Binocular Telescope Mirrors
I received an order to
make matching focal length mirrors for a binocular, and the project intrigued
me so much I took it on. Originally, the plan was to use a single tool to grind
two mirrors going back and forth between them to keep the focal lengths as
equal as possible. But a different plan hit me. It may work, it may not, but I
have to try it. If it fails, Ill return to the standard method as mentioned
above.
The order was for two
matching 6 f/7 paraboloids. My plan
is to place three 6 disks onto a curve-generated 13.1 mirror blank with
blocking pitch and then grind them as one single optic. The 13.1 mirror blank
was curved to an ROC of 85 inches, and so when I placed the three blanks onto
it their fronts will have an ROC of about Ύ shorter, just over the 84 ROC
needed. I curve-generated a 10 tool to an 85 ROC. I then fixed-posted ground
the 10 tool with the 13.1 mirror blank. If the image below you can see each f the mirrors attached to the lower glass are tilted.
Next I used blocking
pitch to secure the three 6 discs onto the 13.1 mirror. They are already
tilted with their optical axes pointing toward the center of curvature of the
13.1 master substrate. The 6 mirrors have an extremely short sagitta, so I did not pre-curve-generate them, but I beveled
their edges well and started slowly. Ill fixed-post grind them as if they were
a 13.1 mirror starting with 60 grit silicon carbide. Once ground Ill progress
through the grits as usual, 120, 220, 500, then on to
aluminum oxide with 12, 5 and 3 micron.
https://www.youtube.com/watch?v=FeB97KscL-Q
https://www.youtube.com/watch?v=4nJBCrpja_U
https://www.youtube.com/watch?v=AOWXVLzm_zY
I wanted to check to see
if there was similar grinding on each blank and if the initial grinding started
in the centers to avoid wedge. There is a bit of wedge developing, but for 6
f/7 I dont think it will be a problem. Larger mirrors might have issues. I
drew Sharpie pen lines on both the mirror set and the tool. The tool is totally
worn away, so it is getting good action. The mirror sets are showing a bit of
tilt.
I do not want to polish
them combined like this, mainly because I dont want to mess with casting a
pitch lap of a 10 tool with three 6 mirrors, thats weird. Also if there were
zoning issues, each mirror would have those zones not surrounding their
individual optical axes but rather the center of the triangular array of the
mirrors. Not cool. So I will separate them and polish them individually.
It is possible that even
though they will have identical ROCs after grinding, they may not have after
polishing. So I will select the two that are closest to each other and send
them to the customer, and keep the third as an in-stock item. The 13.1
mirror under them will also be ground, polished, parabolized and sold.
If this works out as well
as I hope it does, I have no reason why not to attempt this with larger
mirrors. As an upper limit in my brain right now, I can see three 22 f/5 mirror sitting atop a 36 f/3 mirror for the same purposes. I suppose a more reasonable set of examples
would be 8 mirrors atop a 17.5 base and 10 mirrors sitting atop a 22 base,
but larger is cool. Putting three 22 blanks atop a 36 blank and having a shop
disaster would be a financial deal-buster. Not ready for that yet. But Im
thinking about it.
I may advertise this as
being a new niche for me if this works out.
I found that the
triangular gap in the center is a place for grit to wash off the work and after
only two turns there is no more grit on the glass. So a huge amount of grt is used to do a tiny amount of work. I was puling mud
off the sides to reapply so to save on grit, but that sucked. Since I have a
large amount of material to grind away in the center of the system that was
going to take forever and use too much grit. So I poured more blocking pitch
into the central triangle to fill it in hopefully to keep this from happening.
Here is a shot of the mirrors after a bit more work and before filling in the
middle.
Notice that one mirror is
grinding well, but two are not placed on the backer sphere perfectly against it
and the mirrors are not grinding from the centers but a bit off axis. This will
result in tilt. If these are 20 mirrors, Id be re-dong this. But tiny little
6 mirrors with slight wedge is not a deal buster, especially since they will
be interferometrically tested. Once all ground, there will be many
opportunities to Sharpie test to make sure they are grinding right and fully everywhere,
hopefully implying a sphere.
Know this all looks bad but Im having a blast
processing this method.
More to come
.