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Mr Magoo
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Reged: 11/05/05

Loc: Franklin, Indiana
Mirror Cell
      #5616962 - 01/11/13 10:13 AM

I was wondering if someone out there has the PLOP numbers for a 13.1", f/4.5, 6 point mirror cell. Secondary size is a 3.1". For one reason or another I've had the worst time trying to use PLOP and was hoping for a bailout by some kind soul.

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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: Mirror Cell new [Re: Mr Magoo]
      #5616980 - 01/11/13 10:27 AM

Supports are at about 60% radius for 6 points. In case of 3 points this is about 40%.
Thickness doesn't matter too much, but if it is 1.5" or so you might get away with only 3 point cell.


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Pinbout
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Reged: 02/22/10

Loc: nj
Re: Mirror Cell new [Re: Mr Magoo]
      #5617024 - 01/11/13 10:53 AM

you didn't state how thick your mirror is.

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Mr Magoo
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Re: Mirror Cell new [Re: Pinbout]
      #5617037 - 01/11/13 11:01 AM

Mirror is 1.1" thick.

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Pinbout
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Re: Mirror Cell new [Re: Mr Magoo]
      #5617070 - 01/11/13 11:17 AM Attachment (33 downloads)

here's what I get. also attached is a pdf that you can print at kinkos for full size.



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Mr Magoo
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Re: Mirror Cell new [Re: Pinbout]
      #5617225 - 01/11/13 12:48 PM

Thanks Danny! Well, I guess I didn't have PLOP too messed up, because this is close to what I got on one of my tries. I need to keep messing with it.

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Starman1
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Reged: 06/24/03

Loc: Los Angeles
Re: Mirror Cell new [Re: Mr Magoo]
      #5617759 - 01/11/13 06:40 PM

At 1.1" thick, I would recommend an 18-point cell. A 9-point cell would be OK. Smaller numbers of support points result in a lot of deformation of the mirror surface.
My 12.5" is 1.25" thick and I use 18 points.


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Pinbout
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Reged: 02/22/10

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Re: Mirror Cell new [Re: Starman1]
      #5617797 - 01/11/13 07:00 PM Attachment (18 downloads)

here's a 9 vs 6...

which one is which?

but 18 did really get better than either of these.


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piaras
professor emeritus


Reged: 01/26/09

Loc: Niagara Region
Re: Mirror Cell new [Re: Pinbout]
      #5617981 - 01/11/13 08:42 PM

I was not the OP but I did a comparison for 6, 9, and 18 point cells using Plop for my Coulter Odyssey 1 (5/8" thick) vs a ZOC that is near completion. See the topic here. Another Plop question

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Mr Magoo
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Re: Mirror Cell new [Re: Pinbout]
      #5618127 - 01/11/13 10:25 PM

I am certainly open to suggestions or outright help. Like I said, PLOP is not my thing. I'm a sheet metal worker/welder. I can build anything given the plans, but this engineering is stumping me a bit. It was my understanding from other posts I had read in the past that a 6 point was better for this size than a 9 or an 18. I know that Dennis at DobStuff suggests a 9 point for the old Coulter mirror scopes like mine. Is the 9 the one on the left? Seems like it because the right one has 6 clearly defined spots. I'm assuming that the big red donut hole in the center of the right one is not good. I'm building this as an experiment to replace the old tailgate on my blue tube Coulter. If it works out well like I plan, I'm making another for a friend.

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Pinbout
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Re: Mirror Cell new [Re: Mr Magoo]
      #5618194 - 01/11/13 11:05 PM Attachment (16 downloads)

Quote:

Is the 9 the one on the left? Seems like it because the right one has 6 clearly defined spots.




yes, the 18 is so much better but the 6 is still to the -6exp. where the 9 is to the -5exp.


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StarDusty
sage


Reged: 10/02/07

Loc: Parsippany, NJ
Re: Mirror Cell new [Re: Pinbout]
      #5618241 - 01/11/13 11:51 PM

PLOP is a great program. I used it for both 13" and 16" home made cells. I used 6 points on both. As I recall the 13" mirror was the same thickness as yours is.

