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Old 30-07-2007, 01:45 PM
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12 " mirror cell

I was going to buy one originaly but found little around ,the UO cell looked ok but i thought I'd have problems trying to mount it to a square frame .
design wise I stayed away away from a sling type cell,and wanted to be able to remove the mirror and cell at once throught the top of the mirror box so didn't go with a fold down tailgate .

So this is what i came up with , six point at 60%
mirror glued or velcroed to cell.
(though i have left enough overhang for edge supports instead) .
the springs may be a little long i think but i'l wait until its bolted together
and have another look , at near full compresion the base of mirror box to bottom of mirror is about 40 mm i think .

I have had a little doubt creep in lately about how well this will work
what do you think ?

take care
graham
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  #2  
Old 30-07-2007, 01:54 PM
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janoskiss (Steve H)
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I think you need more degrees of freedom so the support points are able to move to conform to a rigid surface. For 6-point cell you'd need 3 pairs of pivoting supports.
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Old 30-07-2007, 02:05 PM
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rmcpb (Rob)
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I back Steve one th free pivot points. Three pooints always define a plane but six can have their ups and downs. If you make some pivot points off your collimation screws then they will always balance out and not stress your mirror.
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Old 30-07-2007, 03:58 PM
CoombellKid
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I go with what Rob say's above. Any reason you wanted to stay away from
the sling mission? it's only two bolts split down the middle with a hack saw,
4 nuts, a length car seat belt. That way you wont have to glue or velcro
the mirror. The two problems I see is Velcro will wear out over time and then
will need to be changed, as well as trying to locate the mirror in the right
possy and have the velcro keep it there. I guess you could use some sort
of locating method.

And Glue hmmm... not sure about that whether it will introduce other probems,
like if you glue on a warm or hot day and you use the scope in the cool night air.
Or every night being a different temperture but never the same as when you
glued the mirror.

I' not an expert yet, but to some degree these might become issues

regards,CS
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  #5  
Old 30-07-2007, 04:39 PM
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I can vouch for how well velcro works on my 14" for the last 3 years.
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Old 30-07-2007, 06:26 PM
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Quote:
Originally Posted by Satchmo View Post
I can vouch for how well velcro works on my 14" for the last 3 years.
Well I guess you would know having been using it for three years, do you
take it out much? off it's cell that is.

I'd be more nervous about it's tear/break free properties. IE; trying to
seperate the primary off pivotal points of the cell, you know that last
bit that grabs and then you give it a bit more and all a sudden your primary
either slips out of your hands or you reef it up into the baffles or
strengthening gussets of the mirror box.

just my inexperienced thought in using velcro for this type of job. I think
in my dob it'll be the sling method for me. Besides I could probably make
the sling setup cheaper than using velcro lol

regards,CS
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Old 31-07-2007, 09:11 AM
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The cell is just a plywood triangle with its sides tangent to a circle at the 60% zone of the mirror and hole cut for fan. Push /pull bolts on the three corners attach the triabgle to the tube. There is about 16 X 1" square pads. Thats a lot of holding power.I've never had any qualms about transporting the telescope with the mirror weight in sheer against the Velcro. It takes two or three minutes of sustained leverage with a piece of 3mm plywood to seperate the Velcro if I need to remove the mirror. Velcro has the advantage it cant warp the mirror like silastic. I'm not saying this is for everyone. It works for me in this size range.
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Old 31-07-2007, 02:11 PM
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Instead of glue or velcro what is wrong with small felt pads and three clips at 120 degrees around the mirror. A 12" mirror would not need a sling with this setup.
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Old 31-07-2007, 05:47 PM
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Yes , I agree that there are more than one way to skin a cat as they say.

We don't know how thick the mirror is , but for a passive triangle type flotation even 9 points are needed for a `full thickness' 12" mirror.

Single ring supports are a unique situation in that the bending of the mirror just causes a distortion that is part of a conic sesction and can be focussed out by a slight change of focus. Technically a 20" X 2" thick mirror will still work admirably on a single ring support ( with essentially infinite points in the ring).

