An update on progress with my 33" by 1 1/2" thick plate glass mirror.
There has been a bit of a set back.
After 20 hours of polishing (by hand) I was not altogether happy due to the fact that a significant number of smallish pits remain in evidence on the outer 1" of the radius.
Even though they were invisible to my naked eye, they were clearly visible using a 10X eye loupe in conjunction with a LED light source shone (at night), at an angle across the optics surface.
I had noted the existence of these artifacts (from the grind), at the 12 hour point in polishing, expecting to see their disappearance before the 20 hour polish time. However the difference in their appearance over the eight hours between the two inspections was IMHO not significant enough, so decided to go back to a regrind.
In fact right back to #220. Since have reworked #400 and #800 (over considerably longer periods than previously). Now ready to restart the "polish".
Concurrently have been working a 25" f4.4 (1 1/2" plate). Well through the polish with this one, using the same 17" pitch lap as used for the 33, but heat pressed to conform to the tighter curvature of the 25. When this is fully polished out will again use this same lap for the 33.
A 33" mirror is a sizable piece of glass. Started out at around 115lbs. After curve was generated (by hand), weight was down to around 95lbs. Anyway took a picture of it, (after the first polish before going back to the grind-looked so nice to the naked eye), sitting on the "hand truck" I use to move it about.
I know that feeling when you put a tile tool down on a polished mirror and you get to grind away all the hard work, it's not a nice feeling at all as you stand at it thinking if you should or shouldn't do it.
I did it two times with my 18" and I couldn't comprehend doing it with a mirror that large.
My 20" f3.8ish is coming along nicely, I'm now ready to polish it out.
My 20" f3.8ish is coming along nicely, I'm now ready to polish it out.
Hi Raymond,
You don't waste any time. Hardly the blank arrives at your place and suddenly you have it half done. Be watching to see how this one turns out. Be a big brother to your 18"f3.3.
I'm acquiring a set of optics soon and from what I understand the primary is very heavy. It's a very thick 12.5" dall kirkham.. Can online here tell me what the chances are that I can remove some of the rear end material from the mirror to reduce the weight? I will get pics when I can but would love some ideas.
You can't remove any glass from the rear of the mirror without it destroying the figure on the front. If you were to make the mirror thinner then a regrind and re figure would be required.
An update on progress with my 33" by 1 1/2" thick plate glass mirror.
There has been a bit of a set back.
After 20 hours of polishing (by hand) I was not altogether happy due to the fact that a significant number of smallish pits remain in evidence on the outer 1" of the radius.
Stephen.
Hi Stephen,
I'm having exactly the same problem at the moment, pits at the edge that just won't disappear, frustrating!
Mind you... my mirror is about 1/5 the size of yours!!!
Hi all, great work everyone! Can't wait to see some of the results at the Queensland star fest, maybe. I obtained a secondary for the now 18" primary from Bintel, they had a diagonal for a 16" f4.5 with enough width to make 20% ratio with main mirror at f4.7. I love the idea of a sphere to test secondary mirrors, I guess as a null test, so might order some glass from Sydney Glass Co. What diameter would I need.
We all seem to be having problems with the outer edge, me too as the 18" started life as 19", but had to give up as the mirror got too thin for comfort to grind it again, anyway it's ok as there isn't much extra weight to carry around. Good luck everyone with your projects.
Cheers, Frank
I may as well post this here. I made a little video for someone else to show how I do my rough parabolizing to get the mirror to the stage of fine figuring. I find it works so well and produces a nice smooth zone free result. The lap is 8" with the edges pointed to help with blending.
This is my current 20" f3.8 project.
Here's a current Ronchi. Still a long way to go.
And here's a video of the way I work in W's that increase in size to smoothly dig the mirror down. https://youtu.be/8SamXXGYKps
that piece of glass is looking pretty. Nice smooth lines in the Ronchi gram. Appreciate the video clip on rough parabolizing. Keep the reports coming. Watching with interest.
I have managed to finish polishing (for the second time), my 33". This time a good scratch and pit free finish. Also the 25"er polished out, (also nice clean finish). Figuring these being the next step. First will have to run some diagnostics (Ronchigrams) to determine strategies for each optic.