You can see these at:

13" Dob:

http://www.clearskyobserver.com/index.php/20-telescopemaking/14-13-truss-tele...

and

16" Dob:

http://www.clearskyobserver.com/index.php/20-telescopemaking/7-16dob


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Mr Magoo
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Loc: Franklin, Indiana
Re: Mirror Cell new [Re: Pinbout]
      #5618255 - 01/11/13 11:59 PM

Yes, that looks excellent even to my uneducated eye. I will have to try and see if I can run the numbers for an 18 point in PLOP and get it correct. Thank you so much for everyones help so far.

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Pinbout
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Reged: 02/22/10

Loc: nj
Re: Mirror Cell new [Re: Mr Magoo]
      #5618278 - 01/12/13 12:17 AM Attachment (27 downloads)

see if this will do ya...

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mark cowan
Vendor (Veritas Optics)
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Reged: 06/03/05

Loc: salem, OR
Re: Mirror Cell new [Re: Pinbout]
      #5618403 - 01/12/13 02:56 AM

Quote:

here's a 9 vs 6...

which one is which?

but 18 did really get better than either of these.




The 6-point is quite adequate at 2nm RMS surface.

Best,
Mark


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starman345
Wait, I'm Thinking
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Reged: 07/06/10

Loc: New Brunswick, Canada
Re: Mirror Cell new [Re: mark cowan]
      #5618586 - 01/12/13 08:23 AM

Ken, take a look here

It helped me a lot to understand what the numbers mean.


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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: Mirror Cell new [Re: starman345]
      #5618729 - 01/12/13 10:14 AM

For this mirror 6 points is really sufficient.

You can theoretically get a better one with 9 points as long as you put the supports in one ring. However, the 9 and 18 points cells are harder to make and require lower tolerance. It is hard to screw-up a 6 point cell on the other hand.


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Mr Magoo
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Re: Mirror Cell new [Re: starman345]
      #5618797 - 01/12/13 10:54 AM

Thanks Brian. That is one of several PLOP threads I have toggled.

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Mr Magoo
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Re: Mirror Cell new [Re: Pinbout]
      #5618802 - 01/12/13 10:57 AM

That is really great Danny. I can't thank you enough for doing that. I'm looking at what I want to do. This will be a temporary home for these optics until I can find the time someday to build a more portable, user friendly truss design scope around them. I will probably try the 18 for the trusser and may still now. I have a laser to cut my parts, so I can be very precise.

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Pinbout
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Reged: 02/22/10

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Re: Mirror Cell new [Re: Mr Magoo]
      #5618879 - 01/12/13 11:35 AM Attachment (22 downloads)

I guess I'm avoiding doing the dishes so I had some avoidance time... all parts from mcmaster's



if you got a laser you can download this txt file [right click attachments and save target as] and relable the .txt extension to .dwg and have the autocad file for the laser.

Edited by Pinbout (01/12/13 11:36 AM)


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Mr Magoo
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Loc: Franklin, Indiana
Re: Mirror Cell new [Re: Pinbout]
      #5618930 - 01/12/13 12:00 PM

Very cool. I'm playing with PLOP this morning. I also just checked my mirror and it is actually 1" thick, not 1.1". Not sure how much that changes things. Do you use the automatic cell design?

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Pinbout
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Reged: 02/22/10

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Re: Mirror Cell new [Re: Mr Magoo]
      #5619016 - 01/12/13 12:53 PM

Quote:

Do you use the automatic cell design?




yeah.

what's really cool is make the design and get the colored plot and go back and make changes and watch the colored plot automatically change/morph.


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Mr Magoo
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Re: Mirror Cell new [Re: Mr Magoo]
      #5619025 - 01/12/13 12:56 PM Attachment (13 downloads)

Okay, I ran this through with better numbers that I should have checked first, my apologies for that. My memory is not as good as I thought. Diameter= 332.74mm, focal length= 1480.693mm, thickness= 25.4mm, obstruction= 78.74. I used the Automatic Cell Design feature and came up with this for a 6 point cell.

