just received the CF focuser plate. I went with 3mm and is very stiff, strong and light as expected but I could've gotten away with 2 or 2.5mm.
it'll be held by sandwiching it between two pieces fixed to the frame with epoxy glue, unless I can drill a 3mm wide slot on the UTA so it slots in. Not sure if there are 3mm router bits available.
Last edited by alistairsam; 16-04-2012 at 01:12 PM.
yes I noticed the side sealant, but did you use spacers while it cured? else won't it tend to pull the mirror sideways?
I found this sealant that works well on glass and aluminium http://www.selleys.com.au/sealants/silicone/glass
specs mention it doesn't shrink while curing. But does mention its not suitable for roofs and gutters as it will cause corrosion. should be fine for Al.
Nup, no spacers, the velcro held it solid. I really didn't need the side supports I reckoned but was just playing safe.
If it will cause corrosion on gutters and roofs which are galvanised then it may not be great for alum either. Silastic cures by absorbing atmospheric moisture using acetic acid as a hygroscopic medium. Alum is not keen on Alkali's ( Try it with NaOH, Caustic Soda and see !! ) but it ain't great with acids either. Galvanising is zinc plate on steel and acids and alkalis attack zinc ( think the old batteries ) thus exposing the steel underneath to rust.
The sealant version cures differently, not sure how but my Aquarium fish appreciate the fact that it is a neutral ph when I fix their tank joins. Hence the sealant version.
I came up with a secondary holder that has its bolts inverted so adjustments are pull rather than push and this makes adjustments very simple and tool less.
(Brendan, I stuck to the 3 point adjustment rather than 4. )
There is no central bolt that holds the mirror holder, the central bolt on the upper holder block instead is the locking bolt. so tightening the bolt locks the assembly and it doesn't move.
I got tired of the old holders where if you loosen two push bolts, the whole secondary is free to rotate.
with this one, the mirror holder is tightly held to a plate which in turn is supported on the upper block.
I drilled a hole through the mirror holder and embedded a nylock nut in the holder, then tightened a bolt through the plate so it doesn't rotate freely.
The bolt holds the lower holder with sufficient compression such that the mirror no longer freely rotates, but if I do want to rotate it, I simply rotate it by hand and it holds.
I decided to go with a wire spider similar to Rolf's design after a few rudimentary tests.
I slotted the carbon fibre focuser plate between the two upper sections. I bought a 3mm Router bit off ebay, cut a straight slot with a 6mm depth using a simple jig and its a tight fit and doesn't move.
I managed to align upper and lower slots so the focuser plate is square to the upper assembly. the carbon fibre plate is very stiff and light, but once I cut out the focuser hole, I expect it will become a lot weaker. so I'll add two vertical supports along the sides to reinforce.
Primary 10" mirror is now siliconed to the 6 point mirror cell. I used Selleys Wet silicone which is neutral cure. I tested it with some offcuts and its very strong. I will add some side supports though.
I will be ordering a 3" orion optics focuser from TS in germany. until then will have to use a 2". once I get the new focuser, I'll just cut up a new carbon fibre plate for the bigger focuser.
Hope to string the secondary, assemble the whole OTA with temporary Al trusses to check measurements and do a star test this weekend.
Once that's completed, I'll dissassemble, and then sand, finish and coat with marine grade black stain to make it waterproof. Will then order the carbon fibre truss tubes and epoxy it to the tube ends.
The total weight so far as per my calculations with a QHY8, St80guidescope, (Till I get an OAG), qhy5, focuser, heavy duty dovetails, carbon fibre truss tubes, etc is around 8 to 9 Kg. One of my major objectives is to keep it under 10kg with everything.
Last edited by alistairsam; 27-04-2012 at 04:15 PM.
I don't think mirrors really float once they are glued down like that . You might as well mount the mirror on a single ring of silastic . I'd lose the big square bars and get the profile of the cell down and beef up the thickness of the backplate.
I don't think mirrors really float once they are glued down like that . You might as well mount the mirror on a single ring of silastic . I'd lose the big square bars and get the profile of the cell down and beef up the thickness of the backplate.
