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Poor man's 4" Televue

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#151 Princess Leah

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Posted 21 April 2025 - 04:35 AM

Perhaps Baader's info is outdated? I have an original 2 inch Zeiss prism made by Baader - it's 100mm. However it's 20 years old.

However so far I have found the table attached accurate.

 

Just a point of interest , 1.25;inch prisms seem easier to produce to a higher standard than 2 inch prisms.

 

Also in my experience the Celestron prism is clearly better than the basic Baader T2 prism, I haven't yet compared it with the Baader Zeiss T2 1.25  prism.

My Celestron prisms are equal to my Baader 2 inch Zeiss prism.

 

It's interested how many people on CN only feel confident using equipment that costs a premium.

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#152 Procyon

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Posted 21 April 2025 - 07:30 AM

Perhaps Baader's info is outdated? I have an original 2 inch Zeiss prism made by Baader - it's 100mm. However it's 20 years old.

However so far I have found the table attached accurate.

 

Just a point of interest , 1.25;inch prisms seem easier to produce to a higher standard than 2 inch prisms.

 

Also in my experience the Celestron prism is clearly better than the basic Baader T2 prism, I haven't yet compared it with the Baader Zeiss T2 1.25  prism.

My Celestron prisms are equal to my Baader 2 inch Zeiss prism.

 

It's interested how many people on CN only feel confident using equipment that costs a premium.

post-226662-0-14623811.jpg

 

Source: TOMDEY

 

PS> William Optics Dielectric Mirror 1.25" 98.8/98.9/99.0


Edited by Procyon, 21 April 2025 - 12:21 PM.

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#153 noisejammer

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Posted 21 April 2025 - 08:04 AM

It doesn't, assuming all of the nosepiece fits into the 2" focuser receptacle.  

 

I've measured the effective optical paths of a few diagonals myself.   The 2" model with the shortest optical path I have come across so far is the Baader 2" BBHS Zeiss-spec Prism #2456117.  I measured it on two different occasions and got 96 mm to the top of the 2" Clicklock clamp.  My estimates of optical path are probably accurate to +/- 1 mm.

Hmmm - I think we're discussing separate issues. This topic (as I understand it anyway) is about converting a regular refractor into an instrument that has properties similar to a NP101. Specifically, around f/5 - f/5.5 and capable of using 2" eyepieces so that the true visual field is maximized.

 

I tried this with my APM 115 (natively f/7.0) and TOA150 (natively f/7.3) some time ago. I used a TOA reducer. I found that - even though both instruments were designed for imaging - the primary issue was achieving focus because the light cone from the reducer converged too rapidly. Solution - bring the eyepiece closer to the reducer.

 

1. The only variable in my system was the distance between the reducer and body of the diagonal. It may be different for other reducers, but my TOA130R has its last optical element 6 mm from the rear rim of its flange. This means the system maximizes the available back- focus for imaging.

 

If you were to install a nose piece on the diagonal, you would need to install a 2" receptacle on the reducer which pushes the diagonal about 30 mm from the reducer. If the diagonal is screwed directly to the reducer, you can do this with an adapter that only eats up ~3 mm.

 

2. I realize that the optical length of a 2" prism is ~15 mm shorter than a mirror. The problem in this specific case (f/5 - f/5.5) is that a prism creates a lot of spherochromatism. This is why I did not suggest a prism (even though I find my T2 BBHS prism produces my highest contrast shallow space images.)


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#154 maniack

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Posted 21 April 2025 - 04:25 PM

No, because the spacing recommendations are for after, or "downstream", of the TSFLAT2.  You are absolutely correct.  I just noticed that whenever I installed the TSFLAT2 I always had to rack the focuser inward about 7-8 mm farther with the same eyepiece than without it.  I definitely had to take that into consideration when deciding whether or not to install one of those Baader M56 Clicklock visual backs on a Celestron Omni AZ 102.  I made some careful measurements of how much remaining in-focus travel I had with the stock visual back, and from those decided that with my "worst eyepiece" regarding in-focus travel requirements, an Orion 38mm Q70 whose focal plane is about 10-11 mm up inside above the level of the outer lip, or flange, on the barrel that rests against the diagonal's Clicklock clamp or 2" compression ring receptacle, I would have a few millimeters to spare. Here is a picture of that scope setup with a Baader 2" MaxBright Dielectric diagonal and the Orion 38mm Q70 (same eyepiece as the Agena 38mm SWA):

 

attachicon.gif 101_3105.JPG

I just tested my TSFLAT2 in the daytime, and I found with my 30mm UFF I had to rack in the focuser 6-7mm inward after adding the TSFLAT2.


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