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
Steven Aggas
sage
Reged: 04/15/08
Posts: 463
Loc: Arizona
|
|
Ok, there's alot of secondary holders making use of RTV to adhere the glass secondary to a stationary position on the holder.... Some holders are wood, others PVC, and still others are metal (mostly likely aluminum).
Who's done stress analysis, with regard to the affects on the frontside accuracy, of the expansion/contraction of the various holder materials that contact the glass secondary which is constrained by the RTV?
Secondly, is the holder typically pressed into a bead of RTV, or, is it more typical to lay a series of beads on the ID and OD of the holder against the glass? (In the first scenario an RTV film resides between the holder and glass whereas in the second the holder material is in direct contact with the glass.)
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: 1166
Loc: Houston,Texas
|
|
Hi Steve, I did FEA analysis when we were working out the mounting of the 20"RC's secondary and determined that for our needs, the backing for the secondary mirror which was Sital needed to be carbon fiber.
In addition we determined that 3 disc of 3M silicon aquarium adhesive .6-.7 inches in diameter placed at 120 degree intervals at the 72% of radius offered the least distortion. The pads of silicon were 0.055" thick.
The secondary mirror was 7 inches in diameter and 1.1 inches thick.
The next best thing was Stainless steel and the worst was aluminum 
I do not however have the calculations with me here in Mexico and will not be returning to Houston for likely about a year.
Hope that helps a bit.
Best Regards,
Preston
--------------------
A few I enjoy,
and a few more in the works ;<)
|
mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2103
Loc: salem, OR
|
|
For secondaries you definitely want to use three pads of RTV to hold them, I've heard of cases using a solid bead that produced warping of the secondary. Under no case would you want to use either a complete layer or a series of beads, and it's not called for by the support requirements. Clean both parts with acetone before gluing up and use fresh aquarium RTV to do the bonding, wait 24 hrs before applying any stress.
I've used PVC pipe for small secondaries (1.5") but my current version calls for a stainless interface to an aluminum structure (see wire spider).
Best,
Mark
|
Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3086
Loc: Northeast
|
|
With secondaries up to 2.6", I pretty much have followed Bryan Greer's suggested method, with a few beads about 1/8-1/10th inch thick; RTV on either aluminum, or spun plywood plugs. Used his included spacers.
For primaries, either particle board (repair/test) or plywood/aluminum backer plates with new Dob construction, RTV @ 1/4" thick, a minimum of 6 blobs at the 70-80% radius, and maybe 3 blobs inside of that, about quarter/half-dollar size. I used 1/4" nuts for spacers, a minimum of 24 hours before removing. Up to 14" size, absolutely no issues at all. Any backers of ply or PB/MDF I made a good effort of assuring stability by checking flatness, and sealing adequately.
Unless I did the analysis Preston did, I would -NOT- go as thin as he did. I have never gone that thin personally, but had to fix some who had. Thinner is better as far as collimation is concerned, but it can be a headache with distortion of the optics. Silicone in the thicknesses I outline seem to be adequate to prevent distortion from ocurring.
Mark
--------------------
So many projects, so little time!
|
PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1166
Loc: Houston,Texas
|
|
Hi Mark,
In using either aluminum or wood you are correct, the thicker thickness of the silicone allows better for the expansion/contaction to the base substrate.
In our case with the Zero expansion Sital and the Zero expansion Carbon fiber mounting base, using anything thicker than we did caused a noticable collimation shift from the vertical to 30 degrees above the horizon...due to the added flexture of the silicone.
The secondary mirror weighted about 5-7 pounds if memory serves correctly and this also caused a lateral shift of the secondary of about 0.001+ inches in thicknesses greater than 0.075" when lowered from vertical to 30 degrees.
All of these calculations were confirmed by Algor FEA software with thier software engineers doing the final confirmation...it was just a bit beyond my level of trusting that I fully understood all of the criteria...
I was however able to get very close in determining that aluminum would have cause about 1/4 wave of an error and the thin silicone with the CF back plate was just about perfect.
Mike Jones defined the error envelope for movement of the secondary and then we became very concerned to get it right the first time and not be chasing after trying to find the errors when using the CCD, for which was the scopes total purpose.
