Porthole glass mirrors
Has anyone actually made a mirror from a porthole glass, ala John Dobson? The reason I'm asking is that I just grabbed a piece of glass on eBay, an old porthole glass, 8-1/2" diameter by 3/4" thick, for $29. I've made a few mirrors from both plate and Pyrex, but have never ground and figured an actual "port-hole" mirror.
I never have. I have wanted to, but never got around to it. Mat -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Chuck Hards Sent: Wednesday, February 15, 2012 5:33 PM To: Utah Astronomy Subject: [Utah-astronomy] Porthole glass mirrors Has anyone actually made a mirror from a porthole glass, ala John Dobson? The reason I'm asking is that I just grabbed a piece of glass on eBay, an old porthole glass, 8-1/2" diameter by 3/4" thick, for $29. I've made a few mirrors from both plate and Pyrex, but have never ground and figured an actual "port-hole" mirror. _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options". ---------------------------------------------------------------------------- This message and any included attachments are from Siemens Medical Solutions and are intended only for the addressee(s). The information contained herein may include trade secrets or privileged or otherwise confidential information. Unauthorized review, forwarding, printing, copying, distributing, or using such information is strictly prohibited and may be unlawful. If you received this message in error, or have reason to believe you are not authorized to receive it, please promptly delete this message and notify the sender by e-mail with a copy to Central.SecurityOffice.Healthcare@siemens.com Thank you
Mat, part of me wants to do the whole "Dobson" experience, and actually use screened beach sand for rough-grinding... No shirt, grind it on a picnic bench with hair in a pony-tail, figure it with only a light-bulb..."eat well, sleep well, work like hell"... and being only 53 and in great shape, I'm sure I could do it... OTOH, having made a few mirrors (and NOT made of the same stuff as Dobson, lol), I'm all in favor of using any and all labor-saving "devices" available... ;-) John Dobson is one-of-a-kind. We won't know his ilk again for several generations. I've never had the bad experience of reaching the figuring stage only to find out that the blank has astigmatism from internal stress due to lack of proper annealing. And I don't want to EVER experience it. But I couldn't pass up an 8-1/2" piece of glass for less than thirty bucks! ATM's could benefit from an intervention. It's a sickness. ;-) On Wed, Feb 15, 2012 at 3:54 PM, Hutchings, Mat (H USA) < mat.hutchings@siemens.com> wrote:
I never have. I have wanted to, but never got around to it.
Mat
John Dobson gave me some porthole glass that I used as a tool to grind my 22 inch. It was 16" in diameter and worked well until it exceeded the plastic limit on my floor one evening. ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Wednesday, February 15, 2012 3:32 PM Subject: [Utah-astronomy] Porthole glass mirrors Has anyone actually made a mirror from a porthole glass, ala John Dobson? The reason I'm asking is that I just grabbed a piece of glass on eBay, an old porthole glass, 8-1/2" diameter by 3/4" thick, for $29. I've made a few mirrors from both plate and Pyrex, but have never ground and figured an actual "port-hole" mirror. _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options".
I'm pretty sure I'd use one of my own ceramic-composite tool foundations for any effort along these lines, unless I could score a second porthole for little cash outlay. Please expand on the plastic limit on your floor, LOL! I'm expecting a good story... On Wed, Feb 15, 2012 at 4:44 PM, Brent Watson <brentjwatson@yahoo.com>wrote:
John Dobson gave me some porthole glass that I used as a tool to grind my 22 inch. It was 16" in diameter and worked well until it exceeded the plastic limit on my floor one evening.
