30 replies to this topic

[RichA]

Seems I remember reading about "complimentary" eyepieces used to correct or improve certain telescope attributes, like spherical correction.  But the Pretoria was the only eyepiece I specifically remember as being designed to do it.

 

[Ernest_SPB]

There are a lot EP-s that compensate some or another aberation of main lens. It is especially usual  practice in microscopy (lateral color correction) and widely used in devises that could not be disassembled (binoculars, spotting scopes, etc.). 

 

"Pretoria" was designed to compensate coma of Newton (not sperical aberration). It was possible just becase all Newtons has the same conditions to compensate the coma.

[Jan Owen]

For those who have never heard of, or seen, a 28 mm Pretoria coma correcting eyepiece, here's mine...

 

[junomike]

Brandon also had a version in 16mm and 20mm I believe.

 

Mike

[Scanning4Comets]

I've looked through one before, (Pretoria). AFOV was small. It wasn't like some kind of revelation took place. It was quite ordinary IMO.

Edited by Scanning4Comets, 11 August 2015 - 03:06 PM.

[Dave Mitsky]

Years ago I had a chance to look through a Pretoria, with similar results.

 

Dave Mitsky

[Starman1]

What killed these eyepieces was that they had narrow fields of view precisely at a time when ultra-wide eyepieces were becoming popular.

And they were expensive.

And those ultrawides essentially had no issues with the central 50 degrees of field anyway.

And, for me, I had other 50 degree eyepieces that were sharper on axis.

[Jan Owen]

I list the Pretoria as having a 55 degree apparent field (don't remember what UO said about apparent field; my estimate came from timing stars across the field from field stop to field stop) in my eyepiece spreadsheet. So, whatever else it is, it isn't a wide field eyepiece...

As it turned out, I used it a lot at first, but as the TV Wide Fields, Naglers, and then Panoptics came out, I bought into them heavily based on their performance, and before long, the Pretoria was retired to my *classic* eyepiece case, instead of my 2 large *working* cases. But because of it's rarity, I kept it for illustrating discussions such as this...

The Paracorr doomed it, too, because images, using a Paracorr & a highly corrected Nagler or Panoptic (preceded by the Wide Fields), are noticeably sharper across the entire field in a Newtonian than the Pretoria is, and the advent of the Paracorr also improved coma-induced edge performance when used with most eyepiece designs (though it doesn't correct eyepiece-induced aberrations), making the Pretoria yesterday's news...

The Pretoria was cool when it first came out, though!!! And briefly, it was nice to have a relatively innovative eyepiece, even if it wasn't perfect, but it was quickly overshadowed by the *eyepiece revolution*, led by *Uncle Al* that had already begun, and soon left the Pretoria behind.

Edited by Jan Owen, 11 August 2015 - 05:46 PM.

[Jon Isaacs]

 

The Pretoria was cool when it first came out, though!!! And briefly, it was nice to have a relatively innovative eyepiece, even if it wasn't perfect, but it was quickly overshadowed by the *eyepiece revolution*, led by *Uncle Al* that had already begun, and soon left the Pretoria be

 

 

Jan:

 

That was an excellent discussion of the Pretoria eyepiece and it's place in history.  It's a keeper because of it's place in history.  It is also a good example of why a separate unit that corrects a particular aberration is preferred over an eyepiece, that eyepiece can only be used in a particular type of scope. 

 

Have you ever used the Pretoria in a refractor?  I used the Paracorr in my NP-101, I could see the reverse coma.

 

Jon

[KaStern]

Hello Jon,

 

 

I used the Paracorr in my NP-101, I could see the reverse coma.

 

  Thank you very much for your statement!

 

I have often said that this would happen, but most people did not believe me.

Did you try it in a slow refractor too? Maybe f/15 or f/12 or so?

 

RichA,

besides the 28mm Klee University Optics sold a down-scaled version of it with 16mm focal length.

And there was the 20mm Brandon.

