Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Some refractor objectives tuned to use prism diagonal?

  • Please log in to reply
15 replies to this topic

#1 Nippon

Nippon

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2651
  • Joined: 22 Oct 2009
  • Loc: Central Florida

Posted 21 July 2019 - 08:59 PM

I have another thread about the Takahashi prism diagonal versus the Baader T2 non Zeiss prism diagonal. During my search for information I read posts and statements that the Zeiss APQ objectives were designed in such a way that a prism was needed for these scopes to perform to their potential. And I think it was the company's own 2" prism diagonal. I also read a claim that CFF refractors tuned for imaging needed a prism diagonal to perform their best for visual. I was not able to find that on the CFF site. The oddest thing I read was that the Astro Physics Traveler performed best with a prism diagonal. I find this all curious but I had read before about the Zeiss APQ and prisms but have never heard off this with CFF and thought the last thing you would want in a fast scope like the Traveler  would be a prism diagonal.

Anyone have correct information about this

Thanks, Paul

 

I have to edit here because I just found that CFF on their site does recommend a prism diagonal for photographically tuned objectives and mirror diagonals for their visually tuned objectives


Edited by Nippon, 21 July 2019 - 09:05 PM.


#2 Starlease

Starlease

    Vostok 1

  • -----
  • Posts: 179
  • Joined: 11 Aug 2018
  • Loc: Rocky mountains.

Posted 21 July 2019 - 09:40 PM

All my apos work best with the Baader T2 Zeiss prism diagonal.

But the APM 2" introduced colors to the edges of planets.


Edited by Starlease, 21 July 2019 - 09:40 PM.


#3 Nippon

Nippon

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2651
  • Joined: 22 Oct 2009
  • Loc: Central Florida

Posted 21 July 2019 - 09:45 PM

All my apos work best with the Baader T2 Zeiss prism diagonal.

But the APM 2" introduced colors to the edges of planets.

From what I have been able to read the prism size, 2" versus 1.25" has an effect on optical properties because of the different thickness of glass involved that the light has to pass through. Quite unlike a mirror where the light is just reflected without having to pass through glass


Edited by Nippon, 21 July 2019 - 09:47 PM.


#4 Gabby76

Gabby76

    Ranger 4

  • *****
  • Posts: 321
  • Joined: 04 Dec 2015
  • Loc: British Columbia

Posted 21 July 2019 - 10:17 PM

The owner of Baader Planetarium posted a note about testing your refractor first before buying a prism. 

When doing a star test if the inside focus first ring has a reddish colour to it your telescope may benefit from a prism.

 

If if I can find the actual post I will post it for you.   



#5 starman876

starman876

    Nihon Seiko

  • *****
  • Vendors
  • Posts: 20285
  • Joined: 28 Apr 2008
  • Loc: VA

Posted 21 July 2019 - 10:37 PM

The owner of Baader Planetarium posted a note about testing your refractor first before buying a prism. 

When doing a star test if the inside focus first ring has a reddish colour to it your telescope may benefit from a prism.

 

If if I can find the actual post I will post it for you.   

I would be interested in reading that also.



#6 Gabby76

Gabby76

    Ranger 4

  • *****
  • Posts: 321
  • Joined: 04 Dec 2015
  • Loc: British Columbia

Posted 22 July 2019 - 12:29 AM

I managed to find the question put to Thomas Baader, here is the question and his reply. 

 

Q:  "Do the Baader prism diagonals with BBHS coating work well with refractors faster than f/7 or f/8? I am wondering if chromatic aberration (or spherochromatism) would be a problem as the light propagates through the glass prism."

 

A: "We do not recommend a prism for refracting telescopes unless the optical calculation of the objective lens calls for using a prism instead of a mirror.
For 100 years - all Carl Zeiss Refractor telescopes were designed to compensate for the added glasspath of a prism. 
And throughout our 25 years of offering this prism we repeatedly found refractor telescopes - especially of very short focal length - where the airy disc went colorless only when adding a prism into the beam of light - as was the case with the "long gone" Astro Physics Traveler.
So please check carefully with your existing mirror star diagonal if the first ring of the airy disc appears reddish when observing a bright star - or better when observing an artificial star at high magnification. if this is found to be the case then a prism will bring a noticeable improvement. 
If the first diffraction ring appears white already than a prism would not lead to an improvement but will induce a color error."


Edited by Gabby76, 22 July 2019 - 12:29 AM.

  • SteveG, Astrojensen, AllanDystrup and 1 other like this

#7 Astrojensen

Astrojensen

    Voyager 1

  • *****
  • Posts: 12035
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 22 July 2019 - 02:35 AM

Strange that this topic should pop up now. I was out with my 102mm f/11 Altair ED last night and found out, when I wanted to compare my two prism diagonals, that it performs better with a 2" Baader/Zeiss prism, than it does with a Baader/Zeiss T2 prism. In other words, the extra glass seems to help. The Altair does indeed show a reddish-purple first diffraction ring on Vega, when the small prism or no prism is used, but with the larger prism the first ring is much closer to neutral. It doesn't quite eliminate the faint purplish hue, but it's much reduced. The brightness of the first diffraction ring is also slightly reduced and the image of double stars is a tad cleaner. The star test is improved quite a bit. 

 

 

Clear skies!

Thomas, Denmark 


Edited by Astrojensen, 22 July 2019 - 02:37 AM.

