The Pretoria 28mm

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Piet Le Roux South Africa
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The Pretoria 28mm

#1

Post by Piet Le Roux »


This eyepiece was designed by two South Africans, in Pretoria, in the mid 80's because fast paraboloids with Dobsonian mounts were becoming popuar amoung amateur astronomers. Big paraboliods with low F ratios results in a lot of coma and other aberrations developing at the eyepiece so it made sense to correct it at the eyepiece. The design goals were to produce coma less than 1 arcminute, astigmatism of 0 diopters, field curvature less than 1 dioptre and distortion less than 15% with a apparent field of view of 50 degrees. The logic was that if you could minimise aberrations you would not just see more detail but get excellent contrast.
Only 150 of these were made by University Optics in Japan and although the advent of coma reducers/correctors like the Tele Vue Paracorr are considered to be the reason for it failing to be a commercial success, I don't agree : the Paracorr were launched after 1990 and the first 150 Pretorias were on the market by 1988 selling for $225 US, none were made again although a 16 and 20mm were also made but in even smaller numbers. There were just not enough people using F4 Newtonian telescopes for visual astronomy that would buy these and then it would not do mutch good in a F10 SCT telescope that were the Most popular type.
DSCF0410-crop.JPG
I and some club members had the chance to tryout a Pretoria in a 15" Obsession and compair it to a Panoptic 27mm with a Parracorr II. This is 10 glass elements VS 6 and about $700+ VS $225. The Obsession has a 1717mm focal length and a 381mm mirror giving you F4.5 but with the paracorr II it changes to a focal length of 1975mm and F5.2, so the field of view is effectively not much bigger with the Panoptic.

After about 4 hours of viewing and comparing I have to give it to the Panoptic/Paracorr by a small margin, it was only a bit sharper edge to edge, but compairing the Panoptic by itself to the Pretoria, the Pretoria totally destroyed it!
Main Equipment : Tele Vue 27mm Panoptic, 7&13mm Nagler, Big Barlow : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal : Camera Fuji XT100
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Ruud
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Re: The Pretoria 28mm

#2

Post by Ruud »


Hi Piet,

thank you! I don't think I've seen a Pretoria before, except for on Chris Lord's eyepiece chart in the compensating eyepiece section:
Compensating_Eyepieces.png
The complete chart is here: download/file.php?id=7809

According to Rutten & van Venrooij "Telescope Optics", a five page article on the eyepiece appeared in Telescope Making:
H. W. Klee and M. W. McDowell. "The Pretoria Eyepiece." Telescope Making 29 (Winter 1986/1987): 4-9.
I don't have a copy, but it is available from https://myscienceshop.com/product/digit ... /tmkpdf029 (at modest cost - of course some of us will have the printed version lying around somewhere).

The eyepiece is also discussed in this free source: http://brayebrookobservatory.org/BrayOb ... enses.html which shows spot diagrams of how a Pretoria performs in a coma free telescope versus a coma rich f/4 Newtonian (look toward the bottom of the article)
7x50 Helios Apollo 8x42 Bresser Everest 73mm f/5.9 WO APO 4" f/5 TeleVue Genesis 6" f/10 Celestron 6SE 0.63x reducer 1.8, 2, 2.5 and 3x Barlows eyepieces from 4.5 to 34mm
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Piet Le Roux South Africa
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Re: The Pretoria 28mm

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Post by Piet Le Roux »


One of the club members that went stargazing with us to evaluate the Pretoria in the 15" Obsession just made me a offer for the Obsession, including the Pretoria and Paracorr, that I cannot refuse.....it looks like I will be parting with them!
Main Equipment : Tele Vue 27mm Panoptic, 7&13mm Nagler, Big Barlow : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal : Camera Fuji XT100
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Re: The Pretoria 28mm

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Post by Lady Fraktor »


I have read that the Pretoria is a fine eyepiece, thank you for the writeup Piet.
See Far Sticks: Antares Elita 103/1575, AOM FLT 105/1000, Bresser BV 127/1200, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS100/1000, Vixen SD115s/885
EQ: TAL MT-1, Vixen SXP, AXJ, AXD
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Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102
Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Takahashi prism, TAL, Vixen flip mirror
Eyepieces: Antares to Zeiss
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Re: The Pretoria 28mm

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Post by The Happy Parrot »


Interesting piece of history Piet. I'd love to try one of those eyepieces on my Z8.

It would be cool if anyone with the proper know how and tools made a Kickstarter to revive interesting eyepieces like the Pretoria. I wonder what other eyepiece beauties have gone extinct in recent times?
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Re: The Pretoria 28mm

#6

Post by DeanD »


Piet Le Roux wrote: Mon Mar 22, 2021 4:49 pm One of the club members that went stargazing with us to evaluate the Pretoria in the 15" Obsession just made me a offer for the Obsession, including the Pretoria and Paracorr, that I cannot refuse.....it looks like I will be parting with them!
Hi Piet, does this mean that you finally got the Pretoria eyepiece (as per previous posts), loved it, and now you are only keeping it for a couple of weeks...? It must have been a good offer! ;)

Do you have any plans to replace the Obsession and Pretoria?

Good luck, and all the best,

Dean
Telescopes: 12" f5 dob, Celestron CPC800, 150mmf5 Celestron achro, Tak TSA102, TV76, ETX125...
Binos: Steiner Wildlife XP 10x26, Swarovski 8x30 Habicht, Zeiss SFL 8x40, Vanguard Endeavour 10.5x45, Fuji FMTR-SX 10x50, Tak 22x60, Orion Resolux 15x70
Eyepieces: way too many (is that possible?), but I do like my TV 32mm plossl, 13mm Nagler T6, 27mm Panoptic and 3-6mm Nagler zoom, plus Fujiyama 18mm and 25mm orthos and Tak 7.5mm LE
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Piet Le Roux South Africa
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Re: The Pretoria 28mm

#7

Post by Piet Le Roux »


Yes its sad...but the thing is that I am passing 60 this year and I don't know for how long I can still handle the 27Kg mirror box....I would have liked to keep it longer but these type of offers dont come along every day. No I will not be repacing it unless I pick up a big Dobson very cheap like I did with the Obsession. I will be focusing on the 8" LX90ACF and doing AP with it.
PS at leased I can say that I made sure that at lease one Pretoria made it to South Africa!
Main Equipment : Tele Vue 27mm Panoptic, 7&13mm Nagler, Big Barlow : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal : Camera Fuji XT100
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