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#1 laserrain

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Posted 18 June 2013 - 02:29 PM

I'm not really happy to post these shots.
Mainly because I've made no more than three small mirrors ranging from 5 to 7". All of them turned out fine.

Who made this mirror, a D = 249 mm F = 1290 mm, most likely had no time to spend in correcting the shape. Or he tried, unsuccessfully.

Each shot was taken moving the carriage (single blade moving source tester) away from the mirror by 1 mm.

Any comment(s)?
Thx in advance
Anthony

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#2 rboe

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Posted 18 June 2013 - 02:40 PM

Sounds like the best approach is to refigure it yourself (and it sounds to me you are more than capable of tackling it) or have a pro mirror polisher do it for you.

#3 laserrain

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Posted 18 June 2013 - 02:49 PM

Unfortunately it's coated, Ron.
And the glass (plate glass 33 mm thick) itself is very ugly. Bad rounded, bad chamfered. Hours for re-edging, re-chamfering et cetera...

#4 MKV

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Posted 18 June 2013 - 03:14 PM

Unfortunately it's coated, Ron.
And the glass (plate glass 33 mm thick) itself is very ugly. Bad rounded, bad chamfered. Hours for re-edging, re-chamfering et cetera...

Okay, you don't like the mirror, you don't care to fix it... apparently you don't or can't return it...So, why did you post this?

#5 Dale Eason

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Posted 18 June 2013 - 04:00 PM

Unfortunately it's coated, Ron.
And the glass (plate glass 33 mm thick) itself is very ugly. Bad rounded, bad chamfered. Hours for re-edging, re-chamfering et cetera...


The coating is no problem it will come off in the first few strokes of the polisher. Don't worry about the edge or chamfer. They probably don't need fixing.

I think the surface looks fairly smooth with just large regions of bumps. So it might not give too bad of views for a novice and would take very little to make it better. I've seen much worse and some from large shops.

Take my advice for what it is worth (about nothing.) I have only made two mirrors but I have tested many. I just wanted to point out that I think your objections are not real road blocks.

Dale Eason

#6 greenglass

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Posted 18 June 2013 - 04:01 PM

do you have a zone mask to check it?

#7 rboe

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Posted 18 June 2013 - 04:10 PM

Coating is not an issue. Most will chemically remove it before polishing.

However it is an issue if you can't send it off to be recoated. You don't list your location so that is something we can't comment on yet. I've shipped a mirror out for recoating from AZ to Oregon and then to Florida. Pretty straight forward.

#8 laserrain

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Posted 18 June 2013 - 04:20 PM

Thank you Dale.

#9 laserrain

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Posted 18 June 2013 - 04:22 PM

Thank you greenglass

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#10 EyeSage

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Posted 18 June 2013 - 04:48 PM

The Strehl ratio of these three measurements averages to about .83 which I'd say just borders on what could be considered acceptable. Personally I wouldn't think twice about refiguring it, but that's me. By the time you break in the new lap you'll have to make the existing figure should be gone and if all goes well you can start refiguring from a sphere.

If you're in doubt about whether to do this and it's already coated why not just build a scope? You can always refigure it later if you decide the existing figure isn't doing it for you. If the blank looks ugly just make it a solid tube so no one will see.

#11 Pinbout

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Posted 18 June 2013 - 04:59 PM

If I can polish this out in less than a day, your mirror is nothing. :grin:

[it is in autocollimation so the error is doubled]

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#12 amicus sidera

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Posted 18 June 2013 - 05:41 PM

Coating is not an issue. Most will chemically remove it before polishing,


Personally, I'd never polish off the coatings when refiguring; at least some of that aluminum/SiO2 will remain in the lap, guaranteed.

Fred

#13 DAVIDG

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Posted 18 June 2013 - 10:05 PM

Coating is not an issue. Most will chemically remove it before polishing,


Personally, I'd never polish off the coatings when refiguring; at least some of that aluminum/SiO2 will remain in the lap, guaranteed.

Fred

I 100% agree. The coating never comes off uniformily so you will change the figure even more in the process to remove it and you also risk sleeking or scratching the surface. A little FeCl2 will usually take the coating off in under an one hour and many time much quicker and you won't be causing yourself more work.

- Dave

#14 dave brock

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Posted 18 June 2013 - 11:17 PM

Hi Anthony.
How did you come by the mirror? If you bought it, I assume you saw it first and at least saw the condition of the blank? Did you test it first? Therefore did you pay an appropriate amount for it?

Dave

#15 Mark Harry

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Posted 19 June 2013 - 06:40 AM

I disagree with chemically removing a coating. Regardless of the stripping agent, I've never seen a chemical etch that did -NOT- affect the glass to a noticeable degree, period.
I usually polish coatings off, or touch-grind them; repolish/refigure the glass. It gets a pristeen mirror surface every time.
(Of course, I'm able to do that- and I'm sure Dave(G) can do the same if he desires to!)
M.

#16 Mike I. Jones

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Posted 19 June 2013 - 07:29 AM

Hey Danny, I like the little Space Invader at the center!

Agree, Anthony's mirror is salvagable - flat center, pretty bad TDE, but nothing a fresh lap and ZOX couldn't cure.

Gotta weigh in on stripping vs. polishing an old coating - I have polished off a coating once and sleeked the devil out of the surface. The aluminum is soft as butter, but the SiO overcoat is harder than glass and does sleek up the surface as it crumbles into powder. It also charges into the lap, and you have to wire brush the lap to get it out. I've stripped a few mirrors and have never seen ferric chloride attack Pyrex (so far). I can't speak for fused silica, Zerodur, etc., though - would just have to try it and see.

Mike

#17 bremms

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Posted 19 June 2013 - 09:17 AM

HCl should remove the coating and not attack the pyrex. I had a coating haze on my old RV6 many years ago. I was 15 and didn't know any better and used rubbing to get the coating off (To cheap to pay the $5.00 to remove the old coating.) Mirror needed to be refigured by Telescopics. Mirror was a little better after refigure so it wasn't a total waste.
Removed a coating on a 8" Cave mirror for a friend with strong HCl once and it worked well. Coated it in my physics lab. Came out pretty good had to coat it three times to get it right. Used a diffusion screen to get an even coating. Used a special interferometer to measure the coating thickness. Clean Clean Clean.... then use Ion discharge before coating. Looked good as any commercial coating

#18 Mike I. Jones

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Posted 19 June 2013 - 10:07 AM

Problem is, what to do with the HCl afterwards? I just dumped the ferric chloride PC board etchant back in the Radio Shack bottle after stripping. However, I have several pesky fire ant mounds I'd like to try HCL on!
Mike

#19 JohnH

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Posted 19 June 2013 - 07:21 PM

Problem is, what to do with the HCl afterwards? I just dumped the ferric chloride PC board etchant back in the Radio Shack bottle after stripping. However, I have several pesky fire ant mounds I'd like to try HCL on!
Mike


I think that would qualify as cruel and unusual punishment.

#20 Mirzam

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Posted 19 June 2013 - 07:24 PM

If HCl works then give Coca Cola a try. I'd like to know what happens.

JimC

#21 ZeroID

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Posted 19 June 2013 - 08:39 PM

A bit of caustic soda (NaHCl) will get rid of Alum coatings real quick. Drain cleaner....

#22 DAVIDG

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Posted 19 June 2013 - 08:59 PM

The problem with Sodium containing compounds are that they will attack the polishes surface on the glass because they form Sodium Silicate which is water soluble. Ferric Chloride does not react with the glass so it won't touch the optical polish but will quickly remove aluminum coating.

- Dave

#23 greenglass

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Posted 19 June 2013 - 09:11 PM

Can ferric chloride be made by mixing chlorine bleach with rouge?

#24 bremms

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Posted 19 June 2013 - 11:20 PM

If you think that is cruel punishment for a fire ant.. Must not have experience there.. One of the most vile creatures. They have very toxic stings. They are pervasive invaders that can kill wild animals, pets and livestock. They seem to delight in stinging. Squamish has no fire ants. SC is infested with these spawn of Satan.

#25 ccaissie

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Posted 23 June 2013 - 01:12 PM

Okay, you don't like the mirror, you don't care to fix it... apparently you don't or can't return it...So, why did you post this?


Just wanted to see if the great MKV would post a helpful reply to a newbie.






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