Is that the mirror support for polishing : a thin square of non slip rubber mat ?
Hi Mark,
the mirror is floated on several layers of wet newspaper(not easily seen in the pictures). Between support board (plywood) and pine trunk, I have used a piece of thick rubber carpet underlay to float the whole mirror/support board assembly, as interface between support board, and log top dont exactly match. In this way I am attempting to avoid astigmatism of the support board itself.
A rough sagitta calculation indicates that the central thickness of a 25" f4.2 made from 19mm float will be around 10mm, and a bit over 12mm for the 22's...
fwiw) The club scope in Bunbury WA has (or had?) a 20" primary made from a slumped 3/4" pyrex (meniscus) blank. The lateral support was effected via a central plug. Whilst it is true that it did form an image of sorts, I cannot say that it was ever a satisfying instrument to use. My old 12.5" f6 with a Parkes mirror produces a more pleasing view on any (and every) night that they were operating from the same location. (and that is being polite)
So I'd imagine that you can only polish on an overcast day. The heating effects of the Sun on the mirrors would make figuring near impossible.
Overcast, cool, (10-15deg C), little or no wind. Mornings and evenings best. May have to wait another month for the right conditions to do a fine tune on the figuring.
I'm currently thinking some more about what F ratio to aim for. My impression is that a high F ratio is much more likely to give me some success as I learn how to do this.
Bob
This is as I understand it , F4 being difficult F10 easy, F6 maybe ?
Would like to see some discussion of the various strokes used? is there someone who can post up a vid (or a linkie)?
Roger
Bob , for a first 8" I'd recommend F7.5 ( or even F8) as the 60" focus is a good eyepiece height for standing and the mirror will only require a small amount of parabolising.
For an interested party I said I would put on a brief log regarding time taken to go through the grind of a 22" float glass mirror blank.
As follows:
Grit #46 Sic. Using a 7"diameter 2.5kg barbell weight disc it took 4.75 hours (TOT) to hog out a sagitta of 5.8mm.(My aim for final sag depth 6.7mm(0.264")).
Grit #80 Sic.Continued with the 7" hogging tool for 3.75 hours (TOT) sagitta to 6.9mm (looks like an overshoot-but not so as you will see).
Grit #120 Sic. Changed to a larger tool 14" ceramic tile tool (50mm ply substrate).1 hour only (TOT). Sagitta back to 6.7mm due to the larger tool diameter moving the surface figure away from the hyperbolic formed by previous smaller diameter hogging tool. So now figure more spherical as action occuring simultaneously over a larger area of the blank.
Grit #220 Sic. Moved up to a full sized (22" diameter by 2.5" thick plywood) tool. This one overlaid with glass tiles (1" squares). Weight applied 15kg to facilitate grinding action.Very short strokes (TOT). Took 4.5 Hours to remove all pits from previous grits. In retrospect I should have spent another hour with #120 to take out all artifacts from #80. Sagitta remains constant at 6.7mm (my target sag). The full size glass tile tool further improves the evolving spherical figure appearing on the fledgling mirror. The sagitta dosent change much at this stage because tool and mirror diameters match and wear is across the whole area of the mirror.
Grit #400 Sic. Same 22" glass tile tool. 3 hours (TOT). All #220 pits gone. Sagitta still 6.7mm.
Grit #800 Sic. This is where the drama started. In all 13.5 hours was spent grinding with a mix of (TOT and MOT). For the first time part way through the grind I noticed scatches appearing. I would barely work one out when another (sometimes several others), would appear. I discovered the reason (towards the 8 hour point), why the scratching was occuring was because of the vacuum generated by mirror passing across the channels of the tool was drawing earlier grit particles up into the swarth. ( My strategy when changing down in grit size was to wash down the tool and scrub out the channels, but it can be difficult to get all of the artifacts from the previous level 100%. So I would paint in all the channels with a waterbased paint to attempt to capture such particles. unfortunatly the waterbased paint was inefective in the capture of all such unwanted particles. So I switched to an oil based paint mixed with a penetrating oil and painted the channels this eliminated the problem and no more scratches. Sagitta depth 6.7mm (target depth).
So in all time taken to fully grind this 22" mirror was 31 hours
I more recently ground a second 22" mirror in 20 hours
Time taken to polish to 99% : 7 hours. (14" lap #55 G and CeO).
Time figuring (first iteration) : 16.5 hours. (5", 8.5", 11", 14", 17" laps).
My method was to start the deepening towards parabola from the center out. Progress through lap sizes from smaller to larger, thus moving the correction outwards as lap size increased.
Noted: when the lap size was small, figure tended to rough and chaotic, but as lap size increased and correction moved out towards the mirror edge, figure began to smooth out and become less chaotic.
