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Old 02-10-2020, 05:57 PM
4333bw (Brad)
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Advice re eyepiece please

Hi All,
First post, please be gentle. I have a Royal Astro 76mm x 1200mm telescope. I am using the original prism and a 11/4" adapter with some cheap eyepieces. I have a GSO 12mm plossl and a shorty 2x barlow. I am in almost inner Brisbane but have had some good views of Jupiter and Saturn but was wondering if I could improve things for Mars. Would a reasonable 6mm or 4mm Plossl work? I am not looking to spend a lot, $100 or so perhaps. At the other end of the scale how far can I go to get a wider view of the heavens? I was hoping to get involved with an Astronomical Society for advice but that has been put on hold.
Many thanks
Brad.
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Old 02-10-2020, 06:20 PM
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The Mekon (John Briggs)
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Welcome Brad. I owned a Royal Astro 76mm many years ago (1970 to be precise). It had very good optics and had a 4mm eyepiece with the scope which I did not use much. I think 300x was asking a lot despite the optics. So as you now have the capability of 200x with your 12mm and barlow - why would you want more as a 4mm will probably deliver less?
If I remember correctly these scopes had a dual barrel focuser - one section you pull out, the other is on a rack and pinion. What I would be looking at is the diameter of the second section and whether it would be worthwhile adapting that more directly to 1 1/4". But I have a feeling that this diameter was less and so this approach may not work. A replacement focuser with 1 1/4 capability would be much better that your current setup which limits the scope's field of view.
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Old 02-10-2020, 06:42 PM
astro744
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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.
Also
Exit pupil = aperture / magnification

Note I have rounded in some of the calcs above including f ratio.
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Old 02-10-2020, 06:49 PM
astro744
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Just read the other post. Do you in fact have a 0.965 focuser and are using a 0.965 to 1.25” adaptor? If so then your maximum true field will be significantly less than the figures I had calculated due to vignetting by using the adaptor since the clear aperture is only 24.5mm absolute max so any eyepiece with 27mm FSD will be restricted. For low power may I suggest a separate shorter 80mm refractor with 2” focuser.
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Old 03-10-2020, 03:38 PM
4333bw (Brad)
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Advice re eyepiece please

Thanks John and The Mekon, The telescope does have a long .965" focuser, one part pull out and the other on a rack and pinion and I am using an adapter to fit the 1.25" eyepiece. I have a cheap 4mm SR that I have used - 'tis pretty dull and objects certainly scoot past. I was just wondering if a better eyepiece, say a 6 or 4mm would be better than the 12mm with the 2x barlow. Thanks for your help, might just stick with what I've got and save for a 8" dobsonian. Brad.
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