The 5" mask was an off-axis mask that used an unobstructed portion of the mirror - no secondary mirror involved. The scope functioned as an achromatic optical system since the image was formed by reflective optics. I also had an 8" on-axis mask, but didn't use it much. It effectively increased the effects of the central obstruction on the image, reducing contrast & resolution. I can't recall if it helped with the size of the star images. It did help to reduce the brightness of the moon.
All I have is the observed effect on the behavior of the star images. The off-axis mask helped reduce the size of the stellar images in the eyepiece, but they shifted around more. This suggests that seeing cells were causing the image through the 5" aperture to show the effects of "twinkling" - shifting the image. At full aperture the stellar image expanded. I interpreted that to be caused by an integration of the several possible 5" apertures produced by the 13" aperture simultaneously.
I've never looked for any papers on the subject.
I did find this S&T article from 2006:
https://skyandtelescope.org/astronomy-eq...he-seeing/
Phil S.
All I have is the observed effect on the behavior of the star images. The off-axis mask helped reduce the size of the stellar images in the eyepiece, but they shifted around more. This suggests that seeing cells were causing the image through the 5" aperture to show the effects of "twinkling" - shifting the image. At full aperture the stellar image expanded. I interpreted that to be caused by an integration of the several possible 5" apertures produced by the 13" aperture simultaneously.
I've never looked for any papers on the subject.
I did find this S&T article from 2006:
https://skyandtelescope.org/astronomy-eq...he-seeing/
Phil S.