I just finished the mirror tonight. I think this one turned out a little better than the 18" as I'm sure the surface is smoother as I didn't need to use a small lap at all. I did the lot with an 8" lap.
Everything went a LOT faster with a LOT more ease! It all seemed to easy really. It'll be interesting to see how it performs but I'll need to build a scope for it first.
Another great result, Raymond. Seems you have something of a "Midas touch",having such success with these large fast projects.
Have managed to do the regrind and repolish work on the 33". This time its come through "pit free", as well as free, (as far as I can tell) from astigmatism.
Like to comment: The two mirrors just polished, (25 and 33), are the first I have done using recycled pitch, (Gugolz). I had around 4kg of old gugolz, retrieved from the cleaning up of old laps and channel cutting. As new Gugolz is expensive, I "took the plunge", by melting down and reusing the old. Was a little concerned that there could have been abrasive contaminants in the recycled product, which would have (if present), been a source for scratches. My concern was unfounded as both mirrors came through their respective polishes scratch free.
Did note that the recycled pitch (#55 Gugolz [when new]), was more like #73+ hardness. Still did the job, even though my working temperatures have been around 7-12deg C.
Adding some pics of current polished 25 and 33 to show size comparisons. Also comparison 3" to 33". (My first reflector (1960/61) was a 3" Newt). The 33" mirror (when completed and coated, will gather 121x the light of a 3"). Also pictured (far right), comparison 6" to 33". (6" size of the first mirror I ground).
One thing I found good for piece of mind was to heat it up nice and hot and poor it through some muslin. It comes out like new. I always get bits of tin foil and plaster in mine and the muslin removes it very well.
I always add a teaspoon or so of turps to keep it soft each time I heat it up. Some times I'll pull a bit out cool it off and test it with the finger nail test.
Rac: is it more or less difficult figuring a fast parabolic of smaller sizes? I'd love an 8" f/3.8 or f/3.3 however from what I know the smaller the surface the harder it is to figure faster and faster optics.
Rac: is it more or less difficult figuring a fast parabolic of smaller sizes? I'd love an 8" f/3.8 or f/3.3 however from what I know the smaller the surface the harder it is to figure faster and faster optics.
I wouldn't know, I've only ever made two mirror and both are not small. It shouldn't be too much of a problem. It comes a point I think where you will need a sub diameter lap to get the correction into the glass, I'm not sure what that point is with an 8". I know with my 18" f3.3 I was having a hard time with an 8" lap so I did it with a 4" lap and it worked great but yet I could use the 8" lap all the way on my 20" f3.8, may have been how I was doing it though.
You learn so much every time and trying is the only way no matter how much you read on the net.
Have one of the 3 current 25" mirror projects ready to silver. This one being f5.295 in its uncoated state tested well under the night sky. Focus snap on moon and stars is good. Doesn't seem to have any noticeable astigmatism.
The only downside of such a long focal length is the height of eyepiece above the ground, (144"[12'] with dob on transport trolly). This puts a 6' observers feet over 6' off the ground when scope pointed at the zenith [vertical]. Ok up to a little over 45deg in altitude. Currently using a ladder may have to invest in a height adjustable platform.
Ronchi image (some air currents affects present). Second picture: getting ready to slide secondary cage (UTA) onto the 3m x 76mm strut poles. I have 3 sets of struts to suit the 3 different focal length 25" mirrors currently being worked on. Dob family in right picture,(14"f5.1, 8"f6, 25"f5.3).
Been juggling my spare time between my 25"f4.5 and 33"f4.1, (both in relatively early figuring stages).
The 33" came through its polish,, (second time) with the usual artifacts, (for me). A wide "rolled edge" and "large" hole in the center, but fortunately was free from obvious "astigmatism". At the moment have arrived at this figure. Been 9 hours work so far, using 8 1/2" and 17" laps. I know there's still a long way to go.
All work is done by hand. The hardest job is lifting the 95lb hunk of glass,(it was 115lb at the start before "hog"), off and on work block. Still good exercise for an "old bloke",(keeps the muscles right).
to see how this one turns out. Be a big brother to your 18"f3.3.
Nice work.
for each optic.
