Recent star testing of my 64cm mirror revealed two areas in the figure that need further work. From the 88% radius, some rolling down of the edge exists, and the central 55-60% of the mirror is still undercorrected. Interestingly enough, when I fitted a 22" aperture mask, (thus hidding the turned down 12% of the radius), I was intrigued by what was seen at the eyepiece, (ES82deg 24mm giving 125x). When viewing through the eye lens center, was unable to get a clear focus, but when viewing through the ocular nearer the edge of the eye lens, a sharp focus was possible, and star images were decidedly smaller than those of my 14" dob, with double stars being separated by a larger margin, when similar magnifications used in both scopes (and my 14" is no slug). Aperture does rule, even when the larger is considerably short of being perfect. Currently I rate this mirror as a 2/10. So some more work on the figuring block. Again I noted how much more sensitive the Ronchi (133 lpi) eyepiece star test is compared to the Ronchi (65 lpi) bench test at ROC.
PS. Currently would rate my other two mirrors: 22.2" f5.36 @ 3.5/10 and 22" f5.23 @ 5.5/10.
I'll add my 2c to the 'any mirror makers' thread. About 14 years ago I made an 8" f/6 mirror as my first project. It was OK but ended up with a TDE and a rough zone inside that where I'd tried to fix it.
The coating has had it, so I decided to have a bother go before I got it recoated. I tried fixing the edge itself, but in the end went back to a sphere and reworked it from there. To parabolise I used a 6" star lap - not really necessary for this mirror, but there's an unfinished 14" next and I was trying a few things out on this one.
The biggest difference since my first effort was that I built a Bath interferometer and used that for monitoring the figuring progress. I have to say, it's a wonderful thing to just take a photo of the mirror, load it on the computer and see the exact shape of the surface. I'ma bit surprised I haven't seen more mention of them on here.
The final result is that I've gone from a rough surface, -0.8 conic and 0.75 strehl to a smooth surface, -0.98 conic, 0.95 strehl, and 1/29 wave RMS. the only downside is a small scratch. You can hardly see it, but it shows up in the photos, and I'm betting it will stand out when the mirror gets coated. I have no idea how it happened, and 2hrs polishing didn't improve it. I decided in the end that I wasn't going back to grinding to get it out, so it's going to get a bit of texta after it's coated.
I've attached the before and after images and reports. I'd like opinions from the experts here, but I think it's done.
Bruce.
Last edited by Brub58; 03-10-2012 at 08:02 PM.
Reason: Adding photos
Looks like a big improvement there Bruce. Well done.
Quote:
Originally Posted by Brub58
the only downside is a small scratch. You can hardly see it, but it shows up in the photos, and I'm betting it will stand out when the mirror gets coated. I have no idea how it happened, and 2hrs polishing didn't improve it. I decided in the end that I wasn't going back to grinding to get it out, so it's going to get a bit of texta after it's coated.
Bruce.
Not sure what you mean by "texta" but if it involves some sort of painting or similar I would urge you to just leave it alone. The light diffracted from 1 scratch is not going to be noticeable.
Thanks for the reassurance. The original plan was to ignore it, but it looked so bad in that photo that I started to think about blacking it out. I'll go back to ignoring it.
Just returned my reworked #1 22.2" mirror to the dob structure, after further figuring work was required, to remove a rolled down edge 88% radius out, and further improve correction over the inner regions.
Star testing, using a 133 lpi Ronchi ocular last night, revealed a vast improvement since test previously carried out, after the original figuring. As a test subject I used Rigel Kent. The test was carried out using the eyepiece focused at various numbers of lines from 1-8. The most useful numbers being in the 6-8 region. The image seen revealed straight,clean,parallel lines edge to edge except at the extreme top and bottom of lines which indicates the existence of some TDE. However as I let the star image transverse the FOV I was unable to notice any bowing of the lines as they came onto/exited FOV. I conclude from this that TDE must be quite narrow. When image was reduced to 1-3 lines seeing caused too much image movement to see the TDE condition.