A more complex variant of the single ring support used that is used professionally , is the multiple ring air bag support . The 2.3 metre Zerodur equivelent mirror of the ATT at Siding Spring, is only 6" thick ( 1: 16 ratio) and supported by two ring linked air bag supports whose relative pressures are controlled by gravity controlled pistons/valves to alter the ring pressure according to the pointing altitude of the telescope.
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Old 31-07-2007, 09:47 PM
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Good thoughts and ideas guy's, there is a lot to think about. I like the sling
idea because to me it is more supportive when the scope is pointed towards
the horizon. The velcro idea as Mark has explained it also has me thinking.
Maybe make the tailgate like a trap door then I could access it and do the
3mm plywood lever job, if and when I need to remove the mirror.

I'm in the design stage for my 16" dob. Just at the drawing up plans stage
along with reading Kriege and Berry's book A Practical Manual For Building....
I must say it's quite an uplifting experience, even at this stage. So it's good
I think to hear more approaches and ideas, for me anyways. I'll forgo the
airbag idea although I think it would be something to see in action, so my
air mattress is safe for now lol

regards,CS
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Old 31-07-2007, 10:30 PM
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thanks for all the input
A sling type cell just didn't seem the best option at the time rob

Sorry I should of made that clear I wasn't planning a full thickness
( thats 6:1 yeah ? ) in this cell .

Mark your mounting on a single point annulas?.. seems a lot more
(secured surface area ) than I had planned.. did I miss something somewheres ?
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Old 31-07-2007, 11:19 PM
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Graham,

Where did you get the attachment for your router to cut circles? for
the secondary cage. Or did you just make one up.

regards,CS
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Old 31-07-2007, 11:23 PM
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Quote:
Originally Posted by nightstalker View Post
Mark your mounting on a single point annulas?.. seems a lot more
(secured surface area ) than I had planned.. did I miss something somewheres ?
Well at approx 7" diameter 1" wide ring you have about 23
square inches. How about 12 1" square pads ?

BTW I'm not talking the cheapo Velcro in the prepacked pads , I'm talking the serious Velcro you buy off the roll , by the metre from the hardware store.
Your mirror being so thick might imstead be supported by 3 large pads of 4 square inches each at the 70% zone.

Another use for Velcro might be a velcro sling with some of the hook pads along the edge. it would certainly stop your mirror from wanting to slip out of the sling.
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Old 31-07-2007, 11:32 PM
CoombellKid
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Mark,

Exactly what I had planned or something to secure the sling when the scope
is pointing towards the zenith. I was thinking if the webbing wasn't rigid enough
I'd sick a couple of those el cheapo velcro pads in there lol.... well at least
they have a use for something lol

That is put a velcro circle on the bottom of the mirror (when the scope
is pointed at the horizon) to secure the sling.

regards,CS

Last edited by CoombellKid; 31-07-2007 at 11:38 PM. Reason: didn't explain
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Old 01-08-2007, 09:01 PM
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mark my maths took a left turn last night , and doubled the area you have secured your mirror on to..adheasive wise what did you use to fix the velcro to the ply and glass .. contact ?.

Rob your welcome to use my router .. I'm done with all the ply and aluminium I'm going to cut with it for now .. It comes with a circle attatchment
which will do a 500 mm OD ring if thats enough for your plans .
Let me know when you need it and I'll get it to you.
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Old 01-08-2007, 11:26 PM
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Graham,

Thank you for your kind offer mate, much appreciated. Actually I've been out
buying tools this week. I brought a Router yesterday along with a Drill Press
and a portable Bench Saw setup. I'm off to a buddies place of mine who has
an old sawmill on his property with alot of old wood stock to build a decent
work bench.

regards,CS
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Old 02-08-2007, 05:59 PM
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Quote:
Originally Posted by nightstalker View Post
mark my maths took a left turn last night , and doubled the area you have secured your mirror on to..adheasive wise what did you use to fix the velcro to the ply and glass .. contact ?.
.
Nightstalker,

The glue is just the normal adhesive thats allready on both sides of the Velcro. As I said you need to buy the good stuff off the bulk roll that actually works
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