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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1168
Loc: Tampa, FL
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I'm putting some finishing touches on plans to build an 8" F5 newtonian astrograph. The mirror cell is being built from scratch and I am trying to determine how much spacing I should have between the mirror plate and the lower mounting plate (that attaches to the inner tube walls). My bolts will be standard 3/8" fine thread.
I would like to have as little space between the plates as possible so as to keep lateral stability higher. There will be no provision for a central support post because my primary is a Royce conical. The three collimation bolts and springs will need to be strong enough such that lateral movement is at an absolute minimum, preferably nonexistent. I've seen several designs just like this that work very well, I'm just not sure of the exact spacing between the two plates. It seems like a spacing of 0.5" or 1.0" should be enough to allow adequate spring compression for adjusting while still being very solid laterally. Basically I'm trying to avoid having several inches worth of space between the plates and the associated risk of sagging.
Thanks for your help.
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Curt
8" F5 Newtonian Astrograph (under construction)
Orion 80ED CUSTOM
80mm F5 guidescope (under construction)
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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RossSackett
sage
Reged: 08/17/07
Posts: 322
Loc: Memphis, TN
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You might consider a push-pull arrangement, pairing each collimation bolt/spring working in tension with an adjacent locking bolt working in compression. This would allow the plates to be rather close together, since all the springs really need to do is to keep them apart while collimating (the rigidity will come from the locking screws). This would let you use short, relatively weak springs. The push-pull arrangement also cuts down on vibration.
Ross
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Ross Sackett
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11 scopes currently on the flight line from 4.25 to 18"; 5 pairs of astronomical binocs 35-80mm. My wife suggests that with just one pair of eyes, this might be excessive.
See my scope pix at http://www.flickr.com/photos/8315630@N04/
Carpe noctem!
Amateur astronomer = A mature moon-starer
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1168
Loc: Tampa, FL
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Thanks Ross. Personally I'm not a huge fan of the push/pull design. My mirror cell is going to be made of 0.25" aluminum with threaded holes for the collimation bolts on the top plate. Three bolts (or in my case probably the kind with thumb knobs on one end) should be adequate with spring tension around each one.
I think I can get away with between 0.5" and 1.0" of spacing. Can anyone confirm?
Or...
Perhaps a more useful question for me to ask - if I wanted to shoot for a spacing in this range, how strong do the springs need to be?
--------------------
Curt
8" F5 Newtonian Astrograph (under construction)
Orion 80ED CUSTOM
80mm F5 guidescope (under construction)
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
Edited by imhotep (08/13/08 01:41 PM)
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1168
Loc: Tampa, FL
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Bump
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Curt
8" F5 Newtonian Astrograph (under construction)
Orion 80ED CUSTOM
80mm F5 guidescope (under construction)
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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RossSackett
sage
Reged: 08/17/07
Posts: 322
Loc: Memphis, TN
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Curt,
Since the only variable here seems to be bolt length, the most direct way to find out is to build it and try different lengths--all you have to lose are a few inches of threaded steel and a few compression springs.
Let us know what works best.
Ross
p.s. I can see those push-pull bolts on your thumbnail, so clearly your aversion comes from experience.  I don't particularly love them myself, but would consider them for an astrograph where stability matters...maybe...
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Ross Sackett
---------------------
11 scopes currently on the flight line from 4.25 to 18"; 5 pairs of astronomical binocs 35-80mm. My wife suggests that with just one pair of eyes, this might be excessive.
See my scope pix at http://www.flickr.com/photos/8315630@N04/
Carpe noctem!
Amateur astronomer = A mature moon-starer
Edited by RossSackett (08/14/08 08:36 AM)
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1937
Loc: Boechout, Belgium
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You could consider belleville springs.
You add as needed.
Embedding springs will make the spring stronger.
Adding spring on top of each other will sepparate the two parts from each other if needed.
These springs need less space compared to regular springs.
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Chris
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RossSackett
sage
Reged: 08/17/07
Posts: 322
Loc: Memphis, TN
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Quote:
You could consider belleville springs.
You add as needed.
Embedding springs will make the spring stronger.
Adding spring on top of each other will sepparate the two parts from each other if needed.
These springs need less space compared to regular springs.
...and they don't vibrate the way helical springs can.
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Ross Sackett
---------------------
11 scopes currently on the flight line from 4.25 to 18"; 5 pairs of astronomical binocs 35-80mm. My wife suggests that with just one pair of eyes, this might be excessive.
See my scope pix at http://www.flickr.com/photos/8315630@N04/
Carpe noctem!
Amateur astronomer = A mature moon-starer
|
imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1168
Loc: Tampa, FL
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Ross, that's my old SN8 in the thumbnail. It could very well be that I'm misunderstanding the terminology so please set me straight if I'm all mixed up. The mirror cell on the SN8 did in fact have six bolts, but only three of them were used for actually adjusting the position of the mirror. The three larger knobs are the collimation bolts which were spring loaded and could move the mirror up or down. The other three bolts were called "locking" bolts or something to that effect in the user manaul. Once collimation is dialed in you can apply a light amount of pressure to the back of the mirror plate with all three locking bolts to help prevent slippage.
My plan was to have a similar approach for the design of my astrograph's mirror cell. Make sense?
--------------------
Curt
8" F5 Newtonian Astrograph (under construction)
Orion 80ED CUSTOM
80mm F5 guidescope (under construction)
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1168
Loc: Tampa, FL
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Chris, I think I've seen you mention Bellville springs before. McMaster Carr sells them and I'm definitely keeping that on my list as an option. I guess I could just order a ton of them and stack/nest as necessary.
--------------------
Curt
8" F5 Newtonian Astrograph (under construction)
Orion 80ED CUSTOM
80mm F5 guidescope (under construction)
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
|
RossSackett
sage
Reged: 08/17/07
Posts: 322
Loc: Memphis, TN
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Curt,
Yep, those are push-pull bolts. You back off the locking bolts, then adjust the spring-loaded collimation bolts, then snug up the locking bolts again.
Some people just use spring loaded collimation bolts, but those aren't as stable.
Belleville washers are great--they can pack a lot of spring in a little gap, and depending on how you arrange them you can vary the stiffness. See
http://en.wikipedia.org/wiki/Belleville_washer
Quite cheap for something so useful.
Ross
--------------------
Ross Sackett
---------------------
11 scopes currently on the flight line from 4.25 to 18"; 5 pairs of astronomical binocs 35-80mm. My wife suggests that with just one pair of eyes, this might be excessive.
See my scope pix at http://www.flickr.com/photos/8315630@N04/
Carpe noctem!
Amateur astronomer = A mature moon-starer
|
|
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