Posts Made By: Jeff Blazey

David:

Building an OTA is not that difficult, especially if the lens cell fits over or into a standard aluminum tube. You can save a TON of money too. You already have the most expensive part, the AP focuser.

What diameter tube does the cell fit over or into?

Jeff

Good old algebra for a straight line. Y=MX+B. B is one half of the fully illuminated field you choose. I recommend 35MM.

Jeff

Having only recently been back in the hobby since 1990, I've found out just how RUSTY I'm at finding things in the sky. DSCs and GOTO are now looking very attractive indeed. What are the pro's and cons of each? It would seem to me that setting either of them up in the field would be similar but the DSCs would be more reliable as you don't have to deal with noisy high torque motors. Or is that not a problem?

There's a lot of experience & skill here and your comments and advice would be most appreciated. I own both the Losmandy 8 & 11.

Thanks.

Jeff

Here's my 1986 vintage AP 130 F6 with the famous "NASA Glass". The lens was made in December of 1986 and was signed on the edge by roland. I've updated the scope with a new backplate (D&G) and AP 2.7" focuser.

The lens is exceptionally sharp and the CA is well controled for such a fast scope. It makes an amazing telephoto lens as well.

That old guy next to it is...well..me.

Jeff

Hey how about a new thread where we post pictures of ourselves with our scopes "in the day" and in the present. Might be fun. For me, it's that 80's show:

Then: ~1982, 28 years old with a 6" F10 Jaegers on a modified Cave mount

Now: 2008, 54 years old with a 1986 vintage 5" F6 AP.

Ok, my questions for the day:

1.Just how accurate are the focal lengths that are stamped on the side of eyepieces? Is 25MM really 25MM?

2. How can I measure the true value of the eyepiece's focal length?

Curious

Jeff

I've a large apeture achromat, 9" F12 to be exact, which needs refiguring. I would appreciate the names of individuals/companies which offer such services.

Thanks.

Jeff Blazey

Title says it all. It has "O.P.S" stamped on the sloped side of the black top. It was obviously done at the time of manufacture. Not sure what it means.

Can anybody help?

Jeff

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

Well, here's a picture of the 11" objective I made for my scope.

It's a doublet with a slight air gap between the elements. The front element is made from O'hare's famous CPI1P (Cast Pig Iron One Pour), the rear element is Shot HD3/8PW (Home Depot 3/8" Ply Wood).

I can detect no spherical abberation of any kind, including spherochromatism. There are NO color crossings as there is no longitudinal color error from the deep dark red to radio frequency blue. Fifth order and all other abberations are zero. It's not sensitive to wedge and very tolerant of de-centering of the elements.

It was cheap to make (about 26 bucks) and NOT imported from China.

Jeff