Best Regards,
Preston
--------------------
A few I enjoy,
and a few more in the works ;<)
|
mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2103
Loc: salem, OR
|
|
Preston,
A secondary that size (7x1.1") it seems would need lateral support, or, as you used, a something like Sitall and zero expansion backer with near-rigid RTV pads. Did you happen to model it for thinner RTV pads as well? Just curious.
Best,
Mark
|
PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1166
Loc: Houston,Texas
|
|
Hi Mark,
Modeled with the 3 pads at 120 degrees at the 70ish percent of radius at the thickness of 0.055" constrained the secondary from a vertical orientation of the whole scope to a horizontal position to a movement of less than 0.00015 inches of laterial movement.
That with no movement of the secondary or backing plate due to thermal expansion and at that point we did not look at anything thinner.
Getting the 3 points to be the same size in diameter and the same thickness proved to be the biggest challange and it took many trials to develope a method to do just that.
Regards,
Preston
--------------------
A few I enjoy,
and a few more in the works ;<)
|
Steven Aggas
sage
Reged: 04/15/08
Posts: 463
Loc: Arizona
|
|
Ok, for those holders which use a piece of tube cut off to 45 degrees, is there a gap between the holder and glass in which the glob of RTV resides (holder not in direct contact)? Given what read above the answer is 'yes' and the thinner the better given the sag of the suspended glass and its effect on the non-rigid RTV, but, thin must only be used on objects that don't expand and contract large amounts. Is this because the RTV will lose adhesion from the repeated expansion/contraction of very dissimilar materials, less so with Carbon Fiber and Glass v. Aluminum and Glass?
So, if a 5" diameter Carbon Fiber tube were used (0.156" wall thickness) and cut to 45 degrees, and, a CF plate (0.100" thickness) adhered to the cut-off, then would many globs, uniformly spaced, on the plate give little to no deformation of the wavefront, relative to an aluminum plate, or what would the dot formation look like on the 5"ma plate to hold a 6"ma, 1.5" thick elliptical mirror? Or would the 5"me plate not be needed and RTV applied only on the 0.1" wall thickness of the 5" ma cut-off tube be enough?
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
|
Charl
member
Reged: 02/08/07
Posts: 81
Loc: Hants, UK
|
|
Here is a useful table for finding a suitable material to adhere glass to. From this table the most practical match for pyrex is wood. Since these two materials have almost the same expansion and contraction properties, the adhesive layer may be kept thin. Silicone adhesive does not adhere well to wood, but this can be overcome by drilling a hole for each pad and allowing the adhesive to fully penetrate the hole. The holes may also be tapped with a thread or stepped beforehand for additional strength.
It goes without saying that the wood must be sealed against moisture.
|
PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1166
Loc: Houston,Texas
|
|
Hi Steve,
Without actually running the FEA but having looked at very similar criteria for the 20inch RC, I would recommend doing just about what we ended up doing...
Thus using the CF plate on the tube definately!!! Otherwise you are only adhering to the edge of the tube.
Then pick the raduis of the elipse at about the 70% region and use a blob at the thickness of about 0.70 inch diameter...every 120 degrees...three abound the parameter should to the trick...
Start the pattern at the 6 oclock position and have the upper 2 at 120 degrees from that...
Try to stay at under 0.0625 inch thickness and practice on something back plate and plate glass to develope a method of getting the diameter of the silicon about the correct diameter...
If you have further questions, just ask...
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
|
|
Thanks Preston. What is the best material to adhere two CF pieces together? I've seen references to thick cyano-acrylate. However, I’m still stuck on a couple points detailed below.
Charl, I'm not convinced that wood would be the best, or CF even though FEA has been done on a configuration using that and it seems to be proven.
Since we are creating a sandwich; glass/rtv/holder, the glass/rtv boundary is of concern for deformation of the glass, and maybe that interface should require the adhesive to match glass’s coefficients (or ceramic if that's what actually used), I say that because when forming a sandwich mirror they don’t use dissimilar materials for the front or back plates and ribs, it’s all the same material. So a coefficient mismatch should be avoided. If it’s avoided, then the adhesive layer can be sufficiently thin. It seems thin is the goal as a thick film can allow the mirror to cantilever out of position as the scope is moved in altitude horizon to zenith, but thin films if mismatched can have great stresses and I believe be a mechanism to cause adhesion failure.
However, it seems we’re forced to use something other than glass-matched materials as the adhesive in this application because secondary mirrors don’t come with anything but a flat back. (To look at the problem differently, it’d be nice to have a secondary with a few glass posts shaped like bolt heads that we might hold them captive in some manner that the front surface isn’t affected by expansion/contraction of the captive method used. These posts could be at the ~70% radius, or something else if another pattern is defined as better. Then the holder just clips onto the mirror back. If I worked with glass I’d try this. This may be a bad idea, or not, let me know.)
But, the other boundary, the rtv/holder interface, should have materials matched to rtv, similar to the sandwich mirror requiring similar materials, so the two move together when contracting/expanding I would think. It would seem any holder material drastically different from RTV could run the risk of adhesion failure since once again a thin layer is the goal.
So, what rtv has the expansion coefficient of glass? If there is one, then wood as a holder material may be a good combination, because then all three match. (Though I really like my newly minted “Secondary with glass mounting posts” idea)
Let me know where I’ve gone astray.
Steven <<<goes astray quite often (sometimes it's the journey, not the destination necessarily, that's of interest)
--------------------
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: 1166
Loc: Houston,Texas
|
|
Hi Steve,
For bonding CF pieces together we found and Mike Jones concured that JB Weld worked very well and is available to us amatuers 
A thin bead on the edge of the tube and then carefully place it the position on the flat plate that you require.
After it has set up, lightly sand the exposed JB Weld and then using a tiny spatula, run a filet of JB Weld around the tube to the plate for added strength.
As far as the CTE goes, if your mirror is pyrex your are at about 2.0 x 10 -6 in/in F
Aluminum is 12.3
304 SS 9.6
410 SS 5.5
plate glass 5.0
porcelian 2.5
Wood Fir 2.1
Silicon 1.7
Invar 0.8
And then CF can be designed anywhere between Negative 0.3-0.5 to around 2.0, but it is not easy for us amatuers.
As to an RTV with the same CTE of Pyrex, no way...they are closer to 20+ typically.
Thus, if it were me I'd be looking at something with the smallest differance for the temperature that I would be using the instrument. ie, you are not likely to be using it at 90 deg F. and more likely to be using it at 40-70 deg F.
Thus, what ever you chose look to do you bonding at the middle of the temperature range of your use without getting into dewing problems during the cure of the silicone to the substrates.
I'd personally go with CF at 90 +45 90 -45 ply with about 8 to 10 plys ending up with the top and botton in the same orientation.
Clean both CF plate and mirror back with acetone and then alcohol, and bond them together.
Any further questions, just ask.
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
|
|
Thanks Preston. With RTV 20+ then it's really mismatched to either glass or practically any material used for a holder, though aluminum comes the closest, and you said it gave the worst results. It's unclear to me how a good result can be had with Carbon Fiber (you) and Stainless Steel (Mark) when they are so different in CTE to each other as a holder, though they both have similar delta CTE RTV to glass (unless the aspect ratio of the two mirrors is significantly different one (you) to the other (Mark), thickness to minor axis).
So maybe the holder material makes not so much apparent difference when there is sufficient aspect ratio of the glass?
Additionally, just piecing the FEA data you mentioned....
Material Stack; CTE's; FEA Commented Result
CF-RTV-Pyrex; 0.3-20-2; Best
SS-RTV-Pyrex; 9-20-2; Next Best
Al-RTV-Pyrex; 12-20-2; Worst
Then, how does the mirror'ed, symetrical mismatch on both sides of RTV, seen above, give better FEA results than to an asymetrical mismatch? Two high-stressed junctions in a mirror'ed system perform better? Do stresses like that cancel, one being compressive and the other tensile? To insert the wood holder sandwich, 2 to 5-20-2, into the list above, it should give better results than SS..., but not as good as CF. (2 to 5 for wood depending on the wood chosen and results have been emirically found to be good)
I know, questions - I got more where those came from....
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
|
mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2103
Loc: salem, OR
|
|
As the secondary gets bigger the support requirements get stiffer (pun). What works fine for a 2.6" quartz secondary isn't necessarily going to work at 8" without good design, similar to supporting an 8" full-thick mirror vs a 20" thin mirror. Recall here (I hope!) that it's not just aspect ratio at work - the effectivness of any multi-point cell goes down roughly as the square of the size for the same aspect ratio - a 200x20 f/4 receives 4x better RMS support than a 400x40 f/4 for the same cell design...
I don't think the CTE of the adhesive makes much difference for flexible bonds like RTV, these are small areas holding to larger pieces. What you want to minimize is the thermal differential stress generated over the much larger area of the secondary and the backing plate, as those will load the secondary through the bond pads. A thicker bond pad takes up more of that stress without loading the secondary, but as the secondary gets larger the bond pads have to not allow much shear in altitude changes, so they have to become proportionally thinner, as Preston explains.
The bottom line is the same as the first sentence: for larger (more massive) secondaries the support has to be better, and CF does the trick for low-expansion substrates. So my 2.6" spider with a stainless "flex" plate is fine, but it will only scale up so far before something more exotic is called for. In the case of the wire spider design, it could indeed be a CF plate mounted to a central pivot.
Best,
Mark
|
Steven Aggas
sage
Reged: 04/15/08
Posts: 463
Loc: Arizona
|
|
Well, what I've found available:
CF Tube; OD: 5.30", ID: 5.00", Wall Thickness: 0.156"
CF Plate; 0.197" Plate Thickness (to be cut to 5.30"ma elliptical shape)
So, for a 6"ma Pyrex secondary, 1.5" thick, three dots of silicone ~0.6 to 0.7" diameter, placed at ~72% of radius, which equates to 4.32" circle, with the lowest dot at 6 o'clock and the other two dots above at 120 degrees with a target thickness of ~0.055" per dot.
Will that plate, at 0.197", be thick enough? The three pads will be nearly to its edge where the tube provides support so I think so. I'll also be putting a central hole pattern in th plate to pass air through the enclosed tube structure and past the secondary backside for cooling.
And, four non-contact mirror clips bolted to the tube and wrapping around the front surface, for peace of mind.
The CF tube at the other end will have a 5"OD 1/2"T aluminum plate plug for the collimation screws to push-pull against for alignment, with the second plate having the threaded rod to attach to the spider.
Any thing here look to be a problem? Elliptically cut tube with elliptical plate and aluminum plug at each end, that construction is a little different than an RC scope holder.... Any suggestions?
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
|
mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2103
Loc: salem, OR
|
|
Steven,
It sounds good to me, since the plate will be joined to a tube at the edge there should be no issues, but I'll be interested in Preston's comments.
Best,
Mark
|
Steven Aggas
sage
Reged: 04/15/08
Posts: 463
Loc: Arizona
|
|
Would, instead of a CF plate, a scrap 5"ma pyrex elliptical blank be better as the surface to adhere with RTV the 6"ma pyrex mirror (using the same dot pattern, thickness, etc)?
The scrap would have at least 1/2" thickness to cement into the slip-fit 5"ID CF tube, and it would have the 1/2" aluminum push-pull alignment plate inserted into its other end.
Thoughts?
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
|
Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2359
Loc: Courtice, Ontario
|
|
Quote:
For secondaries you definitely want to use three pads of RTV to hold them, I've heard of cases using a solid bead that produced warping of the secondary.
That's the truth! I did the complete bead the first time I mounted a secondary and astig was so bad in focus I was looking a two stars! 3 blobs of silicone fixed it up nicely. That it how I mount my secondaries now and haven't seen any stig since.
--------------------
Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
|
PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1166
Loc: Houston,Texas
|
|
Hi Steve, Everything sounds good until you get to the end talking of an aluminum plug in both ends of the tube...
You are not talking of putting aluminum plugs inside of the 5 inch ID CF tube are you 
Do you have any way to draw what you are describing so that we are all clear on exactly what you are asking???
If not, I'll look at your description over the weekend and make a simple drawing to post to try and clarify exactly where you are currently.
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, the circular end would have the Al plug as part of the alignment plates section. The 45 degree end would have CF plate. Would a 5"ma blank be better than CF plate?
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
|
|
10 registered and 5 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: 602
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
