A question on coma (Newts)

[Ags]

For the same true field, does a fast newtonian show more coma than a slow newtonian? For example, assuming I use a 24 mm 68° ep in a 6" f6 scope and a 16mm 68° ep in a 6" f4 scope, thus showing the same true field, would I see the same amount of coma?

[John]

I'm happy to be corrected but I think there would be less coma showing in the F/6 scope than in the F/4. 

Edit: If I am corrected it will be by a coma corrector 😁

 

Edited June 20, 2023 by John

[vlaiv]

First expression is in mm second is in inches

(I misread explanation on the page - first expression is PV error, second is RMS error, different numerical constants go for mm / inches - can be found on page)

D is diameter, F is F/ratio, alpha is field angle and h is linear height in focal plane.

Maybe best to use h as we have field stop diameters. For 16mm it is 18.2mm or h=9.1mm and for 24mm we have 27.2mm field stop diameter or h = 13.6mm

So we have

38 * 9.1 / 4^3 = 38 * 9.1 / 64 = ~5.4 waves of coma at the edge of the field with 16mm 68 degree EP at F/4 telescope vs

38 * 13.6 / 6^3 = 38 * 13.6 / 216 = ~2.4 waves of coma at the edge of the field with 24mm 68 degree EP at F/6 telescope

Coma would be about half with F/6 scope

source:

https://www.telescope-optics.net/newtonian_off_axis_aberrations.htm

 

Edited June 20, 2023 by vlaiv
error ...