RE: ATM Spider Advice

["Bawden, Michael" <Michael.Bawden@spr.gases.boc.com>]

Hi,
	I think the problem with a curved spider is the supports to the tube
need to be longer. I read the S&T article a while ago and I think they use a
flat piece of metal. I'm going to use one in  my 10" and will make the end
supports about 3X the length of the center of the curve. Sorry I'm not
describing this very well.
	regards
	Mike
 


-----Original Message-----
From: Dennis Woos [mailto:wooscons@sover.net]
Sent: Tuesday, 13 August 2002 10:33 PM
To: atm@shore.net
Subject: ATM Spider Advice



Hi All,

My son Doug and I just returned from Stellafane, where we showed our
brand new 10" f5.6 flex-mirror dob.  The tube is .25" sonotube (doubled
.125").  The 1.83" secondary is mounted in a 3-point collimating holder
we made, with the mirror bonded to a 1.75" dowel.  We built a
curved-vane spider, based on Gary Seronik's recent S&T article.  The
major difference is that we bolted a 5/16" coupling nut to the curved
vane, and thread the secondary holder into it.  Well, as it currently
stands, the spider is a flop.  It vibrates a lot, and at 150x even
focusing causes the secondary to wiggle.  We got a lot of input at
Stellafane, but wanted to see what folks here think.

The easiest fix to try is shortening the distance from secondary mirror
to vane as much as possible (we can take off .25"), and adding a second
curved vane.  Another thing which seems to help is adding a
counterweight equal to the secondary assembly weight and sticking out
towards the front of the tube.  However, we have done more research
about spiders, and are starting to have second thoughts about the curved
vanes.  I don't know if we will ever be able to get it really secure, as
it cannot be tensioned the way a straight-vane design can.

Any input?  BTW, the flex-mirror is made from .875" plate glass
(Casarro), and works great!

Dennis



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