[ATM] Flat grinding the back of the primary mirror

["Terin Lyr D'Amico" <terin.damico@verizon.net>]

Hello,

 

This will mark my first post to the ATM list since subscribing.

 

I have two - six inch mirror blanks I'm about to grind (f/10 or f/8 -
haven't decided), and wanted to get some opinions on flat grinding the back
before starting the grinding process.

 

I have read some sources on the subject, but they lacked detail.  It appears
that some "purists" would fine grind the back flat, and even grind the side,
with differing opinions on the merits.

 

Does it add value?

 

Or, is it something you would normally reserve for larger apertures?

 

If worth doing, does anyone have any recommended techniques for grinding the
backs flat?

 

I've read where you can just get a large (much larger than the primary)
piece of flat glass, and just grind back and forth with grit, rotating the
blank.

 

If someone were to grind the back flat, to what degree would they fine grind
the back?  To what grit size?

 

If anyone has a link to any articles on the subject, I'd appreciate it as
well.

 

My plan is to build two dobs with either wooden octoganal OTAs, or the
Evolution design, and thinking the longer focal length (f/10) may be better
for planetary viewing than a shorter (f/8) focal length.  One would be for
me, and the other for my brother.  We're both 6' tall, so a 60(+) inch long
OTA would not pose a challenge for us, although I also see merits in the
shorter OTA of an f/8.

 

If anyone wants to share opinions on design concept, please feel free to
chime in.  These are both "first scope" projects, and I thought I'd be
better off to start with six inch mirrors before taking on bigger
mirrors/projects.

 

Thanks,

 

Terin Lyr D'Amico

Camarillo, CA

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