Telescope Diaries

Done with 120 Grit posted on 06 Oct 2009 04:34

It has been a long time since I posted, but I have been busy grinding the mirror with 120 Grit.

It took much longer than I expected; about 10.5 hours of actual grinding. But I am happy with the end result. I can't find any large pits even with a magnifying glass.

Coincidentally, I spent the same amount of time on 80 Grit too.

Effort so far
Activity Time Calendar (2009)
80 Grit 10.5 hours June 12 to Aug 19
120 Grit 10.5 hours Aug 20 to Oct 5

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A fresh start posted on 07 Aug 2009 15:49


The mirror I was making for the last three years met an untimely and tragic end, sometime in May 2009.

I had just got the mirror coated and was gluing it to the mirror cell, when the incident happened. I did not get the chance to see even once with the coated mirror :(

Anyway, I don't want to dwell too much over what happened. After a brief period of melancholy mulling, I decided to start again. Thanks to all my friends who didn't let me stay depressed for long (esp, the sagacious sagar, Anna, Pratap, the Pretentious Pervert Pavan, and Atul sir).

Hopefully, I will be able to build a better mirror with the wisdom of hindsight. The new mirror will be to the same specifications as before (8.5" aperture, about f/8 focal length)

Mirror blanks

Getting the new blank took longer than it should have. I basically spent quite some time in procuring it from nearby my hometown. But the glass cutters themselves convinced me to not shoot my own foot, but procure them from Bangalore.

So. Back to Bangalore. Shivaji Nagar. Balaji glass works. And ofcourse, Lassi at Shreeraj lassi bar :)


This time round, I am lucky to have a completely empty hall for myself and my contraptions. There's ample space and minimal clutter which is a big help.

For comparison, the last mirror, underwent grinding on 3 different open terraces, under dusty conditions.


There was a lot of fine grit remaining from last time. I only had to get the rough grit. I didn't spare any expense here. Got 2kgs of 80,120,220 320 grit couriered from Pune. Costed me about 1200 Rs.


The grinding has started. I am aiming for f/8.5 (72 inch focal length), 1.6mm sagitta. I have dug about 1mm sagitta already with 80 grit, after 6.5 hours of grinding, spread over 2 months.

Again, another difference from last time is that I have wires of precisely measured thickness this time. No need to second guess the value of sagitta.

I hope to finish grinding completely in the next 2 months.

Watch this space for more!

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