As Stephan has already seen on "the other forum" and by email ( and who deserves big thanks for selling me the fine ground blank and giving me the (rather out of date now but they worked!) silvering chemicals.) I decided that my 550mmf4.6 was done. i read in a book that opticians often don't declare a mirror done, it is just that the mirror gets taken away from them. It was the second time (well, third if I count the time I was close except for a terribly turned outer 25mm.) that it was ready to go. I measured it at about 1/8 wavefront error a few weeks back and hubristically decided I could better that, and ended up overcorrected so had to go backwards a bit. Then I proceeded slowly, just 80 strokes and then testing, pretty much every day for a week or two.
Now it's nominally 1/5 wavelength, but a lot of that is a bit of turned down edge. maybe when I have more experience (and hubris) I'll attempt to fix that at some future recoating occasion.
yesterday was home silvering in the garden. Lots and lots of scrubbing with cotton balls, precipitated chalk and demineralised water. and then another scrub. Finally the tin chloride spray, a thorough rinse, then the magic couple of minutes with the silver solution and reducer in hand spray bottles.
There's a patch of frosty coating, so I need to redo the coating, but we had a little first light celebration with a friend and couple of beers and managed to see alpha crux through a tiny cloud hole, as well as thoroughly inspect the red anticollision lights on the nearby radio tower a couple of km away while collimating and star testing.
what a journey. I can't wait for some proper observing!
To anyone who is contemplating attempting their first mirror, go for it. one key thing I learned is the importance of having somewhere to be able to work on the mirror without needing to set up each time. I progressed a lot faster then. Also, procrastination is easy.
Yes, first mirror. pretty crazy to take on such a monster. Originally I was looking for a 16 inch blank but saw stephan's trademe post. I have a spare 22 inch blank completely unground. I liked to think it was my insurance against accidentally dropping the one I was working on, although not sure my wife would let me start again. :-) I don't think I will use it to make a matching mirror for a 22" bino (although never say never).
She is happy that the test rig has left the living room at last, and the polishing bench will soon leave the spare room.
I'll take some pictures of the coating and post soon.
Thanks for that encouragement. I have three Pyrex mirror blanks given to me to work on. A rough ground 100mm f5 for practice . An f7 200 mm which I would like to regrind to F4 . And finally , once I can work out what I am doing , a blank 12 inch which may be sometime before I can get to.
I have just received my grinding materials, 80 , 220 , 600 silicon carbide , 25 micron and 14micron aluminium oxide as that was what I could get locally.
I have made a mirror cell for an old finished 8 inch f7 so I can the tube assembly and secondary assembly process sorted out.
Any tips starting out or advice about my mirror cell?
The 200f7 will probably be fine with simple three point support and two lower edge supports at 90 degrees, such as in dobson's original instructions. For dob mount this is ok, as the gravity load is always in the same direction. If it may be used on an equatorial mount, then this doesn't work. I had to redo my cell for my 200f6 dob as I also use the tube on my CGEM equatorial mount. I used a design by Gary Seronik , which is a backing disk of wood, three small 90 degree brackets around the mirror circumference and the mirror fixed with nice thick but not to large in area blobs of silicone to the disk, plus small blobs between the brackets and mirror edge.
Ensuring 2-3mm of clearance (which is filled by the silicone) means dimensional changes of the wood don't result in stress/deformation of the mirror. It seems to work ok, star test is still good and symmetric. If it was stressing, the star test would probably appear 3-lobed.
The self-weight of a 200mm mirror seems to be not too critical in terms of support. Main think is to be able to collimate it.
An added benefit of the double-plate cell is that mirror distance along the tube (for example to get a bit more back focus at the focuser) can be tweaked a little by having the acorn nut stick up higher and using longer collimation bolts.
I have recoated the mirror. the frosty patch is gone, but I'm still not 100% pleased. There is a faint haze, like looking though cirrus, when looking directly at the mirror under strong lighting. Looking down the tube it isn't visible and through the eyepiece (at the local radio tower red beacon, was so far too cloudy for stars) the performance looked no different to the previous coating.
I suspect the haze may be costing a few % in reflection and extra scattered light.
disturbingly, more tiny scratches are visible under strong lighting. I'm worried that perhaps my chalk supply is also containing some silica. It is labeled pharma grade so should be ok I guess. I will check this by dissolving a sample with dilute hydrochloric acid, which should dissolve calcium carbonate but not silica (or alumina if it was contaminated with clay).
If so, bugger. At this stage I will not regrind and refigure!
Quote:
Originally Posted by hamishbarker
Yes, first mirror. pretty crazy to take on such a monster. Originally I was looking for a 16 inch blank but saw stephan's trademe post. I have a spare 22 inch blank completely unground. I liked to think it was my insurance against accidentally dropping the one I was working on, although not sure my wife would let me start again. :-) I don't think I will use it to make a matching mirror for a 22" bino (although never say never).
She is happy that the test rig has left the living room at last, and the polishing bench will soon leave the spare room.
I'll take some pictures of the coating and post soon.