 

Here is some information on the Pretoria designed by Klee (you have to scroll down):

http://brayebrookobs...low lenses.html

 

Edit:

Jan, can you post a picture of the 28mm U.O. and for example a Televue Plössl,

to compare the size?

 

Cheers, Karsten

Edited by KaStern, 12 August 2015 - 05:49 PM.

[BDS316]

Another example of the downfall of University Optics, a company that flourished in the day of the f/15  Unitron refractors,  f/10 Schmitt Cassegrains, and 6 inch f/8 Newts, all on driven mounts  that would allow nice viewing with Orthos, Konigs, Widescans, etc.

 

What killed UO was the Dobsonian revolution, the eyepiece revolution, and the short focal ratio apo refractor revolution.  ie, John, Al, and Roland....

Edited by BDS316, 13 August 2015 - 01:34 PM.

[Starman1]

Rumors of their demise are somewhat exaggerated:

http://www.universityoptics.com

Edited by Starman1, 13 August 2015 - 03:00 PM.

[junomike]

Rumors of their demise are somewhat exaggerated:

http://www.universityoptics.com

 

Don, I think he meant their demise as a top producer of classic eyepieces and not the current cheapo re-brands they mostly carry now.

 

Mike

[Starman1]

They were always an importer, never a manufacturer that I know of.

[junomike]

They were always an importer, never a manufacturer that I know of.

 

They were always an importer, never a manufacturer that I know of.

I believe that's correct Don, but they could Import things like this instead of this.

 

I missed the glory years of UO as I wasn'tactive in the hobby back then.  Luckily I was able to find them used (Pretoria, 25/32/40 MK-series, UO abbe's, UO HD's)

 

They even place a "classic" eyepiece pic on the eyepiece page yet don't offer it even though It's still available.  Agena has picked these up now.

 

Dunno, Maybe they couldn't be bothered anymore or the cost/profit wasn't worth it.

 

Either way,  IMO there is a HUGE difference between UO then and UO now!

 

Mike

[Jan Owen]

As requested, here's the UO 28 mm Pretoria next to a TV smoothie 7.4 mm Plossl...

As you can see, the Pretoria isn't exactly tiny, but I have several of Uncle Al's glass marvels that are considerably larger than the Pretoria... Heheheh!!!

Edited by Jan Owen, 14 August 2015 - 11:31 PM.

[KaStern]

Hello Jan,

 

thank you very much for this picture.

Since it is a non-widefield this 28mm Pretoria is a huge eyepiece.

Can you tell me how long it is from the eyepiece shoulder

to the end of the barrel?

 

Greetings, Karsten
 

Edited by KaStern, 20 August 2015 - 04:40 PM.

[RichA]

They were always an importer, never a manufacturer that I know of.

That's how Meade started out, but UO never evolved beyond it.

[Jan Owen]

Hello Jan,
 
thank you very much for this picture.
Since it is a non-widefield this 28mm Pretoria is a huge eyepiece.
Can you tell me how long it is from the eyepiece shoulder
to the end of the barrel?
 
Greetings, Karsten

It's 58 mm (2.28") from the shoulder to the end of the barrel... Overall length is 5" (127 mm)...

Edited by Jan Owen, 21 August 2015 - 11:10 AM.

[KaStern]

Hello Jan,

 

thank you very much   

To me it seams as if the Pretoria eyepiece would fit to my 8"f/6 ATM dob and my 6"f/4 Newt.

I should really try to get one. 

 

Cheers, Karsten

 

[csrlice12]

Some interesting reading on optical history with a really interesting read on the Pretoria eyepiece:

 

http://www.brayebroo...low lenses.html

 

interestingly enough, microscope optics are designed to remove coma....donno why....

Edited by csrlice12, 21 August 2015 - 04:55 PM.

[HandyAndy]

Hi,

 

Thanks for the link csrlice12.

 

I was lucky enough to be given a 28mm Pretoria in exchange for a lost 9" blank by Frank Hourigan working at Wise Optics.