Have ended up, (for the first iteration, yet to be tried as an uncoated optic under the night sky). The figure is fairly smooth, (no high or low zones), a bit of TDE/rolled edge. Possibly a little over correction.
I have made two aperture masks, to give 1) 19" f6.21 (this takes out the edge defects, and should provide reasonable performance for solar system viewing). 2) 17.5" f6.74 ( experimental only). Am in process of making a 20.5" mask as well.(this may be sufficient to hide edge defect).
Currently figuring second 22" primary. Will take a similar time, but no appreciable edge defect at this point in time.
As for the first mirror: If it dosent perform , then another figuring iteration wil be in order.
Hi Steve.
When using small laps for figuring I've had success with circular strokes to get a smooth figure. I've just finished a 14.3" f/4.4 using a 4" lap.
What strokes do you use?
Thanks for your imput. For the first of the 22"ers I polished with a sub diameter (14") lap (TOT). Stroke "Rosetta", with approx 3" over hang (F-B), coming to the edge side,(no over hang). Took 7 hours to achieve a 99% polish.
The second 22"er, (currently working on), I used a full sized (22") lap. Stroke straight through the center, with a 2"-2.5" over hang (B-F). Took only 3 hours to reach a 95%-99% polish. Used both (TOT) and (MOT), but mainly the former. This was actually very hard work for a 67 year old,(a real good workout).
In the figuring strokes I used all laps from 4" to 17" (TOT). With the smaller laps 4" and 5" a variety of strokes, including, very short straight, to circular (tight and not so tight), to take down high centers, and high zones. The larger laps from 7"- 17", long straight strokes across, but with an off center bias to avoid forming zones.
Love your nice pics.Im not having a lot of joy with my camera in this respect, always seem to look rougher than the visuals. If I can clean up my act (photography), will post something.
As I had heard of other ATMers, using 1" plate glass for mirror blanks, I contacted Sydney Glass Pty Ltd, in respect to the 25mm plate sheets, they currently have in stock.
I received the following reply:
We have got some 25mm clear float, but is so badly annealed it is impossible to cut it conventionally.
We use a waterjet and often supply panels of this to telescope enthusiasts.
(I was then provided with prices for the various sized discs I had enquired about-waterjet cut-edges look flat ground-no bevels).
(I also asked about cut sheets ie. Full sheet cut in half/quarter.)
The reply continued:
There is no point in providing you with "stock" glass because of the cutting problem. (end of reply).
I wont give the name of the correspondent.(Writer).
I agonized, over whether I should post this information, or not, but felt if not posted, many ATMers may fall into the trap, of trying to make a mirror, out of the above.
If one is going to use cheaper glass, glass by the Pilkington process, (float glass), is generally ok, if it comes from a reliable source. Good float, is relatively free from strain, has acceptable anneal, reasonably flat (usually within 4 to 5 waves of flat), and wedge free. (front/back parallel).
I feel sorry for those, who may have tried mirror making, and have failed, due to poor anneal, of the substrate they selected, as a blank.
Just come inside from first light experience of my recently finished 22" f5.36 (fl 3000mm), mirror, (as yet uncoated). Unfortunatly my sky was 90% plus cloud covered and my dob structure has yet to birth a finder scope, so views were confined to the moon (when I eventually located it) as it briefly appeared for a minute or so from time to time.
Using a GSO 30mm 2" Superview (67deg FOV), the magnification 100x. The 91% Gibbous moon took most of the field of view. Focus came up nice and sharp, with small craters and surface wrinkles standing out with good contrast. At the boundary where the moons bright limb meets with the dark background of deep space, I was pleased with the sharpness of the boundary going from the brightness of the lunar fully lit surface to the sudden blackness of empty space.
Even though an uncoated mirror reflects around 5% of the light falling on it, a mirror of 22" (uncoated) still captures the light of a 5" coated mirror, so not going to be great on DSOs. So next step is to put on a coat of silver, and it should be ready to go.
PS. Would have liked to try more magnification, but with the limiting viewing, and difficulty in locating a target object without a finder decided to call it a night. My finder is going to be a Skywatcher 4.5" f4.44 newt.
Thanks for the report . You really need to test your mirror on stars as that will tell you something about your mirror. A quality 100 lpi grating at focus will give you even more detail about overall correction and zones. It is hard to tell anything much by looking at the Moon in between sharp and blurry at 5 X magnification per inch of aperture.
does anyone know , the "w" stroke as shown there seems not to "flow" looks a bit odd or is that how it should be done?
it is impossible to cut it conventionally.
due to poor anneal, of the substrate they selected, as a blank.
