My copy of "How to Make a Telescope by Jean Texereau" arrived today. Second hand off Amazon.
Not sure if I've missed it in the various threads but is anyone able to comment on options for getting a mirror aluminised. I'm Brisbane based.
Ballpark prices for different sizes, who does it etc.
Still not quite decided if I'm going to try pushing some glass and that's one part of the process I've not seen any info on.
Bob
Hi Bob,
there is a guy in Brisbane that does it, the company is called Optical Coating Associates with HQ based in sydney. I think he can do up to 20" mirrors.
cheers, Frank
Hi all,
I have a question for someone who has made their own mirror. I have figured a 17" blank of 19mm plateglass, and put it into the future telescope assembly. I can view the moon sharply at 840x magnification, Jupiter to about 350x (due to bad Sydney seeing at present), and defocussed stars have fairly equal rings both sides of focus, with no sign of astigmatism. Does anyone know if this is a good enough indication that the mirror could be coated? Mel Bartels says it should magnify to between 30 and 50 times per inch of aperture.
cheers,
Frank
I think you have answered your own question in that if you can do the moon at 840x that equates to better than 49x per inch of aperture.I would definitely go ahead with the coating. If I can figure my current mirrors (22" and 25" float glass 19mm thick) to do even 30x to the inch I would be well pleased. I also have almost completed my dob structure and hope to test the first of my uncoated mirrors (22") sometime after the new year, when we get a little more night down under. Im doing my figuring outside and the temps are not to my liking (too warm).I like to work in cooler temps (around 10-15deg C with no sun), so may have to wait a month or so to fine tune the final figure.
I think you have answered your own question in that if you can do the moon at 840x that equates to better than 49x per inch of aperture.I would definitely go ahead with the coating. If I can figure my current mirrors (22" and 25" float glass 19mm thick) to do even 30x to the inch I would be well pleased. I also have almost completed my dob structure and hope to test the first of my uncoated mirrors (22") sometime after the new year, when we get a little more night down under. Im doing my figuring outside and the temps are not to my liking (too warm).I like to work in cooler temps (around 10-15deg C with no sun), so may have to wait a month or so to fine tune the final figure.
thank you very much for your reply Stephan as I hadn't actually realised it was at that level of mag. I have read that it if there is any slight problem it looks really bad after coating , so I am not confident. Is the moon a real test or should it always be something more subtle such as Jupiter? Also I have looked around for a cheap mirror coater and as I can't find one I guess the next thing is to build my own coater. I have already priced the fabrication of a chamber and it seems to be about the same price as getting one good coating done. So another project!! As this is my first time on these Ice in Space thread things (or any others), is there a mirror making thread? Hoping to hear from you and thank you again, cheers Frank
"We have 2 chambers at 2 locations. The smaller (500mm) original system is based in SE Qld and the other (590mm dia) is based in Sydney." - the SE Qld office is in Southport.
I have to confess to a mistake with my numbers in previous post. The 840x was OK for the moon but the 350 for Jupiter should have been 270, which gives about 17x per inch before the start of image breakdown. Also the moons do look like tiny balls. My 12" commercial mirror (F5) is also only useful to about 17x per inch on Jupiter as well. I suppose the real question for you is this, can you test a mirrors magnification using the moon and is this a definitive test.
Cheers Frank
No, a visual observation of the moon or Jupiter is not a meaningful test. Sure it is satisfying to know you are getting an image, but it tells you nothing about how good the figure is, nor what you should do to correct it.
You could make these measurements with the mirror in a telescope pointed at a bright star, with a knife edge at focus and no eyepiece you can do something similar to a Foucault test - however its very awkward to do this in practice, and atmospheric turbulence makes it impossible most of the time on mirrors more than 6" or perhaps 8".
You should be setting up the mirror in a place where you have still air and using one of the various tests described in the ATM books - a Foucault test, Ronchi, Ross null test, Dall null test, caustic test or perhaps interferometry. Plenty of sources on the net describe these.
These tests will tell you how good the figure is,and also show you where the surface is high or low.
My mirror making days were long ago... Maybe one day I'll try again.
No, a visual observation of the moon or Jupiter is not a meaningful test. Sure it is satisfying to know you are getting an image, but it tells you nothing about how good the figure is, nor what you should do to correct it.
You could make these measurements with the mirror in a telescope pointed at a bright star, with a knife edge at focus and no eyepiece you can do something similar to a Foucault test - however its very awkward to do this in practice, and atmospheric turbulence makes it impossible most of the time on mirrors more than 6" or perhaps 8".
You should be setting up the mirror in a place where you have still air and using one of the various tests described in the ATM books - a Foucault test, Ronchi, Ross null test, Dall null test, caustic test or perhaps interferometry. Plenty of sources on the net describe these.
