Jump to content


Photo

Mirror Cell

  • Please log in to reply
34 replies to this topic

#1 Mr Magoo

Mr Magoo

    Surveyor 1

  • *****
  • Posts: 1945
  • Joined: 05 Nov 2005
  • Loc: Indiana

Posted 11 January 2013 - 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.

#2 Arjan

Arjan

    Mariner 2

  • -----
  • Posts: 203
  • Joined: 21 Jan 2009
  • Loc: Netherlands

Posted 11 January 2013 - 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.

#3 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7543
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 11 January 2013 - 10:53 AM

you didn't state how thick your mirror is.

#4 Mr Magoo

Mr Magoo

    Surveyor 1

  • *****
  • Posts: 1945
  • Joined: 05 Nov 2005
  • Loc: Indiana

Posted 11 January 2013 - 11:01 AM

Mirror is 1.1" thick.

#5 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7543
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 11 January 2013 - 11:17 AM

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

Posted Image

Attached Files



#6 Mr Magoo

Mr Magoo

    Surveyor 1

  • *****
  • Posts: 1945
  • Joined: 05 Nov 2005
  • Loc: Indiana

Posted 11 January 2013 - 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.

#7 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 22578
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 11 January 2013 - 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.

#8 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7543
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 11 January 2013 - 07:00 PM

here's a 9 vs 6...

which one is which? :grin:

but 18 did really get better than either of these.

Attached Files



#9 piaras

piaras

    Viking 1

  • -----
  • Posts: 810
  • Joined: 26 Jan 2009
  • Loc: Niagara Region

Posted 11 January 2013 - 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

#10 Mr Magoo

Mr Magoo

    Surveyor 1

  • *****
  • Posts: 1945
  • Joined: 05 Nov 2005
  • Loc: Indiana

Posted 11 January 2013 - 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.

#11 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7543
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 11 January 2013 - 11:05 PM

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.

Attached Files



#12 StarDusty

StarDusty

    Ranger 4

  • -----
  • Posts: 341
  • Joined: 02 Oct 2007
  • Loc: Parsippany, NJ

Posted 11 January 2013 - 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.clearskyo...3-truss-tele...

and

16" Dob:

http://www.clearskyo...emaking/7-16dob

#13 Mr Magoo

Mr Magoo

    Surveyor 1

  • *****
  • Posts: 1945
  • Joined: 05 Nov 2005
  • Loc: Indiana

Posted 11 January 2013 - 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.

#14 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7543
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 12 January 2013 - 12:17 AM

see if this will do ya... :o

Attached Files



#15 mark cowan

mark cowan

    Vendor (Veritas Optics)

  • *****
  • Vendors
  • Posts: 3987
  • Joined: 03 Jun 2005
  • Loc: salem, OR

Posted 12 January 2013 - 02:56 AM

here's a 9 vs 6...

which one is which? :grin:

but 18 did really get better than either of these.


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

Best,
Mark

#16 starman345

starman345

    Wait, I'm Thinking

  • *****
  • Posts: 9245
  • Joined: 06 Jul 2010
  • Loc: New Brunswick, Canada

Posted 12 January 2013 - 08:23 AM

Ken, take a look here

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

#17 Arjan

Arjan

    Mariner 2

  • -----
  • Posts: 203
  • Joined: 21 Jan 2009
  • Loc: Netherlands

Posted 12 January 2013 - 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.

#18 Mr Magoo

Mr Magoo

    Surveyor 1

  • *****
  • Posts: 1945
  • Joined: 05 Nov 2005
  • Loc: Indiana

Posted 12 January 2013 - 10:54 AM

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

#19 Mr Magoo

Mr Magoo

    Surveyor 1

  • *****
  • Posts: 1945
  • Joined: 05 Nov 2005
  • Loc: Indiana

Posted 12 January 2013 - 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.

#20 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7543
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 12 January 2013 - 11:35 AM

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

Posted Image

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.

Attached Files



#21 Mr Magoo

Mr Magoo

    Surveyor 1

  • *****
  • Posts: 1945
  • Joined: 05 Nov 2005
  • Loc: Indiana

Posted 12 January 2013 - 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?

#22 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7543
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 12 January 2013 - 12:53 PM

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.

#23 Mr Magoo

Mr Magoo

    Surveyor 1

  • *****
  • Posts: 1945
  • Joined: 05 Nov 2005
  • Loc: Indiana

Posted 12 January 2013 - 12:56 PM

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.

Attached Files



#24 Mr Magoo

Mr Magoo

    Surveyor 1

  • *****
  • Posts: 1945
  • Joined: 05 Nov 2005
  • Loc: Indiana

Posted 12 January 2013 - 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, ...?

#25 Pinbout

Pinbout

    Cosmos

  • *****
  • Posts: 7543
  • Joined: 22 Feb 2010
  • Loc: nj

Posted 12 January 2013 - 03:35 PM

Save the txt file the rename the extension to dwg






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics