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Equipment Discussions >> ATM, Optics and DIY Forum

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ZeroID
sage


Reged: 04/21/10

Loc: Auckland, New Zealand
Re: Bad mirror new [Re: Mirzam]
      #5930118 - 06/19/13 09:39 PM

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

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DAVIDG
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
Re: Bad mirror new [Re: ZeroID]
      #5930152 - 06/19/13 09:59 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


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greenglass
scholastic sledgehammer
*****

Reged: 01/22/06

Loc: Hamilton. Ontario, Canada
Re: Bad mirror new [Re: DAVIDG]
      #5930174 - 06/19/13 10:11 PM

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

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bremms
Carpal Tunnel
*****

Reged: 08/31/12

Loc: SC
Re: Bad mirror new [Re: Mirzam]
      #5930406 - 06/20/13 12:20 AM

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.

Edited by bremms (06/20/13 12:30 AM)


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Bad mirror new [Re: MKV]
      #5936483 - 06/23/13 02:12 PM

Quote:


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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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Bad mirror new [Re: Dale Eason]
      #5936518 - 06/23/13 02:35 PM

Quote:


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




Yeah! mount it and view with it. You'll have a complete scope, and when you think you'd like to get a better view, refigure or replace the mirror. Might give fair images, and suck in a lot more light than your smaller scopes.

You can compare the Foucault readings with the star test impressions and have fun with that. Try a mask...all kinds of neat science projects here...

If you refigure, polish off the coating. Then take a brush and paint each lap facet with really hot pitch. That cures the contamination issue, and generally renews laps that have gotten too crusty, hard, or damaged. Or use pads at first. or shower curtain plastic (Neale Howard-Standard Handbook of Telescope Making),

Looks like slow polishing, and consistent temperatures will work wonders for that mirror. Looks like somebody was in a hurry...


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laserrain
member


Reged: 09/07/12

Re: Bad mirror new [Re: ccaissie]
      #5968828 - 07/13/13 10:59 AM Attachment (10 downloads)

Quote:

Quote:


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




Yeah! mount it and view with it. You'll have a complete scope, and when you think you'd like to get a better view, refigure or replace the mirror. Might give fair images, and suck in a lot more light than your smaller scopes.

You can compare the Foucault readings with the star test impressions and have fun with that. Try a mask...all kinds of neat science projects here...

If you refigure, polish off the coating. Then take a brush and paint each lap facet with really hot pitch. That cures the contamination issue, and generally renews laps that have gotten too crusty, hard, or damaged. Or use pads at first. or shower curtain plastic (Neale Howard-Standard Handbook of Telescope Making),

Looks like slow polishing, and consistent temperatures will work wonders for that mirror. Looks like somebody was in a hurry...





I believe you are right both...
Here's my old orange 25 cm ota, bad mirror inside (the 254 mm f/4.2 of unknown origins works much better in its new truss on a dob mount). Notice I added 4 wood spacers, after a hand-made cut (a miracle - the cut was a success, with no drift). The secondary mirror is a 70 mm m.a., the same of the dob.

To be honest and serious, my first Saturn at 80x was not so encouraging. Because I know how well my old telescopes work, I carefully adjusted the primary mirror. The secondary was dramatically ok.
Low powers show small stars, nice clusters, saw the Polaris' companion, splitted the two epsilon Lyr but...

...

After I mounted a shorter eyepiece, a slight defocussing highlighted the effects of such a darkened zone on the mirror surface, as shown in my first post picture.

There's a floating bright sector of a ring inside the defocus'd img that sprinkles so much light in the eyepiece field.
Intra and extra foc are very, very different (what's new ? This is not my home brewed 7" ).

Saturn is sadly flat. Cassini's div isn't as dark as I expect from my other scopes. It's really blurred. Short glimpses of good seeing don't help.

What I learnt (what everyone and possibly myself know): a lambda/3.5 ptv on the wavefr. mirror with many steepy and zoned slopes works much worse than a lambda/2 smooth mirror with a good hyperb of revolution (it's my old 25 cm f/4.2, which offers acceptable planetary views, in good seeing conds).


I'm afraid I've to refigure it.
Coarse grit needed for chamfering and edging, and elbow grease needed...I should fix some scratches also.

My lost holidays, chapter 2 to follow.

Sure, that old tm was in a hurry. Perhaps one day I will tell you why, under his living friends' agreement only...


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