#Diy on Hill's Space

12" polishing update: Oblate no longer?

hillexed@email.com (hillexed) — Sat, 10 May 2025 17:47:25 +0000

Five or so 30 minute long sessions of long parabolizing W strokes have finally turned my center hill into a center hole. The dip at around 80% to 90% diameter remains, while the 90% to 100% diameter edge zone is turned up.

I made two changes to get better pitch contact: I am filling a plastic container with hot water and pressing the pitch to the glass underwater, and I am only polishing for 30 minutes at a time instead of going for a full hour. I think it’s working!

Hopefully now that it’s no longer an oblate spheroid, the standard 1/3 strokes will work properly and allow me to fix that bumpy outer zone and its turned up edge.

Creating a mirror cell

hillexed@email.com (hillexed) — Thu, 08 May 2025 13:23:13 +0000

I have made a contraption called a “mirror cell”

it traps naughty glass so I can examine its sins

hillexed@email.com (hillexed) — Tue, 22 Apr 2025 02:31:02 +0000

More wide parabolizing strokes are bringing my 12" mirror closer and closer to a sphere…

I have a slight worn down edge, overall a tiny bit oblate, as well as a tiny central hill, but it’s all very small.

Here’s a Ronchi test picture of the mirror today compared to two weeks ago. As a reminder, straight up and down is spherical.

Big Baffles

hillexed@email.com (hillexed) — Tue, 15 Apr 2025 18:55:12 +0000

My 8" telescope has been upgraded with some new printed shields to block unwanted light near the top.

Pics of said light blocking baffles. Also added a mount for my pifinder.

Polished out?

hillexed@email.com (hillexed) — Sun, 09 Mar 2025 13:35:11 +0000

My mirror is now reflective enough to see the entire surface and get my first Ronchi test image! But I’m not done with polishing - it may look reflective by eye, but the same region under a microscope reveals tiny pits left over from 5 micron grit are still there. I’m not polished out until all those are gone.

Crack Coverup

hillexed@email.com (hillexed) — Wed, 06 Nov 2024 18:29:20 +0000

I’m slowly dealing with the crack’s aftermath. Slowly, I ground through the big hole I made when burnishing the crack. As I ground it down over the week, the crack wasn’t visible under the loupe unless I put a flashlight sideways and used a finger to cover the top of the mirror, illuminating from the side. After watching the hole get smaller and smaller, I took a look… and the crack was still barely visible. It’s there in the last image as a slight diagonal line below the dot.

After the last picture, I ground for another hour and I can’t find the crack anymore. I hope that means it’s gone and I can move from size 320 silicon carbide grit to 30 micron aluminum oxide grit!

Total time spent grinding out the crack: 7.4 hours

Crack Tragedy!

hillexed@email.com (hillexed) — Sun, 27 Oct 2024 04:12:33 +0000

Tragedy!

I dropped my tool onto the glass from an inch up. I thought I was fine, but close inspection revealed a crack a millimeter long. I was advised to try to burnish it out because the crack could travel very deep and spread.

I ended up putting a screw in a drill and using the head as a bootleg buffing wheel with my 30 micron water mix to try to dig out the crack and its subsurface damage. Now I have an ugly 4 square mm divot on my mirror, which is otherwise polished down to 30 micron abrasives. Sigh…

Divot Gone

hillexed@email.com (hillexed) — Mon, 23 Sep 2024 00:25:40 +0000

Looks like after 3 hours of grinding at #220 grit, the divot that has been in my glass since the start has been ground out! Yay! I’m glad I went back to coarser grit; this could have easily been six hours had I stuck with #320.

A mirror with a short focal length will make objects look brighter because they’re zoomed out, but is much harder to parabolize at the very end of the mirror making process. I was hoping my grinding would increase the focal length from 36" back to around 40". I measured and it’s… 37.5", around f/3.1. Oh well.