Purpose of this section is to show how to achieve the nice spherical surface free of any defect and zonal irregularities before moving to figuring or prabolization. It is a very important and critical activity.
If you have been good in your fine grinding and polishing strokes, you will end up with a nice spherical surface free of any defects. Note that it is an over expectation to see your mirror spherical when you first test it. Usually, you will see defects on the surface which you need to fix and make the mirror as spherical as possible before you start the Figuring process.
For testing, you need…
Make them by clicking on links above.
Test for a Spherical Surface
Before reading any further, you should know how to read Ronchigrams and Foucault Shadowgrams. Please check the below links to learn and understand about these two tests and come back.
- Ronchigrams: https://stellafane.org/tm/atm/test/ronchigrams.html .
- Shadowgrams:
The best and quick way to check the surface defects is to do a Ronchi test. Setup the mirror on the stand and use ronchi tester to look at reflected light through the screen at very near to CoC position. If your mirror is spherical, Ronchi lines will be straight both inside and outside of CoC. Best position to judge the surface when you see 3-5 lines both inside and outside.
When I first tested my mirror, it was oblate spheroid i.e. it had slightly longer curvature in the middle portion of the mirror than the remaining surface.
So I had to correct it i.e. deepening the center portion. I used long parabolizing strokes and overshot it 😀 The next test shows it having a deeper curve at middle (called hole)
My mirror also have some turned down edge (TDE). If you look closely, you can see the ends of Ronchi bands inside and bend outside near the mirrors edge. It would be reversed in the case of turned up edge.
At this point, I decided to take help from the experts in ATM community and made an account on Cloudy Nights (https://www.cloudynights.com/) with name AstroKhan (so you know why this site is named as such)
On CN, I quickly got the advice to remake the lap. As I mentioned earlier, I was using a sub-diameter lap. So I made a new lap. Dogbiscuit* suggested the pattern and my lap came out matching with the pattern B which he had suggested on another thread.
Image Credit*: Dogbiscuit
I continued the work using the new lap with 1/3 D CoC strokes and it started showing the good results. In the next Ronchi test, mirror surface became flatter and lines became straight. There was still a slight dimple at the center left of the mirror.
Further, I did 3 sessions of 20 minute each with 1/3 D CoC and found the mirror was going oblate again!
This was also the stage when I took the first good KE (knife-edge) image of the mirror using the Foucault tester. You can see the mirror is oblate sphere at this stage i.e. a hill or bump at the center of the mirror. Overall mirror profile is smooth and there are no other surface defect identified.
Image showing an Oblate i.e. hill or bump in center 
Same image with increased brightness to show better surface details
As the surface profile was smooth, Dogbiscuit* suggested to start the parabolizing which would ultimately remove the oblate. He suggested following 3 strokes to add the corrections. See the suggested position of the hands while doing these strokes.
Credit*: Dogbiscuit for all the patterns above. Used with permission.
So as suggested, I did the following.
- Applied thin layer of polishing mix on the lap and cold press for 15 min.
- Started the first (1/4 D – 3/4 D) 7 stroke zig-zag W pattern. Target was to completed it 12 times with Mirror turning 30 degree counterclockwise after each complete stroke. Total time 4 minutes (less or more). That means 20 second for full 7 strokes and one mirror turn. That means around 2.85 or 3 seconds for a single stroke! (center to forward, back and center again). You see how slow it was but it was what the mirror needed.
- Repeated the same with second (1/3 D – 2/3 D) and third (4/5 D) in 4 minute each.
- Completed all three patterns in 12 minutes around.
Did further testing and found these strokes worked like a charm! Central hill was gone and mirror was inching towards the flat/spherical surface profile. In below images, you can see ronchi lines are going straight. There is also some turned down edge (TDE) visible at the edge.
Knife Edge (KE) is also showing a flatter mirror profile at this stage.
Then another parabolizing stroke suggested by dogbiscuit to add correction in the center. Did single 4 minute session in the same manner as mentioned in the point 2 above.
Image credit: Dogbiscuit
This improved the ronchi results and made the surface profile looking smoother in KE. Below image will also show very fine scratches in middle lower and left portion of the image. They are barely visible on the mirror surface.
As a nice spherical surface without any irregularities is achieved, We can move to Figuring or Parabolization stage.
Keep in mind that if you are making a smaller and slow mirror (say 4 inch F/10), You may not require to parabolize the mirror as the difference between the curved surface of a parabola and sphere is negligible at this size. A small spherical mirror can perform well within the 1/4 wave rating without being parabolized.
Astro Khan Telescope Making 