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Equipment Discussions >> Eyepieces

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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: UPDATE: Prism Diagonals new [Re: Fomalhaut]
      #6250765 - 12/13/13 11:38 AM

Quote:


According to Rutten vanVenroij 1st ed.p.312-313, "a reflecting prism behaves as a plan-parallel plate, ...causing overcorrected spherical aberration."
Well reputed (even top-notch such as Tak) apos are slightly undercorrected, presumably i.o. to work better with zenit-prisms. => The prism's overcorrection then tends to neutralize the objectives' undercorrection.
On astro-foren.de, Taks and Zeiss APQs and others have been tested with and without prisms and many of them work better with and others without prisms. Zeiss even produced some of their famous APQs to be used with prisms and others (of same type, aperture and f-ratio) to be used with mirrors.
Mr. Rohr found their measured system-correction was indeed in accordance with the goal of their design.
Chris




I'm sorry. Sometimes a post just goes a little too far off into La-La Land for me to let it go.

1) No manufacturer would purposely build undercorrection into its scopes. They couldn't know if the instrument was going to be used with a prism, a mirror, a camera, or straight through (common in Japan). IF undercorrection is present, it is because the lenses weren't made or spaced correctly.

2) Ditto on the Zeiss refractors.

3) Rutten and vanVenrooij's discussion of prisms referred to oblique angles through a prism, i.e. short f/ratios. It's fairly well known that prisms aren't suitable for short f/ratios due to, well, "the prismatic effect". They mention that a weak positive lens in front of the prism can offset that. This is also well known.


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pga7602
sage
*****

Reged: 06/11/09

Loc: Los Angeles, CA
Re: UPDATE: Prism Diagonals new [Re: Fomalhaut]
      #6250877 - 12/13/13 12:46 PM

If I wanted to try a 1.25 prism diagonal in my TV85, is it just a matter of sticking a 1.25 eyepiece adapter ahead of the diagonal? Or is there a diagonal 2-1.25 adapter made for this purpose.

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Fomalhaut
scholastic sledgehammer


Reged: 08/16/08

Loc: Switzerland
Re: UPDATE: Prism Diagonals new [Re: Starman1]
      #6250913 - 12/13/13 01:18 PM

Quote:

Quote:


According to Rutten vanVenroij 1st ed.p.312-313, "a reflecting prism behaves as a plan-parallel plate, ...causing overcorrected spherical aberration."
Well reputed (even top-notch such as Tak) apos are slightly undercorrected, presumably i.o. to work better with zenit-prisms. => The prism's overcorrection then tends to neutralize the objectives' undercorrection.
On astro-foren.de, Taks and Zeiss APQs and others have been tested with and without prisms and many of them work better with and others without prisms. Zeiss even produced some of their famous APQs to be used with prisms and others (of same type, aperture and f-ratio) to be used with mirrors.
Mr. Rohr found their measured system-correction was indeed in accordance with the goal of their design.
Chris




I'm sorry. Sometimes a post just goes a little too far off into La-La Land for me to let it go.

1) No manufacturer would purposely build undercorrection into its scopes. They couldn't know if the instrument was going to be used with a prism, a mirror, a camera, or straight through (common in Japan). IF undercorrection is present, it is because the lenses weren't made or spaced correctly.

2) Ditto on the Zeiss refractors.

3) Rutten and vanVenrooij's discussion of prisms referred to oblique angles through a prism, i.e. short f/ratios. It's fairly well known that prisms aren't suitable for short f/ratios due to, well, "the prismatic effect". They mention that a weak positive lens in front of the prism can offset that. This is also well known.





1) Before introducing their FSQs Takahashi designed their Fluorite FCs and FSs mainly for the (visual) use with their own dedicated zenit-prisms. Together with the FSQs they introduced an expensive and extremely accurate short-building 2-inch mirror-diagonal (for low-straylight applications deliberately and explicitely renouncing dielectric coatings).

2) I suggest some people perhaps write a little less and read a little more, e.g.
http://www.astro-foren.de/showthread.php?6084-Verzeichnis-optischer-Berichte
on several APQs with or without glass path

3) Yes, very well known over here in LaLa-land .
I bought and started reading that book over and over in 1990.
Nevertheless there were chromatic differences visible in my former f8.8 Vixen-Fl-102 with/without zenit-prism...

