Go Back   IceInSpace > Equipment > ATM and DIY Projects

Reply
 
Thread Tools Rate Thread
  #1  
Old 09-05-2022, 03:52 PM
mura_gadi's Avatar
mura_gadi (Steve)
SpeakingB4Thinking

mura_gadi is offline
 
Join Date: Apr 2019
Location: Canberra
Posts: 678
Why is my mirror polishing outside in on MoT.

Hello,

I have a D8F9 that I have got to the polishing stage again, I have polished with MoT the whole time.

After my first 90minutes I noticed the edges were polishing out first. I completely reset the lap even though contact looked really good. Another 180minutes of polishing on the new lap and I pass the laser test on the whole mirror except for the centre 30%. (two laps same issue, my centre squares flatten out about 3 fold from channel press squares to mirror pressed squares.)

With MoT I should be polishing from the centre out, I had issues with shoulders left behind in the hog. I returned to 220/400 and took those out once already, so I'm guessing its not shoulders.

Thinking TDE is caused by over run I have been running closer to 1/4 strokes over 1/3 strokes mostly on the polish stroke. But that's about it.

But I'm stumped as to why I am polishing from edge in... What are my most likely mistakes for a rookie here?


Thanks

Last edited by mura_gadi; 09-05-2022 at 04:05 PM.
Reply With Quote
  #2  
Old 09-05-2022, 09:26 PM
Rod
Registered User

Rod is offline
 
Join Date: Sep 2006
Location: Mount Martha
Posts: 359
Hi Steve

If you are using a short strike, that’s possibly your problem. Short strokes polish the outer zones more than the centre. Try to stick to 1/3 stroke. If you are worried about a turned edge, try more of a centre over centre stroke or a Narrow 1/3 W stroke.

Check contact carefully. A good way is to warm press some some nylon fly screen into the lap and make sure you get an even imprint of small squares all over the lap.

Is your lap microfacetted?

The other possibility is an incomplete grind. Perhaps from too short a stroke combined with insufficient time at one or more of the early grades.

Hope that gives you a few ideas.

Rod.
Reply With Quote
  #3  
Old 10-05-2022, 06:17 AM
mura_gadi's Avatar
mura_gadi (Steve)
SpeakingB4Thinking

mura_gadi is offline
 
Join Date: Apr 2019
Location: Canberra
Posts: 678
Hi Rod,

Thanks for the replies, I'll go back to a 1/3 stroke, I was about 1/8th overhang or a little more. Basically about an inch in from the edge for my marker I think.

What's the best way to check to see if I have two curves in my mirror?


Thanks
Steve

Hadn't tried the flyscreen this time, I got a steel wire brush from the two dollar shop. The brushes seemed stiffer than the brass BBQ brush and the wires better secured. Really like the way it takes the gloss off the squares and slapping the lap gets you some nice pits and well as the scratches...

My micro factettes last about 50minutes, the pads are completely smooth, almost glossy (Channels get the Stanley as well). I am replacing microfacets after ever session and the lap needs a complete new channel after the three 45 minutes sessions. (well, it wouldn't survive a 4th session anyway)

I have a floating lap, I leave that face down in warm water to help get some fast warmth at the start. Definitely a +1 for wooden laps on that score.

I seldom skip or grab at the start, and contact is solid. I find that if I don't keep a slightly damp hand I have great trouble moving the lap. With a drop of water smeared over my hands, its 5 "I" strokes at about 1 secs each, and turn tool/mirror repeat.

I get good action within a couple of minutes of the start of each session using the water warmed lap and a warmed blank at the start. the 45minutes normally starts about 5-8minutes in when I get good cutting action, and doesn't count the wash or prep cleans. So about 70-80minutes a pop before I'm pooped.


*My Ronchi screen and tester are a little fail atm. I'm off to buy some parts this morning to make a better tester. The 125/200lpi I have now is just too many lines... Hoping to make the fishing line slide at 80lpi today.

Using "210x.000550 / (1/2xFL)" for the line count, seems to line up to Mel Bartels Ronchi simulator really well.

Last edited by mura_gadi; 10-05-2022 at 08:58 AM.
Reply With Quote
  #4  
Old 10-05-2022, 08:57 AM
Rod
Registered User

Rod is offline
 
Join Date: Sep 2006
Location: Mount Martha
Posts: 359
Hi Steve

125 LPI should be fine for an F9 mirror. Make sure you understand how to do the test before wasting time on different line frequencies. 200 LPI is too much.

