bwilson
super member
Reged: 10/09/05
Posts: 187
Loc: Texas
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Here are my pictures of the once made UO. Pretoria Eyepieces.
The 16mm was the rarest, only 50 made to my knowledge.
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Barbara
20" f/4 SpaceWalk #004 Galaxy Serial # 19 ServoCat/Argo Navis
13" f 4.5
11" Celestron SC G11
8" f/6 Meade Newtonian
7" f/5 Starmaster "Ollie" Raycraft optics
WO 66 ED Triplet (Black Beauty)
Nagler 1 4.8,7,9,11,13
Nagler 2 20mm
Nagler 6 3.5mm
13 mm Ethos 8mm on its way
Clave 5mm 6mm
TMB Mono 8mm
All the Televue Plossls
Denkmeier 2 w/ powerswitch 24 Pans.
Pretoria 28,16mm
Nikon Diplomats
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bwilson
super member
Reged: 10/09/05
Posts: 187
Loc: Texas
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Both eyepieces, I have 2 28 mm. But picture is of one of the 28's and the 16 to show their size differences.
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Barbara
20" f/4 SpaceWalk #004 Galaxy Serial # 19 ServoCat/Argo Navis
13" f 4.5
11" Celestron SC G11
8" f/6 Meade Newtonian
7" f/5 Starmaster "Ollie" Raycraft optics
WO 66 ED Triplet (Black Beauty)
Nagler 1 4.8,7,9,11,13
Nagler 2 20mm
Nagler 6 3.5mm
13 mm Ethos 8mm on its way
Clave 5mm 6mm
TMB Mono 8mm
All the Televue Plossls
Denkmeier 2 w/ powerswitch 24 Pans.
Pretoria 28,16mm
Nikon Diplomats
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bwilson
super member
Reged: 10/09/05
Posts: 187
Loc: Texas
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From the top to show the coatings.
They are different. One is greenish, one is blue/purple.
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bwilson
super member
Reged: 10/09/05
Posts: 187
Loc: Texas
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top down
--------------------
Barbara
20" f/4 SpaceWalk #004 Galaxy Serial # 19 ServoCat/Argo Navis
13" f 4.5
11" Celestron SC G11
8" f/6 Meade Newtonian
7" f/5 Starmaster "Ollie" Raycraft optics
WO 66 ED Triplet (Black Beauty)
Nagler 1 4.8,7,9,11,13
Nagler 2 20mm
Nagler 6 3.5mm
13 mm Ethos 8mm on its way
Clave 5mm 6mm
TMB Mono 8mm
All the Televue Plossls
Denkmeier 2 w/ powerswitch 24 Pans.
Pretoria 28,16mm
Nikon Diplomats
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bwilson
super member
Reged: 10/09/05
Posts: 187
Loc: Texas
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28mm Pretoria
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Glassthrower
Vendor - Galactic Stone & Ironworks
Reged: 04/07/05
Posts: 17931
Loc: Oort Cloud 9
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I've never heard of these? What kind of design is it?
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Michael Gilmer - Member of the Meteoritical Society & Collector of Falling Stars.
☄ ⒼⒶⓁⒶⒸⓉⒾⒸ ⓈⓉⓄⓃⒺ ☞ www.galactic-stone.com
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Mike Hosea
Postmaster
Reged: 09/24/03
Posts: 6148
Loc: "Metrowest" Boston
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It's called the Pretoria, actually. It's a 6-element coma-correcting negative-positive design (with f/4 Newts in mind) by Klee and McDowell from the 1980s. I had a scan of the article describing it once, but I don't know where to find it now. Bottom line: Plossl-sized field, about a diopter of field curvature designed in (the article specifically notes this and argues that it is not too much). Interesting because of the coma-correction, but not a viable product in this day and age, which is why it went out of production so quickly. The combination, however, makes it collectable. I'd never seen even a photo of the 16mm. That's a great-looking piece.
