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Posts Made By: Ed Jones

May 12, 2004 04:53 PM Forum: Telescope Making

Re: Turpentine dissolves stuck pitch

Posted By Ed Jones
Andrew,
Yes you'll probably not see this in print anywhere. I never stick my mirror to my lap but did once years ago. I would stick them in a plastic bag in the frig. for an hour and then in the freezer. Try to separate them every half hour or so, they will eventually separate. I always strip my pitch laps I'm done with by sticking them in the freezer. When pitch reaches freezer temperature put your hot hand on it and it cracks and pops like ice in hot water. Often you can take it off with your fingers, if not then a putty knife will take it off easily. When done empty the bag into your pitch pot. Keeping it in a bag keeps the mess down.
Another trick I know is my "pizza method" of making a pitch lap. If you're interested I can describe it for you.
Ed Jones

May 15, 2004 02:00 AM Forum: Telescope Making

Step 3

Posted By Ed Jones
Chuck,
No these are 4 inch+ corrector lenses for my window telescope I told you about. They were the only thing I had laying around ready to polish that I could use for a demo.

Ed

May 16, 2004 03:05 PM Forum: Telescope Making

Re: Pizza Method

Posted By Ed Jones
Andrew,
Opaline is available through Universal Photonics (I think)and www.rhodia-ec.com. It's not cheap but it's worth it.
No I haven't made a refractor, I'm strictly a reflector person mostly because I like to built unique scopes even bizarre. There's not much new in refractor designs.
You should be able to use a potters wheel for polishing, I've seen a simple polisher which is just a simple motorized spindle with a lever arm holding a spindle point allowing a free spining tool on top (or mirror). You wouldn't want to hold the tool or mirror on top by hand and not allow it to rotate.
Ed

June 18, 2004 11:06 PM Forum: Telescope Making

Test Results on Secondary

Posted By Ed Jones
Floyd,
You are interpreting it correctly, it is 1/13.3 wave peak to valley. As far as predicting the effect on the Strehl ratio I'm not sure I can do that, it will depend on how much of the area of this flat the axis uses. I would guess that the axial area on the flat is probably near twice as good as the total PV. One thing to keep in perspective is that the certified flatness of the transmission flat is only 1/10 or 1/20 wave (Zygo has sold both). My bet is that you'll have a hard time seeing any effects due to this flat even on a night of perfect seeing.
Ed Jones

June 20, 2004 02:54 PM Forum: Telescope Making

Homemade EQ Platform for Dob

Posted By Ed Jones
David,
Build one! They are so convenient, inexpensive and easy to build. Here's one that I did. I used two 1/15 rpm synchronous motors driving .370 inch knurled shafts with teflon pillow blocks. The wooden sectors were done on a table saw with a sanding disk tilted to the correct angle. I adds about 5 inches to my scope but it is well worth it.
Ed Jones

August 10, 2004 10:55 PM Forum: Telescope Making

how to make a sunspotter telescope?

Posted By Ed Jones
Alvaro,
Just grind and polish a very long focal length lens say 50 to 80 feet, mount it on a board and use a diagonal mirror to feed the sun into it. See May S&T.

Ed

September 29, 2004 01:50 AM Forum: Equipment Talk

Maksutov precision doubt

Posted By Ed Jones
Andre,
By definition the Strehl ratio is not dependant on obstruction only wavefront aberration. It is 1.00 in a perfect Maksutov.
The point spread function however would be worse because of the obstruction compared to a refractor for example.

Ed Jones

December 6, 2004 11:09 PM Forum: Telescope Making

polishing with aluminun oxide

Posted By Ed Jones
Phil,
I think that's what the old-timers (me) called pre-polishing. I've never tried it but I have used 0.3 micron alumina powder for polishing hard glass types and hard stuff. It will polish faster too. It sounds like you had some success with using 5 micron but you might run the risk of getting some sleeks if you don't scrub the lap really good afterwards. I would recommend instead to use .020 urethane polishing material, you can just press it into your lap to polish it fast and when fully polished it peels off your lap easily. You then pess your lap and contine polishing. It's great stuff.
Ed

December 24, 2004 03:58 PM Forum: Equipment Talk

Interferogram vs. Foucault vs. Ronchi

Posted By Ed Jones
Joe,
At least you didn't start another APO vs reflector fire.
These are all good tests but the advantage interferometry has is that it gives you a quantitative 3D (X-Y and height) view of the mirror surface. Other tests lack one of these attributes, i.e. qualitative or one dimensional. For this reason interferometry is great at detecting astigmatism or asymmetry that is hard to detect or quantify in other tests and astigmatism is often the biggest contributor to a lower Strehl ratio. I often chuckle when I hear a Srehl of near 1.0 quoted for a mirror. But interferometry of parabolic mirrors must be done with a null of some sort and this is where the problems can come in (Hubble). Even so interferometry is the choice of the pros.

Ed Jones

February 23, 2005 11:03 PM Forum: Telescope Making

Equatorial Platform

Posted By Ed Jones
Garrick,
If you are good at woodworking you can make it quite inexpensively. I used two 1/15 RPM clock motors to drive mine and I bet I don't have $20 in it. It's not as pretty as some but tracks really well.

Ed Jones

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