Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

To silicone or not to silicone?

  • Please log in to reply
20 replies to this topic

#1 calypsob

calypsob

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,955
  • Joined: 20 Apr 2013
  • Loc: Virginia

Posted 31 July 2020 - 03:01 PM

I have a 12" F4 Newtonian which has an unusual cell, there are 3 pivot points spaced 120 degrees around the cell with 2 pads and additionally beneath the mirror clips are threaded nylon screws that are designed to keep the mirror centered.

 

I am not a fan of mirror clips because they make additional diffraction spikes.   I would like to remove the plastic/rubber pads that are currently resting on the pivot points and replace them with silicone by shimming the mirror at an elevated height with a 2mm piece of metal while the silicone dries.

 

Ideally this will let me remove the mirror clips. I will come up with some way to secure the primary during transport so it is not relying on silicone alone.  I also hope that with a silicone secured primary there will be no shift of primary at all during my imaging sessions. 

 

With that said I my question is how should I space the silicone blobs out so they do not introduce astigmatism? 

 

In the image below, the allen head screw can see above the central triangle is where the pivot point is located. there are 3 points in total each with two 2mm thin plastic/rubber pads maybe .5" in diameter  

 

thumbnail_IMG_8761.jpg


Edited by calypsob, 31 July 2020 - 03:02 PM.


#2 mac57

mac57

    Viking 1

  • *****
  • Posts: 579
  • Joined: 19 Feb 2013
  • Loc: DeLand of Oz, Florida

Posted 31 July 2020 - 03:21 PM

I can tell you that silicone will hold very well.  I re-mounted my secondary mirror with three dots and couldn't pull it apart. Use flat toothpicks as spacers. Mark


  • jim kuhns likes this

#3 calypsob

calypsob

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,955
  • Joined: 20 Apr 2013
  • Loc: Virginia

Posted 31 July 2020 - 05:38 PM

I can tell you that silicone will hold very well.  I re-mounted my secondary mirror with three dots and couldn't pull it apart. Use flat toothpicks as spacers. Mark

Yea, I mounted my smaller 6" f4 with silicone. It worked well. I know its possible to introduce stig on a larger thinner mirror so Im hoping someone can chime in. I have been reading about the PLOP method a bit and wonder if I should try to optimize the design of my cell with some 3d printer discs that can hold the silicone like you see on aurora cells 



#4 calypsob

calypsob

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,955
  • Joined: 20 Apr 2013
  • Loc: Virginia

Posted 01 August 2020 - 01:42 PM

anyone else care to chime in? 



#5 coinboy1

coinboy1

    Apollo

  • *****
  • Posts: 1,473
  • Joined: 03 May 2011
  • Loc: Tulsa, OK

Posted 01 August 2020 - 02:44 PM

Silicone could cause astigmatism and pinched optics in a primary mirror. I wouldn't do it. You could always make a circular retaining ring/mask to cover the mirror clips so you don't get the diffraction patterns.


  • careysub and clang02 like this

#6 SteveV

SteveV

    Mariner 2

  • *****
  • Posts: 220
  • Joined: 26 Nov 2006
  • Loc: Ore.

Posted 01 August 2020 - 05:30 PM

Your cell isn't all that unusual.  It looks derived more or less from a typical 6 point PLOP cell from the bottom.  The edge supports as is or what you propose are not going to pass muster with the thinking in the current 'edge support' thread.  

 

But if you like it as is, why not trying gluing?  What do you have to lose?  Some time and effort.  Glue it up put it to work.  If it works you're good.  If not you learned something.


  • calypsob likes this

#7 lphilpot

lphilpot

    Surveyor 1

  • *****
  • Posts: 1,659
  • Joined: 15 Oct 2005
  • Loc: Central Lousiana, USA

Posted 01 August 2020 - 05:51 PM

Will you wash the mirror with the entire cell attached (by removing cell and all from the scope)? Disassemble the cell and remove just the attached parts? Or just cut the silicon and re-glue each time? I personally don't care for glued-on mirrors, for that very reason.