Chris


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BillP
Postmaster
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Reged: 11/26/06

Loc: Vienna, VA
Re: UPDATE: Prism Diagonals new [Re: pga7602]
      #6250951 - 12/13/13 01:38 PM

Quote:

If I wanted to try a 1.25 prism diagonal in my TV85, is it just a matter of sticking a 1.25 eyepiece adapter ahead of the diagonal?




Yup.


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pga7602
sage
*****

Reged: 06/11/09

Loc: Los Angeles, CA
Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6250998 - 12/13/13 02:03 PM

Thanks Bill.

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Ant78
member
*****

Reged: 09/23/11

Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6251197 - 12/13/13 03:56 PM

Quote:

Quote:

How many of you have cleaned your star diagonals regularly (like every 6 months or more)?





Every time I do serious planetary observing, I clean my diagonal.

I give everyone that asks about getting a dielectric diagonal to get one, and I explain that the main reason you should use a dielectric is simply because you can clean it for every serious planetary observing session and not damage it.

And this is one of the things I dislike about prisms. While the coatings on prisms are pretty hard (not as hard as dielectric), they are much harder to clean than a mirror.



I recently used the method you suggest on my dielectric diagonal and it worked a treat, cheers edgie


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BillP
Postmaster
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Reged: 11/26/06

Loc: Vienna, VA
Re: UPDATE: Prism Diagonals new [Re: Fomalhaut]
      #6251693 - 12/13/13 08:42 PM

Quote:

Quote:

Yup...the more I am using that T2 Prism, the more it seems to be distinguishing itself from the field.




Bill,
Since thera are three versions of the T2 prism, I would like to ask you which it is that you have:
http://www.baader-planetarium.de/sektion/s28/s28.htm#t2_90_grad_ohne
Chris




According to the owner, both Baader prisms are the Zeiss prisms. The T2 is fitted with the 1.25" eyepiece clamp and a 2" nose piece. I will take some pics tomorrow when get some daylight and post.


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Eddgie
Postmaster
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Reged: 02/01/06

Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6252371 - 12/14/13 10:06 AM

There are people that think there T2 diagonal has a Zeiss prism, but there are indeed two very different T2 Prism diagonals sold and only one of them has a Zeiss prism.

The $129 unit does not have a Zeiss prism. People often get the impression that it does becuase some of the sales info posted on vendor pages is cut and pasted from Alpine Astro or Baader and taken out of context of the full Baader writeup.

When one reads the full writeup on the Alpine Astro website, all of the writeup is for the Zeiss prism.

But when you go to the order page, you see that the standard prism is only 32mm of clear apture, while the Zeiss prism unit has 34mm (I have the standard Priam and it is only 32mm apeture). And on the price page, it specifices that the T2 01B has a Zeiss prism, but makes no such claim for the T2-01C.

Also, the light path is considerably shorter in the standard prism vs the Zeiss prism unit.

I have addressed this many times on the binoviewer forum where people that had the T2-01C ($129) diagonal thought that it had a Zeiss prism, but nowhwere will you find this documented. Almost all of the sales data that says the $129 unit has a Zeiss prism I beleive is in error and has used the writeup for the much more expenisve Zeiss unit.

I do not think this is any attempt to deceive anyone. I think it is an honest mistake.

But I have looked all over for it, and in no instance on any Baader literature has it said that the T2-01C has a Zeiss prism, but it always says that for the T2-01B.

So, a lot of people seem to think that their $129 diagonal has a Zeiss prism, but it does not.

Not sure what your friend has. Maybe he has the T2-01B. Maybe not.

But I know a lot of people thought they bought the T2-01C beliveing they had a Zeiss prism.

And I have to say this as well.

I have the T2-01C and I also have the Max 1 Maxbright Dielectric as shipped with the Mark V binoviewers.

I have compared the performance of both diagonals in my EdgeHD 8" and found it difficult to tell them apart. I would say that even the $129 diagonal is a really excellant quality part.

I also had the Tak 1.25" prism, and I never found it to be all that. Compared it to a 2" Televue dielectric and found it to be about the same in performance.