You check for astigmatism with the ronchi test and look for ‘clocking’ as you move the tester from inside to outside of focus. Clocking refers to the bands moving from one orientation to another. You can also sometimes see a S shape in the bands if the astigmatism is severe. There’s a photo and a description here:

https://stellafane.org/tm/atm/test/ronchigrams.html

Final testing on a slightly defocused star to ensure it remains circular inside and outside of focus is a good idea before coating the mirror. The definitive test though is likely a Bath Interferometer. I haven’t tried that yet.

Just watch for rust with the steel brush. You don’t want that falling into your lap.

Rod.
Reply With Quote
  #5  
Old 10-05-2022, 02:40 PM
mura_gadi's Avatar
mura_gadi (Steve)
SpeakingB4Thinking

mura_gadi is offline
 
Join Date: Apr 2019
Location: Canberra
Posts: 678
Thanks Rod,

I have just built a simple box for both the knife test and the Ronchi screen. Just snapped together with balsa atm, it will look better after it dries and gets a sand. Rubber bands for clamping the front to the back, Coles still had 40w incandescent light bulbs so I'm trying that for a light source. Pretty much a copy of Mr Bartels. If it doesn't work, I can stick a LED and battery pack in there instead.

All I want is to check sphere atm, so, I'm off to make a knife edge shortly. The lateral movement test is next on the list of addons.


Steve
The brush does not get used till the lap is dry and is stored in a "do not touch bag" now.
Attached Thumbnails
Click for full-size image (IMG_8805.jpg)
191.2 KB24 views
Click for full-size image (IMG_8803.jpg)
189.3 KB16 views
Reply With Quote
  #6  
Old 10-05-2022, 10:32 PM
Rod
Registered User

Rod is offline
 
Join Date: Sep 2006
Location: Mount Martha
Posts: 359
Ok. It can be very simple for the ronchi test. It will be clearer for you if you block off all the light that does not go through the the ronchi screen. You want it so light goes through one part of the screen and you observe the mirror through the other side.

Rod
Reply With Quote
  #7  
Old 11-05-2022, 05:51 AM
mura_gadi's Avatar
mura_gadi (Steve)
SpeakingB4Thinking

mura_gadi is offline
 
Join Date: Apr 2019
Location: Canberra
Posts: 678
40w was way too strong! Will have to try a 15w incandescent. Or think about getting a little MES fitting for indicator lights maybe.

Back to the LED which is fully contained and only lights up the screen. Though I did find getting the open box a lot easier to setup, seemed to find the return beam on the screen quicker.

**************

As a testament to the generous nature of the people in astronomy, helping to understand all aspects of the science, I have been offered the loan of a bona fide tester! Which I am stoked about, about to get 10 days home alone, so a decent chance to hammer the caveman work again if need.

Many thanks to the person who reached out btw.

Last edited by mura_gadi; 11-05-2022 at 08:19 AM.
Reply With Quote
  #8  
Old 15-05-2022, 07:48 AM
mura_gadi's Avatar
mura_gadi (Steve)
SpeakingB4Thinking

mura_gadi is offline
 
Join Date: Apr 2019
Location: Canberra
Posts: 678
Thanks all for the replies,

Long strokes got the polish evenly across the mirror centre. I trashed myself yesterday working at the face and forgetting the time. No laser reflection left, on the Ronchi stand, nice fat halo around the 30% zone.

SWMBO is away, so, back to 400 and this time, using the longer strokes over centre.

I'm planning on progressing to the finest grits I have (can use) before moving to polishing. The article I posted about the chemical shear layer deposit and removal rate caught my interest.

After my first 8+ hours of polishing, going back to the smoothing etc seems easy ...

I'm at 141 of 144 inches on the ROC, I'm after longer over shorter so the reface was going to happen anyway. I just wanted to try a polishing spell and setup useable Ronchi test to get some experience. I'm not expecting to get the mirror I want for a few iterations. 'Tis fun, Caveman work to reading and working at fractions of a visible wave length all in one hobby...
Reply With Quote
  #9  
Old 15-05-2022, 09:28 AM
OzEclipse's Avatar
OzEclipse (Joe Cali)
Registered User

OzEclipse is offline
 
Join Date: Jun 2012
Location: Young Hilltops LGA, Australia
Posts: 1,014
Good luck Steve,

Don't be too accurate about those stroke lengths. Remember that the key to a good even spherical surface is randomised stroke lengths.

Joe
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 01:49 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2022, Jelsoft Enterprises Ltd.
Advertisement
Testar
Advertisement
Limpet Controller
Advertisement
Bintel
Advertisement
ScopeDome Australia
Advertisement
Astronomy and Electronics Centre
Advertisement