Why f-5.5 is better than f-4.5

I was running my 13.25 inch f-5.6 scope up against a very fine 12.5 inch f-4.5 at low power.

At f-5.6 the ammount of coma visible is minimal its hard to notice at all the number of people who have commented on this amazed me. All are use to our fine 12.5 inch f-4.5 newt. With a 40mm pentax xl (65 degree field) I get near the best poss deep sky exit pupil at 7mm!!

granted the tube or trusses are longer and a ladder is the order of the day for the vertically challenged like myself at 5 foot 9.

The f-4.5 is a fine mirror and works well at high power but the coma really detracts from the image at lower powers. Even with a paracorr it does'nt match the uncorrected f-5.6 images.

Then there's the figuring even a slight mistake in an f-4.5 is far more devistating than an f-5.6.

the 12.5 inch f-4.5 as a sphere has over 3 waves of spherical aberration to remove and the 13.25 inch at 5.6 its 1.6 waves.

If the respective parabolas are 10% undercorrected they read for the 12.5 inch approx .3 wave

and for the 13.25 inch at f-5.6 .16 wave well under diffraction limited.

I struggle to see reasons to go for a fast parabola in fact the idea scares me.

I have to admit I have the upmost respect for those of you out there that pull it off and make a good deep parabola. It still needs a paracorr or similar to give the good edge definition.

I will be using a ladder on the 16 inch f-5.5.

Convince me I'm wrong I'm interested to see what people think on this one.

You are not wrong, but I 've used a 16" f/6, and it doesn't get used much. It's awfully hard to work with. The result has been that I've dropped the idea of a 16" and am vaguely pursuing the idea of a 14.5" f/5 or a 12.5" f/6.

For many eyepiece designs, the astigmatism of the eyepiece swamps the coma in the mirror, so what is often taken for coma, is not.