Thanks for the comments Mark. I guess I was a bit generous with the silicone.
Most of the 6 point ATM cells I found mentioned the silicone blob to be the size of a dime, mine is around a 20c piece.
what I don't understand with the commercial ones is how the mirrors are secured without pinching at the sides, this one for example, http://www.orionoptics.co.uk/OPTICS/...irrorcell.html
I'm guessing the side clips prevent the mirror falling off so there's no silicone at the floatation points.
I was trying to secure the mirror without side clips or having the primary support at the sides.
if I used smaller blobs, would the mirror be secure enough?
I went with the big square sections to get the contact area the size of a coin, this one's 25mm. I could use smaller ones and that would lower the profile.
But do you recommend lowering the profile to prevent lateral movement?
the mechanics of the 6 point actually prevent sideways movement and any movement would be more due to the elasticity of the silicone.
anyways, I will research more and redesign or might just get the one in the link above.
Thanks
That calculation is for a 3/4" mirror (i.e. plate), and scope pointing horizontal.
Hi James,
My mirror is 26mm thick at the edges and is the Suprax glass (3/4" is 19mm), not sure how different its density is from plate glass so if that calculation is correct, mine could suffer similar effects.
anyways, I've already redesigned the mirror cell and will go with a combination of smaller silicone blobs for the 6 points, around 10mm and edge supports as well.
Is there any way of visually verifying cell induced errors with star tests or is it not worth the hassle cause I will test the mirror visually the way it is before moving it to a different cell.
Thanks
Agree with Mark - that amount of adhesive will probably negate the advantage of 6 point mounting - your redesign should help.
The analysis that you found of the glued mirror includes a cubed scaling term, so you can divide the results by half to get something applicable to your system - and then ask yourself how many times you will be viewing at the horizon.
Suggest that you leave it as is and try it out - change it only if you can detect any real effects. I have a glued 12 inch GSO mirror in a 9 point cell and can just see the effects of adhesive mounting in out-of-focus star testing. However, there have been two occasions where the mirror crept sideways enough to run into side supports that were put there as a fail safe - then the resulting astigmatism was well and truly visible - even in focused images. Equatorial mounting will rule out most edge support systems and glueing will probably be better than anything else you can do. regards Ray
Suggest that you leave it as is and try it out - change it only if you can detect any real effects. I have a glued 12 inch GSO mirror in a 9 point cell and can just see the effects of adhesive mounting in out-of-focus star testing. However, there have been two occasions where the mirror crept sideways enough to run into side supports that were put there as a fail safe - then the resulting astigmatism was well and truly visible - even in focused images. Equatorial mounting will rule out most edge support systems and glueing will probably be better than anything else you can do. regards Ray
Thanks Ray
Had a few questions
How did you use glue in your nine point cell, just small blobs around the 9 supports?
When you say crept sideways, was that due to the silicone's elasticity?
Any harm in adding silicone at the sides?
What should I look for to see effects of adhesive mounting?
Could you explain the relation between eq mounting and edge supports and why it won't work?
Thanks
Mine sits on the 9 point support and then i have the silicone blobs holding the mirror so its fully supported, and locked in so the mirror doesn't slide around. I have no issues.
But do you recommend lowering the profile to prevent lateral movement?
the mechanics of the 6 point actually prevent sideways movement and any movement would be more due to the elasticity of the silicone.
anyways, I will research more and redesign or might just get the one in the link above.
Thanks
Yes , I thought the high profile puts more load on the rear of the cell which looks fairly thin. Experiments with silicon always have to be carefully monitored. If you are not getting nice round extra focal rings at high power you may be compromising resolution on your images.
Mine sits on the 9 point support and then i have the silicone blobs holding the mirror so its fully supported, and locked in so the mirror doesn't slide around. I have no issues.
Hi Brendan,
are your silicone blobs around the 9 support points or at the sides?
Yes , I thought the high profile puts more load on the rear of the cell which looks fairly thin. Experiments with silicon always have to be carefully monitored. If you are not getting nice round extra focal rings at high power you may be compromising resolution on your images.
