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Problem With SS 83mm Jaegers Objective

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#1 gerfmon

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Posted 02 February 2013 - 01:53 PM

Hi all, just received a Surplus Shed 83mm Jaegers objective. I'm having a problem where all stars and Jupiter have a large halo around them. I have the lens elements oriented per the SS instructions. I have the alignment marks aligned. I've tried reversing the orientation of the crown element. This only made the image worse. The flint element is oriented with the concave side pointing out (towards the sky). I have the supplied spacer installed.

The halos look very much like an optic element is fogged. The halo around bright Jupiter is about 3 times the size of the planet. I can see no detail on Jupiter. Sirius focused down to a bright dot, but was surrounded by twinkling spikes jutting way out from the star. I get a nice round out of focus image on a star, but no diffraction rings can be seen.

I checked the entire optical train and nothing is fogged. I tried a different diagonal and a couple of different eyepieces with no luck.

BTW, the scope was cooled down, had been outside for a couple of hours.

I also have an 80mm Towa lens and when mounted in the cell and same OTA I get beautiful images. One difference, the Jaegers has a slightly longer focal length and I have to rack the focuser way out, several inches further out than with the Towa objective (for which the OTA was originally designed). The focuser is just an inexpensive Synta R&P focuser with a long drawtube.

Could there have been some really weird seeing conditions for me last night? If so, I've never seen anything like it in months of observing. Last night I didn't think to try a different scope.

Any suggestions before I call Surplus Shed?

Thanks

#2 neo

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Posted 02 February 2013 - 02:42 PM

Is it an uncoated lens which SS sels now?
I got one of those but coated a while back togather with an aluminum cell and tested out beautifully on autocollimation (at least to my eye). But I had to tweak it a bit (rotate the lens one against the other), also the spacing wasn't really spot on. The markings on the edge weren't very clear to me. I'm not sure if the markings were made by jaegers or SS.
I'm also not sure how careful are the guys from SS on testing these lenses before shiping so anything is possible. I guess this is the real price for being surplus.

#3 gerfmon

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Posted 02 February 2013 - 03:15 PM

I hadn't heard from Surplus Shed for a few day after I placed my order, so I wrote them a quick email asking for a status update. This is a direct quote from the email I received back from them: "We do take our time with the Jaegers lenses to ronchi test them and make certain you get the quality you expect".

So I guess who knows for sure? Guess my next step would be to rotate the crown element a few degrees at a time and see if I can hit a sweet spot. I do have an artificial star I can aim at across the yard.

#4 GlennLeDrew

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Posted 02 February 2013 - 03:19 PM

If there is no gunk causing scatter, your description seems like it could be spherical aberration. As you rack through focus on a bright star, this sequence will be seen, if the optic is undercorrected.

Well inside focus, most of the light will lie withinin a ring, with a dimmer center.

Getting close to focus, the image will be a more or less uniformly bright disk.

At sharpest focus, there will be a tiny, bright central point, surrounded by a small halo.

Continuing to rack outside focus, the intensity of the central point will diminish as more light is put into the halo.

If overcorrected (not so commonly found), the sequence will be reversed.

What is the objective's focal length or f/ratio?

#5 Sean Cunneen

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Posted 02 February 2013 - 03:22 PM

You have the elements in the wrong way. I know you said you checked, but I have put together many and from what you describe, that's what's happened. I am pretty sure you probably have the two lenses the right way but you have them flipped around in the cell.

If you had a bad lens, you wouldn't see halos anyways, so we know it's the orientation, not the quality.

#6 ed_turco

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Posted 02 February 2013 - 03:26 PM

I'd send it back and ask for another one. I don't think that lens got tested at all.

SS makes claims, that sometimes I really wonder how they got the numbers, except by guessing; however, they are totally golden on returns.

I would have a hard time going anywhere else; they are that good for 99.9% of what they sell.


