Grinding, lesson #2

The second lesson on mirror grinding is over.

Sessions #2

Here is an update on our second session with mirror making.


A good breakfast before we start grinding.

We started out with checking the sagitta on the primarys after our homework. There was a big difference in how close the mirrors where to correct sagitta.

We used almost all day, working with grit #180 to get the sagitta correct and removing pits from grit #80. Remember this is the very first time we try this.

When so many amateur astronomers are working in the same room, it is very easy to get interrupted or take a break to talk our hobby. So the time we actually where working on the mirrors, could be a lot more effective. But we are having a great time.

A the end of the day, everybody was ready to go to grit #320 and some us work around 10 wets with grit #320 before calling the day.

Checking for pits!

So the status is now:
All the mirrors are close to correct sagitta and we are ready to finish grit #320.

We have done pencil test on all the mirrors and they all have very good contact.

One of the things that are giving us a little headache is the secondary. The secondary sagitta is close to the primary, but until now we haven’t succeeded in getting them really close. I have talking with David about it, and he gave us some advice, we will try. So nothing to worry about yet, I think.

When we get to grit #380 we will measure the ROC with foucault testing.

What we learned:

We all know what “Walking the barrel” is now.

Get your own pace when grinding.
Correct strokes, it is easy for a beginner to get the strokes to long, to short or forget to press. It is really good to have an instructor to help out and give advice.
How to measure sagitta and calculate ROC, and we also have time to test one of the home built Foucault testers with a 10” and 6” mirror.
Patience is important.

A wet with grit #180

Thanks for all the advice, tips etc. from CloudyNights members.

Explanation of ROC, Focal length, Diameter and Sagitta
 

Making new "grips" for the grinding table.

Bubbles between the disks. More work to be done.