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Workscope for a bum lens

Started by 1953, 12/18/2008 07:43AM
Posted 12/18/2008 07:43AM Opening Post
Hey all:

A while back I posted the test results of a 9" F12 lens I got second hand:

http://www.astromart.com/forums/viewpost.asp?forum_post_id=630309&poll_id=&news_id=&page=2

I posted the following on CN to get advice for how to proceed and get a sanity check before I spend a bunch of money and perhaps waste the opticians time:

About 6 months ago I picked up a 9" F12 lens made by the Herron company, perhaps in the 70's. It was advertized as a Baker design due to the large airgap of the lens. It sat around for months before I actually got a chance to do a star test, which came out truly horrible.

I sent it to a couple of vendors to see what was going on and got an interferogram. The test results confirmed what I saw at the eyepiece..truly awful with a 4" diameter zonal hole in the middle and a slightly turned edge. Fortunately the glass looks ok...but that's part of the problem, the glass is undocumented.

Here are the measurements for the elements:

R1= 61.964 inches
R2=-38.489 inches
thickness= .7485 inches

R3= -38.930 inches
R4= -175.584 inches
thickness= .538 inches

airgap= .334 inches

One of the vendors used these measurements to do some Zmax runs assumming BK7, F2 and F4. These came out looking nowhere near a C-F corrected achromat.

Then I scaled a known Fraunhofer design using BK7 & F2 to a 9" F12. I got radii that match R2 & R3 within .5% and R1/R4 within 5-7%. Subsequent Zmax runs by the vendor assuming BK7, F2 & F4 with reduced airgaps of .035" & .013" respectively show what appears to be a decent Fraunhofer design using either F2 or F4. I'm just assumming the front element is BK7 as that is such a common glass.

Not being a lens designer and not having access to lens design tools, I'm looking for independant verification of these results before I give the go ahead to repolish the lens in an attempt to save it. So if some of you guys with the design chops would like to take a crack at it, please do. I'd be especially appreciative of and interested in your thoughts as to why the airgap would appear to be all wrong for BK7, F2 & F4 glasses...or am I all wet concerning glass type assumptions?

With regards to the workscope for the lens, here is what we have come up with as a start and again, suggestions are welcome, however, to reduce costs, no test plates will be made.

1. Measure and document wedge
2. Examine the Newton rings formed by R2 & R3. If they are nicely centered and uniform, that tells us that they're probably not the offending surface (s). If they're off badly off centered and pinched, that says R2, R3 or both may have a problem.
3. Test R3 just like a mirror to see what's going on with it.
4. Reassemble the lens with a narrow air gap and restest under autocollimation to see what happens to the zone.
5. Retest in autocollimation with one of the elements rotated at 90 degree intervals to document how the zone behaves. The thinking here is that as the zone is so uniform, it may be confined to one surface, hopefully R1 or R4 as many lens makers use those surfaces to figure a lens. Rotating one element might give us a clue to the offending surface(s)and who knows, we might even get a null in the process.
6. Digest the results of steps 1-5.

The focus of this preliminary workscope is to try to identfy the offending surfaces to target the polishing to reduce reclaimation costs. Any thoughts on this would be appreciated but please keep in mind that I'm not going to turn this into a science project and throw a lot of money at it.

Jeff

"Don't sweat the petty things and don't pet the sweaty things" 8O