Replacing the ronchi eyepiece with my regular 2" eyepieces (ES30,24,TV Nagler 20) I viewed Rigel as the spectacular double it is, nice separation. Did note at this point that collimation is off by quite a bit,(not easy to collimate an uncoated optic with no center spot), as star images flared somewhat to one side. Also looked at the 63% waxing moon, which gave nice sharp high relief images in both ES eyepieces, but a smidgen off being absolutely sharp in the Nagler. Placing on a 20.7" aperture mask cleaned that up somewhat. This removed the TDE, slowed the focal speed,and probably compensated to a degree for the miscollimation.
Overall the test revealed a great improvement in the mirror figure. Previously I rated this optic as a 3.5/10. Now at this point I would rate at a 7/10. Excluding the TDE section I would think mirror would be close to 1/4 wave accuracy.
Congrats on mirror improvement. Just use a whiteboard marker to make a temporary collimation dot in the center- it will wipe off when you need to. I use them to mark areas to polish in normal figuring operation.
Quote:
Originally Posted by sopticals
Just returned my reworked #1 22.2" mirror to the dob structure, after further figuring work was required, to remove a rolled down edge 88% radius out, and further improve correction over the inner the inner regions.
Star testing, using a 133 lpi Ronchi ocular last night, revealed a vast improvement since test previously carried out, after the original figuring. As a test subject I used Rigel Kent. The test was carried out using the eyepiece focused at various numbers of lines from 1-8. The most useful numbers being in the 6-8 region. The image seen revealed straight,clean,parallel lines edge to edge except at the extreme top and bottom of lines which indicates the existence of some TDE. However as I let the star image transverse the FOV I was unable to notice any bowing of the lines as they came onto/exited FOV. I conclude from this that TDE must be quite narrow. When image was reduced to 1-3 lines seeing caused to much image movement to see the TDE condition.
Replacing the ronchi eyepiece with my regular 2" eyepieces (ES30,24,TV Nagler 20) I viewed Rigel as the spectacular double it is, nice separation. Did note at this point that collimation is off by quite a bit,(not easy to collimate an uncoated optic with no center spot), as star images flared somewhat to one side. Also looked at the 63% waxing moon, which gave nice sharp high relief images in both ES eyepieces, but a smidgen off being absolutely sharp in the Nagler. Placing on a 20.7" aperture mask cleaned that up somewhat. This removed the TDE, slowed the focal speed,and probably compensated to a degree for the miscollimation.
Overall the test revealed a great improvement in the mirror figure. Previously I rated this optic as a 3.5/10. Now at this point I would rate at a 7/10. Excluding the TDE section I would think mirror would be close to 1/4 wave accuracy.
I'm glad to read this Stephen, you certainly have put your time in on this one, nice to see it coming together for you. It must be a bear to collimate with no center spot
Congrats on mirror improvement. Just use a whiteboard marker to make a temporary collimation dot in the center- it will wipe off when you need to. I use them to mark areas to polish in normal figuring operation.
We need some sort of revival of amateur telescope making, nice to see it's still alive to some extent back home. Does anyone know how Graeme Loftus is doing? He must be really old now, almost 90? Haven't heard he died.
Funny, Graeme got me into making mirrors years ago. Now I've worked on a bunch of the world's most interesting telescope projects in some capacity, designed and (optics) contract managed a few into existence, now contract manager for E-ELT optics (fab for M1 segments through M4 adaptive mirror and M5).
Thanks to Graeme.
A number of the top professionals I've met in the field of optics over the years started their carreers as teenage ATMs, and there's probably more opportunities for employment in this area than ever now, keep that in mind all you young guys thinking about making a mirror. Go for it!
Hi All,
Like Andrew I am one of the many whom Graeme introduced to atm.
Graeme is still going (87) however stopped making telescopes 1-2 years ago. I dropped in to see him yesterday and even though his body has slowed down his mind is as sharp as ever!
Merry Christmas all.
Hi All,
Like Andrew I am one of the many whom Graeme introduced to atm.