Ed

#7 gerfmon

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Posted 02 February 2013 - 03:35 PM

Ok, maybe I'm reading the SS instructions wrong. Sean, are you saying the flint element should go on the outside, the crown on the inside?? The opposite direction of most achromatic lenses? The instructions read:

"The crown with the slight curve faces the sky, then the spacer ring, then the flint with the negative side goes on top of the spacer" So going in the direction away from the scope, towards the sky it should be "crown - spacer - flint? Sorry, I'm confused. :)

#8 Sean Cunneen

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Posted 02 February 2013 - 03:59 PM

These instructions are just for Jaeger's objectives, other prescriptions might be different.

You have a concave element and a convex element. the curves are different on both sides of both elements. One concave side and one convex side match, those face each other. The other concave side is almost flat and is the back of the objective. The last convex surface is the front of the objective and points towards the sky.

These curves are not as obvious as most would have you think. Also, some cells load from the front and some for the back. There are so many variations among manufacturers I no longer berate myself when I assemble my objectives incorrectly. I now consider it part of the process. I mean, If I don't know what the images look like wrong, how will I know what they look like right?!?! Right? Anyways, that's what I tell myself. Yeah, that's it, I assemble them incorrectly on purpose, that's my story... :cool:

#9 gerfmon

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Posted 02 February 2013 - 04:16 PM

I'm going to go ahead and try facing the flat side of the flint glass pointing to the outside, to the sky. The concave side of the flint will face in, then the spacer, then the most convex side of the crown facing the spacer. It kinda makes more sense that way, according to the SS instruction sheet.

Every two element acromat I've dealt with has had the flint concave side mating with the crown convex side. BUT, every acromat I've dealt with has had the crown on the outside, facing the sky and the flint on the telescope side.

I do seem to remember reading in a thread somewhere about an objective that required the flint to be on the sky side. I can't seem to find it now with a search.

#10 DAVIDG

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Posted 02 February 2013 - 04:50 PM

I have couple of these, they are the standard crown forward Fraunhofer design and the mounting instructions are correct. A flint forward design is called a Steinheil.
As Glenn stated your seeing a large amount of spherical aberration. Either one or more of the surfaces is not spherical, the radius is off, or the two elements are not for the same design. They might have mixed up elementsfrom two different focal lengths. Rotating elements against each other isn't going to fix the problem.
I've had good luck with Surplus Shed over the years but they do make mistakes. A number of years ago I purchased some long focus 60mm achromats were the elements were air spaced and came unmounted. Two of the pairs were correct for each other but the third pair had an element that was very wrong. I sent it back and they refunded the price without any problems.

- Dave

#11 gerfmon

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Posted 02 February 2013 - 05:09 PM

Ok, I have the flint pointing out now. A quick view at some terrestrial objects wasn't encouraging. The view still seems to have a "foggy" look. I wish I had a second lens cell so I could switch the Jaegers and objectives lens back and forth. Oh, another thing I'm seeing, both last night and today..color..a lot more color than a f/15 should show, IMO. The F/15 Towa lens shows almost no color at all, except on the very edges of Jupiter. Just now looking I could see a purple fringe on every tree branch.

Well, the real test will be when the sun goes down. If the star images are still crud, the Jaegers will go back to SS...

Thanks guys for all your input and advice..

#12 gerfmon

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Posted 03 February 2013 - 02:14 AM

Ok, I turned everything around, flint glass to the sky, crown glass to the telescope. Images are much better, pretty sharp focus on stars, Jupiter is showing some detail now.

But oh my, CA is the worst I've seen in any telescope. The CA on Jupiter is horrible. I put on my minus violet filter plus my #8 yellow filter and the CA was still bad. CA isn't as bad on my 6" f/5 750mm refractor!

Any ideas on what I'm doing wrong now? Or does this puppy need to go back to the Shed?

I put my 80mm f/15 Towa lens back in the scope. Beautiful tight stars again. Looked at the trapezium for a bit, over to Jupiter where I saw at least 3 cloud belts and some hints of detail. Just barely a hint of purple halo. M81 and M82 were easy to see. And I'm sitting here right now waiting for M51 to clear my neighbor's house. Again, thanks all for the help today. :)

#13 bremms

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Posted 03 February 2013 - 07:26 AM

Jerry, Cotact Fred, I did get one about three weeks ago that was about bad as the one you have. Something was surely wrong, as the FL was 2.4 inches longer than stated.Fred swapped it out and the replacement is very good.






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