#8 calypsob

calypsob

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,955
  • Joined: 20 Apr 2013
  • Loc: Virginia

Posted 01 August 2020 - 06:00 PM

Silicone could cause astigmatism and pinched optics in a primary mirror. I wouldn't do it. You could always make a circular retaining ring/mask to cover the mirror clips so you don't get the diffraction patterns.

So are you saying that no matter what, if I shim and let the silicone dry, remove the shims, or duplicate the aurora silicone holding pads, there is no way to avoid astigmatism? 

 

Your cell isn't all that unusual.  It looks derived more or less from a typical 6 point PLOP cell from the bottom.  The edge supports as is or what you propose are not going to pass muster with the thinking in the current 'edge support' thread.  

 

But if you like it as is, why not trying gluing?  What do you have to lose?  Some time and effort.  Glue it up put it to work.  If it works you're good.  If not you learned something.

I am going to do it, but its such a pain to remove I want to have some sort of guidance before I do it.  

 

Will you wash the mirror with the entire cell attached (by removing cell and all from the scope)? Disassemble the cell and remove just the attached parts? Or just cut the silicon and re-glue each time? I personally don't care for glued-on mirrors, for that very reason.

Yes, I really dont wash it alot, it has a shroud.  The cell comes off easily, it is a truss setup. 



#9 MitchAlsup

MitchAlsup

    Aurora

  • -----
  • Posts: 4,776
  • Joined: 31 Aug 2009

Posted 01 August 2020 - 06:19 PM

I made my mirror cell in such a way that there are 4 screws pushing on ball transfers at CoM in order that the mirror is held on the optical axis. The first picture is using dial indicators to limit mirror movement to 0.004:

 

assembly01.JPG

 

The next figure shows me attaching the front clips such that the mirror will not (at any angle, including pointing straight down) allow the mirror to leave the cell. For the most part, I got the clips with the same 0.004".

 

assembly02.JPG

 

At this spacing, the mirror clips hold on the bevel of the glass and do not interfere with the parabolic section.

 

The reason for choosing 0.004" is that this is how much differential contraction between the mild steel cell and frame and the supermax mirror from 100ºF to 0ºF the telescope might see. With 0.004, the mirror will never be pinched, and will never be able to leave cell, and will still not cause any additional diffraction effects.


  • SteveV, Mike Lockwood and calypsob like this

#10 calypsob

calypsob

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,955
  • Joined: 20 Apr 2013
  • Loc: Virginia

Posted 02 August 2020 - 06:32 AM

Wow that is a very precise procedure. How does she perform?

I want to avoid clips all together for imaging purposes. A mask will slow down my aperture which I also want to avoid. when I drive to my dark site, the mirror has a tendency to want to lean forwards and rest against the clips while the Newtonian sits in my car, which is one reason the silicone option is appealing 


Edited by calypsob, 02 August 2020 - 06:32 AM.


#11 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 25,114
  • Joined: 22 Feb 2010

Posted 02 August 2020 - 08:32 AM

I would run some beads... let them dry and see how much they deform from the weight of your mirror. then make sure you use spacers at least that thick while letting the sillycone dry while the mirror is resting on the cell.

 

I use toothpicks for spacers while gluing my 2ndry's to the stalks.


Edited by Pinbout, 02 August 2020 - 08:33 AM.

  • SteveV and calypsob like this

#12 MitchAlsup

MitchAlsup

    Aurora

  • -----
  • Posts: 4,776
  • Joined: 31 Aug 2009

Posted 02 August 2020 - 10:51 AM

Wow that is a very precise procedure. How does she perform?

 

The scope performs great.


  • calypsob likes this

#13 calypsob

calypsob

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,955
  • Joined: 20 Apr 2013
  • Loc: Virginia

Posted 02 August 2020 - 05:34 PM

I would run some beads... let them dry and see how much they deform from the weight of your mirror. then make sure you use spacers at least that thick while letting the sillycone dry while the mirror is resting on the cell.

