Re: ATM 75mm 3" or 100mm 4" eyepieces

[epabcc@epa.ericsson.se (Bratislav Curcic)]

Andy Saulietis wrote :

> This doesn't make sense. Eyepiece coma or parabolic porimary coma?

Yes, it does (see discussion below). And I'm talking primary's coma,
asuming perfect eyepieces (or at least good enough not to overwhelm
the primary's coma with  its own aberrations).

>> Coma is a LINEAR function of off-axis distance.

> I agree, however the linear off-axis
> distance being viewed is a function of the eyepiece focal length,
> and is limited by the field stop, which in the case of a 100mm 4"
> eyepiece would be about 2" off axis!

Look it this way :
in a 5 mm eyepiece, we place star image at the edge of the field of
view (say that was 2mm off axis, for the sake of calculations, i.e.
field stop was 4mm). Say that 2mm off axis, primary's linear coma is 5
microns. Let's assume this is not resolved by human eye as a comatic
image, because 5 microns seen thru 5mm eyepiece look as ~3 arc minutes
blur - just resolved by normal eye, but still considered excellent.
Now scale everything 20 times : we have 100mm eyepiece, with 80mm field
stop, and we see 100 microns comatic image that is 40mm off axis. 
In tthe eyepiece it subtends, guess what - about 3 arc minutes again !!!  
We still do NOT see coma.

See, it was the eyepiece's field of view that dictated the angular size
of the comatic blur - if we use now 5mm eyepiece with 8mm field stop,
star image as seen thru eyepiece would subtend at 6 arc minutes - large
enough to discern it as coma.

Makes sense now ?

Bratislav