#Telescopemaking 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.

On to 220

hillexed@email.com (hillexed) — Sat, 21 Sep 2024 00:08:45 +0000

I’ve gone to 200 grit from the finer 320 grit. I’m glad I did; an hour of grinding later and my last tiny divot still has’t ground away. Had I stuck with 320 grit, I’d need to spend twice as long to get to where I am now. Hopefully another hour will get it out.

Focal length flattening

hillexed@email.com (hillexed) — Sun, 15 Sep 2024 13:05:10 +0000

A problem! After 6 hours of total grinding, I measured my mirror’s focal length and found it was 37". That’s weird because before it was 39" - maybe my hands were applying pressure in the center of the tool? Lower focal length means more zoomed out (and brighter) telescope views, but makes the final stage of parabolizing harder. If focal length / mirror diameter is less than 4, you also get “coma” that distorts the stars towards the edges of the view.

So… should I move forwards with my fine #320 grit, accepting my shorter focal length? Or should I lose progress and start using a coarser #220 grit to speed up glass removal and make the curve shallower, so there’s less coma and parabolizing is easier?