Posts Made By: Tom Rodstrom

I have a couple Mak's and I am not quite sure how to determine the actual focal length with different configurations. As I understand it the true focal length changes when a mirror is moved to change focus.

Both of these scopes (Questar 3.5 & 7) have turrets which change focal length so I need to take many readings. I have an inspection scope that lets me measure exit pupil's accurately. I believe that if I focus the scope at infinity and then measure the exit pupil of a known eyepiece, I can calcuulate an accurate focal length of the scope.

Is this a good plan?

I finally have too many telescopes and telescope projects (I think). There are at least 18 scopes and lenses 80mm or larger so I am going to have to sell some of them. It's difficult since they are all different and all have unique attributes. Most are in the picture. Starting from left to right they are:

1. Perkin Elmer 8" F5 four element Astro lens.
2. Military 5" F7.5 four element OTA with full 5" diameter focal plane.
3. Questar 7 with fast camera focuser.
4. Celestron C90 armored spotting scope.
5. Questar 7 surveillance model with focal changer and filter cassette (643mm to 4826mm efl at camera focus).
6. Antique (around 1900) French made 5.5" objective that once belonged to the Vatican observatory.
7. Home made 55mm 6 element eyepiece with 58mm eye lens and 62mm field stop.
8. Questar 3.5 surveillance model (605mm to 2416mm efl at camera focus).
9. Strange zoom lens with 6' f5 achromat, 5" negative fl triplet and 4" achromat.
10. Astro Rubinar 106mm F10 Mak with custom rotating 2" diagonal and finder scope.
11. Home made 100mm f4 wide field scope.
12. Zeiss 6' f7 triplet astrograph.
13. Jaegers 4.25" f5 refractor.

Not shown

14. Celestron C4.5 Newtonian (disassembled).
15. 4" f5 lens (only).
16. 4" f4 lens (matches 11).
17. Apogee 80mm f6.25 ED lens with tube & focuser.
18. Camera lens 400mm fl f7 with 12" diameter corrected focal plane.

For mounts, I have an eq3, two Polaris mounts, two CG5's, and a JMI prototype with 1.5" shafts.

I just can't seem to part with any of them. I think I need Telescopes Anonymous. What should I sell?

P.S. I'm broke and, yes, I'm single.

Tom

Is there a recommended length for dew shields. I need to make dew shields for a 4" refractor, a 5" refractor, and a 7" Mak.

I just bought a nice 6" f8 1960's era mirror with cell by Edmund Scientific. I want to make a custom Dob using a bunch of Bosch "erector set" type aluminum extrusions. The main support for the eyepiece would be a very stiff 40" long 1.6" x 1.6" extrusion.

I have a 1" diagonal which is a little small for the mirror. I plan to compensate for this by making a linear focuser that moves the secondary and the mirror up and down rather than moving the eyepiece in and out. This would allow me to put the eyepiece as close to the secondary as possible.

There will be no tube, but a thin ring of fiberglass would provide a dark background for the eyepiece. The spider would be two stainless steel strips attached to a four screw push-pull adjuster. The spider would be fixed, but mirror and cell could be moved left and right to align the optical axis with the focuser travel. The cell has the standard three screw adjustments to align the mirror.

Any comments from those more experienced?

I just picked up a Graphlex 4x5 camera, some Polariod film holders and a couple boxes of 800 speed 4X5 B&W film. I'm thinking about trying some astrophotography with the lens board removed. I would use a 5" f7.4 refractor with large coma corrector on a CG5 with a 4" guidescope.

Any comments or suggestons would appreciated ("you're nuts" is an acceptable comment).

Thanks
Tom

This is a nice picture that was posted by Graeme Hill on the Off Topic Diccussions forum "Posting an image" string.

When I look at this picture, I get the same effect I sometimes have looking at the moon at very high magnification through a scope. The topology looks inverted so the craters look like hills. By looking carefully at the small crater on the left, I am able to flip things in my mind so everything looks normal.

I've never seen this effect in a picture before. Does anyone else see the same thing?

I have a Mitutoyo binoviewer from a tooling microscope. It has a 26.5mm clear aperture and UWF10X eyepieces. I tried it in a 90mm F4.3 refractor and was really pleased with the sharp views.

Unfortunately, it needs too much in travel (4.44" compared to a Meade 3000 25mm Plossl). I had to remove the ring from the diagonal and the sleeve from the binoviewer to get it to focus. In order to use it normally, I need to pick up at least 1.75" of that in travel.

Any suggestions?

I have an 8" F5 telephoto lens that I finally put a focuser on and did a star test. It's not too bad (very bright) up to about 80x but there is too much color after that. It is optimized for film and gets a blue green haze at high visual magnifications.

I want to make an optical bench to test it. I have a monochromator that will give me a precision colored source. I plan to run the light from the monochromator to an aperture plate that goes down to 0.002" with a fiber optic cable. The plate would be put at the focal point of a Questar 7 set for infinity.

The Q7 would be the target. It would be mounted on a tilt/raise plate attached to a large rotaing ring that reads to arc minutes. The base would be a pair steel rails with commercial Bosh "tinkertoy" extrusions attached to the top. NRC optical plates would be attached to the extrusions where needed and the whole thing would be on vibration pads or possibly bubble wrap.

Does anyone see any fatal flaws in this approach.

I have a 8" F5 telephoto lens. It is marked "DISTORTIONLESS TELEPHOTO AERO LENS 40 IN. (1016MM) F/5.0 T5-Y6-068 MADE IN THE U.S.A. BY THE PERKIN ELMER CORPORATION, NORWALK, CONNECTICUT"

The only reference I can find to a PE T5 lens is a lens that was used at the end of WWII to film atomic bomb explosions.

Does anyone know where I can find more information on this lens?

I added a picture. The Perkin Elmer is on the left.

Here is a picture of a drill press I use as a small mill. It is a cheap Ryobi 10" with an X-Y table I bought used for about $100. The key is the black knob on the left. It locks the quill while I am actually cutting. Without it, the quill is too loose and you cannot get a clean cut.

I had to remove the quill to drill and tap a #10/32 hole for the locking shaft. I used a bronze screw for the shaft so I don't ding she quill itself.

With aluminum and a 1/4" end mill, I can cut about 0.020 at a time. The procedure is to loosen the quill just enough to move it down to start the cut and then lock it while moving the table. There is some chatter while the quill is loose and you are starting the cut, but it is manageable.

It's a jury rig that works quite well for me, but I make sure to wear eye protection, not wear loose clothing, and do everything slowly and carefully.