Thought others may be interested.
Thanks to Stephen I have started working on a 365mm F3.something mirror.
Last nights rough grinding brought it below f4. With a 6mm sagitta.
I'm completely new to all of this. We got our first telescope a month and a half ago. A Nova 4inch reflector. Which I have now made a table top mount for.
To me I enjoy the challenge of being able to make something so perfect as a parabolic mirror only using the most basic of tools and tests.
Once I get to the end of fine grinding I plan to build the scope, or at least most of it so that I can test the mirror during polishing.
The glass isn't perfect, the back has two large chunks out of it, so no idea what the end quality will be. But it's all part of the learning process. Next piece of glass is also on its way from Stephen.
Rough grinding complete.
12hours, about 140 wets. about 20 wets per mm of sagitta.
at 365mm F3.3 shape. 1200mm focal length.
It's been a good learning experience. Although it has taken longer than it could have with other methods, I now have a very smooth curve that is looking really good with fine grinding. Past 120 and finishing up 220 now. Discovering how hard it is to gauge what tiny pits are from what grit.
Sure I will learn.
Thanks for the 3rd piece of glass Stephen. Will have to work out what I do with it.
I have decided the next project will be grinding the other side of my tool into a constant thickness 10 or 12 inch mirror. It will be quite thin. Maybe 20mm.
The last rough cut blank, (one you have just received) you could tidy up by getting it water jet cut to 13",or thereabouts. Was 14 1/4" rough cut? So then it could be made into another mirror. Just my thoughts.
Needed to re-tile my 14" glass tile tool after working 3 by 18" to 24" mirrors.
Old tile depth down from 7-8mm original thickness to about 1.5-2mm, so obviously wouldn't have had a "dogs show" of lasting the distance grinding out my current 22"f3.5 mirror project.
As the curvature on the worn tool was near enough from previous project, an 18"f4, it was a simple matter to apply a commercial tile adhesive over the old surface, and lay on the new sheet of glass tiles.
The adhesive used normally dries in 24hr,(at 23degC), but because of the low winter temperature, took 3 days to cure. Just finished cleaning out the grooves, (between tiles), used tools seen, (old haksaw blade and paint brush).
See pics: including the 14" tool "astride" my 22" tool (used for 30"and 33" mirrors)
So much discussion in this thread it's difficult to find a starting point.
I have to convince myself I can test a mirror before buying a blank etc.
Just looking for confirmation that I am doing things correctly.
I have built a foucault tester based on the Stellafane model and have a silvered completed 8" GSO mirror to test.
From the attached images can anyone tell me if:
1. The position of the LED looks OK. Just above the slide hole for full passage of light to the mirror.
2. I have aligned the returned mirror reflection (small circle) back to the knife edge.
3. I have moved the knife edge to the radius of curvature (twice the focal length). On my mirror focal length 1200mm/47.244", f/5.9 making ROC at 94.49 inches.
4. I have observed the returned beam at its smallest and most focused to confirm its at ROC. I have moved the tester closer and further away from the ROC.
Problem is, I am not getting the expected shadow either side of ROC.
At exact ROC the light is very bright.
Is this to be expected when testing on a completed silvered mirror?
I have the same question as I just read on a previous post. Just completed a faucault tester and the exact position of the light source is not clear on many examples. Some use a torch light some distance to the side, some use a light on a fixed post separate to the knife edge. I have used a LED light as per the Stellafane models shining through the bottom, middle of the 2" slide hole close to the knife edge.
Last edited by nobby2; 09-01-2017 at 03:48 PM.
Reason: I was expecting it as an answer to a specific thread and found it out alone with not explanation.
Hi nobby2 (Neil),to exploring the prospect of mirror making ATMing.
No body has chimed in at this point so thought it might be of interest to you to see a very simple rig made from scraps of ply, a 133LPI Ronchi screen, a cheap LED torch (has multiple LED's, covered all except one), a small piece of green cellophane wrapper paper, (through which I pass the LED beam), a few screws etc.
Pics: side view, front view, and a shot of Ronchi taken of my 33"f4.1 mirror,(still a work in progress). Photo of R-gram taken by free hand held Canon Power Shot A1200 digital camera.
I will let others show their test rigs for Foucault,(will be much more sophisticated than my simplistic Ronchi rig).
Thanks sopticals for your reply, much appreciated. Mirror making is obviously not a big thing in Australia. I think its time I moved on to the Ronchi Screen.
Did you make your Ronchi screen or order it. I notice the US publisher Willmann-Bell sells screens at various lines per inch. I read that 100 is a good starting point.
I Think I have the structure of the tester working OK, just looking for some confirmation that I have the basics sorted.
Hi Neil.
First off, don't get the willmann-Bell screens, they count the lines wrong and are 1/2 what they state. Get them from http://ronchiscreens.com/
With your tester, it's a bit hard to tell from your pics but the light seems in the correct position with the knife edge cutting 1/2 way over it. The return beam also needs to be cut into with the knife to get the shadow (above the light). Start say 1/2" inside where you think the RoC is and there should be a dark, sharp shadow coming from the same direction as the knife. Move the knife back until the shadow forms the doughnut pattern. Continue moving the knife back and the shadow will come in from opposite the knife.
A possible problem I can see is you need to have your eye right in close behind the knife edge which might be difficult with your set up.
One more thing.
It is brighter with a coated mirror but the shadows are just as visible and it can be troublesome if the coating is badly deteriorating or very dirty/dusty.
Thanks Dave. Just the information I was looking for. I can now move on and order the screens.
I was wondering if my silvered completed mirror was too reflective to get the shadow. It looks like I was doing all the correct things. Yes, every grain of dust makes viewing more difficult. Will have to try again with a closer eye to the knife edge. Much appreciated.
Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,344
Hi Neil,
Foucault testers are generally illuminated to be bright enough for diffraction patterns to be visible on an un-silvered polished mirror - reflectivity 1-4%. A 96% mirror will therefore be very bright.
What you are seeing could be correct. I just can't tell from that photo. You could be seeing some minor un-eveness in the figure and a Foucault shadow. It's hard to tell from that photo.
My tester is a dual ronchi / Foucault tester based on Jean Texereau's original model. It uses a razor and a very fine brass mesh at each end of a rotating arm so you can position the ronchi grating or Foucault blade at the optical axis. Keep the light source and viewing point as close together and on the optical axis as possible. The horizontal strands have no optical effect because of the vertical slit over the light source only diffracts with the vertical strands of the mesh.
I'll post some pics of my tester when I get a chance.
There are many ways to determine the exact ROC. You can use the sun to determine the focal length with a piece of card attached to the end of a long stick. Point the stick toward the sun, slide the mirror along and mark the position on the stick where the suns image comes to a focus on the card. With your tester and a a silvered mirror, place a piece of tracing paper at the viewing point. Adjust the position of the tester until the slit comes into sharp focus.
Joe
Last edited by OzEclipse; 13-01-2017 at 02:53 PM.
Reason: adding photos
to obtain good quality glass at a reasonable price.
with interest.
