13 replies to this topic

[Magnus Ahrling]

Hello!

 

 Anyone who has the APM diagonal prism with quick release 2"? How do you like it? I have had a 2" WO mirror diagonal since 2005. Nothing wrong with it, I like it. But I have read many good things about prism diagonals and I would like to try one. I will use it with my 120 f/7,5 ED refractor, 6" f/12 Mak and 8" f/12 Classic Cassegrain. Mainly for observing multiple stars, planets and globular clusters. I will use 1,25" eypieces.

 

Cheers!

Magnus

Edited by Magnus Ahrling, 06 March 2025 - 06:53 AM.

[Ernest_SPB]

Do you mean APM 2 inch Erect-Image-prism?

They specify 46mm unobstructed diameter. So, optical pass in the prism glass 78mm. In F7.5 optics it only introduces CA 1/2λ or reduce Strehl ratio till 78%

I am not sure if it is a good idea having in mind "multiple stars, planets and globular clusters" especially using 1.25" eyepieces. 

[jimeh]

Do you mean APM 2 inch Erect-Image-prism?

They specify 46mm unobstructed diameter. So, optical pass in the prism glass 78mm. In F7.5 optics it only introduces CA 1/2λ or reduce Strehl ratio till 78%

I am not sure if it is a good idea having in mind "multiple stars, planets and globular clusters" especially using 1.25" eyepieces. 

He's probably referring to this one:

 

https://www.apm-tele...oadband-coating

 

OP would be better off with the reduced optical path and chromatic aberration of a 1.25" prism.

[mountain monk]

Magnus,

 

A search will turn up many posts on this topic. I have both prisms and mirrors. I follow the general “rule’ to use prisims above f/7 or f/8 and mirrors for faster scopes. I very much enjoy my Baader ASTRO AMICI prism in my Tak 100DZ. I use a TV mirror in my NP-101. They’re both great. I don’t know anything about the APM except that some folks here on CN like it.

 

Dark, clear, calm skies.

 

Jack

[Magnus Ahrling]

Thanks for advices Ernest SPB and jimeh Yeas I mean the prism your link shows jimeh. Optically I really don`t need a 2" diagonal but mecanically. It sits safer on the visual back on my refractor and CC8 than a 1,25" diagonal. It has only 1 screw to secure it, resulting in irritating rotation very often. Two screws on the 2" visualback hold the 2" diagonal rocksteady. Hope you understand what I mean......

Also the refractor and CC8 have no threads so I can use a Baader ClicLock.

 

All the best!

Magnus

[Magnus Ahrling]

Ah thanks to you as well, mountain monk Seems like a f/12 telescope will be a good choise for a prism....

 

Magnus

[Oldfracguy]

I use a 2" Baader BBHS Zeiss-spec Prism Diagonal with an 8" f/12 Classical Cassegrain:

 

 

 

and most recently with a 120mm f/7 ED Triplet refractor.  The advantages of the prism over what I am thinking is your 2" William Optics Dielectric Mirror Diagonal, of which I've had two before, are reduced residual light scatter and the ability to show smaller and "tighter" stars with a darker sky background.  Views of the planets, the lunar surface and especially double stars are better using the prism diagonal.  

 

However, since you have specified that you will be using 1.25" eyepieces, I highly recommend the Baader T2 BBHS Zeiss-spec Prism diagonla with the Helical Focusing Eyepiece Holder. Here is one on an SCT (I used the exact same configuration on a 150mm f/12 Mak):

 

 

 

and here it is on an APO refractor:

 

Edited by Oldfracguy, 07 March 2025 - 09:06 AM.

[Magnus Ahrling]

Intressting arrengment you got, Oldfracguy. Thanks for thoughts and pictures! Will consider that but Baader Zeiss will for sure be an expensive diagonal. Worth it? I don`t know yet as I have never tested.

Yes my WO 2" diagonal is a William Optics but not dielectric, I think that was rather rare 2005. I have also  a WO 1,25" dielectric diagonal bought 2011, can`t notice any difference between dielectric and non dielectric really.

 

Magnus

[Spikey131]

Intressting arrengment you got, Oldfracguy. Thanks for thoughts and pictures! Will consider that but Baader Zeiss will for sure be an expensive diagonal. Worth it? I don`t know yet as I have never tested.

Yes my WO 2" diagonal is a William Optics but not dielectric, I think that was rather rare 2005. I have also  a WO 1,25" dielectric diagonal bought 2011, can`t notice any difference between dielectric and non dielectric really.

 

Magnus

I don’t think you will see much difference with a prism diagonal, either.

[Oldfracguy]

I don’t think you will see much difference with a prism diagonal, either.

I agree that a prism diagonal will not improve the views with your 6" Mak and 8" Classical Cassegrain, as those scopes already have two internal light-scattering mirrors.  On the other hand, I do believe the APM Prism diagonal will provide sharper views with less light scatter and a darker background sky in your 120mm f/7.5 ED refractor.  I used to have one of those, a Sky-Watcher 120ED Evostar.  Using the Baader T2 Prism diagonal (I didn't have the 2" Baader Prism at the time), I had noticeably better views than with the 2" WO Dura-Bright Dielectric Mirror diagonal I had at the time.  I only used that one with 2" eyepieces for wide-field viewing.  For planets, the Moon and double stars I always switched to the Baader Prism.

[drspiv]

I have the APM 2" prism, I can compare it to the smaller t2 standard baader prism and the Astro-tech 2" dielectric.

I've come to the conclusion that in my refractors it's somewhere between pointless and bad (f7 to f5), and that's the right place for a mirror. It's slightly worse than the t2 at f7, probably because there's just more space between the glass surfaces and therefore a little more CA. The t2 is maybe a little sharpener than the dielectric at f7, but with a hint of fringe on the right object.

At f12 (mewlon) you get the advantages of being a little sharper and more contrasty without any fringe. I can't really tell the difference between the t2 and APM in that situation except that the APM let's me put in very silly wide eyepieces.

In a c8 I think the telescope was the primary contribution to contrast loss and I couldn't tell much difference between anything.

What I actually got it for is a big f10.5 RC, which is still in progress, and I just can't help but want to try new things. I expect it will be similar to the mewlon story though...

[Magnus Ahrling]

Thanks all for help and advice.  But for now I stay with my 2" William Optics mirror diagonal before I can test a prism diagonal.

 

Magnus

[ABQJeff]

A 2" prism is great in an F/7.5 refractor for low mag DSO viewing, like M31, big nebula (Veil, North America) or big open clusters.  High mag planets or bright stars, yes do a head to head.  My F/7 and faster refractors show false color on bright stars with prism diagonals.

 

My 11" SCT even in F/7 mode does not, curiously.  So it is 2" Amici prism for DSO mode or 1.25" T2 BBHS prism with 1.7x or 2.6x GPC and binoviewer for planet mode.

[WilRobinson]

I have the 2in. APM prism, I just bought a2in. AP Max Bright and I have a 2in. Astro Tech Quartz diagonal, I plan on testing them against each other for the next few months then I will sell one off.  I like the weight of the APM and the AP, they are a good counter weight for the triplet lens.   

  I have owned the Astro Tech since I bought my TMB in 2008 and it has survived this kind of test before against a couple Williams Optics diagonal, a Televue and an Intes Micro.  

  I did a quick test a few nights ago with all three on the crescent moon with my 100/8, all the views were sharp and absolutely color free, this should be an interesting comparison.