Graeme is still going (87) however stopped making telescopes 1-2 years ago. I dropped in to see him yesterday and even though his body has slowed down his mind is as sharp as ever!
Merry Christmas all.
Hi Mark,
Great to hear Graeme Loftus is still with us. What a inspiration he has been to so many a modern day amateur mirror maker in "down under" Oz/Nz.
Seems the price for 19mm float glass in NZ has taken a big hike over the last two or so years. Just been quoted NZD$930.90 by Metro glass for the supply of a 640mm (25.2") diameter circle of 19mm float. The last circle I purchased this size was NZD$168 from the same source. A price jump of 554% seems a bit over the top.
Seems the price for 19mm float glass in NZ has taken a big hike over the last two or so years. Just been quoted NZD$930.90 by Metro glass for the supply of a 640mm (25.2") diameter circle of 19mm float. The last circle I purchased this size was NZD$168 from the same source. A price jump of 554% seems a bit over the top.
Sounds like they don't want to do it so have quoted a price that makes it worth the effort if they have to do it.
Might be worth shopping around. Maybe ask for a square cut quote.
hello everyone,
I have just had my 17" mirror coated and thought I should report in and thank those who offered advice for this newby glass pusher. The field tests so far give great views and stars are quite small dots (proportional to magnitude). Sydney has terrible seeing and so accurate tests outside are impossible these days. I did do a ronchi test and lines appeared straight with the usual TDE appearing in the outside 1/2". Like Stephan, I found 6 - 8 lines were easier to read, and showed up the TDE which seemed to get masked with just a few lines. Looking at the trapezium in Orion I can regularly see 6 stars there, and the gas cloud gives plenty of detail. The LMC looked fabulous and Jupiter shows details in the bands and the moons are round and bright. One thing I noticed after using it uncoated for many months was that the coating made no visual difference to the images only they were brighter. I was worried that after coating the images would be far worse so I was relieved when this wasn't the case. So overall it seems to be a reasonable mirror and I am happy with it.
Frank
Congratulations. Great effort. You will be feeling good after all your hard work, and perseverance having borne fruit. There is something special about the view through a scope, when the scope, and mirror has been forged from ones own personal vision, and effort.
I am still working on getting my 25" mirror to work in my dob. Still having edge support bothers. I can put the mirror into the cell and have "stig",(star images as streaks, which change orientation when moving through focus), take it out and put it in again and "no stig", (star images point like, and inter/extra focal images similar). Put it (mirror) in again last evening for optical testing in scope, some stig is back, but still did a good job of separating the a1/a2 (4sec arc),components of Alpha Crucis with plenty of dark sky between. In fact a much clearer separation than the 14" dob gave set up beside. The 25 is yet to be coated, this I intend to do myself *silvered*, when I get the edge support issue sorted.
I'd say hello fellow mirror makers, but I am yet to finish my first mirror. I am fine grinding at the moment.
I am making a 150mm F6 Newt (at least I hope it is an F6). While it was wet, I held a 40mm eyepiece up to it at arms length and tried to focus on the neighbour's roof. I figured that as it was around F6, and a sphere is close to the Raleigh Criteria, I would get a reasonable image off axis if my arms were long enough.
It surprised me that I could focus on a near object holding the objective at arm's length. I could not get it to focus at infiinity, which was a relief, because I don't know how easily i could figure an F4 or F3.
It is also amazing how much light is reflected off glass during the daytime. I am considering leaving it uncoated for use as a drag and drop lunar telescope.
Anyway, I am glad that there are still mirror makers out there and this thread has had some good tips.
Thanks Stephen for the congrats. I get a little astig sometimes too, but according to a thread at Cloudy Nights, as you mentioned, the edge support makes a difference. I use packing tape, approx 10mm wide for lower 180 degrees of mirror. I have large air vents in the bottom of the mirror cell for cooling and can check that the tape is sitting dead centre. This seems to help for me. I notice temperature seems to move the glass around a fair bit, I guess as various stresses come in to play as I only used thick sheet glass from Sydney Glass, but it doesn't seem to make a lot of difference to the image, however as it moves from say under corrected to over corrected during the evening there is image deterioration if you are observing planets. As I want this for looking at faint fuzzies I am not worried.