The plastic limit is the point beyond which if you deform a material it will not return to its original shape. The porthole glass exceeded that limit upon reaching the floor after having experienced gravitational acceleration through about three feet. In other words, I dropped the tool. John Dobson calls that booboo number two. Booboo number one is dropping the mirror. I purchased a 16" diameter piece of 1/2 inch plate glass to continue. I glued it to a 1" thick plywood disk using RTV. That piece of glass got pretty thin and so I purchased a second piece of glass. I thought it would be easy to separate the original glass from the plywood, but NOOOOO. I ended up putting a band saw blade in a vise and having a friend stretch it out. I then moved the tool across the blade, acting as a human band saw. It took about an hour because the RTV would clog the blade, and the blade would stick in between the glass and the plywood. After the separation was complete I epoxied the two pieces of glass together. I had not heard of the plaster of paris and tile tools. However I did make one of those later on to grind the back of the mirror slightly concave to eliminate astigmatism. I learned a lot grinding the 22. ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Wednesday, February 15, 2012 5:07 PM Subject: Re: [Utah-astronomy] Porthole glass mirrors I'm pretty sure I'd use one of my own ceramic-composite tool foundations for any effort along these lines, unless I could score a second porthole for little cash outlay. Please expand on the plastic limit on your floor, LOL! I'm expecting a good story... On Wed, Feb 15, 2012 at 4:44 PM, Brent Watson <brentjwatson@yahoo.com>wrote:
John Dobson gave me some porthole glass that I used as a tool to grind my 22 inch. It was 16" in diameter and worked well until it exceeded the plastic limit on my floor one evening.
_______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options".
Lol! I know what the plastic limit is, but the way you worded it, I wasn't sure if it was the glass or your floor! On Feb 15, 2012 6:39 PM, "Brent Watson" <brentjwatson@yahoo.com> wrote:
The plastic limit is the point beyond which if you deform a material it will not return to its original shape. The porthole glass exceeded that limit upon reaching the floor after having experienced gravitational acceleration through about three feet. In other words, I dropped the tool. John Dobson calls that booboo number two. Booboo number one is dropping the mirror.
I purchased a 16" diameter piece of 1/2 inch plate glass to continue. I glued it to a 1" thick plywood disk using RTV. That piece of glass got pretty thin and so I purchased a second piece of glass. I thought it would be easy to separate the original glass from the plywood, but NOOOOO. I ended up putting a band saw blade in a vise and having a friend stretch it out. I then moved the tool across the blade, acting as a human band saw. It took about an hour because the RTV would clog the blade, and the blade would stick in between the glass and the plywood. After the separation was complete I epoxied the two pieces of glass together.
I had not heard of the plaster of paris and tile tools. However I did make one of those later on to grind the back of the mirror slightly concave to eliminate astigmatism.
I learned a lot grinding the 22.
________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Wednesday, February 15, 2012 5:07 PM Subject: Re: [Utah-astronomy] Porthole glass mirrors
I'm pretty sure I'd use one of my own ceramic-composite tool foundations for any effort along these lines, unless I could score a second porthole for little cash outlay.
Please expand on the plastic limit on your floor, LOL! I'm expecting a good story...
On Wed, Feb 15, 2012 at 4:44 PM, Brent Watson <brentjwatson@yahoo.com
wrote:
John Dobson gave me some porthole glass that I used as a tool to grind my 22 inch. It was 16" in diameter and worked well until it exceeded the plastic limit on my floor one evening.
_______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy
Send messages to the list to Utah-Astronomy@mailman.xmission.com
The Utah-Astronomy mailing list is not affiliated with any astronomy club.
To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options". _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy
Send messages to the list to Utah-Astronomy@mailman.xmission.com
The Utah-Astronomy mailing list is not affiliated with any astronomy club.
To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options".
The porthole glass arrived today and looks like it could make a great mirror. A shallow chip on one face that will obviously be the backside. For less than thirty bucks, I think I got a deal. It would also make a terrific tool, providing one doesn't "pull a Brent". ;-) I recently found in long-term storage a 12.5" full-thickness Pyrex blank, and I think I have a 16" plate-glass blank around here somewhere that's at least 3/4" thick. I sold a few small blanks to Jay last month but the big stuff is still hidden in either my basement or mom's garage. I shouldn't even think about grinding another mirror, at least for a long time yet. I have at least a dozen finished mirrors that need to have a telescope built around them first.
participants (3)
-
Brent Watson -
Chuck Hards -
Hutchings, Mat (H USA)