Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Can I use 19mm thick plate glass to grind 14 inch mirror ?

  • Please log in to reply
32 replies to this topic

#26 ed_turco

ed_turco

    Vanguard

  • *****
  • Posts: 2,486
  • Joined: 29 Aug 2009
  • Loc: Lincoln, RI

Posted 19 January 2020 - 11:19 AM

Someone pointed out that a great amount of experience is required for such a project.  I have 60 years of experience as an ATM and I wouldn't touch such a project -- ever.



#27 GTom

GTom

    Ranger 4

  • -----
  • Posts: 343
  • Joined: 19 Nov 2016

Posted 19 January 2020 - 11:31 AM

Someone pointed out that a great amount of experience is required for such a project. I have 60 years of experience as an ATM and I wouldn't touch such a project -- ever.


I don't mind wasting $120 worth of plate glass :D. ATM is not a for-profit operation anyway. Obviously I'd approach a $1600 pyrex blank with more care.
  • brave_ulysses likes this

#28 Pierre Lemay

Pierre Lemay

    Apollo

  • *****
  • Posts: 1,416
  • Joined: 30 Jan 2008
  • Loc: Montréal, Canada

Posted 19 January 2020 - 01:36 PM

Someone pointed out that a great amount of experience is required for such a project.  I have 60 years of experience as an ATM and I wouldn't touch such a project -- ever.

Ed, I have almost 40 years of ATM experience including the fabrication of more than half a dozen mirrors, up to a 20 inch f/3.9, 1/2 inch edge, conical mirror. I know going into this project, eyes wide opened, that my chances of successfully parabolizing a smooth, 1/4 wave curve are slim. It will be a frustrating struggle, including the fabrication of the 7 inch m.a. diagonal.

 

However, whether I am successful or not, it will be fun and educational. And if I succeed?: The prize is a 100 pound, 28 inch quasi-flat footed telescope that will be a pleasure to transport, store and observe with.


  • brave_ulysses and sopticals like this

#29 macdonjh

macdonjh

    Aurora

  • *****
  • Posts: 4,942
  • Joined: 06 Mar 2006

Posted 19 January 2020 - 03:05 PM

Just curious: is the temperature used to slump also hot enough for annealing?  If somebody had a strained piece of glass and slumped it, would it emerge from the kiln stress-free if it was cooled properly?



#30 GTom

GTom

    Ranger 4

  • -----
  • Posts: 343
  • Joined: 19 Nov 2016

Posted 19 January 2020 - 04:16 PM

Ed, I have almost 40 years of ATM experience including the fabrication of more than half a dozen mirrors, up to a 20 inch f/3.9, 1/2 inch edge, conical mirror. I know going into this project, eyes wide opened, that my chances of successfully parabolizing a smooth, 1/4 wave curve are slim. It will be a frustrating struggle, including the fabrication of the 7 inch m.a. diagonal.

 

However, whether I am successful or not, it will be fun and educational. And if I succeed?: The prize is a 100 pound, 28 inch quasi-flat footed telescope that will be a pleasure to transport, store and observe with.

Here is a project aiming for a 1" thick, 28" f3 Borosilicate mirror, not slumped (sry, only German). He hasn't finished it to diffraction limit that time and complained about support issues with the conventional 18pt cell.

 

Just curious: is the temperature used to slump also hot enough for annealing?  If somebody had a strained piece of glass and slumped it, would it emerge from the kiln stress-free if it was cooled properly?

AFAIK slumping is done at 650°C, annealing needs only around 500.


Edited by GTom, 19 January 2020 - 04:18 PM.


#31 Stathis_Firstlight

Stathis_Firstlight

    Stathis Firstlight

  • -----
  • Vendors
  • Posts: 13
  • Joined: 28 Dec 2019
  • Loc: Germany

Posted 20 January 2020 - 12:54 PM

 Hello Samir,

 

17 years ago I made two 360 mm diameter with 20 mm thick boroslilicate glass (14" x 0.8") f/4.5 mirrors.
Details see here:
http://www.stathis-f...ingscopeeng.htm

With one mirror an ATM friend build an all aluminum ultralight travel dobson:
http://www.stathis-f...g_14incheng.htm

Although at that time I had no access to interferometric measurements to measure for astigmatism as well, the mirrors got really good with no astig visible.

So, it can be done this thin, if you follow consequently the rules of large thin mirror making. As you use plate glass, be prepared to spend more time with figuring, or accept to get an inferior accurancy.

 

 

Here is a project aiming for a 1" thick, 28" f3 Borosilicate mirror, not slumped (sry, only German). He hasn't finished it to diffraction limit that time and complained about support issues with the conventional 18pt cell.

Among the most experienced and talented mirror makers I know of, he is THE ultra thin mirror extremist. When he started this mirror, I was pretty sure, that this would work as a light bucked only with flaw Images at high power. He showed me, that I was wrong.



#32 Jack Day

Jack Day

    Ranger 4

  • *****
  • Posts: 301
  • Joined: 04 Mar 2005
  • Loc: Wenatchee, WA

Posted 20 January 2020 - 01:09 PM

My first mirror was a 14" f/5.4.  It turned out well, but I am not sure I would want to go much below that  if I did it again.  F/4.8 to F/5 seems like a good compromize. 

 

Good luck!



#33 GTom

GTom

    Ranger 4

  • -----
  • Posts: 343
  • Joined: 19 Nov 2016

Posted 20 January 2020 - 05:26 PM

 

Among the most experienced and talented mirror makers I know of, he is THE ultra thin mirror extremist. When he started this mirror, I was pretty sure, that this would work as a light bucked only with flaw Images at high power. He showed me, that I was wrong.

Amazing work indeed! As I gathered from the astrotreff forum, the scope well surpassed "atmosphere-limited" quality.

 

I was wondering, which, approximately same weight f3 28" design would sit better in the cell while pointing on the horizon:

 

- Conventional flatback, 11mm thin in the center but 1" strong rim touching the edge support

- Slumped, e.g. 19mm thin everywhere

- Mel Bartel's type "overslumped" with thicker center and thinner edge, again, using 3/4" material. Maybe try that with a 4" hole and Cassegrain type center support?




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics