Click here if you are having trouble logging into the forums
Privacy Policy |
Please read our Terms
of Service | Signup and
Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1170
Loc: Houston,Texas
|
|
Hi Steven,
Thanks so much for the very nice drawing 
This makes it so much clearer for all...
Once you get to the size that you are dealing with for the CF tube a 5 inches plus and the amount that aluminum expands ...I for one would be concerned with stressing the end of the CF tube with the cycled expansion/contraction of the aluminum to CF tube that could result in either movement of the collimation or failure of the CF tube by cracking...
In addition you do know that any contact between CF and aluminum must be isolated to prevent galvanic corrosion...it have a layer of epoxy on either the CF or aluminum.
We did an CF back plate that had SS spherical nut heads that were slotted and had a wire running through the slot in the head to prevent them from turning as one tightened the collimation nuts...for the back plate...
I think the use of a CF back plate with the fastners being SS and the hole being slightly larger than the diameter of the shaft of the collimation bolt shaft would work very nicely in your situation...
I can go back to our thread or perhaps do a simlpe drawing if this is not clear...
Otherwise, I think your good to go...
Best Regards,
Preston
--------------------
A few I enjoy,
and a few more in the works ;<)
|
Steven Aggas
sage
Reged: 04/15/08
Posts: 463
Loc: Arizona
|
|
Hi Preston, thanks. I may , however, need a picture that describes what you're talking about with regard to the SS Acorn(?) nuts and wire. Let's see the SS/wire first. In the previous picture the CF tube could be cut so that some number of 'tangs' fit as a sleeve over the Al plug with one holding screw per tang.
However, would the CF plate (at both ends) glued into the CF tube combined with a smaller, circular Al plate bolted to the circular CF plate be better since the CF plate would be in the CF tube and the Al plate, bolted to the CF plate, would give a substatial push-pull plate for alignment, and, since it's not in the CF tube it would not cause the stress? I've made another drawing of this.
I can see that the galvanic corrosion would occur if the carbon is exposed, we're working with CNT's as a transparent conductive film at work.
Thanks, Steven
--------------------
Mr. Wizard
Walking on a Dob: http://darkskyobserving.com/MtGraham.html
Elements in Harmony I, an 8"f6 German Equatorial - Stellafane Winner,
II a 20"f4.2 Newt-Dob - Astrofest Winner,
III a 6"f3.5 Finder/Newt-Dob, and
IV a 36"f4.5 Newt-Dob - "If it's up there, it's in here."
Apache-Sitgreaves Center for Astrophysics
www.DarkSkyObserving.com
|
Steven Aggas
sage
Reged: 04/15/08
Posts: 463
Loc: Arizona
|
|
What's not shown in the picture are the holes in the CF and Al materials and fans for cooling the glass, as well as, the thin-film heaters and all wiring....
Steven
--------------------
Mr. Wizard
Walking on a Dob: http://darkskyobserving.com/MtGraham.html
Elements in Harmony I, an 8"f6 German Equatorial - Stellafane Winner,
II a 20"f4.2 Newt-Dob - Astrofest Winner,
III a 6"f3.5 Finder/Newt-Dob, and
IV a 36"f4.5 Newt-Dob - "If it's up there, it's in here."
Apache-Sitgreaves Center for Astrophysics
www.DarkSkyObserving.com
|
Steven Aggas
sage
Reged: 04/15/08
Posts: 463
Loc: Arizona
|
|
One thing that concerns me about a CF tube with two CF plates, one elliptical, the other circular (with or without an Al plate for the alignment, or, SS acorns and wire) is the cantelever stress of having a ~6 pound mirror suspended out that far and its effect on the CF joints.
I had planned on wrap-around clips from the tube to the mirror's front as a safeguard measure. In either of the two revisions we've discussed, where the CF tube has two CF plates, another clip safe-guard structure would be needed should the joining epoxy fail at the circular CF plate. That led me to wonder....
If the tube were instead Al, and, it had an Al plug used for the push-pull alignment at one end, and, a CF plate as the elliptical plate at the other end using SS flathead mounting hardware, the Al structure is substantial from the threaded rod all the way to the tube and the mirror is adhered to CF for warp-free use.
It seems no solution is perfect....