  • Don Taylor and Gabby76 like this

#8 Mark9473

Mark9473

    Voyager 1

  • *****
  • Posts: 11201
  • Joined: 21 Jul 2005
  • Loc: 51°N 4°E

Posted 22 July 2019 - 05:39 AM

Thomas, if I'm not mistaken you reported on that scope having some spherical aberration. Does the prism do anything for that?
  • Astrojensen likes this

#9 Astrojensen

Astrojensen

    Voyager 1

  • *****
  • Posts: 12035
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 22 July 2019 - 06:11 AM

Thomas, if I'm not mistaken you reported on that scope having some spherical aberration. Does the prism do anything for that?

Apparently, it does indeed. The star test close to focus is visibly better with the large prism, than with the smaller. I should investigate it more, but I was too busy enjoying the superb seeing yesterday.

 

 

Clear skies!
Thomas, Denmark


  • Mark9473 and AndresEsteban like this

#10 Nippon

Nippon

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2651
  • Joined: 22 Oct 2009
  • Loc: Central Florida

Posted 22 July 2019 - 07:12 AM

This is interesting. While I was comparing my Tele Vue Everbright 1.25" to my Baader T2 1.25" prism mainly for reduced scatter with the prism on Jupiter I do believe there was a small improvement in contrast. I did look at Vega with the prism looking for any increase in CA and the star was white including the first diffraction ring. Since reading some of this stuff, including the above quote from Thomas Baader, I have looked at Vega with the TV mirror and did indeed see a slight reddish hue in the first diffraction ring. The scope I am using here is a Vixen ED103s. My 103 has very good color correction. I do not see any color on Jupiter or the Moon and can't say I really noticed the red in the first ring until I actually looked for it. But perhaps it explains the reason I have found that the 103 has a very slightly warmer tone on the Moon compared to my 4" triplet.

My Baader has collimation issues that I was able to shim to being close but not perfect. That is why I was asking opinions of the Tak prism versus a new T2 in another thread.



#11 Nippon

Nippon

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2651
  • Joined: 22 Oct 2009
  • Loc: Central Florida

Posted 22 July 2019 - 05:10 PM

I carefully read the section titled "Lens Correction" on the CFF Telescopes site. It is very interesting.  



#12 Blueox4

Blueox4

    Ranger 4

  • *****
  • Posts: 398
  • Joined: 26 Nov 2015
  • Loc: Upstate New York

Posted 22 July 2019 - 07:43 PM

All my apos work best with the Baader T2 Zeiss prism diagonal.

But the APM 2" introduced colors to the edges of planets.

Not surprising. BAK4 prism in the Baader Zeiss and K9/BAK7 in the APM. BAK4 is a much better optical quality glass though BAK7 is very good and much cheaper. 



#13 Don Taylor

Don Taylor

    Viking 1

  • -----
  • Posts: 980
  • Joined: 12 Jul 2009
  • Loc: Broken Arrow (Tulsa Area) Oklahoma

Posted 22 July 2019 - 10:34 PM

Strange that this topic should pop up now. I was out with my 102mm f/11 Altair ED last night and found out, when I wanted to compare my two prism diagonals, that it performs better with a 2" Baader/Zeiss prism, than it does with a Baader/Zeiss T2 prism. In other words, the extra glass seems to help. The Altair does indeed show a reddish-purple first diffraction ring on Vega, when the small prism or no prism is used, but with the larger prism the first ring is much closer to neutral. It doesn't quite eliminate the faint purplish hue, but it's much reduced. The brightness of the first diffraction ring is also slightly reduced and the image of double stars is a tad cleaner. The star test is improved quite a bit. 

 

 

Clear skies!

Thomas, Denmark 

Oh Thomas! Thomas! Thomas!    Every time you do some observing it hurts my bank balance!   I was just thinking of trying the 1.25" Baader Zeiss prism and now this! Oh agony!


  • Astrojensen likes this

#14 Astrojensen

Astrojensen

    Voyager 1

  • *****
  • Posts: 12035
  • Joined: 05 Oct 2008
  • Loc: Bornholm, Denmark

Posted 23 July 2019 - 02:05 AM

Oh Thomas! Thomas! Thomas!    Every time you do some observing it hurts my bank balance!   I was just thinking of trying the 1.25" Baader Zeiss prism and now this! Oh agony!

My sincerest apologies to your bank account. flowerred.gif  grin.gif

 

 

Clear skies!

Thomas, Denmark


  • Don Taylor and AndresEsteban like this

#15 Nippon

Nippon

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2651
  • Joined: 22 Oct 2009
  • Loc: Central Florida

Posted 23 July 2019 - 02:13 AM

I got a chance tonight to compare my TV Everbright with this time a well collimated Celestron 1.25" prism diagonal in my Vixen ED103. And as described by Thomas Baader in the comments provided by Gabby76 I confirmed that Vega shows a slight reddish hue in the first diffraction ring at high power that is completely gone when using the Celestron prism diagonal. According to Baader's comments that should improve color correction as well as less scatter. SA correction seems unaffected as the scope displays it's usual excellent star test with either diagonal and I did not see any increase in bluish color. Vega just appeared white with the prism.


  • AndresEsteban and Gabby76 like this

#16 Nippon

Nippon

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2651
  • Joined: 22 Oct 2009
  • Loc: Central Florida

Posted 23 July 2019 - 11:06 AM

All my apos work best with the Baader T2 Zeiss prism diagonal.

But the APM 2" introduced colors to the edges of planets.

What are your APOs? doublet ED or fluorite, or ED triplets?




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics