Image of the day

Captured by
Troy Carpenter

2024 MAY 9 - Giant Sunspot AR3664

My Account

New to Astromart?

Register an account...

Need Help?

Posts Made By: Preston Engebretson

September 6, 2005 01:57 PM Forum: Refractors

Magic Glass (8inch F/7 Simi-Apo)

Posted By Preston Engebretson

Hello Keith, Ok I took the bait and loaded it into ATMOS...why would you even want to make this lense????

Yes, the spot in green light is great, at .01549 but Red and Blue are greater than 30mm and the coma is really bad...

Am I missing something???



Keith Lawson said:

This is for the lens designers out there. Just something to play with. The design uses all common glasses except for one. Can anyone tell me if the "magic glass" can be made? If not, why? Just trying to learn something so please don't flame me 8) BTW, the design at F/9 is an APO. If anyone is interested I will post the data.

December 6, 2005 01:16 PM Forum: Telescope Making

Making a Newt mirror from 16" f4 sphere

Posted By Preston Engebretson

[QUOTE]Steven Gorkowski said:

"""I picked up a 16" f4 mirror 3" thick. """

Hi Steve, At 3 inches thick...your are going to have cool down problems!!!

I would recommend correcting the edge and using it for a testing sphere or sell it for that purpose...

JMHO as I'm working with a 36" diameter mirror at 6" thick and it normally takes until 4:00 am before the seeing is any good at all..

Good Luck,


January 4, 2006 01:40 AM Forum: Telescope Making

Black rouge?

Posted By Preston Engebretson

Hi Doug, I have some of it that I ball milled and have used that on BK7 is slower and cleaner than red Rouge...and a bit slower than ball milled cerium...

I doubt that I would use it on Pyrex for figuring by machine as it is slow enough now with the ball milled 50-D-1 or ZOX CE-89.

But with soft lense materials the ball milled Black Rouge works well...without ball milling it does not wet well for me...

Hope that helps,


March 13, 2006 01:27 AM Forum: Telescope Making

In search of a good coater

Posted By Preston Engebretson

Hi Micheal, I had the 12.5" F5.6 that we figured coated by Spectrum...just fantastis...and just purchased another 12.5 F5.6 From Carl Zambuto...he uses Spectrum and now Nova due to the lesser shipping cost to Florida vs Oregan I think...

Both top notch all the way...

Best Regards,


October 1, 2006 04:50 PM Forum: Telescope Making

Cellular Mirror Portrait

Posted By Preston Engebretson

Hi Bill, Is that one of Jean-Guys mirror blanks? It looks just like the 14 inch that I got from him several years ago. Polished out to a sphere, with print through...will be slowly working to correct that this Fall if time permits...

Best Regards,


Bill Byrd said:

[COLOR="Purple"]I had the 18.75" cellular mirror out of the scope tonight while I worked on the mirror box. Thought I'd post a pic since I haven't shown the whole thing here before. That's a 6" ruller in the right front of the pic for scale.

November 10, 2006 10:06 PM Forum: Telescope Making

Scored a GREAT deal on a mirror grinding/polishing

Posted By Preston Engebretson

Hi Robert,

It looks like the overarm would have a pin that would go into a hole or socket in the center of the back to your tool or lap.

There is a Yahoo Group "Zambuto mirrors" or something like that that Carl Zambuto has a great deal of information on machine polishing and figuring of mirrors...

He has a great tutorial, and many well writen examples of figuring sessions and what to do next...

Hope that helps,



Anyway, I'm not quite sure how I would attach a tool to it, so I thought I'd post some pics and see if I get any recommendations. If any of you live near me and are willing to help me out, you are welcome to use it for your own project as well. Thanks!![/QUOTE]

November 29, 2006 04:13 AM Forum: Telescope Making

20" F/3.5 Mirror + Extras

Posted By Preston Engebretson

Hi Bill,

I believe that your decision to put this off is a wise one...

Take that I paid Carl Z $1250 for a 12.5 inch f5.6 and you are looking at a 20 inch f3.5...

A good precision annealed blank would be $1800 and the finished mirror that is any good in that size $8000+

I think you made a very good move!!!


Best Regards,


October 11, 2007 12:28 AM Forum: Telescope Making

Zambuto Mirrors

Posted By Preston Engebretson

Jack, goto Yahoo groups, Zambutomirrorgroup and send Carl an email...

He will reply...

If your looking for an 12" f5.6, I've got a fine one that Carl selected just for's for sale for the price I paid for it...and has never been used, just measured to confirm that the one I figured is good...

I'm the one that got him to show the puplic just how he figures his mirrors...and that was a fine time and still is...

Best Regards,


February 13, 2008 01:53 AM Forum: Telescope Making

10" mirror

Posted By Preston Engebretson


Not likely as most cassagrains are primary of F2-f4 and then the secondary magnification multiplies that by 4 to 6 times..

Additionally, coring a finished mirror would like ruin the figure...

Sorry...if you want a Cass, sell this mirror and buy a cass...

If you are interested, I think I have a 12.5 inch Cass set somewhere around idea what state it's in as I purchased it here on Astromart...and have not tested it yet...

I'm traveling starting tomorrow to Monterrey Mexico for the next 2 weeks, but if interested, drop me a note and I'll respond when I get back...

Best Regards,