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Old 02-10-2020, 07:42 PM
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astro744 is offline
Join Date: Apr 2007
Posts: 1,008
Welcome! Nice ‘scope you have there, what mount do you have and do you have tracking in RA?

Your telescope is 1200mm focal length , 76mm aperture giving f15 focal ratio (1200/76).

At the low end you have three choices; (All 1.25” with 27mm field stop diameter giving identical true fields). Max true field for your telescope = 27 x 57.3 / 1200 = 1.3 deg.

40mm Plossl, (43deg apparent field of view) gives 30x and 2.6mm exit pupil.
32mm Plossl, (50deg AFOV) gives 37x and 2.1m exit pupil.
24mm Panoptic, (68deg AFOV) gives 50x and 1.6mm exit pupil.

Unfortunately you are limited in your max true field by the focal length of the telescope and the fact that you cannot use 2” eyepieces to get a longer focal length Plossl, say 55mm giving you 21x and 2.2 deg true field with 3.6mm exit pupil, (46mm field stop diameter). The exit pupil is how bright the image will appear. You want to operate in the range of 0.5 to 7mm typically although for a refractor the rules are not so stringent as there is no central obstruction and you can ignore comments on wasting light.

At the high power end (small exit pupil), images are dim and eye floaters can be bothersome. Also unless you have tracking, the objects will move very fast across the field.

At 1200mm with a 6mm eyepiece you get 200x and 0.4mm exciting pupil. This is workable and would likely be your maximum for a 76mm refractor. The only way to get a bigger exit pupil at 200x is to get a larger aperture. Note a 6mm Plossl has a very small eye lens and short eye relief. A more comfortable option would be a 12mm Plossl and 2x Barlow or 18mm Ortho or Kellner and 3x Barlow (I’ve never seen an 18mm Plossl but that doesn’t mean they don’t exist). Note at f15 poorer design eyepieces will work quite well but there’s no need to go out their looking for them, any eyepiece will work fine if of reasonable quality.

A 4mm eyepiece will give you 300x and 0.27mm exit pupil. Seeing will typically have to be superb to allow 300x but then the 0.26mm diameter image will be a bit on the dim side.

Exit pupil = eyepiece focal length / telescope focal ratio.
Exit pupil = aperture / magnification

Note I have rounded in some of the calcs above including f ratio.
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