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Double-sided foam tape for primary- bad idea?

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#1 elstargazer12

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Posted 17 June 2019 - 07:50 AM

Hi all,

 

I gather from these forums (and from looking at what manufacturers do) that silicone'ing a small to medium sized, full thickness mirror to a mirror cell is ok.  

 

How about double-sided foam tape?  Specifically, I have a 10" mirror with 1 5/8" thickness (so quite thick).  I also have a ton of foam tape handy, so I'm thinking maybe I'll just tape it to the cell bottom.  The said cell bottom is a flat plywood surface.  

 

Is this a terrible idea?  Is foam tape inherently better or worse than silicone?  The tape thickness is about 1/16".  

 

Thanks!

Roger

 



#2 PirateMike

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Posted 17 June 2019 - 08:00 AM

I would go with the tried and true silicone, no use to buck the trend.

 

A small tube should cost only about $2.

 

 

Miguel   8-)

 

.


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#3 Richard O'Neill

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Posted 17 June 2019 - 08:13 AM

 Double sided tape is a disaster waiting to happen. It'll work until you invert the tube and leave it in that position for any length of time. Gravity will eventually be the mirror's undoing. I wouldn't take the chance with tape.


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#4 wrvond

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Posted 17 June 2019 - 08:14 AM

Many mass produced Newts use foam tape to affix the secondary to it's holder and it works just fine. I don't know how well foam tape will adhere to plywood, but assuming adhesion isn't an issue, I don't see why you couldn't use foam tape under the mirror. I'd probably lay it out in concentric circles or some such to provide maximum weight distribution. Any inequality is likely going to result in uneven compression of the supporting foam leading to an unsatisfactory collimation.


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#5 photonhunter

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Posted 17 June 2019 - 08:51 AM

Having used foam tape to mount ethernet switches and having them come down after a time there is no way I would consider foam tape as a safe mounting mechanism for a secondary unless there was also a shroud holding it in place.

If manufacturers are using a type of foam tape on scopes I'm sure it's not the standard consumer grade stuff. I've used a black rubberized double sided tape that won't ever let go, but silicon is the normal method for attaching the secondary so not sure why you'd deviate from that.

Sent from my SM-N950U using Tapatalk

Edited by photonhunter, 17 June 2019 - 08:51 AM.


#6 PirateMike

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Posted 17 June 2019 - 09:01 AM

If you are worried about the silicone sticking (and staying stuck) to the wood there is a simple solution...

 

1. Give a very light sanding to the spots where you want the silicone to stick to the wood and blow off the wood dust.

 

2. Mix the silicone with a little acetone to make a liquid silicone/acetone mix. You may need to add a little acetone every once and a while as it evaporates quite quickly.

 

3. Put a little of this mix on the sanded spots. The mix will be absorbed by the wood and then the acetone will evaporate, leaving a silicone infused wood surface. Wait for the acetone to evaporate which should take only a minute or three.

 

4. Put a dab of silicone on each of the silicone infused spots of wood and place the mirror in position.

 

5. Let this stand for at least 24 hours to cure.

 

 

Miguel   8-)

 

.


Edited by PirateMike, 17 June 2019 - 09:03 AM.

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#7 perfessor

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Posted 17 June 2019 - 09:27 AM

So I have just one question to ask:  What problem are you trying to solve, by "improving" from silicone glue to tape?



#8 wrvond

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Posted 17 June 2019 - 09:50 AM

<snip>

If manufacturers are using a type of foam tape on scopes I'm sure it's not the standard consumer grade stuff. I've used a black rubberized double sided tape that won't ever let go, but silicon is the normal method for attaching the secondary so not sure why you'd deviate from that.

<snip>

I can assure you it is nothing special. While RTV silicone may be an accepted method of attaching the secondary, I certainly wouldn't be so bold as to characterize it as the normal method. At least not without some kind of research to back that up.


Edited by wrvond, 17 June 2019 - 10:09 AM.


#9 wrvond

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Posted 17 June 2019 - 11:20 AM

To be clear, I am not endorsing the use of double sided foam tape over RTV silicone adhesive. Just saying that in and of itself, there's no reason to dismiss it's use out of hand.

However, none of use know the particulars of your assembly, and it's up to you to determine what will work best for you. Perhaps you would be better off without any kind of adhesive at all. Three metal tabs with rubber pads screwed to the board could be all you need to secure your primary.

If you do want to use an adhesive, I'm liking Miguel's suggestion.



#10 KerryR

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Posted 17 June 2019 - 12:12 PM

My DGM OA-4 (4" off-axis newt) came with the primary affixed to the plastic cell with double-sided foam tape. However, it's backed up with duct tape on the sides. It's been nearly 20 years, and the stuff is still holding. The tube is stored vertically in the Dob mount, mirror facing up.

The 10" mirror is much heavier than my little 4". I'd be worried about it being subjected to strong shearing and peeling forces while it's transported, laying horizontally, in a vehicle. If the mirror was never going to be stored or transported near or past vertical, then I foam tape might be fine. My inclination would be to back it up mechanically.

Another possible consideration is ventilation around the mirror. The foam tape, being 1/16" might reduce air-flow around the mirror, and slow thermal equalization. Silicone blobs could be taller, say 1/4", and still hold while perhaps getting you a little more air space around the back of the mirror.


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#11 wrvond

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Posted 17 June 2019 - 12:17 PM

My DGM OA-4 (4" off-axis newt) came with the primary affixed to the plastic cell with double-sided foam tape. However, it's backed up with duct tape on the sides. It's been nearly 20 years, and the stuff is still holding. The tube is stored vertically in the Dob mount, mirror facing up.