 

I use toothpicks for spacers while gluing my 2ndry's to the stalks.

So in your opinion is a thinner shim better than a thick one?  I like the idea of running the beads, where on the cell would you put them? 



#14 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 25,114
  • Joined: 22 Feb 2010

Posted 02 August 2020 - 05:51 PM

No. not thinner the better.  Too thin is trouble, too thick you need proper edge support. You have to find that thickness for your mirror by testing. My opinion 


Edited by Pinbout, 02 August 2020 - 05:51 PM.

  • calypsob likes this

#15 calypsob

calypsob

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,955
  • Joined: 20 Apr 2013
  • Loc: Virginia

Posted 02 August 2020 - 06:06 PM

No. not thinner the better.  Too thin is trouble, too thick you need proper edge support. You have to find that thickness for your mirror by testing. My opinion 

sounds like a plan !



#16 calypsob

calypsob

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 5,955
  • Joined: 20 Apr 2013
  • Loc: Virginia

Posted 05 August 2020 - 04:53 PM

Ok one more question here. Would there be any negative to using double sided heatsink tape

instead of silicone? Like this stuff, .25mm thick. Keep in mind I do have edge support via delrin/nylon bolts spaced 120 degrees around the cell

 

AIYUNNI Double Sided Thermal Conductive Strong Adhesive Tape 10mm x 25M x 0.25mm, Self Adhesive Super Durability Cooling Tape for Heat Sink, SSD Drives, Computer CPU, GPU, LED, Coolers https://www.amazon.c...i_DMYkFbC90461W



#17 avarakin

avarakin

    Vanguard

  • *****
  • Posts: 2,307
  • Joined: 13 Jul 2009
  • Loc: Parsippany NJ, USA

Posted 07 August 2020 - 07:56 PM

I believe that conventional wisdom is saying that gluing mirror to cell is ok for mirrors up to 10", after that you get astigmatism if mirror is tilted.

How about making round clips? This should prevent diffraction spikes.



#18 Oberon

Oberon

    Gemini

  • -----
  • Posts: 3,496
  • Joined: 24 Feb 2013
  • Loc: Hunter Valley NSW Australia

Posted 07 August 2020 - 08:31 PM

The key to minimizing diffraction effects of mirror clips is minimizing obstruction. Better to have a tiny straight clip than a larger rounded clip.


  • Pinbout likes this

#19 Sam Danigelis

Sam Danigelis

    Ranger 4

  • *****
  • Posts: 366
  • Joined: 22 Apr 2018
  • Loc: Excelsior Springs, MO

Posted 07 August 2020 - 08:51 PM

I would recommend Gary Seronik's wisdom here. I believe fresh silicone can be a very tough, durable solution to securing a primary without clips.

https://garyseronik....your-reflector/
  • SteveV and danmdak like this

#20 danmdak

danmdak

    Viking 1

  • *****
  • Posts: 522
  • Joined: 03 Oct 2007
  • Loc: NE Ohio

Posted 14 August 2020 - 04:45 PM

Yes, see Sam Danigelis post just above this one. Gary has silicone on the back AND the sides. It gives a very firm bond. My 10" f/4 was mounted this way. I have since gone to double sided carpet tape on the back and silicone blobs on the sides....no particular reason other than I didnt want to mess around with spacers while the silicone cured.



#21 Sam Danigelis

Sam Danigelis

    Ranger 4

  • *****
  • Posts: 366
  • Joined: 22 Apr 2018
  • Loc: Excelsior Springs, MO

Posted 15 August 2020 - 07:25 PM

I will add that I used Gary's method on my 6" full thickness mirror. I was very careful to clean the glass first with alchohol, and use a tube of fresh GE silicone. I had to remove the mirror from the cell once and can testify that the silicone blobs did NOT want to let go of the glass. It was very tough to saw through with monofilament line. But it holds everything with no pinching or stress.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics