited October 19, 2021 by PadrePeace

[Adam J]

42 minutes ago, PadrePeace said:

I’d strip the stars out using Starnet or Straton and then reduce the red and blue equally in PS. 
 

skinning cats here I think.

Might make the stars look green if you drop the other two channels bit not sure what you would end up with if you then ran HLVG. Ok think it's worth a try. 

Edited October 19, 2021 by Adam J