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jasonharris
sage
Reged: 09/16/06
Posts: 248
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Hi all, I was going to put my secondary in place when I realised I might not be doing it quite right..
My secondary is round 6" for my RC. I had decided to use a neutral curing silicone as I had read the acidic cure types might give me a problem, worse case do something bad to the coating.
Now, I was going to put three felt pads on the secondary holder and then puts some dabs of silicone around and push the mirror in flat against the felt pads and allow it all to dry. I was going to leave the felt pads in like a 3 point cell.
I have since read that this may not be the way to do it. my pads should come out so that that silicone is the only points of contact. Is this correct?
Also, if this is correct, is it ok to have the gap between the mirror and the cell at around .15" or should it be more like half of that?
Thanks,
Jason
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sixela
Postmaster
Reged: 12/23/04
Posts: 10855
Loc: Boechout, Belgium
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Yes, you want three support points in total. Anything more and you risk straining the mirror (though felt is usually rather benign given sceondaries are usually THICK for their size).
I tend to leave 1mm, 2mm tops. 0.15" looks a bit on the wide side, although it depends on the silicone. On some softer types, it certainly might give you some collimation instability.
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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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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1170
Loc: Houston,Texas
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Hi Jason,
Three round spots about .75 - 1.00" in diameter at about the 70% radius that are between 0.050 and 0.625" thick is just about perfect.
Try to get the 3 spots as close to the same size as possible...
You should practice on some clear window glass first to get the technique down...you will be glad that you did, trust me 
Best Regards,
Preston
Ps...clean the mirror and base substrate with rubbing alcohol several times before doing the final bonding, and mark the secondary's location for the silicone spots with a majic marker first.
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A few I enjoy,
and a few more in the works ;<)
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jasonharris
sage
Reged: 09/16/06
Posts: 248
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Hi Preston, I am a little confused, the three round spots you are referring to would be the silicone right? do you really mean .625" thick? Thats hanging the mirror a fair bit out?
So I wouldn't be using any permanent felt pads etc, just as spacers perhaps?
Thank you as well Sixela, you seem to be suggesting silicone only, no felt. That sounds right. I will look at some washers to be the spacers, they should only lift the mirror up 1.5mm.
Thanks,
Jason
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MitchAlsup
super member
Reged: 08/31/09
Posts: 184
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When I did my 3.7" secondary (0.5" thick), I used 3 blobs of silicone ~0.5" in diameter as it comes out of the tube. Then i used three nails (1.5mm-2mm thick) and positioned the secondary base holder onto those three blobs. The blobs flattened out close to 0.7" in diameter and 1.5-2mm thick.
I happened to use the 50% radius as an accomodation between POLP which indicates a 40% radius is ideal for the primary supported by 3 points, and the conventional wisdon of using 70% radius. For whatever reason, it works just fine 7 years later.
Previously on the primary I had used silicone and felt pads. I disrecommending using both, one way or the other.
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PrestonE
Pooh-Bah
Reged: 04/29/05
Posts: 1170
Loc: Houston,Texas
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Sorry Jason that should have been 0.0625" max thickness 
Best Regards,
Preston
--------------------
A few I enjoy,
and a few more in the works ;<)
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jasonharris
sage
Reged: 09/16/06
Posts: 248
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Thanks all, I reckon I am good to go on this now!
Thanks,
Jason
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sixela
Postmaster
Reged: 12/23/04
Posts: 10855
Loc: Boechout, Belgium
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Quote:
Hi Jason,
Three round spots about .75 - 1.00" in diameter
That's on the large side. I've had issues with different curing that made such blobs pull on mirrors in the middle differently from at the edge, and that's Not Good (tm). I usually advocate just 8-10mm for each blob.
--------------------
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: 1824
Loc: Central Texas
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Quote:
Quote:
Hi Jason,
Three round spots about .75 - 1.00" in diameter
That's on the large side. I've had issues with different curing that made such blobs pull on mirrors in the middle differently from at the edge, and that's Not Good (tm). I usually advocate just 8-10mm for each blob.
That is enough silicone to securely hold a 6" mirror?
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David
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sixela
Postmaster
Reged: 12/23/04
Posts: 10855
Loc: Boechout, Belgium
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Depends on the weight and the glue.
Of course, my secondary is only 3"; I missed the size of the secondary (for that secondary I'd certainly go to blobs 10mm wide, which would probably grow to 12-14mm when the secondary is seated).
The glue I'm using can hold 140N (14kg) per square centimeter. Three blobs of one square centimeter each would hold 42kg. That's four times the weight of my 16" primary.
But yes, the blob sizes I'm using mean that rapid deceleration of a scope hitting the ground will cut the secondary loose. I know it, because I've tried it (fortunately the secondary slided aluminised face up onto carpet.)
But I don't know what size blobs you'd need to get insurance against that; I think I'd rather glue some extra strings (slightly loose in normal usage) glued with epoxy if I was worried about that.
--------------------
400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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