Steven
--------------------
Mr. Wizard
Walking on a Dob: http://darkskyobserving.com/MtGraham.html
Elements in Harmony I, an 8"f6 German Equatorial - Stellafane Winner,
II a 20"f4.2 Newt-Dob - Astrofest Winner,
III a 6"f3.5 Finder/Newt-Dob, and
IV a 36"f4.5 Newt-Dob - "If it's up there, it's in here."
Apache-Sitgreaves Center for Astrophysics
www.DarkSkyObserving.com
|
PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1170
Loc: Houston,Texas
|
|
Hi Steve, I like this version much better than the following one with the aluminum tube and CF plate screwed to an aluminum plate.
As far as the JB Weld holding the CF together, after all of our testing and Mike's CF guru's blessing I really trust the bond if everything is cleaned well first and then one wears latex or poly gloves so as not to get oils, acids and salts from ones hands onto the mating surfaces to be bonded together.
We also did have the light baffle act as our safety net, however the bond of the 3M aquarium silicone to our test plate glass and CF plate really made a believer out of me....
We broke the 1 inch thick plate before the silicone would give and ended up with the area where the 3 blobs were fully adheared to the glass and CF plate!
The other nice thing with all of the CF is it tracks the ambient temp much better than the aluminum.
Where your fastners go through the CF plate to attach it to the aluminum ring...they make some little washers that look like a small tophat with the middle removed...
If you get some of these, they fit into the hole and the fastner goes through the center isolating the galvonic effect potential problem. MSC carries them.
The one thing that I am unclear of in the drawing is the threaded rod with a ball end. If the aluminum plate has a hole in the center, what does the ball end seat into 
Best Regards,
Preston
--------------------
A few I enjoy,
and a few more in the works ;<)
|
Steven Aggas
sage
Reged: 04/15/08
Posts: 463
Loc: Arizona
|
|
Hi Preston, thanks. While waiting for anyone to comment on the design variations I looked them over and actually chose the previous one to draw to scale. In all previous dwg's the pivot point was never shown accurately, but this one it is.
This Rev A of the CF tube and CF plates design shows all the clips, for the mirror and the CF plate, so should something fail the reflecting brick will remain suspended. The pivot point is also shown as I've built it in the current holder although I've used a acorn nut not a sphere (limits within Excel). A bolt passes through the acorn and threads into the rod. The acorn sits in a chamfered, enlarged hole for allowing the one plate to angle relative to the other. The four alignment screws thread through the rod plate and press against the mirror holder Al plate, thereby forcing the chamfer against the acorn.
Thanks for the tip on top-hat washers, I'll look for them.
Let me know what you think.
Steven
If someone else does it I don't know if they screwed it up, but if I do it myself, I know.... Red Green
--------------------
Mr. Wizard
Walking on a Dob: http://darkskyobserving.com/MtGraham.html
Elements in Harmony I, an 8"f6 German Equatorial - Stellafane Winner,
II a 20"f4.2 Newt-Dob - Astrofest Winner,
III a 6"f3.5 Finder/Newt-Dob, and
IV a 36"f4.5 Newt-Dob - "If it's up there, it's in here."
Apache-Sitgreaves Center for Astrophysics
www.DarkSkyObserving.com
|
PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1170
Loc: Houston,Texas
|
|
Hi Steve,
That looks great 
You can add lightening holes in the CF plate that attaches to the Mirror Back and the CF and Aluminum Plates to aid in air circulation for getting everything to ambient temperatures quicker.
For attaching the little retaining clips, you can bond some small 4-40 screws to the CF tube by drilling a hole and pushing the screw through from the ID leaving the threads exposed on the OD. Then put a blob of JB Weld over the screw head.
We made our safety clips from thin aluminum and then slotted the center. That allows you to adjust the clip up close to the face of the secondary. We bonded a small piece of felt to the edge of the aluminum to protect the mirror in the event of pending danger.
Please post pictures of the build progress for all to enjoy.
Best Regards,
Preston
--------------------
A few I enjoy,
and a few more in the works ;<)
|
|
15 registered and 8 anonymous users are browsing this forum.
Moderator: Don W, Mike I. Jones
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 625
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
[Re: 
Edit
Reply
Quote
