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8" f/5.8 Mirror support design

ATM Beginner Equipment Dob Mirror Making
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#1 Knight Sky

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Posted 16 April 2022 - 03:44 AM

Hello fellow CNers!

 

I have been struggling to understand and implement a decent mirror support design. I saw a similar thread started a few days back by another member but didn't want to ruin that thread with my own questions/queries and hence decided to start a separate thread.

 

I have a thin mirror and my cell and support design was largely based on stellafane design. Mirror details:

 

Plate glass

200mm f/5.75 (197mm clear aperture)

standing 12mm at the center

 

So, it is on the thinner side. It suffers from minor astigmatism but provides decent views up to 280X (tested), at which point it is slightly on the softer side. Part of this whole analysis could be marred by the fact that I have not been able to properly collimate it and that the glue, so it seemed to me, distorted the figure quite considerably to be noticeable at higher mag. I realized that I had put in too much glue at too many places without thinking over the expansion rates at all. Glue is RTV silicone adhesive. Does the quality of the glue matter? I bought it at a tool shop (motor vehicle) for very cheap $0.5. It seems to be decently stable and reliable and yet easily removable. Clips do press the mirror a bit to gain better resistance, but that distortion is visible in star test. Mirror padding material is car trunk liner cloth/fabric.

 

So the questions I have are:

 

- How to construct the cell support points?

- Shape and size of support points?

- Where to place them?

- What material is best to use for support?

- Is gluing ok?

- Any specific glue?

- Hook vs resistance clips? Would resistance clips work close enough to hook clips?

- Any specific arrangement of the clips apropos the OTA? Right now clips are 120° apart.

 

Thanks in advance for your valuable suggestions!

 

Text in green is not legible - "3 clips with no hooks, padded with fabric for resistance fit."

Attached Thumbnails

  • IMG20220415153629.jpg
  • IMG20220415153915.jpg

Edited by Knight Sky, 16 April 2022 - 04:28 PM.


#2 Pinbout

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Posted 16 April 2022 - 11:21 AM

I use nylon cap screws as the points on alumna

F83C9EE4-9DCA-4930-8745-50A99862FD6D.jpeg


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#3 Knight Sky

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Posted 16 April 2022 - 04:43 PM

I use nylon cap screws as the points on alumna

attachicon.gifF83C9EE4-9DCA-4930-8745-50A99862FD6D.jpeg

That almost sounded a little...ahem!  wink.gif

 

Do you mean aluminium? Do you have a photo with the nylon cap screws? So basically mirror rests on these nylon screws? How about gluing and the hooks on the clips? If I got this right then in the photo that you posted your mirror rests on 6 nylon screws, right? How to find the optimal number of resting points and the locations? I think I might use Mel Bartel's web resource for that.



#4 Dale Eason

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Posted 16 April 2022 - 11:17 PM

Most use the program PLOP that will help you compute the best mirror support points and number of points.  Search the recent CN ATM forum threads for a post about it.

 

If you scope uses a DOB base then no need to glue the mirror.  All it needs are two edge support points 90 deg apart.  Then some clips that don't touch the mirror to keep it from falling out of the cell.

 

 

 

Dale



#5 Dale Eason

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Posted 16 April 2022 - 11:37 PM

Here is a post that shows my 10 inch mirror cell.  The mirror rests on the heads of nylon screws.  The two lower edge supporst  along with a 3rd at the top keep the mirror in it's cell.   They do not touch the mirror.  Only one is shown in the drawings and no posts are shown in the actual picture.

 

https://www.cloudyni.../#entry11479432



#6 Knight Sky

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Posted 17 April 2022 - 06:54 AM

Thanks a lot Dale and Pinbout for excellent suggestions and resource links!!

 

I think I have a much better understanding now than I had at the beginning of this thread. Here's what my take is from your responses and please correct me if I am wrong or if you think additional information is necessary:

 

- Mirror, by and large, rests on points (more or less), since these nylon screws are kind of dome shaped.

- Spacing and number of points can be calculated using PLOP app, which optimises for minimal distortion.

- Aluminum support is better than plyboard base (as is my case) for better cooling??

- 90° separation for lower clips minimizes for clip support distortion.

- Hook only on the 3rd clip to prevent it from tipping over, while not touching the mirror.

 

Based on these however I have these questions which, in the first place, propelled me to glue the mirror.

 

- How to make sure collimation is not disturbed?

- Traveling seems bound to cause mirror to rock around and lose collimation.

- Does PLOP work on Macbooks? I have had a hard time finding suitable ATM apps that work on Macs. I am hesitant to install WINE or even create boot camp just to run my ATM related apps.

 

How do you address these points?


Edited by Knight Sky, 17 April 2022 - 06:54 AM.


#7 chantepierre

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Posted 17 April 2022 - 08:08 AM

Plop runs fine via Wine (though I was on linux, not osx) but there's a OSX fork by someone on this forum, brebisson I think.

You can also use Virtualbox and download legit windows VMs on microsoft's website, they give some for browser testing. But they expire every 30 days.

Edited by chantepierre, 17 April 2022 - 08:10 AM.

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#8 Pinbout

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Posted 17 April 2022 - 11:15 AM

Try to just type alum. And automessup makes it…

 

https://youtu.be/1RZCgcWufww

 

but this is a better design

 

cell.JPG


Edited by Pinbout, 17 April 2022 - 12:52 PM.

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#9 Knight Sky

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Posted 18 April 2022 - 02:53 AM

Plop runs fine via Wine (though I was on linux, not osx) but there's a OSX fork by someone on this forum, brebisson I think.

You can also use Virtualbox and download legit windows VMs on microsoft's website, they give some for browser testing. But they expire every 30 days.

Lucas, thanks for that suggestion! The main reason not to use Wine is to avoid exposing my machine to Windows security weaknesses. Some would say that it's an unfounded fear but I would not want to expose my one and only work machine to any risk. I do have a Linux machine (Windows machine converted to Linux wink.gif ), that seems to have died, I would try to revive/have it repaired and see if Wine could be installed on it.

 

VMs could be tried if from legit MS website.



#10 chantepierre

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Posted 18 April 2022 - 03:34 AM

Lucas, thanks for that suggestion! The main reason not to use Wine is to avoid exposing my machine to Windows security weaknesses. Some would say that it's an unfounded fear but I would not want to expose my one and only work machine to any risk. I do have a Linux machine (Windows machine converted to Linux wink.gif ), that seems to have died, I would try to revive/have it repaired and see if Wine could be installed on it.

 

VMs could be tried if from legit MS website.

Here is MS's link. VMs last for 90 days and not 30 ! They're specifically made for testing websites on Edge or IE @ various windows versions but you can install anything as on a regular windows VM.

 

https://developer.mi...edge/tools/vms/

 

 

Mac users will need to use a tool that supports zip64, like The Unarchiver, to unzip the files.
The password to your VM is "Passw0rd!"


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