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Sarkikos
Postmaster
*****

Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6252412 - 12/14/13 10:31 AM

Quote:

When one reads the full writeup on the Alpine Astro website, all of the writeup is for the Zeiss prism.

But when you go to the order page, you see that the standard prism is only 32mm of clear apture, while the Zeiss prism unit has 34mm (I have the standard Priam and it is only 32mm apeture). And on the price page, it specifices that the T2 01B has a Zeiss prism, but makes no such claim for the T2-01C.




I've also noticed this. If a company - or private seller on CN or AM, for that matter - does not say specifically that a product has such-and-such a quality or includes this-or-that, it's much safer to assume that it doesn't. Even if the product is said to have this-or-that specialness, be skeptical if the price seems too good to be true.

Mike


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Roy McCoy
Pooh-Bah
*****

Reged: 10/13/05

Loc: Glendale, AZ
Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6252470 - 12/14/13 11:19 AM

Quote:

So, a lot of people seem to think that their $129 diagonal has a Zeiss prism, but it does not.




I would think less than half the price would tip people off to the fact that it does not have the Zeiss prism it did for me. As Mike says above "if the price seems too good to be true".

Ive ordered from Alpine and you have to send an email so clicking the wrong box is not a possibility.

Maybe Bob will read this thread and add the designation (Baader Prism) to the T2-01C.

Edited by Roy McCoy (12/14/13 11:23 AM)


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Don Taylor
professor emeritus


Reged: 07/12/09

Loc: South TEXAS
Re: UPDATE: Prism Diagonals new [Re: Roy McCoy]
      #6252537 - 12/14/13 12:04 PM

I'm looking to get a Baader T2-01C as well and understood that it does not have a Zeiss prism - but the short optical path solves some unique issues for my application.

BTW - it is also available from Agena (along with the other Baader diagonals)- although neither Apline nor Agena have any in stock at this time.


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BillP
Postmaster
*****

Reged: 11/26/06

Loc: Vienna, VA
Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6252672 - 12/14/13 01:20 PM

Interesting Eddgie. Important for people to take note of when purchasing.

I removed the 1.25" eyepiece adapter so I could get nearer the prism, then illuminated a pattern laser throught the other end and measured the illuminated FOV and got 34.2mm. Plus the purchase prise was way more than $129 So given both these facts, Zeiss I'd say.


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Eddgie
Postmaster
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Reged: 02/01/06

Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6253393 - 12/14/13 08:39 PM

Measure on the front side. The rear side of the $129 prism is 34mm, but the front side is 32mm.

Here is the Baader page showing both. They look a lot a like, but the nose opening of the $129 (non Zeiss) is only 32mm...

Baader page showing both diagonals

So, if the front of the unit has a 32mm opening, this is the non Zeiss. The Zeiss is a full 34mm at the front and rear.

The non-Zeiss is 32mm at the front and 34mm at the rear, but the front opening determines the true aperture.

So, measure at the front to know for sure.

And notice again, on the Baader page, they are very clear that the T2 01B has the Zeiss prism, but no mention is made of having a Zeiss prism in the standard diagonal.

Not saying you do or don't have one.

My main point here is that everyone thinks that the standard T2 diagonal has a Zeiss prism and nowhere is that ever mentioned on the Baader page.

But it does show up on a lot of web sites and again, I think they have made an honest error and simply used the T2-01B sales info by mistake.

I want to be sure to re-enforce this again. the standard Prism is a superb prism. As best as I can see, identical in performance at f/10 to the Televue and Baader dielectric diagonals.


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Tamiji Homma
Post Laureate
*****

Reged: 02/24/07

Loc: California, USA
Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6253438 - 12/14/13 09:10 PM

Here is several Baader diagonal side-by-side photos:

From left to right:

Baader T2-01C prism, T2 mirror, Amici prism, T2-01B (Zeiss), Prism-2 (2"):




Here is T2-01B (Zeiss) and Takahashi prism side-by-side:


Tammy


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leonard
scholastic sledgehammer
*****

Reged: 10/19/07

Loc: West Virginia
Re: UPDATE: Prism Diagonals new [Re: Tamiji Homma]
      #6253540 - 12/14/13 10:11 PM

Hello Tammy ,


It looks to me that the diagonal has screws to let one remove the prism from the housing for cleaning , is this true ?
It was mention that prisms are a little hard to clean , have you done it ?