 

On the 1m F3.8 telescope at Puimichele in France it showed nice round stars across the field unlike a Nagler.

 

My friend who does a lot of variable star observations with a 20" f4 says its the best eyepiece ha has used.

He had to give it back.

 

My friend had a small Astroscan type scope and it inspired me to actually buy a telescope to suit it so I got a GSO 8" F4 as a nice compact corrected system. The Moon shows up nice and crisply.

 

I would love to buy the shorter FL's.

 

I have a Parracorr V1 so I will try a comparison one day. A 32mm Panaview should be about the same FL in one. And similar amounts of elements.

 

Cheers. Andrew.

[Eddgie]

Seems I remember reading about "complimentary" eyepieces used to correct or improve certain telescope attributes, like spherical correction.  But the Pretoria was the only eyepiece I specifically remember as being designed to do it.

I am not aware of any eyepeice that was ever sold that corrected spherical aberration.    That would limit it's use only to scopes that had a known amount of spherical aberration and would degrade the performance of all other scopes it was used with.

The Pretoria was as is indicated a design that was built to deal with a problem common to many different scopes (SCTs and Newtonians, which hold a very large share of the market for scopes over 6").

 

Now, it is in interesting topic because the telescope and eyepeice do indeed work either together, or against one another.

 

The best example of this is with field curvature.    Most telescopes have a field that curves away from the observer.

 

Most eyepieces have a field that curves towards the observer.

 

The reason this is interesting is because these curves can cancel each other out.   If you use a telescope with a sharply curved field and an eyepiece with a flat field (which is rare, and  which is in fact undesirable) then you will not be able to focus across the entire field..  Focus on the inside and the outside will be out of focus (and perhaps appear to have astigmatism, which is really almost always there in most eyepieces, but being defocused just makes it easier to see).

 

If though, you used an eyepeice with an inward-curving field of the right curve, it could exactly offset the curve of the field turning away from the observer and give you pinpoint focus all the way across the field.

 

Just my theory, bu I think this is why off axis performance reviews of wide field eyepieces is all over the place.  If the fields of the eyepeice and scope are not complimentary, you can get a very bad result.

I pretty much ignore off axis eyepeice tests if the tester was using a small refractor or SCT.  Both have badly curved fields, and people will see the defocused blur and think it is the eyepeice, when most of the aberration is in fact coming from the scope being used.

 

I recommend the book Telescope Optics.  It has a  whole section on eyepeices and how they behave with telescope aberrations.  It also has some ray traces showing how several difference eyepieces can combine with some different scopes to either improve or lower the off axis performance.

 

An excellent book.   If you are serious about knowing more, that is the best source you will find.   A masterpiece of inforation on how telescopes and eyepeices work, and even how they work together.

[KaStern]

Hello Eddgie,

 

 

The Pretoria was as is indicated a design that was built to deal with a problem common to many different scopes (SCTs and Newtonians, which hold a very large share of the market for scopes over 6").

 

do you speak of the coma that both SCT and Newts have off the optical axis?
If so, could the Pretoria eyepiece compensate for the coma of an SCT ?

 

 

I pretty much ignore off axis eyepeice tests if the tester was using a small refractor or SCT.  Both have badly curved fields, and people will see the defocused blur and think it is the eyepeice, when most of the aberration is in fact coming from the scope being used.

 

To how many diopters will the field curvature of a small refractor result when used with a 2" widefield flatfield eyepiece?

Could a young observer accomodate it?

Could it be that the age is the maun factor rather than the amount of field curvature of the telescope?

 

https://en.wikipedia..._4_modified.svg

 

 

I recommend the book Telescope Optics.

 

Me too! And the author is not only an expert but a very nice person too

 

Cheers, Karsten

Edited by KaStern, 24 August 2015 - 06:39 PM.

[Eddgie]

A young observer might be able to accommodate 2 to 3 diopters.   An older observer maybe half a diopter.