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Posts Made By: Preston Engebretson

February 8, 2003 05:15 PM Forum: Telescope Making

Machine Parabolizing?

Posted By Preston Engebretson


We've just finished polishing to a sphere our 12.5" f3.8 by machine in about 3 hours with an 80% lap TOT using cerium oxide. Testing shows beautiful straight lines to the edge with 3 bands showing.

My question is, how would one parabolize at least to start on a at f3.8 there's a lot of material to be removed?

The machine is a Zeiss type, but we have only been using one stroke arm driving to simplify the motion.

Should we look at something like a star lap offset to one side of center?

Thanks in Advance


March 21, 2003 04:27 PM Forum: Telescope Making

Need recommendation 4 Webcam 4 Ronchigram

Posted By Preston Engebretson


I'm wanting to capture pictures of the Ronchi image of a 12.5 f3.87 . I have the Ronchi setup but was wondering if anyone had any experiance or recommendations with using a webcam to capture pictures of the images. The real reason is that I've got something going on with the figuring of the mirror that I don't quite understand and I'd like to be able to post the images here to get some advise.

Thanks Preston

April 22, 2003 12:48 PM Forum: Telescope Making

Mirror cell Help???

Posted By Preston Engebretson

I have an Astro Sitall lightweight dish-type mirror and am wondering if the design used in typical cells to support flat backed mirrors will work with this non-flat back mirror. It's back appears to have been ground to a conical shape from a center thickness of 2.5 inches to an edge thickness of 0.75 inches with a center hole diameter of 4.5 inches. Also, will the calculations from a program like PLOP still be valid for supporting such a mirror.

Thanks for any ideas in advance,


June 12, 2003 11:49 PM Forum: Telescope Making

Re: Shott/Schott? Glass

Posted By Preston Engebretson


Schott Glass provides a multitude of glass types that one would primarily use in a refractor or eyepiece type project...or perhaps a mirror flat...

Very Good company, but what are you looking for in glass???


August 9, 2003 10:33 PM Forum: Telescope Making

Opinions on Ronchi images of 12.5" F3.8, Please

Posted By Preston Engebretson

We have been working on an 12.5" f3.8 and after getting a nice
sphere polished out on the machine went on to figuring...
A batch of TOO soft pitch and and over eager helper resulted in the following images...after about 2 hours.
I believe we now have a slight hole in the center and a badly turned down edge of about .75 inches.
First image is almost null. With a 100 line/inch screen
Second image is +.200"
Third image is -.200" ...Cannot seem to get this to load???

Tester has the LED reflecting through the upper part of the
Ronchi screen and moves as a unit...

Thanks for any comments or corrective advice.

August 9, 2003 10:34 PM Forum: Telescope Making

Opinions on Ronchi images of 12.5" .200+

Posted By Preston Engebretson

.200 plus
Thanks for any comments or corrective advice.

August 27, 2003 08:30 PM Forum: CCD Imaging and Processing/Solar System

Another...My first Mars

Posted By Preston Engebretson

Mars was very nice last night after midnight inColumbus Texas.

We imaged for 4 hours getting about 80,000 frames in 45 2 minutes sets, with a TouCam675 at 10fps and 1/50 second taken starting at 12:06am CST.

Scope was a C14 at f33 and seeing was a 5 of 10.

This is the first of the images processed, 200 hand picked out of 1796 stacked in Registax, and little processing in Adobe 5.0...need to work on the color...

But, for our first attempt...I happy.

Any comments welcome. Thanks for looking.


December 27, 2003 02:02 PM Forum: Telescope Making

Grinder picture for Chuck Fellows

Posted By Preston Engebretson

Here's the link to the buried in the
Great Web Page thread...

Regards Preston

January 14, 2004 05:03 PM Forum: Telescope Making

Re: 18" starstructure solid edge cell part 5

Posted By Preston Engebretson

Micheal, Very nice overall design. You might want to look at "Reflecting Telescope Optics II" by R N Wilson pages 259-272 have quite an indepth and current discussion on edge support and the requirements.

I believe that your current setup is similar to Schwesinger's case "e" page 265 where the support is discribed as followssadV-support: The astigmatic mode disappears if the angle 2e=90 degree. Thus the right-angled V-support is almost optimum and gives a big improvement over a pure edge support).

However, I would look closely at you supports contact points close to the mirrors surface. From the research presented in the above referance, it concludes that support should come from the center of the edge in mirrors with f ratios greater than about 3.

Very nice workmanship...Keep it up.

Best Regards Preston

January 16, 2004 01:07 AM Forum: Telescope Making

Re: Solid edge mount (PRESTON)

Posted By Preston Engebretson

Thank You Micheal, You've got my gist...although the current thinking would place the support roughly in the middle thickness of the mirror...and this is a SIMPLE SOLUTION!!!

The higher studies look to have a push/pull system as in the ESO 8m....very differant situation, I'll agree.

Again, I'm not sure what your angle between the supports was now...but the optimum by the referenced material was 90 degrees...

Your a Bright Fellow Micheal...I'd urge you to look at the latest data available to help when possible...they have MUCH MORE MONEY TO SPEND ON RESEARCH THAN WE Do...They get Government grants!!!!!!!!!!

Thanks for the Kind thoughts, I was just trying to help you...

Very Best Regards,


ps...Your construction and fab skills mirror ours...hope to be able to meet up some day.