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RE: [ATM] 4 point mirror cell
Hello Rod,
I just finished a rebuilt of my mirrorcell for a similar mirror (12,5 inch,
2 inch thick pyrex f 4,8.)
Several people on the ATM-list have calculated this mirror in PLOP and they
all found that a 3-point cell is sufficient !! Though there is need for
very small refocus.
The three points are placed on 0,41*R (65mm) from the centre of the mirror.
Lately I finished this 3 point cell and the first results look very
promising. Advantage of a 3-point cell is that it is easy to built and
therefore very rigid and not too sensitive for building-tolerances.
In my design I used one fixed point and two points which are adjustable. For
the adjustable points I used two guitar tuning-mechanisms, which allow to do
collimation from the side of the mirrorbox (instead of underneath). Another
advantage is that the collimation can be done very precise.
I will place pictures of the mirrorcell on the internet soon.
Jan van Gastel is using a 4-point-cell for thinner mirrors.
This design is also easy to make.
Good luck
Harro Treur
-----Oorspronkelijk bericht-----
Van: Rod Brackenridge [mailto:rjbr@lycos.com]
Verzonden: maandag 14 februari 2005 10:15
Aan: atm@atmlist.net
Onderwerp: [ATM] 4 point mirror cell
Hi,
I am in the process of converting my 12 inch F5 Dob to Mel's scope drive
system. I want to create space inside the rocker box for the azimuth drive
(similar to James Lerch's design for his 8 inch binoculars).
To help create this space I would like to re-build the mirror cell. It
currently has a 9 point flotation cell which works OK as is but the cell
sits rather high. I thought if I switched to a 3 or 4 point cell sitting on
metal crossbars (Krieg and Berry style) I could reduce its height and save
some weight (by shortening the mirror box). My mirror is 12 inches in
diameter and 2 inches thick (pyrex). The diagonal is 3 inches in diameter
(too big really, one day I will buy a smaller one)
I have some questions:
Am I right in thinking a 4 point cell is adequate?
Mark Holm's site describes a 4 point cell for a plate glass mirror 2.125
inch thick. I am wondering whether the cell specifications given for it
would be OK for mine: the radius upon which the pivot bar sits is 0.357 and
the radius on which the support points lie is 0.505. Would that also be OK
for a 12 inch pyrex mirror?
I would like to RTV the mirror to the cell if possible. Would four blobs of
silicone support the weight of this mirror?
Thanks,
Rod Brackenridge
Mount Martha
AUSTRALIA
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