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mtb54703
sage
Reged: 11/12/08
Posts: 205
Loc: Eau Claire, WI
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I've been surfing for information on mirror cell construction and have collected quite a bit, but...
I currently own a 12" Lightbridge where the primary is glued to the mirror cell and additionally secured with clips and collimation is done with the screws on the back. So the mirror is pretty stuck in place. The collimation screws pass through heavy compression springs that provide the push/pull tension (standard sort of stuff) and it all works slick.
Mirror cells for larger primaries appear to float the mirror on the cell in combination with a sling type support. But I don't see any springs on the collimation screws, and the cells don't appear to use a pair of screws to provide the push/pull, so how does the collimation adjustment work?
And just exactly how loose is the mirror? It looks like could just flop out.
Maybe the answers are in the Kriege/Berry book...
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18" F/4.11 AstroSystems Telekit under construction
Meade 12" Lightbridge
Garrett 100mm F/5.3 45º Binocular Telescope
Vixen Foresta 7x50
Chippewa Valley Astronomical Society
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bbasiaga
professor emeritus
Reged: 05/10/06
Posts: 724
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Well the weight of the mirror provides counter tension on the bolts, and there is little to no vibration during operation of the scope so they are not likely to come move on their own.
The sling is tightened to keep the mirror in position above the pads, so it's not going to side out the bottom. There are also retainers that act like your clips, but in most kreige type desings they aren't actually touching the mirror. Clips that touch the mirror if improperly adjusted can pinch it, causing degredation of the image. The mirror on a kriege design can flop forward a fraction of an inch towards the retainers, but usually only very close to the horizon when viewing is not likely anyway.
-Brian
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Vixen 80EDSF
Stellarvue 102ED2
Orion Atlas 11 EQ-G
15" Astrosystems Telekit w/ Discovery Optics
Lust for something Larger
Lust for something Larger than that
Past Lovelies:
Oberwerk 20x100 binocs
Meade AR5
Meade LX10 8" SCT
All sold to a good home
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sixela
Postmaster
Reged: 12/23/04
Posts: 10873
Loc: Boechout, Belgium
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Quote:
Mirror cells for larger primaries appear to float the mirror on the cell in combination with a sling type support. But I don't see any springs on the collimation screws,
There are two possible arrangements:
-bolts going through the back and threaded in the cell, with springs to constrain the bolt head/knob to the back.
-bolts threaded in the back and connecting in the cell without threads (usually with the cell constrained to one location on the bolt by the use of two locking nuts). Springless.
The advantage of the first method is that the collimation bolts never stick out. With the second method, the collimation bolts stick out more or less (from the tailgate or the back) depending on where you are in the collimation range (which is often smaller). It's popular in tailgate designs that have the mirror box extend beyond where the bolts are.
There are variations on the first design that are springless (like a bracket on the side of the mirror box and two locking nuts holding a collimation rod and threads on the cell that make the cell ride on the rod closer to one end of the bracket or the other; one advantage is that the collimation rod can be long without any impact on collimation stability, because it's the short bracket close to the cell that constrains the rod. It's frequently used for collimation rods that go to the top of the mirror box).
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4968
Loc: Zuni Mtns, NM
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Dobs should never point down. So they can just float on the cell, a 1/2 the mirror sling goes on the bottom edge so gravity holds it in place.
With an EQ mounted newtonian, this won't work, the mirror has to be silcon adheasively adheared to the cell. All floating points must have some sort of tension on them the amount should equal the weight of the mirror adding up all points involved.
Timely post, I am modifiy'n an awesome double floating cell for a 30" primary from dob to eq right now. 
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sixela
Postmaster
Reged: 12/23/04
Posts: 10873
Loc: Boechout, Belgium
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Some illustrations.
http://members.ziggo.nl/jhm.vangastel/Astronomy/30cmscope/collimationboltdetails.htm
Top illustrations are for bolt arrangements that have threads in the cell and no threads in tailgate, bottom illustration has the other variant (thread in tailgate, no thread in cell).
Both designed to get the bolt head on top, and both constrain the unthreaded part with a locking nut on one side and a rubber plus locking bolt on the other (compressing the rubber more or less allows you to tune the resistance for turning the collimation bolts.)
Design with bracket, nuts constraining the collimation rod and a thread in the cell:
http://www.binodob.de/b_16z/jtg.jpg
(Note the attention to detail: where the threaded rod goes through a part in which it's not supposed to be threaded, there is a brass bushing...)
I think that last method is actually the bees knees. It's stiff (the bracket is attached to the mirror box side so both the load on the edge support and on the mirror cell points is carried with ease as it's all parallel to the mirror box side), light and minimal, and lets you get rid of lots of stuff (the system means you need neither tailgate nor bottom for the mirror box).
Unfortunately I can't use it on my scope easily because my mirror's too light: removing so much (tailgate, mirror box bottom) makes the mirror box too light unless I make it a lot taller and attach bearings higher or make them (too) large.
But if you have a larger mirror that has more mass relative to your typical UTA stuff, it's just perfect.
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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davidpitre
Carpal Tunnel
Reged: 05/10/05
Posts: 1825
Loc: Central Texas
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Quote:
Design with bracket, nuts constraining the collimation rod and a thread in the cell:
http://www.binodob.de/b_16z/jtg.jpg
(Note the attention to detail: where the threaded rod goes through a part in which it's not supposed to be threaded, there is a brass bushing...)
I think that last method is actually the bees knees. It's stiff (the bracket is attached to the mirror box side so both the load on the edge support and on the mirror cell points is carried with ease as it's all parallel to the mirror box side), light and minimal, and lets you get rid of lots of stuff (the system means you need neither tailgate nor bottom for the mirror box).
Unfortunately I can't use it on my scope easily because my mirror's too light: removing so much (tailgate, mirror box bottom) makes the mirror box too light unless I make it a lot taller and attach bearings higher or make them (too) large.
But if you have a larger mirror that has more mass relative to your typical UTA stuff, it's just perfect.
This is the basic idea I used on my 18" f4.2, and it came out beautifully. It pulls the collimation pivot points out as wide as possible which makes each turn of the collimation knob less sensitive. Precise collimation is easier. It also allowed me to put the collimation knobs on top of the mirror box where they are more accessible.
It is a stiffer system all around compared to the old school Kreige/Berry design.
It does take a little more thought though. You can't just build it stiffly and bolt it on. The connection between the cell and the box mount must have lateral adjustment or there will be undo tension on the collimation bolts.
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David
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