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Mike
- 7" f/6.7 home-built Newt and equatorial platform
- 120mm f/8.3 home-built grab-n-go Newt
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Glassthrower
Vendor - Galactic Stone & Ironworks
Reged: 04/07/05
Posts: 17931
Loc: Oort Cloud 9
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Interesting.
So if it has a diopter of field curvature intentionally designed into the view, what would this do to the final image (visually) in a refractor or other instrument that is essentially coma-free?
Regards and clear skies,
MikeG
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Michael Gilmer - Member of the Meteoritical Society & Collector of Falling Stars.
☄ ⒼⒶⓁⒶⒸⓉⒾⒸ ⓈⓉⓄⓃⒺ ☞ www.galactic-stone.com
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bwilson
super member
Reged: 10/09/05
Posts: 187
Loc: Texas
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Mike,
Actually my understanding as to why they went out of production was an article post back in 1996 from Gary Hand who stated that one of the glass types specified was no longer being made by the major glass houses.
I can't remember the glass type I will have to look it up in the ATM article that specified the glass prescription.
Not sure why you state that this is not a viable product today? Is it because of the Paracorr?
McDonald observatory has one 28. I know a few people who have them, but they are rarely ever sold.
I personally have never seen another 16mm, glad that I have it.
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Barbara
20" f/4 SpaceWalk #004 Galaxy Serial # 19 ServoCat/Argo Navis
13" f 4.5
11" Celestron SC G11
8" f/6 Meade Newtonian
7" f/5 Starmaster "Ollie" Raycraft optics
WO 66 ED Triplet (Black Beauty)
Nagler 1 4.8,7,9,11,13
Nagler 2 20mm
Nagler 6 3.5mm
13 mm Ethos 8mm on its way
Clave 5mm 6mm
TMB Mono 8mm
All the Televue Plossls
Denkmeier 2 w/ powerswitch 24 Pans.
Pretoria 28,16mm
Nikon Diplomats
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Tom Trusock
Reged: 02/26/02
Posts: 33846
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Quote:
So if it has a diopter of field curvature intentionally designed into the view, what would this do to the final image (visually) in a refractor or other instrument that is essentially coma-free?
Field curvature of refractors typically goes in the other direction than that of newts, so while they would induce a diopter (or whatever) of field curvature from the eyepiece, it would be added to the field curvature from the refractor. These were designed for newts. I suspect that's one of the reasons they weren't as viable as other designs.
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You do not need a parachute to skydive, you only need a parachute to skydive twice...
Edited by Tom Trusock (10/02/07 01:04 PM)
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Tom Trusock
Reged: 02/26/02
Posts: 33846
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Barbara,
Thanks for posting those. I'd heard of these, but not ever seen any. Even any photos.
T
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You do not need a parachute to skydive, you only need a parachute to skydive twice...
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bwilson
super member
Reged: 10/09/05
Posts: 187
Loc: Texas
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Actually according to Klee's article in Telescope Making 29,
The field curvature is less than < 1 dioptre.
Performance Requirements:
Spherical aberration Diffraction limited
Axial Color Diffraction limited
Coma < 1 arc minute
Primary lateral color < 1 arc minute
Astigmatism 0 dioptres
Field Curvature < 1 dioptre
Distortion < 15%
Eyepiece focal length 28mm
Field of view 2 x 25 degrees
Exit pupil diameter 7 mm
Eye Relief >20mm
Primary Mirror paraboloid (f4) (designed for)
From the article:
Field curvature and astigmatism are related aberrations and their state of correction is open to controversy. Some prefer a flat tangential field, others a flat sagittal field, while others, including the authors, would prefer a curved field without astigmatism provided of course that the field curvature is not excessive.
They go on to talk about how the eye can accommodate and render the effects of astigmatism and field curvature less noticable.
They note that a young person can accommodate over 13 dioptres, while someone over age 55 can only accommodate by about 1 dioptre.
We therefore decided to design the eyepiece to show zero astigmatism and -1 dioptre field curvature while using a 6" f4.