Hi Mark
I initially started off with an 18mm backplate, but then changed to the 12mm as the load is widely distributed and not concentrated. Moreover, the mirror weighs only 2kg's so the backplate remains stiff even at 12mm.
with the redesigned cell however, I'll be using 18mm ply and the support points will only be around 5mm from the backplate against the current 30mm or so.
since my primary interest is DSO imaging, would it make much of a difference as opposed to high power planetary imaging?
mine are around the perimeter of the mirror, and just resting on the 9 point support
Brendan,
I presume this is with the 9 point Orion optics cell and you added the silicone in on the 3 bottom corners? is it primarily an edge support cell when shipped from the factory? http://www.orionoptics.co.uk/OPTICS/...irrorcell.html
Thanks Ray
Had a few questions
How did you use glue in your nine point cell, just small blobs around the 9 supports?
When you say crept sideways, was that due to the silicone's elasticity?
Any harm in adding silicone at the sides?
What should I look for to see effects of adhesive mounting?
Could you explain the relation between eq mounting and edge supports and why it won't work?
Thanks
hi again
i have used a variety of adhesives - the current brew is double sided adhesive tape, since the silicone de-adhered from the GSO supports (which turned out to be thermoplastic). While it was attached, the blobs were about 1cm across and about 3mm thick - it worked well from an aberration viewpoint. the sideways creep was due to an earlier version of double sided tape that softened in the heat - silicone should not have that trouble.
I haven't tried silicone on the mirror edge, but the astigmatism from a wrongly placed hard edge mount was pretty bad.
The optical effect that I interpret as being due to adhesive mounting is a vertical gradation of the appearance of an out of focus star image - the image remains nice and round, but the pattern looks a bit different top to bottom.
All of the edge mounting techniques that I have seen are aimed at Dob mounts, where gravity has a fixed orientation wrt the OTA. on an EQ mount, the direction of "down" varies with where the scope is looking, so it is not possible, for example, to put supports at the recommended plus and minus 45 degrees from the bottom of the mirror - as soon as you move to another part of the sky you have to rotate the OTA to get the supports back in the right orientation. And the last thing you want is a hard edge support in the wrong place.
regards ray
Yep thats the one I have, though me being me it isn't as from factory. When i recieved it the poor thing was in a bit of a state semi modified from somebody else it didn't fit the requirements.
I have three 3mm flat ally plates that are about 20mm wide that come up the sides much the same as what is there now. but they come upwithin a few mm of the actual mirror. there are blobs of silcone on those. So basically what i did was, set the mirror up so the 9 point support was sitting flat and everything was honky dory, then put the silicone in and waited for a few hours before i put it all back together.
My collimation holds after long trips with minor tweaking from the cats eye equipment and holds perfeclty in a eq setting.
Progress has been slow, but I've completed the string spider. I'm really surprised with how stable it is. Thanks to Rolf for his advise.
I did incorporate a 4mm offset, hope I got it right.
I avoid over tightening by testing the note made by each string!
I used two carbon fibre off cuts to terminate the strings. they're fixed with two bolts that run through the upper holder, that way they remain stiff. the CF provides stiffness with hardly any weight.
As rolf suggested, its possible to collimate the secondary mirror with the string adjustments alone, but I prefer to keep their tensions fixed and make minor adjustments with the 3 spring loaded central bolts.
Central brace turned out pretty strong as well. I will add two more bolts through the dovetail and wood that way both pieces are held firmly to the dovetail.
I will have to test the strength with the camera and mirror sections, but it should stay sufficiently stiff for the 6kg odd load.
I will add one more dovetail to the opposite end so it can fit in my fork mount as well.
That looks great Alistair! You probably found that the strings don't have to be particularly tight in order to hold the secondary stable, which is nice. I was surprised with how little tension they need. People often get the impression that it's pure tension that holds a wire spider in place, where in reality it's the geometry (at least when you make the geometry right!)
I see you used the same system of pivoting guitar machine heads as I made for mine. Have you adjusted it so the wires are squared yet or is this just a test? It can be a little tricky to adjust exactly but you only have to do it once
me being me it isn't as from factory. When i recieved it the poor thing was in a bit of a state semi modified from somebody else it didn't fit the requirements. 
