18 replies to this topic

[Nippon]

I was wondering with all the discussions of optical quality on the refractor forum about the Lick and Yerkes refractors. How good are those objectives? Do they have excellent figure and polish or was size the main thing in their day. I know that big observatory mirror telescopes took over because they could be made much larger and did not have chromatic aberration. I visited Yerkes last year but they were not giving views through the 40" at the time. 

[james7ca]

I seem to remember reading a report that after the construction of the 40" they found that the weight of the objective glass caused deformations in the optical figure of the lens which somewhat limited the performance of the scope. In fact, this is one of the reasons why they never tried to make a refractor that was any larger than the 40 inches.

Edited by james7ca, 23 September 2019 - 12:49 AM.

[Jon Isaacs]

I seem to remember reading a report that after the construction of the 40" they found that the weight of the objective glass caused deformations in the optical figure of the lens which somewhat limited the performance of the scope. In fact, this is one of the reasons why they never tried to make a refractor that was any larger than the 40 inches.

 

I remember Roland Christen said that the difference in focus between the colors was about 1/4 inch, that you could focus the different colors.  

 

Jon

[Nippon]

I read about the optics sagging but also read that the two elements being one positive and one negative cancelled out figure problems since the sags effect to figure in one was compensated by the sag in the other. 

I think what I read about that was from Astrojensen.

Edited by Nippon, 23 September 2019 - 10:14 AM.

[Alan French]

"The Telescope," by Louis Bell, figure 187 "Hartmann Tests of Telescopes" includes the Yerkes' Clark.

 

Sky & Telescope's "Gleanings for ATM's" column in the March, 1982, issue, "Optical Designs of Some Famous Refractors," by John Church is interesting reading. 

 

It includes the Lick and Yerkes instruments. 

 

Clear skies, Alan

[AndresEsteban]

"The Telescope," by Louis Bell, figure 187 "Hartmann Tests of Telescopes" includes the Yerkes' Clark.

 

Sky & Telescope's "Gleanings for ATM's" column in the March, 1982, issue, "Optical Designs of Some Famous Refractors," by John Church is interesting reading. 

 

It includes the Lick and Yerkes instruments. 

 

Clear skies, Alan

Thanks for the S&T March 1982 indication, Alan!

This issue - S&T March 1982 - is available at:

https://archive.org/...ope_1982-03-pdf

 

pages 302 to 308 March 1982 S&T - Gleanings for ATMs - "Optical DEsigns of Some Famous Refractors"

There's also an interesting reference in the April 1981 S&T issue, page 299

 

 

Clear skies for us all!
Andy

[Mitrovarr]

I read about the optics sagging but also read that the two elements being one positive and one negative cancelled out figure problems since the sags effect to figure in one was compensated by the sag in the other.
I think what I read about that was from Astrojensen.

Seems unlikely this would cancel out completely since the two lenses aren't the same thickness nor are they the same material, so they should not sag equally. Also sag is in the direction of gravity which means it is off-center if the scope is not pointed straight up. This is going to create complex, asymmetric effects.

[Astrojensen]

Seems unlikely this would cancel out completely since the two lenses aren't the same thickness nor are they the same material, so they should not sag equally. Also sag is in the direction of gravity which means it is off-center if the scope is not pointed straight up. This is going to create complex, asymmetric effects.

Nevertheless, it's happening. That is at least what I've read so far, from those who have actually used such scopes. Both the Lick and Yerkes refractors are diffraction limited and have been used (on rare occasions of perfect seeing) at magnifications in excess of 3,000x and to resolve double stars near 0.15". Both scopes were commonly used at around 1,000x magnification. 

 

 

Clear skies!
Thomas, Denmark

[Vla]

It may be "diffraction limited" in the optimized wavelength, but its polychromatic Strehl (photopic) is 0.36, with near perfect central line correction. If central line correction is, say, 0.80 in the central line, it drops to 0.29. This is still much better than what its nominal secondary spectrum implies, but polychromatic Strehl in achromats changes with the square root (closely) of the nominal secondary spectrum, not with it.

[spatzek]

The Lick refractor was refigured 30 years ago. We - five amateurs from Denmark had a night together in 1991 with Gene Harlan and the 36" under crystal clear skies with perfect seeing. No optical problems whatsoever. Beautiful night. We saw M57, M17, M13, NGC6210, Uranus, Saturn etc. The central star with direct vision, no problem. Titan was a disc looking as Mars in a 3-4" telescope.... Saturn, looking like the Hubble photos - the best view ever with a telescope. Uranus was this green-white globe with 4 moons. Hands down, it can deliver views out of this world if the conditions allows. 

[Chris Cook]

"The Telescope," by Louis Bell, figure 187 "Hartmann Tests of Telescopes" includes the Yerkes' Clark.

 

Sky & Telescope's "Gleanings for ATM's" column in the March, 1982, issue, "Optical Designs of Some Famous Refractors," by John Church is interesting reading. 

 

It includes the Lick and Yerkes instruments. 

 

Clear skies, Alan

Good read. Thanks for posting Alan.

[daquad]

I remember Roland Christen said that the difference in focus between the colors was about 1/4 inch, that you could focus the different colors.  

 

Jon

From Sidgwick's "Amateur Astronomers Handbook," p.94:  Referring to his observation that achromats greater than 7" will have secondary color in excess of the tolerable limits (due to practical restrictions of focal length) , he writes:

 

"In this connexion it is of interest that the H(alpha) and D2 foci of the Lick refractor are 81.5 mm apart."  That's 3.2 inches!

 

Dom Q.

Edited by daquad, 29 September 2019 - 09:53 AM.

[gnowellsct]

"The Telescope," by Louis Bell, figure 187 "Hartmann Tests of Telescopes" includes the Yerkes' Clark.

 

Sky & Telescope's "Gleanings for ATM's" column in the March, 1982, issue, "Optical Designs of Some Famous Refractors," by John Church is interesting reading. 

 

It includes the Lick and Yerkes instruments. 

 

Clear skies, Alan

Good lord what a memory

[noisejammer]

Did  read those lens specs correctly ... R₄ was 1.2 kilometers?

 

I did some FEA on behalf of Peter Wise when he was considering a 50" dialyte. Even a BK7 singlet deformed very appreciably. (Sorry - I can't remember how much it was.)

 

It also struck me that the resulting strain might lead to localised changes in the refractive index.

[Alan French]

Did  read those lens specs correctly ... R₄ was 1.2 kilometers?

 

[SNIP]

Which lens?

 

Clear skies, Alan

[noisejammer]

The Yerkes - R₄ is down as 1,219,000 and then later as 1,209,290 (millimetres). That's 1.2 kilometers(!) I see the Lick has a 1 km radius on R₄.

 

As a occasional ATM, I'm curious how you'd even grind this (let alone measure it to better than 1 part in 10⁶.)

[Alan French]

I'm still puzzled. Where did you get R4?

 

In the Gleanings for ATM's column in the March, 1992, Church has R4 as 96000mm for the Yerkes and 180000mm for Lick, both long but shy of 1km. 

 

It any of these cases, it would be interesting to see how critical the value of R4 is. 

 

Clear skies, Alan

[noisejammer]

Those are the numbers in the table of the March '82 Gleanings. Table 1, pp 303

[Alan French]

Those are the numbers in the table of the March '82 Gleanings. Table 1, pp 303

I should have looked at page numbers. I have the article photocopied and in a loose leaf binder and had only seen the table of indices on 304 and the numbers Church derived later on. The first two pages apparently stayed together when I leafed through. 

 

Thanks, and clear skies, Alan