Backfocus of the Celestron F6.3 Reducer/Corrector is listed as 105.0 cm but there has always been confusion on my part as to where to measure from. I contacted Celestron and they indicated that the measure was “from the rear surface where the flange of the T-adapter meets the end of the reducer” (i.e. bottom of the threads). I had been measuring from the surface of the lenses, a difference of 10 mm. This actual gave me a backfocus of 115.0 mm.
Much of the online discussion was regarding the F-stop at varying backfocal distance, which did not concern me so much. I was more “focused” on the impact on coma correction. Using the proper backfocus, my coma correction was indeed better.
What I didn’t appreciate is the impact on sharpness closer to the center of the image. This is a side-by-side comparison of backfocal distance (left being correct, right is 10 mm too far). The images are both essentially the same: integration time, pre-processing, and histogram stretch. Background values and S/N are nearly identical. Even though the sub-exposure length is 5 minutes for left image and 3 minutes for the right, I would not expect too much difference in the fainter portion of the galaxy given that the total integration times are nearly identical.
As one can see, the left-hand image is much sharper even though the guiding was slight worse for that image. I’m guessing that the focus across the image is better, allowing more light from the fainter portions of the galaxy to be gathered.
I was also testing the use of a longer sub-exposure time (5 min vs 3 min) to see if light pollution would play a role since many people seem to think that they need short exposures (30-60s) because they are under heavy light pollution. I think the results speak for themselves. As long as you aren’t blowing out your stars or galactic center, you can get the benefit of longer sub-exposures. NO LPF WAS USED and the blue filter should be worse-case for LP.
Let me know what you think.
The comparison Image can be found on my astrobin page at https://www.astrobin...hbpc8v/?nc=user