I found it online and I think it was right here on Cloudy Nights.
I was trying to find the size of the obstruction in a ETX-125.
And I must have spent the entire evening (raining anyway) on the subject because
the baffle around the secondary mirror is causing confusion, it flairs out larger then
the secondary spot it's self.
Also the primary is actually 5.5" in diameter, adding to the confusion.
Some are saying if we say 5" for the primary and INCLUDE the secondary baffle,
40% central obstruction.
And I can say there is no way the ETX-125 has a 40% obstruction, it's too good.
If we use 5" aperture and the actual secondary spot diameter 1.55" = 31%, that's better
but what about the true size of the primary 5.5" and the true size of the spot?
28% obstruction. I feel this is more inline with what I see in the eyepiece but it
probably isn't correct.
It could be 40%. I tested my 2045 with pieces of masking tape around the co and at the edge and marked the limits when I placed my eye at the focus without an ep. Easier with an sct but maybe stretch masking tape across the whole aperture
I found the aperture was 99mm and the co 45.5mm giving a huge 46% but the image quality is still impressive although it would be put in it's place by a good 3'' refractor. The DX-8 seems to have 35/36%.
I wonder how much difference it would make to a Questar 31/2 if a 40% paper disc was stuck on. I expect it could be seen.
Edited by davidc135, 19 February 2021 - 07:41 AM.