The Mirror

During the Spring of 2025, I had to go to a sphere on my 17.5" f/3.4 to fix TDE, After that, I asked Mel if he could help mentor me in parabolizing my mirror, and much to my delight, he agreed. We initially planned for me to go to his shop on the 30th of June 2025 so that he could help me cross the finish line, but this turned out not be necessary as I was able to obtain snap focus, albeit with an imperfect figure, with a week to spare.

When Mel agreed to mentor me, he said let's conspire to make the mirror equally as good as the structure that I built for it, which was encouraging coming from Mel! But, what does this look like? I can't zoom Mel because I use a signed language and he uses a spoken language. What does collaboration look like? How can I collaborate with Mel, but at the same time, learn from Mel in a meaningful way? Constantly posting to the old OSW thread back and forth would be too discontinuous which is not a seamless way of communicating especially on a project as involved as this. So, what's the answer? Google Docs, as it turns out!

I decided to keep a parabolizing log, linked to at the top of this message, which made it easy to see what was happening from a quick glance, especially right in between sessions since there is a before/after column that shows side by side comparisons of ronchigrams, to make it easy to see changes at a glance. It was time consuming to take RGs after each session and process the images, no doubt, but it was the only way I could think of to help Mel mentor me. As someone learning from Mel, it's my job to facilitate his mentorship and not vice versa. So, it was a responsibility I gladly embraced.

It took 81 sessions to parabolize my 17.5" f/3.4, and the first star test was after session 65 which failed to get snap focus as the center wasn't deep enough. Mel and I emailed back and forth hundreds of times, and I pasted his analyses into the Mel's calculator column in my log for the sessions he gave analysis for.

It wasn't until the 4th star test, as noted in my log, that I was certain of snap focus.

Retrospective on parabolizing the 17.5” f/3.4

List the good, the bad and the ugly as lessons to learn for the next parabolizing process

The good:

• Taking before/after RG’s- VERY HELPFUL! Not only helpful to me, but tremendously helpful for Mel to give accurate analyses 

• Experience working under Steve Swayze did not go to waste- his techniques helped me here

  • 1. Frugal with polish, don’t over do it. 

    2. Never hold things like the spray bottle over the polish bowl- avoids contamination 

    3. Nice and slow, even drag, don’t work against the drag, work with it. 

• Arrived at snap focus by star test #4.

• Solved TUE but took a while. FOT for 7 sessions, which reduced TUE wave but did not solve it until later sessions.

• MOT is useful for reducing scratches- particles sink into pitch or fall into the channels

• Mel’s sage advice- not only did he guide me but he helped develop my understanding of mirror working by teaching me to visualize the “edge perspective” or the “center perspective”.

• Developed an understanding of what high and low means on the mirror.

• Improvisation leads to bad results- need to take the edge perspective or center perspective when working on a problem like a zone.

  • 1. Example: a low 75% zone can’t just be polished out. The inner zones have to be brought down, meaning the center has to be brought down first, then the edge.

    2. Don’t be impulsive. If a problem brews, wait for Mel’s analysis.

• Whenever I thought things were starting to go sideways, Mel’s analysis usually revealed the opposite- see the bad. 

• Mel’s ronchi calculator (software null) is a especially useful tool to inform the strategy for the next session

• Mel’s ronchi null is better than the Ross Null for the novice mirror maker 

• Surface is smoother than it was after Steve and I left off with the mirror, a result of using a full size lap.  

• The mirror can go up to 400x without losing focus. 

• Did not cause TDE during the parabolization process.

• Mirror figure is stable despite strain.

• Parabolization went well, despite strain.

• Did MOT for almost all sessions involving a lap.

• Full size lap works to parabolize a large fast mirror, which is surprising considering how used I am to sub-dia work.

• A full size lap helps control for surface roughness.

• Shrouds are important- I can see thermals in the ronchi after rotating the mirror and going back to the tester. Takes about 15 seconds to settle. 

The bad:

• Time consuming process of taking RGs, cropping, desaturating/exposure adjustment, then posting. 

• Did not note time and temperature in my parabolizing log. Should do this next time. 

• Don’t rotate the mirror at the end of strokes- rotate in the middle of the stroke, or rotate as you progress through the stroke. Rotating at the end of the stroke can change the frequency of the pitch and cause the mirror to be dug in at one specific region. 

• Didn’t confirm good contact after first session- Mel advises to always check for contact through the mirror back. 

• Didn’t always make sure the lap was channeled and microfaceted. VERY important! Always make sure the lap is well channeled and microfaceted. 

• Not channeling the center of the lap after the first 10 sessions or so, led to the center being dug out too quick. 

• Bad stroke feels bad on first stroke- usually needs pressing- stop immediately. Didn’t do this on 1st session and 29th session. 

• Sub-dia tools cause roughness when used too much and without feathering during the stroke.

• Whenever I thought things were starting to go sideways, Mel’s analysis usually revealed the opposite- need to look at what Mel sees and pay closer attention to the Ronchi and trust the what the null says instead of relying on how the RG “looks”. 

• Sessions too long at the beginning- resulted in driving a divot into the center. Go slow and shorten the sessions.

• After session 65, the edge was good but no snap focus. Should have left the edge alone and focused on deepening the center. After this session, it might have been good to suspend the 50-100 zone on the lap.

• During the initial sessions, the central half of the lap got pressed in, so no channels were left. This resulted in the parabolization process going too fast in the center. I achieved 100% parabolization in the 0-65% zone within the first ten sessions. Too fast. 

• Miscalculated 1/4 stroke overhang during session 20. I did 4 3/8” overhang because I forgot to divide by 2, so overhang should have been closer to 2”.

• Misunderstood Mel when he said 1/3 or 1/4 or 1/5 stroke. This was noted in the parabolization log. 

The ugly:

• On the 2nd machine session when returning to sphere prior to starting the parabolization process, I did not follow Steve’s procedure and turned the machine on using the on/off switch with the dial already set at 40 RPM from the previous session. Bad idea. The tool retainer flew off and impacted the mirror at 3 points. The scratches were not visible to the naked eye but showed in the RG. Whoops. 

  • From now on, always first turn on/off using the dial, not the on/off switch. 

• The mirror has considerable strain that hasn’t proved to be fatal