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Re: [ATM] questions about the condition of this mirror
Roman,
I did not realize that you had not fine ground the mirror. I would stop at
this point and go back to the fine grinding stage. I usually start with 220
Silicon Oxide, then 320, 500, 22.5 micron Aluminum Oxide, then 15 micron,
12, 9, 5 and finally three. I spend about 45 minutes to an hour at each grit
size and I clean up meticulously between grits to avoid contamination of
smaller grits with larger grits. (Based upon what Mr. Mulherin, a
professional optician who runs a telescope making company that just sold a
research quality 60 cm telescope to the government of Turkey, says, I may
be engaging in overkill. By the way, check out his website,
www.opticalmechanics.com )
Fine grinding serves two important purposes, It removes pits and scratches
from machining the balnk and larger grades of abrasives, and it serves to
smooth the figure of the mirror to something approaching a sphere, depending
on your skill.
Best regards,
Francis J. O'Reilly
----- Original Message -----
From: "jcmulherin" <jcmulherin@opticalmechanics.com>
To: "'Roman Toledo'" <rtoledo2002@yahoo.com>; <atm@atmlist.net>
Sent: Sunday, July 08, 2007 3:38 PM
Subject: Re: [ATM] questions about the condition of this mirror
> Hi Roman:
>
> If I understand correctly you started polishing the mirror without first
> fine grinding it? If this is correct, then you definitely have to do some
> fine grinding first. You can start with #600 silicon carbide or 18 to 20
> micron aluminum oxide.
>
> A blank that has been curve generated may look smooth enough to polish to
> the naked eye but it definitely has some tool marks if you inspect it with
> a
> magnifier.
>
> James
>
> James Mulherin
> President/Lead Optician
> Optical Mechanics, Inc.
> jcmulherin@opticalmechanics.com
> www.opticalmechanics.com
> Tel (319) 351-3960
> Fax (319) 351-3943
>
>
> -----Original Message-----
> From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
> Of
> Roman Toledo
> Sent: Sunday, July 08, 2007 2:09 AM
> To: atm@atmlist.net
> Subject: [ATM] questions about the condition of this mirror
>
> Mark it looks like it's on the surface, that old saying "if you don't
> use
> it you loose it" is at work here, I forgot some useful things I learned
> last
> year and since I put it all away for a while I forgot them.
>
> First mistake is I did not check the surface when I got it, mainly
> because
> this mirror looked so smooth, it never occurred to me to look for sleeks.
> I
> got
> up this morning and decided to polish out the back some more, and got it
> looking really good but not 100%, I probably ended up adding some more
> sleeks
> but i was very careful, for what ever reason I took apart a eyepiece t use
> a
> loupe glass and inspected it and sure enough the scratches specially the
> one
> that looks like a V or Y and bunch of others are on the top. the
> scratches
> look like the Grand Canyon so fine grindind looks to me like 320? or a lot
> of
> 600?
>
> So my dilema is I never made a fine grinding tool, I went straight to
> pitch
> (73 modified with some linseed oil )so I may have to do one.
>
> can I do a fine grind by using pads on top of pitch? and then discard
> it
> and make a new pitch lap or should I just do what I think I know must be
> done,
> make a new tool, and should I buy smooth porcelain tiles ?
>
> Mark Holm thank you.
>
> anyone please feel free to comment
>
> PS: Greetings go out to Richard Schwartz and David Harbour have not seen
> much
> in here from you gents lately
>
>
> Regards
> Roman
>
>
>
> regards,
> Roman Toledo
> Celestron Nexstar GPS 9.25
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