11 replies to this topic

[Jeff B]

I figured I would post this topic here instead of in the "Classics" forum as this is where I post all of my auto-collimation evaluations (AKA: DPAC).

 

A good friend of mine here just gave me an old 6.5" achromat that originally came from the Lohmann Brothers.  They were in business in the early 20th century in Greenville Ohio, which is less than an hour and a half from where I live.  Here is a link for an interesting and informative article on the brothers:

 

http://www.antiquete....org/20thc.html

 

They made both telescopes (refractors and reflectors) and mountings (which bear more than a passing resemblance to Warner & Swasey mounts, also in Ohio).  Of note, is the fact that while they made some of their own optics, but they also sourced some of the larger aperture achromats (up to 12") to folks like the Clarks.  This particular objective has a clear aperture of 6.5" (165mm) with a focal length in green of  83.75" (2127mm) for an F12.9 focal ratio and a chromatic ratio of 2.

 

I got the lens and cells first.  The brass main tube will arrive in the near future.  The objective cell is made of brass with a front brass retaining ring held in place with three radial, countersunk brass screws.  The cell threads into a light weight, two piece AL counter-cell, which will slip into and bolt to the brass main tube.  The AL counter cell, may or may not be original.  Aluminum only became a common readily available, cheap industrial metal in the late 19th and, especially, early 20th centuries.  But it  certainly looks and fits the part as it's cast and looks old and well used....kinda like me.

 

So like any self respecting engineer who chronically tinkers.....

 

I took it apart.

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[Jeff B]

Look at the size of that air gap!

 

It measures ~10mm to 11mm thick, which is around 6% or so of the clear aperture.

 

Huge.

 

And very Clark-like.

 

There were no visible alignment marks on the element/spacer edges so I used a fine Sharpie to add some.

 

The waxy stuff on the element edges made it easy to handle each element as I cleaned them with cotton balls, 91% rubbing alcohol, and my breath.  The uncoated lens elements cleaned up beautifully, no worry about harming coatings here.  The glass looked really good too, with excellent bevels and readily seen bubbles.  There is a small scratch at about the 60% span on the front element though (and in my mind's eye I see some folks recoiling in horror).

 

The cell and lens elements seem really well made.  The elements had a nice fit to the cell and that wax-type edge coating really helped to smoothly reassemble the objective with no binding of the elements.  I did do one "modification" to the cell though (more folks recoiling!).  I put three, equally spaced, clear tape tabs on the cell's inner lip where the objective sat.  I just don't like the idea of that brass to glass contact and any uneven pressure points that contact may apply to the rear element.  

 

Once reassembled, there was, like before, just the slightest of rattle when I would gently shake the assembly.  It's well made.

 

Now, one of these days, I'm going to get myself a nice, accurate set of guages and fixtures to measure element thicknesses and radii.  That would provide entertainment for the folks here with design software.  

 

Next I stuck the objective in my lower resolution DPAC set up for a quick look.  This set up is rather crude but works well for a preliminary assessment on slower objectives.  I buted the cell right up against the flat, held it in place with foam and blue tape, then used the old, beat up light box with 88 LPI screen to hunt around "downstream" to find the approximate focal length.  Once done, I stacked cardboard boxes and wood scraps there to the right height so I could slide the box back and forth through focus, centering the return image just above the top of the box.   Red Green   would be proud of me.

 

Jeff

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[Daniel Mounsey]

   Nice work Jeff. You're not mess'n around. 

[Jeff B]

Thanks Daniel and that's a nice cue for some DPAC photos.

 

These shots are all "preliminary" and a bit lower in resolution than usual because of the 88 LPI screen used, but they tell a story. 

 

And it's good story.

 

The at focus shot in green shows small/medium scale circular polishing marks/zones with some overlaying, uniform, small scale roughness.  I suppose by today's standards, you could argue the lens is not fully polished out, but it is a figure of revolution.  There is also a very, very narrow, odd "something" at portions of the edge. 

 

The inside/outside of focus shots show very straight lines indicating excellent spherical correction, and the polish is not all that bad IMO, certainly compared to some modern scopes such as the shot of the Meade 10" SCT.   In my experience, the subjective effect of such a polish is rather subtle, showing up as, for example, some scatter/fuzz around bright stars at high power and subtle loss of contrast on low contrast features on Jupiter.  

 

But the inside/outside images, especially the higher contrast extra-focal image, also show that very thin "something" at the edge as a very thin, uniform circle, 1-2mm thick, if that.   I believe it's real and not some diffraction effect, but this also points out how revealing DPAC can be as I seriously doubt it could be detected in a star test.  But I'll mask it anyway when I get the brass tube, assemble the system and retest it.  