Thanks for any info. Leonard


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Roy McCoy
Pooh-Bah
*****

Reged: 10/13/05

Loc: Glendale, AZ
Re: UPDATE: Prism Diagonals new [Re: leonard]
      #6253558 - 12/14/13 10:26 PM

Quote:

It looks to me that the diagonal has screws to let one remove the prism from the housing for cleaning , is this true ?





Yes it is, and you can, and I have.


From Ed's link above and from Tammy's picture, it looks as though the T2/Zeiss has a silkscreened logo while the T2/Baader has an embossed logo.


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BillP
Postmaster
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Reged: 11/26/06

Loc: Vienna, VA
Re: UPDATE: Prism Diagonals new [Re: Eddgie]
      #6253622 - 12/14/13 11:21 PM

Man...you guys are such doubting thomases Beating this issue to death.

OK. The laser pattern method very accurately projects the 34mm stop of the prism as I did it the last time. However, just to reinforce this, I directly measured that aperture stop opening at both front and rear, top and bottom, left and right, up and down (depends on how you orient the thing ) and.....

....drum roll.....

....just as the owner said and just as the laser projection produced....

all apertures are 34mm

ZEISS!!!!!!!!!!!!!!!!!!!!!!!!!!



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warpsl
member


Reged: 07/04/12

Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6253813 - 12/15/13 03:34 AM

hello.my 2 cents worth! I have both the baader zeiss 34mm,and the baader 32mm aperture prisms.i actually wanted only zeiss,but I was told they were not available,so initially I bought the jap made baader.then late the zeiss.i use them with my skywatcher 6 inch bino,s.there is no colour on d s o,but there is some on the moon and planets.but this is dependant on the eyepieces I use.the denk 14mm giving the cleanest views.i struggle to see any difference in performance between these 2 prisms.both have excellent anti-reflection coatings,and give excellent contrast.regards.

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Eddgie
Postmaster
*****

Reged: 02/01/06

Re: UPDATE: Prism Diagonals new [Re: BillP]
      #6254069 - 12/15/13 09:39 AM

Oh forgive me.

I was not doubting you so much as trying to make sure that people knew that the $129 diagonal does not have a Zeiss made prism.

Becuase so many web pages say this, I think everyone believes that the $129 diagonal has Zeiss too.

Did not know what you had. Did not question it so much as to tell you how to know for sure.

But my main point is still this. The $129 digaonal, in spite of the many web pages that say differently, does not have a Zeiss made prism. At least Baader itself does not say so.

I just don't want someone to read your review and go out to buy the $129 diagonal and think they are getting Zeiss prism.

But I do want them to feel confident that the $129 prism is also of excellent optical quality, and if they really want a prism diagonal, they can rest assured that they are getting an excellent one.

I was not all that impressed with the Tak prism. In particular, I hated the compression ring eyepecie holder.

But optically, having owned both, and having compared them all to dielectric diagonals at f/10, I found no difference it quality.

Based on my experience with the f/7 refractor though, I would lean towards a mirror. I know that the prisms in the binoviewer caused pretty severe chromatic aberration at f/7.

The Max mirror though gives amazingly sharp images, and I think perhap at f/7, better than the prism.

Any, my primarly goal was to alert people that the T2 standard does not have the same prism as the T2 Zeiss, and just helping to know how to tell the difference.

Enjoy your testing.


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roadi
Pooh-Bah
*****

Reged: 08/18/07

Loc: Great Grey Spot "Denmark"
Re: UPDATE: Prism Diagonals new [Re: Tamiji Homma]
      #6254288 - 12/15/13 11:23 AM

Quote:

Here is several Baader diagonal side-by-side photos:

From left to right:

Baader T2-01C prism, T2 mirror, Amici prism, T2-01B (Zeiss), Prism-2 (2"):




Here is T2-01B (Zeiss) and Takahashi prism side-by-side:


Tammy



Nice arsenal Tammy By the way isn't the last one in your picture a zeiss also?
I had a 2" which were a zeiss prism in a baader housing!


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