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Old 01-03-2021, 09:26 PM
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OneCosmos (Chris)
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Televue 55mm Plossl

Prompted by one of these advertised. Has anyone used it with a fairly fast mirror and noticed any really obvious problem with the resulting extremely large exit pupil produced?

I’d use it with the 28” f/3.8 SDM
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Old 02-03-2021, 06:04 AM
astro744
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How dark is your sky? If not very dark then you will see the secondary shadow even at night. The Tele Vue 55mm Plossl is intended for SCTs with their f10 focal ratio to get you a larger half decent exit pupil size but also for maximising field of view. I have one but still prefer the 41mm Panoptic on the C9.25 due to apparent field being more pleasing to me. The 55mm can also be used in refractors as there is no upper limit to exit pupil size because the image is as bright as it can get once you exceed your eye’s pupil diameter.

A 41mm Panoptic would be a better choice but still best for darker skies. I also use a 41mm Panoptic at f6.4 and f5.5 suburban/fringe/semi rural skies I am yet to try on my 16” f4.5 but have used a Meade 40mm 2” Plossl on it and hadn’t noticed a dark spot in the centre under same skies. I most often use the 31mm Nagler now again for the pleasing apparent field.

I often observe with a Paracorr (not always) and with your f3.8 telescope, I would think that a Paracorr is mandatory. Is f3.8 before or after Paracorr? If 3.8 is before then the exit pupil will be 1.15x smaller after Paracorr, (your f ratio is then 4.37).
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Old 02-03-2021, 07:01 AM
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Quote:
Originally Posted by OneCosmos View Post
Prompted by one of these advertised. Has anyone used it with a fairly fast mirror and noticed any really obvious problem with the resulting extremely large exit pupil produced?

I’d use it with the 28” f/3.8 SDM
Let's do the math.
The exit pupil would be 14.47mm.
If your secondary mirror is 25% of the primary in diameter (likely at f/3.8), then the secondary shadow would be 3.62mm wide !
Assuming your dark adapter pupil diameter is, say, 6mm (common in people older than 40), then the shadow of the secondary would be 60% as wide as your pupil!
You would see a big black hole in the image.

Normally, you wouldn't go larger in focal length than 6 x 3.8 = 22.8mm focal length, but I think it's almost always OK to sneak a little past your maximum exit pupil.
My dark adapted pupil is only 4mm, but I use an eyepiece with a 5.2mm exit pupil and it works fine, so I will assume you can use an exit pupil up to 3.8 x 7 =26.6mm focal length eyepiece.
But seriously, focal lengths longer than that make no sense and significantly reduce the clear aperture of the scope and lead to visible secondary shadowing.

Don't even be tempted unless you have a scope of f/8 or longer.
Don
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Old 03-03-2021, 01:16 AM
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OneCosmos (Chris)
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Quote:
Originally Posted by Don Pensack View Post
Let's do the math.
The exit pupil would be 14.47mm.
If your secondary mirror is 25% of the primary in diameter (likely at f/3.8), then the secondary shadow would be 3.62mm wide !
Assuming your dark adapter pupil diameter is, say, 6mm (common in people older than 40), then the shadow of the secondary would be 60% as wide as your pupil!
You would see a big black hole in the image.

Normally, you wouldn't go larger in focal length than 6 x 3.8 = 22.8mm focal length, but I think it's almost always OK to sneak a little past your maximum exit pupil.
My dark adapted pupil is only 4mm, but I use an eyepiece with a 5.2mm exit pupil and it works fine, so I will assume you can use an exit pupil up to 3.8 x 7 =26.6mm focal length eyepiece.
But seriously, focal lengths longer than that make no sense and significantly reduce the clear aperture of the scope and lead to visible secondary shadowing.

Don't even be tempted unless you have a scope of f/8 or longer.
Don
Thanks Don,

I have SIPs so my real focal ratio is 3.8x 1.15=4.37. Multiplying that by 7 gives me 30.59. I have the 31mm Nagler which may therefore be as long as I can go. Even the 41mm Panoptic would be pointlesss. I’ll hold off and just try it one night when a fellow observer has one to try.

Chris
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