 

Jeff

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Edited by Jeff B, 13 February 2023 - 11:51 PM.

[stevew]

Too bad it doesn't have a better polish, but the s/a looks promising.

Great find, and worthy project.

Please keep us updated,,

[fate187]

Your DPAC set up is ever so slightly more and more improving  . With all these tests there should be a pinned with a collection of the posts!

[R Botero]

Lovely glass and test results 👍😎

[Jeff B]

Thanks folks....but wait, there's more.

 

I changed out the green LED light bulb for a white light bulb, adjusted my camera's white balance, then took an outside of focus image.  

 

The result is really revealing.   

 

First, it's obviously and achromat with the color fringing around the Ronchi lines.  Second, the fringing is not R/L uniform, which I believe might be due to some azimuth misalignment of the box (Hey! I said it is a crude set up.), and/or.....a function of the design (a little more on that later)  

 

The color montage image shows exactly what you might expect from a decently made slow achromat, namely, color focus differences dominate.   Note though, the straight Rochi lines across the spectrum, which, is clear indication that spherochromatism is very well controlled and basically negligible from blue through red.   Ignoring the polish and tiny edge artifacts, the only meaningful aberration on axis is longitudinal color "error"....color focus differences. 

 

In terms of "color balance", the lens is not quite the classic for the time, C/F, color balance, meaning the blue and red have a common focal point that's just slightly longer than green .  The blue has been "let go"  a bit in focus position in this lens, favoring the red being closer to the green focus.  

 

So, this lens has a very wide air gap, which is very "Clark-like" as the Clarks employed large air gaps, a sort of signature feature, in most of their designs.   Large airgaps can basically eliminate spherochromatism, which is what I see here, but it also introduces some lateral color, which may be partially why the color fringing around the Ronci lines in the white light image are not uniform.  Misalignment of the Rochi screen might affect that too, but, when I can assemble the lens into a proper OTA, I can sort that out.  

 

But historically, with the large airgap, this is a very Clark-like design (Baker, who also used wider airgaps, was born, I believe, in 1914.) and as the Lohmann Brothers would sometimes use Clark lenses, well...........

 

Another thing I find pretty cool, is that the Clarks used a form of autocollimation (DPAC) to figure their lenses.  Now, I don't know if the Lohmann Brothers used DPAC, but if either/both did,  I'm looking at exactly what they or the Clarks were looking at on their test bench over 100 years ago.   I find that pretty cool.

 

Jeff

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[Alan French]

Interesting. I had a 5-inch f/15 Lohmann Brothers for many years and was happy with it. It was eventually reunited with its mount, which had a mechanical clock drive and was owned by a friend.

 

I am reasonably sure the refractor mounted on the side of a 12.5-inch scope pictured on page 441 of King's "The History of the Telescope" is a Lohmann Brothers. 

 

Clear skies, Alan

[Jeff B]

A little bit of an update.

 

I masked down the aperture by ~3mm by installing a ring of black, sticky backed hobby foam on the I.D. of the front retainer ring.  I wanted to see if that eliminated that thin "something" going on at the edge.  

 

And it seems, for the most part, it has as you can see in the original and "Redo" green images taken outside of focus.  

 

Jeff

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[Jeff B]

But I also took the opportunity to reshoot another white light image.  I then broke it down to the individual red, green and blue colors and made a montage. 

 

What struck me this time was how good the overall longitudinal color correction is on this lens.  As the achromat has a "chromatic ratio" of 2, I was expecting to see a lot of color error in the blue.  I did not.  Aside from what may be lateral color and obvious polish differences, this objective, color wise, reminds me a lot of my now gone 6" F10 oiled triplet that I named the Phoenix (which was based upon a pre-ED AP 6" F9).  You can see that in the two color montages.  As Roland related to me, the Phoenix is similar to a 6" F30 achromat, which is a chromatic ratio of 5.  The old Lohmann achromat has a chromatic ratio of ~2.

 

Hmmmm......

 

Now, I'm really curious to actually have a look through this 100 year old objective!

 

Jeff

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[Kitfox]

I’ve always wondered how these old glass lenses test without any anti reflection coatings.  Double Pass is brutal at exacerbating all issues; man is it sensitive!  How much of the apparent lack of “polish” do you think could actually be attributed to internal and air gap reflection (and internal imperfections/bubbles and variable refractive indices within the blanks) and not polish. My experience with these lenses is that they got the polish right.  The best guys of that era were very good at making the absolute best with what they had.