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A Jaegers 80mm F/5

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#1 hypergolic

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Posted 18 October 2020 - 09:48 AM

I have an A. Jaegers 80mm achromat (I think it is f/5) that is at least 40 years old. I am rebuilding an OTA for my daughter. Just out of curiosity, what was the consensus of the achromat quality from AJ in that time period?

 

Thanks.

 

Fleep



#2 markb

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Posted 18 October 2020 - 10:40 AM

I've had 6 Jaegers from about 1968-69. 3 5" and 6" f5s, a 6" f8, a 4" f12? and as as yet unbuilt 3.2 "48 in fl" as they termed it, a Baker design spaced with thick wire in a loop, supposed to be one of the first computer designs. It bench tested extremely well.

 

As with all refractor parts, verify the elements are correctly spaced as designed and are not flipped. One 3.2" element was flipped by a former owner, and tested great after it was rectified. It is very easy to flip elements in cleaning. The front of the flint (rear convex-concave, concave towards front) is a near curve match to the deeper convex rear curve of the front crown glass lens, so they 'stick' together by trapped air, and will show Newtons rings under monochrome light (diffused laser glow works, as do many fluorescent bulbs). Here is an acceptable diagram. http://www.myoptics....as/doublet.html most doublets including Jaegers are Fraunhofers, very few, and no Jaegers, are Steinheils so ignore them. Even my Steinheil binos are Fraunhofer designs.

 

The 4" had the Jaegers cell enclosed in glued pvc, and the maker flipped the entire cell! He forgot to disclose the odd abberations and it took a decade for me to mount it: once I did the problem was obvious.  I tried simply flipping the cell and, wow, stunning views and testing.

 

All are great lenses, excellent figures and smooth polish. All star an Ronchi tested well. IIRC some of the 6in f10s were inconsistent, possibly made by the less experienced workers? The f5 bigger glass were great, I assume more care was used, the 6in f5 sold in the 70s and 80s for the equivalent of $2000 plus in today's dollars, as I recall looking up last year.

 

The SuplusShed liquidation closeouts, usually without cells, were dicier, as many were seconds or were not the original serial numbered and indexed matched pairs. But many were just fine by reports.

 

Squaring the focuser (matching optical and mechanical axes) AND lens collimation is critical to excellent performance. This is true of both SCTs and refractors in my experience.  Claims that focuser squaring/alignment is not important are, in my experience, simply wrong. These alignments are routine for Newts.

 

Jaeger cells are a pain to collimate since they slip in or over the tube and are often held by grub screws,, but a simple cheshire (the refractor Cheshire sold by AgenaAstro was the best and easiest I've used (after the crosshairs are threaded out), beats my other two) gets collimation spot on, at least after you get done fighting grub screws. Other folks swear by careful squaring of the end of the tube; I still had to tune mine.

 

I worked out an external incremental collimation method but implementing it is two months down the road as my cross country move shakes out. I'll post steps after I get it done and confirm it works.

 

I squared my focusers by use of a holographic concentric circle laser, but making an accurate paper disk with a central pinhole "centering target", for a regular laser collimator, that tightly fits the cell will work too.

 

Jaegers crown glass is fragile so do not overtighten! 


Edited by markb, 18 October 2020 - 10:41 AM.


#3 hypergolic

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Posted 18 October 2020 - 11:58 AM

Yep...this is a Jaegers cell. Slip on type. I was thinking of removing the lens from the cell, but as I do not have a proper tool, I thought  better of it and will just leave it alone and only clean front and back.




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