JayTee wrote: ↑
Wed Jul 29, 2020 4:15 am
I had written a lot more about this article, but the more I wrote the more I realized that I didn't fully understand all that was being said. @notFritzArgelander, or anyone else, if you'd like to weigh-in to interpret/decipher, that would be greatly appreciated.
PS, I think I just opened a can of worms! jt
Let's discuss the blue line first. Coma
increases with distance from the center of field. So the blue curve in the graph gives the distance in the image from the center of field where there is 3 arc minutes of visual coma
as a function of the f ratio of the primary (the x axis).
The eye notices coma
if it is more than 5 arc minutes when viewed from the eye. So the red line corresponds to when the eye can resolve the coma
and it becomes obnoxious.
I use the ideas behind the graph to decide whether to use a coma
corrector or not with an eyepiece. To use the graph (or the equations behind it) one needs to know the field stop of the eyepiece and the f ratio of the scope. Specifically I use a simpler method of knowing the FS (field stop) of the eyepiece and computing the following quantity:
Newtonian_limit = square-root(2*FS)
If that number is larger than the f ratio of the scope a coma
corrector is needed. The need for a coma corrector depends on the eyepiece being used.
There is no magical f ratio that is good for all eyepieces / magnifications. If your eyepiece DOESN'T have a field stop then use the size of the barrel. If you can't find the mfr's FS specification you can estimate the FS from
FS = (fl
Here's some worked examples:
Televue Panoptic 41mm fl
, FS = 46mm. Newtonian_limit = square-root(2*FS) = 9.6. So this could be used without a coma
corrector in a Newtonian at f9.6 or slower.
Televue Panoptic 19mm fl
, FS = 21.30. Newtonian_limit = square-root(2*FS) = 6.5. So this could be used without a coma
corrector in a Newtonian at f6.5 or slower.
Televue Plössl 11mm fl
, FS = 7mm. Newtonian_limit = square-root(2*FS) = 4.3. So I could use this without a coma
corrector in my f5 Z12 but things would get hairy around the edges in my f4 AT 8".
Higher magnification eyepieces have smaller field stops that block out the part of the FOV
that has coma
I no longer have the AT8 f4, but when I did a coma
corrector was essential. I favored the TV Paracorr since it was so very easy to use. It has a built in helical focuser with markings so that if you know the setting for your eyepiece you can just dial it in and use the focuser on your scope. TV provides the settings in its specification page for eyepieces.
If you prefer a different brand of eyepiece, no worry. Just experiment.
The above formulas are pretty conservative. I find I could get by ignoring the coma
with a 24mm Panoptic in my f5 Z12. At f4 I never am without the coma
PS If you need references to how/where I got these formulas, that's another post.