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Barbara
20" f/4 SpaceWalk #004 Galaxy Serial # 19 ServoCat/Argo Navis
13" f 4.5
11" Celestron SC G11
8" f/6 Meade Newtonian
7" f/5 Starmaster "Ollie" Raycraft optics
WO 66 ED Triplet (Black Beauty)
Nagler 1 4.8,7,9,11,13
Nagler 2 20mm
Nagler 6 3.5mm
13 mm Ethos 8mm on its way
Clave 5mm 6mm
TMB Mono 8mm
All the Televue Plossls
Denkmeier 2 w/ powerswitch 24 Pans.
Pretoria 28,16mm
Nikon Diplomats
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csa/montana
Den Mama
Reged: 05/14/05
Posts: 76331
Loc: montana
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Barbara: Thanks for posting the photos of your mint eyepieces! I also had not heard of them; interesting to hear their story.
Carol
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Carol
AstroTech 16" Dob (Thanks ASTRONOMICS!)
Vixen 80MF
Masayumas 7.5,15,25W,35
Pentax XW5,10; XL7
TV22Pan
ES30
Paradigm 8,12,18
DreamCatcher Dobservatory
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bwilson
super member
Reged: 10/09/05
Posts: 187
Loc: Texas
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Thanks for the comments Carol and Tom.
I posted the pictures at the prompting of Preston (LivinDixie) as he had never seen them either, and wanted to see pictures.
In another post I wrote a story about them on the "Future of eyepieces" thread.
B.
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Barbara
20" f/4 SpaceWalk #004 Galaxy Serial # 19 ServoCat/Argo Navis
13" f 4.5
11" Celestron SC G11
8" f/6 Meade Newtonian
7" f/5 Starmaster "Ollie" Raycraft optics
WO 66 ED Triplet (Black Beauty)
Nagler 1 4.8,7,9,11,13
Nagler 2 20mm
Nagler 6 3.5mm
13 mm Ethos 8mm on its way
Clave 5mm 6mm
TMB Mono 8mm
All the Televue Plossls
Denkmeier 2 w/ powerswitch 24 Pans.
Pretoria 28,16mm
Nikon Diplomats
|
Mike Hosea
Postmaster
Reged: 09/24/03
Posts: 6148
Loc: "Metrowest" Boston
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Quote:
Not sure why you state that this is not a viable product today? Is it because of the Paracorr?
I don't remember the glass types being so exotic, but that explanation might be correct insofar as the price was going to have to go up to stay in production.
By "viable product", I really meant suitable for sustained production. This requires substantial and steady demand. What you have now is "curiosity demand", which isn't met by current supplies. That makes it feel like a better product than it is until you start doing the math.
The Paracorr is an extremely significant development, as its predecessors added significant spherical aberration, and its presence in the market, combined with many desirable state-of-the-art eyepieces on the market today which do not correct coma themselves, dramatically cuts demand for an eyepiece like the Pretoria with its pedestrian 50 degrees. The significant field curvature of this eyepiece also prevents it from presenting an image which is effortless to view.
This is not to say that I wouldn't like to see more coma-correcting eyepieces on the market. I just would not hold my breath. What I always used to wish for, since a Paracorr was a clumsy attachment to my 10" f/5 Teleport, was thread on coma correctors for my favorite eyepieces. But I had to concede that such a thing was unlikely from a business perspective. You could not make the thread-on coma correctors cheap enough that, adding up the cost for a few eyepieces, it wouldn't make optical sense just to attach a Paracorr and be done with it, and not many people would be as motivated to avoid the Paracorr on account of weight as I was.
--------------------
Mike
- 7" f/6.7 home-built Newt and equatorial platform
- 120mm f/8.3 home-built grab-n-go Newt
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bwilson
super member
Reged: 10/09/05
Posts: 187
Loc: Texas
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" The significant field curvature of this eyepiece also prevents it from presenting an image which is effortless to view. "
Interesting comment. I guess you need to look through one. I have never found a problem with focus from center to edge with these eyepieces due to field curvature. The whole field is in focus for me.
Klee said he designed them to work with 55 year old eyes or older that can easily accomodate the <1 dioptre field curvature.
