#Meniscusmirror on Hill's Space

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?