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Mr Magoo
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Re: Mirror Cell new [Re: Pinbout]
      #5619269 - 01/12/13 03:25 PM

For some reason I can't save that as a .dwg file. I'm not even getting the option to. I have a design question for the masses. What do you use as the support point? Furniture pads, nylon screws, ...?

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Pinbout
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Re: Mirror Cell new [Re: Mr Magoo]
      #5619298 - 01/12/13 03:35 PM

Save the txt file the rename the extension to dwg

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Mr Magoo
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Re: Mirror Cell new [Re: Mr Magoo]
      #5619341 - 01/12/13 04:02 PM Attachment (13 downloads)

Here is the info I used from what I ran.

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Mr Magoo
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Loc: Franklin, Indiana
Re: Mirror Cell new [Re: Mr Magoo]
      #5619354 - 01/12/13 04:08 PM Attachment (10 downloads)

This is what I drew from that for the layout. Does this look correct?

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Pinbout
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Re: Mirror Cell new [Re: Mr Magoo]
      #5619530 - 01/12/13 05:48 PM

Great, it's cool to do it yourself. Stupid iPod won't let do a gremlin.

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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: Mirror Cell new [Re: Mr Magoo]
      #5619791 - 01/12/13 08:44 PM

Quote:

Seems like it because the right one has 6 clearly defined spots. I'm assuming that the big red donut hole in the center of the right one is not good.



The center of the mirror is in the shadow of the diagonal, so it doesn't matter.


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Mr Magoo
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Re: Mirror Cell new [Re: Pinbout]
      #5620057 - 01/13/13 12:16 AM

My AutoCad skills are a little rusty. Took me a bit. I'm going to try and do the 9 and 18 also. Can't do the 3D although I do have the full version. Never got that far in school.

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Pinbout
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Reged: 02/22/10

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Re: Mirror Cell new [Re: Mr Magoo]
      #5620319 - 01/13/13 09:03 AM

I do my 3d work in 3dmax. I use to be able to work 3d in autocad but that was a long time ago.

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Ajohn
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Reged: 12/03/07

Re: Mirror Cell new [Re: Pinbout]
      #5621060 - 01/13/13 03:28 PM

Never used one of these types of supports as when I made mirrors the blank thinknesses were ok. A thought has struck me though, Be interested in comments. As I am probably going to be making another in the near future.

Taking ferric stainless as that has a low coifficient of expansion it's 10um/m/C. Aluminium is a bit more than twice this. If the support bars are 50mm long they change at 1/20 of that or 500nm / C so if there is a temperature difference of 1/250 C there is 2nm difference in the support height and it's hard to imagine them all being at the same temperature to that sort of accuracy and plop goes the mirror. I have also heard of people testing mirrors mounted like this and running into problems even when they are lying flat which sort of suggests the problems are more than 2nm. I also feel it would take a pretty thick backing plate to limit deflection to that sort of extent so also just how thick does that have to be?

John
-


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Don H
sage


Reged: 12/28/06

Loc: Midwest US
Re: Mirror Cell new [Re: mark cowan]
      #5621724 - 01/13/13 09:29 PM Attachment (8 downloads)

I made a 13" f/5.3 a few years ago and used PLOP for the cell design. The pads were nylon floor glides. It seemed to work nicely.

Regards,
Don


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Mr Magoo
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Re: Mirror Cell new [Re: Don H]
      #5621840 - 01/13/13 10:30 PM

That looks great Don! Thank you for the picture. I would love to see if others want to post them. I'm especially interested in what you use for the pivot points on the bars or triangles.

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Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: Mirror Cell new [Re: Mr Magoo]
      #5622748 - 01/14/13 02:04 PM

For a single bar pivot point: 1cm dab of silicone.
Mirror can slide loosely though, on ball bearings.
You build the cell on the back of the mirror, use about 2mm thickness of silicone.


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