The 10" mirror is much heavier than my little 4". I'd be worried about it being subjected to strong shearing and peeling forces while it's transported, laying horizontally, in a vehicle. If the mirror was never going to be stored or transported near or past vertical, then I foam tape might be fine. My inclination would be to back it up mechanically.

Another possible consideration is ventilation around the mirror. The foam tape, being 1/16" might reduce air-flow around the mirror, and slow thermal equalization. Silicone blobs could be taller, say 1/4", and still hold while perhaps getting you a little more air space around the back of the mirror.

A quarter inch would be entirely too much gap. I wouldn't recommend any thicker than a round toothpick.

I'm actually moving away from the idea of any kind of adhesive on the back of the mirror and thinking more of cork strips and three clips 120* apart around the circumference of the mirror.


Edited by wrvond, 17 June 2019 - 12:20 PM.


#12 KerryR

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Posted 17 June 2019 - 01:28 PM

A quarter inch would be entirely too much gap. I wouldn't recommend any thicker than a round toothpick.

I'm actually moving away from the idea of any kind of adhesive on the back of the mirror and thinking more of cork strips and three clips 120* apart around the circumference of the mirror.

Why?
 

I suppose lateral sag could be an issue with a large thick mirror (like this one), but this shouldn't be an issue if the blobs have large enough diameter. I've had no issues with a 6" supported by 3/16" tall blobs, though I recognize the 10" would be much, much heavier.

I tend to prefer non-adhesive supports and edge clips myself, if only because it makes it easier to remove the mirror for cleaning and re-coating, and, of course, because non-sticky solutions can't 'lock in' distortion the way an improperly glued mirror can. It can be shockingly tough to cut through silicone blobs, especially when they're really thin.


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#13 wrvond

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Posted 17 June 2019 - 01:37 PM

You are correct on all counts. I was thinking of blobs the size I used on a secondary, and lateral sag would be an issue for sure, but as you say, larger diameter blobs would be less susceptible to that. Hard to say which would be better, a whole lot of small blobs or a relatively few large blobs.

As you point out, mirror removal for routine maintenance would also be an issue.



#14 PirateMike

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Posted 20 June 2019 - 08:10 AM

Ok, since we are muddying the waters...

 

Why not put the mirror in a cable sling?

 

howie-glatter-howie-glatter-cable-sling-kit-3-8-sh.jpg

 

 

Miguel   8-)

 

.

 


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#15 N3p

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Posted 20 June 2019 - 11:16 AM

I think too the foam tape is not a terribly good idea, it's going to fail eventually and the secondary will fall, most likely, on the primary.

 

A great recipe to sell another telescope but that's about it for me.



#16 dave brock

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Posted 20 June 2019 - 04:56 PM

If you do go with tape or silicone, don't use circles of it on plywood. I've seen that done and the ply twisted and caused distortion in the mirror. Use three blobs only.

#17 precaud

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Posted 21 June 2019 - 08:17 AM

Many mass produced Newts use foam tape to affix the secondary to it's holder and it works just fine.

 

If by "works just fine" you mean, it holds the mirror in place, then I agree. But there is no doubt that it distorts the mirror. See:

https://www.cloudyni...-mounting-woes/



#18 wrvond

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Posted 21 June 2019 - 08:49 AM

If by "works just fine" you mean, it holds the mirror in place, then I agree. But there is no doubt that it distorts the mirror. See:

https://www.cloudyni...-mounting-woes/

When I first read the question I took it as asking about the strength and adhesive quality of the tape. So yes, you are correct. However, I now see the question is open to the suitability of the two materials. So now I am in agreement with Miguel.

Neither is a good choice in this particular application. A sling or even an actual cell would be a better choice for a variety of reasons.



#19 tim53

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Posted 21 June 2019 - 10:34 AM

I think the best solution is a mirror cell that the mirror can't fall out of, regardless of orientation.  That way you're covered when the cleaning lady points the tube toward the living room floor while you're at work.

 

I remember a conversation with Ed Beck about 30 years ago.  He hated getting mirrors to refigure (and send off to coaters) that had silicone adhesive on them.  He said the coater hated cleaning them before putting them in the vacuum chamber, because sometimes it wasn't obvious there was any, when there was.

 

I just don't have the patience to wait for it to cure.  So, what have I used?

 

On my 10" mirror, I used double stick foam tape - the stuff from 3m that's used for affixing weatherstripping on the side of a car.  BUT, the cell also supports the sides of the mirror as well as the back, and the foam tape is also around the sides of the mirror, so getting the mirror to come out would involve shearing the foam tape.  I haven't had any problems with it in the 8 or so years it's been like that.

 

I chose the stuff because, when I stupidly broke the windshield on my VW Eurovan about 10 years ago (by putting 12' 2x10s on the dash and closing the back door...), I had to have the windshield replaced.  The guy had to take the air dam off the front of the van (looks like this one:  https://uploads.tapa...e5892034112.jpg).  But he just stuck it back with the old adhesive, and it started to come off.  So I bought some of the 3m tape, cleaned the mating surfaces with acetone, and stuck the air dam back on the van.  It hasn't shown any signs of separating in 10 years and about 100K miles of driving.  I like to drive 75mph on the interstates, and it's often over 100 degrees f out.  And I've had some serious headwinds from time to time over the years.  So there's some serious force against it for sustained periods of time.

 

Still, I'd never rely on it solely to hold a mirror in place.  Clips might be annoying, but not when they're needed and do their job.

 

If you're going to use tape on plywood, polyurethane the plywood first.



#20 wrvond

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Posted 21 June 2019 - 11:26 AM

I once removed that 3M tape from the doors of a truck. Took me two days to get it off.




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