64 replies to this topic

[clamchip]

Well here is the working telescope for my ebay lens.

The lens at high power is very good but I seem to have a problem that is going to take some time to sort out since it is something I haven't had to deal with in the past, I just take the scopes apart, clean and reassemble.

The problem I think is the lens element spacing. Inside focus I have a purple fringe, outside focus is a yellow/green fringe, even farther outside focus Y/G fringe with red center on star pattern.
All this is fine except at focus I still have a purple fringe and when I defocus to the outside the image is CA free but out of focus. So I think I need to move the purple closer to the objective, away from in focus. I hope this can be done with spacing.
I'm a little out of my expertise.

But here is the scope and mount I've been working on, all this for a 150.00 ebay lens.

Robert

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[clamchip]

I built the RA drive around a vintage C8 Byers gear set and copied the Edmund Scientific clutch design with a central tension screw pressure plate and cork linings, no springs. Works better than hoped and tracks perfect.

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[clamchip]

It's a beautiful big scope and real nice to use. Views of Jupiter and lunar detail have been very promising for this old lens. I just hope I can move the CA away from sharp focus.
Has anyone experimented with spacing? I know a lot of the old scopes we restore can be missing spacers and the thickness can be unknown at times, I think in my case the previous owner had not known the correct spacing for Brandt lenses which are a different design than the standard Fraunhofer.

Robert

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[DAVIDG]

Very nice job ! I hope you can get the color problem fixed with spacing but it sounds to me that one of the radii is off or possibly an element is flipped. The first thing I would do is use a green filter and a ronchi screen with at least 100 lines/ inch and see how straight the lines are when viewing a bright star. If they are straight, then with the spacing you have now is correct for the least amount of spherical abberation in the green. Moving the spacing will then add spherical as you try to adjust the color.
If the lines are not straight then take a hard look at the lens elements and see if there are any indication that one maybe flipped or the whole assembly is backwards. Don't laugh I have seen this happen many times and a backward lens will give an OK image. Once you rule that out try adjusting the spacing until the lines are straight with the green filter and hopefully the color issue will also be corrected.
If you can get use of 4" or larger optical flat it will make adjusting your lens much easier, since you can set it up in double pass mode which increase the errors by 2X so when the correction looks right on the test stand any errors left are only 1/2 when used on the stars.
An excellent book on star testing refractors is "On the Adjustment and Testing of Telescopic Objectives". Digital copies are available free on the 'Net.

- Dave

[clamchip]

Thanks Dave you gave me something to work with. I also downloaded the T.Cooke & Sons book you recommended. I forgot to mention a severe ghost problem at low power, Jupiter for example centered in the FOV has a ghost at the field edge. Not sure if it's a reflection between R2-R3 or from the field lens of the low power ep. But the ghost is the least of my worries right now. No coatings on the 4 surfaces of this lens.
I have this old ronchi screen eyepiece and a green filter I'll try.

Robert

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[tim53]

Robert:

That turned out great! The mount in particular looks beautiful. Your machining skills are showing!

-Tim.

[clintwhitman]

Robert, The New Scope looks great amd the Drive is very cool, Nice use of a good gear. Somone told me these were great but yours really did come out perfect.. As to your optical problems I have only seen what your describing once and it was in a Brandon triplet that was flipped. I worked on a doublet, 1860s Clark that would not show round rings inside and outside of focus, It had concentric Amibas! But I did not notice any reall issues with the color. Well I could not figure this one out so the lens was sent to Barry at D&G. He said the spacing was wrong and he fixed it with the correct size. Now the lens shows one of the best star pattens I have seen to date.
Barry is a real good guy, a note to him about what this lens is doing might set you on the path to correct it in a hurry....
Keep posting and good luck!!
(aveman...

[Fred Ley]

I have one of those Coulter sets, never used, still wrapped in paper and in its plastic container along with the instructions.

[mikey cee]

Well you learn something new every day here on CN. What other size lenses did Coulter sell? :question:I went with Brandt at the time....he was running a 30% off promotion and did not hesitate. :Mike

[clamchip]

I tried out the Coulter ronchi screen eyepiece years ago on a newt but haven't used it on a refractor.

Mike do you know approx thickness of the spacer on your Brandt 8"? I thought I remembered you saying it is a ring spacer and fairly thick?

I know I'm to expect some CA with a focal ratio of 13.3 and a 6" aperture but if I move just slightly out from focus the CA disappears but now out of focus.
My D&G 6"f12 at focus is very CA free, in fact I use the CA fringe to help focus, purple on the inside of focus green on the outside when the fringe is gone focus is spot on. I would expect the Brandt lens to be as good as Barry's lens he was a master lens maker, although the D&G is an exceptional refractor and it really shows when you use one.

Robert

[mikey cee]

Robert.....I only had the lens apart once maybe twice. That's been several years on the last cleaning. But I can tell you this much....it's thicker than regular paper or foil spacers. If it weren't it would sag in your fingers. I'd say like the cover on notebook paper.....it's plastic not rubber or neoprene. ;)Mike

[clamchip]

This 6" is about the same, thicker than a file card almost cardboard.
I'm just going to follow Dave's steps and see what I can find. I had the elements out of the cell once just to see if any markings on the edges and I was careful to put it back together the way it came apart. I didn't pay any attention to the design but I did think it was spaced quite far apart.
No mention of the design by Brandt but looking at a cross sectional drawing it appears to be similar to the Littrow, this could explain the ghosts especially since it's uncoated. I could also have a flipped crown if R2 is slightly different radius than R1 to correct ghosting where as the Littrow has a equiconvex crown.

