8吋鏡 - 拋光/球面化時遇到的問題

[cytangjoseph]

焦內:


焦點:


焦外:

發現中央有山, 及邊沿有低邊.

請問師兄師姊, 可以用甚麼方法修正?

[Chanlunlun]

You have a huge amount of turned down edge, difficult to cure by pitch lap.

Best suggestion : go to #1200 fine grinding and do polishing again with hard pitch. What is the f/no. of your mirror ?

[cytangjoseph]

Hello Chanlunlun,

it is 8 inch, f/6

thanks for your tips!!

[cytangjoseph]

So I think the turned down edge is top issue for me to tackle first.

Will be it like going back to #1200, and grind with 1/3 W stroke?

[Chanlunlun]

Back to #1200, mirror on top using 1/3 center to center stroke.

[cytangjoseph]

Thanks Mirror god!!

Past few days, as a bit too busy [ or lazy ]to go back to #1200 and making a new pitch lap, I spent a few more hours with the pitch lap I am now using:

some ways to try to improve the rolled edge:

1. more frequent cold press for good contact

2. hands placed in the center of the glass, and not on the edges.

3. Keep movemnts horizontal and slow. Push ( and pull) the glass with whole body weight ( back straight, bend your knees and ankles). As in Tai Chi

4. 1/3 COC. Alternate Mirror on Top, and Tool on Top

5. stop the polishing after 10 to 15 minutes, and take a break of 5 to 10 minutes, not to let the glass and pitch to warm up

焦內(inside focal poiint):





焦點 (focus):




焦外 ( outside focal point):






So far the rolled edge is much reduced. Still some turned down edge.

Next I am going to tackle the remaining turned down edge, and the hill in the center

thanks!!

[edited for mixup of inside focal point and outside focal point pics]

[Chanlunlun]

The mirror is not smooth. Cold press can do very little towards matching the surface of mirror to pitch surface.

You should hot press the pitch lap every time you start polishing again. Best of all is that the polishing should be done all in once, avoiding pressing the pitch lap from time to time.

Pitch, especially those bought from local, deteriorate upon repetitive heating and pressing.

[cytangjoseph]

Well noted Chanlunlun,

Thanks for reminding me, that to start a polishing session, it must begin with a good hot pressing first. I also do a cold press after a 15 minutes polishing, for 10 minutes or bit more.

Just updated the progress:

inside focal:



outside focal:


The edge is now more polished

But the turned down edge still remains

The centre region is now closer to sphere shape, and seems to be smoother.

Found that this pitch is better suited in lower temperature, around 20 deg C. As now is summer, in the 30+ degrees room temperature, need to have air conditioning on. Otherwise the edge region is very hard to get polished out

[Kelvin W.]

加油

[cytangjoseph]

Thank you.

Here is some update of today.

inside focal:


focal:


outside focal:





The turned down edge is much reduced. Now I am thinking what should be next.

To further smooth out the zones ( how ? ), or to going for figuring as parabola?