That does sound disturbing. Perhaps , like my SCT, a few scratches and defects in the finish are nothing one can see . The Mel Bartels site suggests that you can live with final scratches.
There is an Indian scientist and mirror maker who suggests that every grade of grinding material he gets requires testing with a method he describes.
Maybe I should try that if there is a risk of problems.
some sensational views of jupiter and saturn and the moon this evening, after a bit of unexpected recollimation. normally this telescope holds collimation even if the mirror has been removed and replaced. Eventually I realised that the need for collimation had been caused by me not screwing the mirror box tailgate completely shut. Thankfully the safety latch was closed and one screw just hand tightened, but that could have been a disaster of a dumped mirror onto the concrete!
the scratches might be making a little scattered light across the field, but overall I am very happy with the performance. No deep sky tonight as scudding clouds had me rapidly switching between jupiter, saturn the moon, alpacrucis as holes in the clouds presented themselves.
Nice report Hamish. Being your own optician is a plus. Wouldn't worry too much about the fine scratches, as you will no doubt at some point in the future do a re figure (too remove the residual TDE) at which time the offending blemishes may disappear (if not too deep).
Excellent news. Good to hear the mirror performs. Always a chance of some problem.
Any idea what to do if the the glass tool sticks to the mirror on 600 grit?
Quote:
Originally Posted by hamishbarker
some sensational views of jupiter and saturn and the moon this evening, after a bit of unexpected recollimation. normally this telescope holds collimation even if the mirror has been removed and replaced. Eventually I realised that the need for collimation had been caused by me not screwing the mirror box tailgate completely shut. Thankfully the safety latch was closed and one screw just hand tightened, but that could have been a disaster of a dumped mirror onto the concrete!
the scratches might be making a little scattered light across the field, but overall I am very happy with the performance. No deep sky tonight as scudding clouds had me rapidly switching between jupiter, saturn the moon, alpacrucis as holes in the clouds presented themselves.
I have just fine ground my first mirror. An F4 4.25 inch. Seemed to work reasonably well except for one very faint wiggly scratch. The earlier worse scratch ground out with aluminium oxide so I suppose I need to try another round.
I am wondering how much mineral oil or other material to temper my black pitch with for the polish lap. It has been used once but is too hard. I tried a few drops of mineral oil, less brittle but the penetrometer test only gave me a mm with 500g after 30 minutes at 19 degrees. I think I need 2.5mm.
My first pitch lap (oversized 4.25 inch. ) on ply as per Bartels. The Stockholm tar worked. Pitch is soft enough to sink a 7mm rounded bar into 2mm after an hour under 500g. Now for the polish. Don’t think the 1 micron aluminium oxide will be right so waiting for Cerium Oxide.
Last edited by Sunfish; 14-07-2020 at 02:54 PM.
Reason: Photo
A few Ronchi of my half polished little mirror inside and near focus and the owl eye outside of focus by nearly an inch.
You can see the outside owl eye focuses on the pyrex mould marks and edge rib on the back.
Sorry about the blurry ronchi.
The in focus look similar to Bartels part polished images except for the strange lines at the very edge . Perhaps the rib on the rear edge. Or the oversize lap , which is a bit of a handful to keep pressed.
Looks like you have a TDE. might be a good idea to post some images inside/outside ROC with less lines (say 4 or 5 dark lines). Inside 90% R looks close to spherical (less lines would give a better picture of where your at). Give higher sensitivity.
When I look through the Ronchigrams on the Stellafane website it looks like a turned up edge gone slightly parabolic which also include those fine lines at the edge. There is also a hint of different turn in lines top and bottom which is problem i think.
I have trimmed up my lap a little reducing the oversize diameter hoping that will help but would like to find out more before i keep polishing.
Here are some images of out of focus owl eyes with less lines and close to in focus. Any further inside focus produces more and more lines as per the previous images.
My images look like this which are parabolic. Not sure I understand that when I am trying to get spherical. Or perhaps over corrected as per the second chart?
I found some old posts here on IS explaining a few techniques for moving along. So I trimmed and faceted my lap more often and pressed it often and took longer one third strokes to get down the turned up edge and it seems to be working.
Those turned up edges are starting to go and the Ronchi lines are starting to straighten up as the edge is more polished. Suppose to finish from centre out but perhaps my Bartels method and the oversized lap has over emphasised the correction at the edges. Still a loong way to go in polishing for such a little mirror but moving forward all the same.
Sunfish - the best strokes to get rid of TDE are very short 1/4 to 1/5 length W stroke with mirror face up , with some randomness thrown in to maintain or head better toward spherical .
Something like a 200mm mirror is much better fit for the hands and more stable , so you are setting yourself a task with a 100mm . The back should be ground flat or slightly concave. If you can see the mirror rock at all while you are polishing tool on top , this can contribute to a TDE .
Being your own optician is a plus. Wouldn't worry too much about the fine scratches, as you will no doubt at some point in the future do a re figure (too remove the residual TDE) at which time the offending blemishes may disappear (if not too deep).
