A while back an individual from another forum challenged me to step my older 180mm lens to the smallest focal ratio which is 2.8 as it would greatly reduce the amount of necessary integration time over the next step at 4.0. Of course this would naturally increase vignetting but this specific lens also has another greater challenge. In getting the tightest star focus it adds a magenta hue to the stars.
Jerry Lodriguss does an exceptional review of the Nikkor 180mm ED AI-S lens. He specifically addresses the focus issue in using the 2.8 focal ratio.
https://www.astropix.com/html/i_astrop/ ... 180mm.html
He discusses the challenge and the potential payback in using the lens at it's optimal focal ratio and focus.
A couple nights ago I had the great opportunity to image closer to the southern horizon while I camped on a Michigan lake. Still I had a very limited window of time to shoot the sky so I am trying what Jerry suggests and am hoping that StarTools has an antidote to the magenta halos.It would come down to the question of whether you want to waste clear dark-sky time shooting more frames stopped down to improve the signal-to-noise ratio, or spend the time in the electronic darkroom fixing the halos on a cloudy night. Personally, I would prefer an image with a higher signal-to-noise ratio because there is no substitute for this in terms of image quality, and clear dark-sky time is precious, whereas I know I can improve the halo problem later in processing.
I can show you what the results appear like. I know that I can't just increase the red rejection because there is some red nebulosity that I'd like to preserve.
Here is the base Fits file prior to the StarTools import in case someone would like to look at the image.
https://www.dropbox.com/s/fgp6nvqj0nc3y ... .fits?dl=0
Here is the resulting image that I processed in StarTools.