These tests will tell you how good the figure is,and also show you where the surface is high or low.
My mirror making days were long ago... Maybe one day I'll try again.
thank you for the info. It passes using my foucault tester, just. I will try your knife edge idea, Cheers Frank
Hi Steve.
I"ve made 50+ mirrors in Hamilton up to 20". Recently completed a 16" Dall-Kirkham refigure (both primary and secondary made by Loftus) and currently polishing a 12" that'll be used in a dob.
Hi Steve.
I"ve made 50+ mirrors in Hamilton up to 20". Recently completed a 16" Dall-Kirkham refigure (both primary and secondary made by Loftus) and currently polishing a 12" that'll be used in a dob.
Dave
Hi Dave,
Looks like you have moved a bit of glass.How do the Graham Loftus mirrors rate in your opinion? there seem to be a number of his mirrors in circulation.Where do you source your glass? Most all the Loftus mirrors are 19mm to 25mm (from what I can ascertain). I have been trying to find a source for thicker suitable glass at a reasonable price, but without success up to this point in time.
Am finding the 25" diameter by 19mm thick float glass mirror (f4.7) a little difficult to figure, but making better progress with a 22" f5.3 of the same thickness. Hope to have the latter up and working in the near future.
Hi Steve.
(hopefully) I've attached a before and after of the Loftus 16". I've seen a few reasonable Loftus mirrors but have also seen a number of others similar to the 16". I've never seen one without a TDE.
The early mirrors I've done were 19mm plate or float glass (Alert or Smith & Smith). My 20" is Pyrex from Newport and have done 3 16" Pyrex from Newport. Other mirror blanks have been accumulated over the years from various people, alot that were half finished. The 12" I'm doing now is actually 320mm, 30mm thick plate from an Auckland Astro Soc. garage sale.
Hi guys, I used to grind mirrors some 20 years ago, sizes from 5 inch to 15.5 inch. In these pics I'm polishing the 15.5 inch with 3 dobs of 6, 8 and 15.5 inch aperture. The little 6" f6 was my first.
Hi Steve.
(hopefully) I've attached a before and after of the Loftus 16". I've seen a few reasonable Loftus mirrors but have also seen a number of others similar to the 16". I've never seen one without a TDE.
The early mirrors I've done were 19mm plate or float glass (Alert or Smith & Smith). My 20" is Pyrex from Newport and have done 3 16" Pyrex from Newport. Other mirror blanks have been accumulated over the years from various people, alot that were half finished. The 12" I'm doing now is actually 320mm, 30mm thick plate from an Auckland Astro Soc. garage sale.
Dave
Hi Dave,
Nice improvement with the 16" Loftus mirror. Is this a float glass substrate, and if so what thickness? What is the f ratio?
For the refiguring, did you use Cerium oxide, or rouge? What pitch hardness? Full sized laps, or sub diameter?
Hi Dave,
Nice improvement with the 16" Loftus mirror. Is this a float glass substrate, and if so what thickness? What is the f ratio?
For the refiguring, did you use Cerium oxide, or rouge? What pitch hardness? Full sized laps, or sub diameter?
Stephen.
Hi Steve.
The 16" primary and secondary were both Float or Plate (I don't know how to tell the difference). I don't recall actually running a tape on the primary edge but I think it was 1". Definitely not thicker. ROC was 2460mm so f/3.075 for the primary, conic was -.555. Final f/ratio for the scope was 6.4!! The owner uses it mainly for imaging with a coma corrector. I just refigured the existing optics, not design the system
The refiguring was done with thin cerium oxide with a 110mm soft lap. See pic. A small amount of polishing was initially done with a 6" lap, which just fit between the outer edge and the central hole but 95% with the smaller one.
Thank you to those who replied to my testing dilemma. FYI, I have tested my 17" mirror in the scope with a ronchi grating and also performed a foucault test with a razor blade. This was stuck in an eyepiece tube unscrewed from a 2" eyepiece. Doing the foucault test in the telescope proved challenging but showed turned edge with the rest of the mirror going dark grey during that brief moment when everything lined up. When right at the null zone the whole mirror just blanked out. It is a very sensitive test to perform! This is about how it tested inside with my foucault tester setup with TDE showing much the same area. The ronchi test showed straight lines except the outer 1/4" which is the TDE. This is a very easy test to perform so I may just stick to that in future. cheers, Frank
Been away awhile. Thought I would post a few pics to do with the progress of what tooling I have been using to grind and polish my three mirrors (2x22"f5.3 and 25"f4.7) currently being worked on.All three now polished, two (22" and 25") at the figuring stage. The 22" nearly there, just waiting for cooler steadier temps in my "out in the open" work area.