As we all know optics are a design compromise, either astigmatism which I hate) , or a bit of curvature in this case. To each his own.
These offer high contrast and uncompromising image quality. I have never experienced any eye strain with these.
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Barbara
20" f/4 SpaceWalk #004 Galaxy Serial # 19 ServoCat/Argo Navis
13" f 4.5
11" Celestron SC G11
8" f/6 Meade Newtonian
7" f/5 Starmaster "Ollie" Raycraft optics
WO 66 ED Triplet (Black Beauty)
Nagler 1 4.8,7,9,11,13
Nagler 2 20mm
Nagler 6 3.5mm
13 mm Ethos 8mm on its way
Clave 5mm 6mm
TMB Mono 8mm
All the Televue Plossls
Denkmeier 2 w/ powerswitch 24 Pans.
Pretoria 28,16mm
Nikon Diplomats
|
Mike Hosea
Postmaster
Reged: 09/24/03
Posts: 6148
Loc: "Metrowest" Boston
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Quote:
Interesting comment. I guess you need to look through one.
Were you under the impression that I had not?
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Mike
- 7" f/6.7 home-built Newt and equatorial platform
- 120mm f/8.3 home-built grab-n-go Newt
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Tom Trusock
Reged: 02/26/02
Posts: 33846
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Barbara,
I'm with you. I detest astigmatism, and would much rather trade off other aberrations.
I was under the impression that field curvature of this eyepiece was subtractive in newtonians. Is it additive?
T
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You do not need a parachute to skydive, you only need a parachute to skydive twice...
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bwilson
super member
Reged: 10/09/05
Posts: 187
Loc: Texas
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Yes, Tom it is subtractive.
Hans Klee's own words since I happen to have his article handy:
The pertinent part:We therefore decided to design the eyepiece to show zero astigmatism and -1 dioptre field curvature with a 6 inch f4 paraboloid.
If used with a larger paraboloid of equal focal ratio, the field curvature will be slightly more negative, because the larger primary mirror will contribute less towards the field curvature compensation, but astigmatism and coma correction will not be affected.
Astigmatism unlike curvature of field can never be focused sharply except for the detail that is parallel or perpendicular to a radius of the field.
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Barbara
20" f/4 SpaceWalk #004 Galaxy Serial # 19 ServoCat/Argo Navis
13" f 4.5
11" Celestron SC G11
8" f/6 Meade Newtonian
7" f/5 Starmaster "Ollie" Raycraft optics
WO 66 ED Triplet (Black Beauty)
Nagler 1 4.8,7,9,11,13
Nagler 2 20mm
Nagler 6 3.5mm
13 mm Ethos 8mm on its way
Clave 5mm 6mm
TMB Mono 8mm
All the Televue Plossls
Denkmeier 2 w/ powerswitch 24 Pans.
Pretoria 28,16mm
Nikon Diplomats
|
sabir
scholastic sledgehammer
Reged: 06/23/07
Posts: 868
Loc: Pune (India)
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Quote:
This is not to say that I wouldn't like to see more coma-correcting eyepieces on the market. I just would not hold my breath. What I always used to wish for, since a Paracorr was a clumsy attachment to my 10" f/5 Teleport, was thread on coma correctors for my favorite eyepieces. But I had to concede that such a thing was unlikely from a business perspective. You could not make the thread-on coma correctors cheap enough that, adding up the cost for a few eyepieces, it wouldn't make optical sense just to attach a Paracorr and be done with it, and not many people would be as motivated to avoid the Paracorr on account of weight as I was.
Hi Mike,
How about the Baader Planetarium - Multi Purpose Coma Corrector (MPCC) 
...would like to know your views on the MPCC 
Thanks,
Sabir
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"Dominate the zeal of your pride, the vehemence of your castigation, the power of your hand, and the sharpness of your tongue. Guard against these vices by restraining all impulsiveness, and putting off all resort to force until your anger subsides, and you regain self-control. But you cannot attain such self-domination without increasing your pre-occupation with remembrance of your return to your Lord." - Imam Ali (as)
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