Well, one of my stated aims for most of my trips is to do something towards my ATMing. So my aim for this trip was to make a mirror. That is grind and polish and if time permited figure it.
My plan was to buy a 14.25" cored blank and grind and polish it with the idea of make an F-16ish Dahl-Kerham Cassigrainian. However after seeing the price of the blank I started to explore other options for accomplishing my aim. It was about a week after I recieved my quote on the 14 incher that a mention was made on the ATM listserv of Surplushed.com selling 8" cass blanks cheap. When I went to their site I found that as mentioned on the list they were pre-cored and ground to radius (F-3). What was not mentions on the list or on the Surplushed site was
that they were not pre-gound but ground and polished with some surface blemishes!!!!
The one I recieved has an incomplete ring of scratches that look like they are metalized to a small extent. Possibly caused by the coring opperation.
You can't actually see the scratches in this image, but they are visible in this one...
The scratch is to the lower left of center.
Well, that's the "blank". Now for the work space that I set up ready to grind this mirror. What I made was a cleat board, which I screwed to the workshop style bench in the basement/laundry.
Atleast I know it is up to the task of polishing the mirror out! The real challenge in this project is figuring an F-3 prolate ellipsiod (K = -0.6695 for those interested).
For those interested here are the design parmeters for this project.
EFL: 2438.4mm
F1: 609.6mm
D1: 203.2mm
E : 250mmv
Df: 52mm
ROCp: -1225.96mm
ROCs: -463.152mm
S: -439.619mm
D2: 66.66mm
M: 3.978
BFL: 689.619mm
Lobst: 32.81%
F/: 12
Ls: 105.14mm
Lp: 213.14mm
Ds: 110.14mm
Dp: 60.38mm
Kp: -0.6685
Ks: 0.0
Well now for an update, I took the mirror into the Chabot Space & Science Center, to test it at the mirror making workshop. Well the results are promising, it's a good sphear with some correction. We didn't take any measurments, I just wanted to know if it was sphearical and weather or not it was smooth.
Here is a pic of the Ronchi test so you can judge for yourself.
You can see there is some correction, but on an F3 that is almost none at all. So I will play it as if it was a sphear and figure it to the 67% correction my design requires.
Next week the polishing lap.
Well it's next week!
Here is a pic of the polishing lap we made a the Chabot, as you can see it is a full size square facetted lap. I have yet to trim the center zone to account for the perforation in the mirror, and I have to open out the channels!
Well as mentioned the center still needs trimming, and here's why.
As you can see there is already, after only 45 mins of 1/3 wide W strokes, done mainly to achieve an even pressing of the lap, a pronounced turned edge around the perforation.
Incidentally you can see the ring segments that make up the incompleat ring of scratchs about 1" out from the perforation.
Well I have now re-channeled my lap and carved out the central portion (2 3/4" diameter), all that remains to be done is wait until next Friday so I can re-test the surface and find out how close I am to the required correction factor.....
This is what the lap now looks like.
Well I have now got about 12 days left till my self impossed dead line, and I can now say I won't get there, but I should still manage to compleat the mirror prior to my departure date....
Just to let everyone know where it's at so to speak, I now have about 100 mins of figuring in the mirror, and all the data points are on the theoretical line or within the 1/8 wave envelope the software gives on the dataplot.
So I have high hopes that I should be finished on the evening of the 13th of December.....
Well that's it at this point. I will update as I go, wish me luck with compleating this mirror before December 10, 2002!!!!
Well I'm sorry to say I missed my self impossed deadline, today is the 15th of December. Although I missed the deadline the mirror is now finished. I finished figuring the mirror during the evening of Friday the 13th, and contacted the gentleman who does the coating for John Dobson's group and for the Chabbot Mirror Makers. He very kindly said he would coat it in the morning. Well this morning I was on the road bad weather and all, at 12:45 pm today the blank I have been calling a mirror really became a mirror.
Here, pictorially is how it happened....
The vacumm chamber.
Getting the chamber ready to coat.
Loading my mirror, incidentally the bevel is just the right size such that the retaining clips don't cover any of the mirror's surface.
The plasma glow durring the discharge cleaning just prior to coating the mirror.
And the coated mirror.
I will post progress reports on the making of the secondary and their twin system as I progress, obviously;)
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