f/4.66 Mirror - First Light 1/7/2019
This is the Mini Maxx two days before first light. I set it up at the Mirror lab where it was rated as acceptable. For this group that is high praise indeed!
This is the difference between the 18" and 16" mirrors. The 18" is 1.75" thick and weigh about 40 pounds and the 16" is about an inch thick. This is also my first attempt at making a plate glass mirror instead of Pyrex. The chips in the edge were from 60 years of moving around. This piece of glass was a tool to grinding a 3+" thick mirror in the 1950s and it was covered with caked on ancient red rouge. It looked pretty bad and was almost trashed. Since it was a tool that also meant that I started it as a convex blank, not concave. Fortunately Mike Davis, a friend and fellow mirror grinder, has a vibrating table machine that flattens out the glass. After I cleaned off the rouge Mike's magic removed about 80% of the curve. This is where my hand grinding and polishing started.
|This is at the end
of 60 grit ready to start 80 grit. The edge needs a little work
yet and the 80 grit will handle that nicely. This is a remnant of
he convex shape that the glass started with The current depth is
203 thousandths of an inch with a target of 207. Our sagitta meter
reads three one thousands shallow so after adding three it ends up at
In the end I needed to go a bit deeper to get all of the edge in the sphere and ended up with f/4.66.
|The mirror is in find grinding here using 25 micron grit. Later, after 12 micron, I made the mistake of attempting 5 micron and the tool came into contact with the mirror and scratched it. back to 12 micron. A few of the tiny scratches remained and I'm hoping to polish them out with the cerium. After all, this is plate glass.|
|This is the beginning of polishing. The edge needs a little better contact. I poured a second layer of pitch over this one when I transitioned from polishing to figuring. That second pour had some turpentine added to soften the 64 pitch.|
|The mirror was finished after 10 Saturdays of polishing
and figuring. The last day, when the above photo was taken, was
after an eight hour marathon figuring session. In the end all axes
checked were better than 1/6th wave. All polishing and figuring
was with a full size tool and the final figuring was accomplished using
a swirl stroke, mirror on top, with the edge of the tool working the
high zones of the mirror based on the previous Foucault test. The
final test results are shown above with a 1/6.37th wave and 0.908
Strehl. The shape of the plotted line is relative to a perfect parabola
for this mirror, which would be a straight horizontal line on
zero. The mirror's surface is very well polished and easily passes
the laser test that checks for first surface light scatter. We had
four mirrors completed at the optical lab in the past three weeks
(10", 12", 12.5" and 16") so we are about ready for
a trip to Spectrum Coatings.
The reflection on the mirror is from the camera's flash bouncing off of the aluminum coating on the Foucault test tunnel just to the right out of the photo.
The mirror returned with a beautiful coating and is waiting for the rest of the telescope to be build...
This is the mirror cell getting its first test fit in the mirror box. The cell was made by JP Astrocraft and he did an amazing job. Later the inside of the box was painted flat black. Since I used 1/2" Baltic Birch I cut off the tips of the 12 screws that hold the mirror cell to the box to that they wouldn't go through the plywood.
The secondary cage is now complete using a Protostar secondary holder and a Moonlite dual speed focuser. The truss pole sockets are also from Moonlite. I haven't attached the dew heater connection yet to the secondary holder in this photo. They are connected and tested with my home made dew heater controller.
|This is the 16" Min Maxx in the wild for the first time at the 2019 Orange Blossom Special Star Party. The skies only allowed a few hours of observing time but the views were really good and the scope worked perfectly. Behind the Sky Commander (small black box in the middle of the scope) is a serial to Bluetooth adapter that allows the The Sky on a PC follow the telescope. Mike's 17.5" dob is in the background.|
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