All I know is it's pretty dang good now and I can only imagine how good once it's set up properly.
If the fun and excitement turns into headaches and tears I'll just send it off to Barry and he can fix it.

Robert

[clamchip]

Being bored today and no future clear sky to test the lens spacing I decided to throw science to the wind and do some gambling.

I removed the .040" spacers and installed .005" spacers and used Newton's rings interference pattern while rotating the crown in case of any wedge.

Really wasn't expecting much so I just set the scope on a chair and pointed it at a insulator on a barn across the valley. Started with low power eyepieces and worked my way up in power.

The scope is completely different now and is nothing short of ABSOLUTELY UNBELIEVABLY FANTASTIC! what did I do? can spacers make that much of a difference? was it SA that was creating the problems?

I brought out some rarely used eyepieces to see if I could find image break down on the insulator numbers:

6.7mm for 303X, walk in the park!
3.8mm - 533X piece o' cake!
2.5mm 811X no problemo!!!!!!!!!!woweeee!!!!

Robert!

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[tim53]

OOo!

Get that bad boy back on the mount and burn a hole in the clouds tonight!

-Tim.

[Don Allen]

Good news Robert. Hope that was it.

[DAVIDG]

Robert,
Your not viewing an object at infinity so the spherical abberation and color correction are different. You might have under corrected the spherical abberation, which is what you want for an object that is not at infinity. Hopefully you improved the problem but a star test is the true test. Also rotating the elements while viewing the interference rings is not canceling the wedge in the elements. The interference pattern is formed in the air gap between the elements. If the pattern is offset it is because the air gap is varying in thickness, which is usually caused by the spacers being of unequal thickness or the element setting slightly higher on one spacer. You need to measure the edge thickness around each element to find the high and low spots and then rotate the elements so they offset.

- Dave

[clamchip]

Oh No Dave! I thought that was a little too easy!

I guess I'll leave it as I changed it and star test, at least I'll know what the minimum air space does to the star. I can go back to the .040" spacers and start over again with the ronchi screen and green filter.

I can see my usual cut-and-try approach is not going to work for this project. I have a few books on the subject I should read while the weather's bad.

Robert

[DAVIDG]

Robert,
The best way to test your lens is with double pass autocollimation but that requires an optical flat which most people don't have. Another method is to use another telescope of known high quality, big Newtonians work well. You place a pin hole light source in the focal point of the Newt. A green LED mounted in a film canister with few thou hole in the middle works well. The light coming out of the Newt, in theory is parallel, just like star light. Now you aim your refractor at the Newt and using an eyepiece and/or Ronchi screen examine the image. If you use a Ronchi, be sure to focus so there is only 3 to 5 lines showing or the sensivity will not be good enough to detect any problems.

- Dave

[tim53]

Interesting suggestion, Dave!

Does a Newt work as well as a flat for autocollimation? It would sure be easier to make, and many of us already have more than one Newt that could be used for this purpose. I suppose the only potential show stopper might be that the test stand would take up more room?

Oh, and it wouldn't be double pass, either.

-Tim.

[tim53]

I just remembered that when I worked at Meade for a few months in early 1982 - by that time, they were making 2080s and 2120s in pretty large numbers - they used an SCT OTA as an artificial star source for testing the new OTAs as they were being assembled.

-Tim.

[DAVIDG]

Tim,
You got it, it is not a double pass test so the sensitivity is less and unlike double pass autocollimation were the flat only needs to optically smooth and can be many waves from flat, the quality of the telescope producing the artificial star is very important.

- Dave

[clamchip]

How far do I need to separate the two scopes, same room?

Robert

[tim53]

I *think* they can be right next to each other, but I'll defer to Dave.

-Tim.

[GeneT]

I don't know about your problem, but you have a beautiful telescope!

[DAVIDG]

Robert and Tim,
In theory it doesn't matter how far apart the scopes are but to keep the effects of air currents down, keeping them close, like a few feet apart is best.
You need to get the light source positioned at the focal plane of the telescope that will be producing the test beam. If not it will be either slight convergering or divergering and the telescope being tested will show spherical abberation to some level.
My advise is to set it up and try it, and see how your lens tests out. This stuff is pretty easy once you get an understanding of it. There is no black magic to lens work. The first thing to do is test the lens in the direction you thing it goes, and then flip it around. In the non correct position you'll see a large amount of spherical abberation which will cause the ronchi lines to really bend. You won't be guessing at what might be wrong and you'll have a method to SEE what effect your having on the correction by flipping an element or changing the spacing. Once you get to a point were your only seeing small errors you can start to worry about the exact position of the light source in the reference 'scope and hopefully you'll be able to cross check your results with a star test. Also try testing a couple of other scopes of known quality as a cross check to your testing method.

Happy New Year,
- Dave