I am working on a long focus 6 inch mirror for a schiefspiegler. RC is 200 inches approx.
After a lot of trouble with all kinds of zones I have a rough hyperbola, looks like an f6 parabola. I am hoping that will be relatively easy to reduce to a sphere.
I am finding the long focus sphere a bit more of a challenge than I expected but it is interesting and I like being able to use the ronchi test as a null test.
Does anyone know anything about a polishing compound called TK 68?
I was given a jar of it by a friend who used to make optics professionally and have used it for polishing this mirror. I read a vague reference to it elsewhere being an aggressive polishing compound in which case it sounds unsuitable for finishing a mirror like this. I have some red rouge so could switch to that.
It surprised me that I could focus on a near object holding the objective at arm's length. I could not get it to focus at infiinity, which was a relief, because I don't know how easily i could figure an F4 or F3.
That's a little worrying because as you focus on closer objects, the focal point moves away from the primary. That is, if the mirror is 36" focal length for infinity then the distance you need to hold the eyepice for a closer object will be, say, 37"-39" for example. Have you done a focal length test?
TK-68 is a very old cerium - I'm not sure if its available any more. I used to use it - don't remeber it being particularly aggressive. if your surface doesn't clean up to a nice sphere quickly maybe your pitch is too hard and you are not pressing enough . I always use polishers on the soft side and get nice smooth zone free surfaces cold pressing fairly regularly. What kind of pitch are you using ?
Mark
Quote:
Originally Posted by Rod
I am working on a long focus 6 inch mirror for a schiefspiegler. RC is 200 inches approx.
After a lot of trouble with all kinds of zones I have a rough hyperbola, looks like an f6 parabola. I am hoping that will be relatively easy to reduce to a sphere.
I am finding the long focus sphere a bit more of a challenge than I expected but it is interesting and I like being able to use the ronchi test as a null test.
Does anyone know anything about a polishing compound called TK 68?
I was given a jar of it by a friend who used to make optics professionally and have used it for polishing this mirror. I read a vague reference to it elsewhere being an aggressive polishing compound in which case it sounds unsuitable for finishing a mirror like this. I have some red rouge so could switch to that.
25.2" mirror back to the figuring block.
work. From the 88% radius, some rolling down of the edge exists, and the central 55-60% of the mirror is still undercorrected. Interestingly enough, when I fitted a 22" aperture mask, (thus hidding the turned down 12% of the radius), I was intrigued
by what was seen at the eyepiece, (ES82deg 24mm giving 125x). When viewing through the eye lens center, was unable to get a clear focus, but when viewing through the ocular nearer the edge of the eye lens, a sharp focus was possible, and star
images were decidedly smaller than those of my 14" dob, with double stars being separated by a larger margin, when similar magnifications used in both scopes (and my 14" is no slug). Aperture
does rule, even when the larger is considerably short of being perfect. Currently I rate this mirror as a 2/10. So some more work on the figuring block. Again I noted how much more sensitive the Ronchi (133 lpi) eyepiece star test is compared to the Ronchi (65 lpi) bench test at ROC.
ocular last night, revealed a vast improvement since test previously carried out, after the original figuring. As a test subject I used Rigel Kent. The test was carried out using the eyepiece focused at various numbers of lines from 1-8. The most useful numbers being in the 6-8 region. The image seen revealed straight,clean,parallel lines edge to edge except at the extreme top and bottom of lines which indicates the existence of some TDE. However as I let the star image transverse the FOV I was unable to notice any bowing of the lines as they came onto/exited FOV. I conclude from this that TDE must be quite narrow. When image was reduced to 1-3 lines seeing caused too much image movement to see the TDE condition.
support bothers. I can put the mirror into the cell and have "stig",(star images as streaks, which change orientation when moving through focus), take it out and put it in again and "no stig", (star images point like, and inter/extra focal images similar). Put it (mirror) in again last evening for optical testing in scope, some 
support issue sorted.
