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Posts Made By: Preston Engebretson

January 15, 2004 01:03 AM Forum: Telescope Making

Re: 18" starstructure solid edge cell part 5

Posted By Preston Engebretson

Good Evening Micheal...I use to have the same ideals...

But learned that asking questions of others, more importantly those that are in the field in the know, will likely reduce by a very great amount of time my investigations...and ...$$$$$

They in general like to answer well founded questions...and reading the latest in telescope technology cannot hurt...

Granted the book is looking at 1 meter plus size scopes, and also looking the the 8.2 meter to 10 meter designs.

But this does relate to out hobby...


Best Regards Preston

January 15, 2004 01:17 AM Forum: Telescope Making

Re: 18" starstructure solid edge cell part 5

Posted By Preston Engebretson

Micheal Great test on the physical movement...our mirror lateral movement is limited to .0005", but we are working with cored mirrors and astatic axial support

But again, with optics in place your lateral supports are very likely to cause a deformation in the mirrors figure that will be seen when lowered past vertical by placing a load on the front edge of the mirror...I'll leave at that...

Good Viewing and Best Regards for some fine construction details...


January 16, 2004 03:16 AM Forum: Telescope Making

Re: Solid edge mount (PRESTON)

Posted By Preston Engebretson

Having been out of Astronomy since my teens, and able now in my 40ties...with funds, and European Clients...4-8meter we have been consulting...

ps...I have had my juices over flowing from some of your post...

Personally, we are working on a 10" and 20" RC robotic observatorys...Then looking to go to perhaps 48"...time will tell...

What is WSP???Out of the loop likely....Our current Research is directed toward amateur...and not commercial...

Very Best Regards, Please keep in Touch...


February 4, 2004 01:35 PM Forum: Telescope Making

New Folded Newtonian Design?!?!?!

Posted By Preston Engebretson

Eric, I got a quote on that for a client. As is $35,000 or finished 1/4 wave or better and coated, with delivery to Texas $95,000......and they will happily make another...

How bout BIG Binoculars???


February 20, 2004 04:16 AM Forum: Home Observatories

In-house observatory

Posted By Preston Engebretson

Hello Gerald, Go to Mark Keitel's post of about 3 Feb regarding his 3rd floor observatory...very nicely may go back further...

Regards Preston

March 1, 2004 02:53 PM Forum: Telescope Making

Apogee 102mm X 550mm Objective??

Posted By Preston Engebretson


I bought the 127mm x 1200mm optic set in a cell from Apogee.
It comes in a plastic non-adjustable cell. The images are good enough for a guide scope which is what we bought it for but it shows the false colors inherant in optics at this price range.

Hope that helps,


March 27, 2004 04:47 PM Forum: Telescope Making

Mirror Polishing Machine Question

Posted By Preston Engebretson

Kwang, The tool in this setup is generally set to a stationary position based on the focal length desired. Moving the tool further out increases the focal length and moving it in decreases the lenght.

Once you are close to the desired focal lenght, you do not move the tool in or out as you will start to develope another sphere either outside or inside the previous one.

In polishing to a sphere, you also keep the lap in the same position as the grinding lap.

One may introduce a bit of ecentric motion to avoid zoning once polishing...

It is only when you began to figure that the arm strokes across the mirror, but at this point the mirror rotation is SLOWED to almost a 3-6rpm versas 40-60 during grinding and polishing...

The following is a good site that supports the MOM "Mirror Matic"

Very useful and active site...

Hope this helped,

Regards Preston

ps...There are other ways to use the machine, but what I discribed is likely the most popular...and Carl Zambuto also uses this basic method.

April 24, 2004 09:27 PM Forum: Telescope Making

Where to find Carbon tubes?

Posted By Preston Engebretson

GY, you might try the following link...but for a truss tube DOB, I don't see the advantage...other than a little weight savings.

The main advantage for carbonfiber composites in scopes is the near zero thermal expansion so one's focus stays put during temp changes. Stiffness is also greatly increased.

Also, one should protect them from UV rays as they are easily paint your pretty carbon rods...

Here's a link:

Hope this has helped,


ps...carbon is not ever resonably priced unless you are looking at a lot of units...price drops from about $25 per yard to $11 but only after some 20+ rolls at 100 yards per roll.

You could likely make them for the price of about 2 rods. If your interested, I could walk you through the process the we use to form tubing from .75" and up.

April 27, 2004 08:54 PM Forum: Telescope Making

Priming/Painting aluminum tube?

Posted By Preston Engebretson

Mike...check your email...also, I've used the product that Don works quite well.

Regards Preston

December 4, 2004 07:27 PM Forum: Telescope Making

Clam Shell Modification

Posted By Preston Engebretson

You could likely set it up in a 4 jaw chuck in a lathe. Otherwise if you were to built a wooden jig to hold the bracket and make a circle guide for a router, by taking small steps down of about .050" per pass could remover the material in about 15-30 minutes...