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

Truly Awful Star Test Of Coulter 17.5" In Truss Dob

  • Please log in to reply
68 replies to this topic

#26 Bob4BVM

Bob4BVM

    Vanguard

  • *****
  • Posts: 2,476
  • Joined: 23 Mar 2015
  • Loc: W. Oregon

Posted 11 March 2020 - 03:27 PM

Tom,

For your mirror support pads, I would not use the nylon acorn nuts unless the back has been ground flat, because a point support like that would tend to settle between the bumps that are on the back of standard unground pyrex back of most Coulter mirrors.  Better to have a wider surface that the bumps can sit on...

Trick is how to locate the flat support points on the triangles in the exact PLOP-calc'd locations. . .

For my new cells on my BT project, I am using these little plastic drawer-glide pads. Not sure what the plastic is, but I can say that it is quite hard and extremely slick against the bumpy bac of my Coulter primaries.  The cool thing is that they have a small pin/brad out the back of the pad, so you can exactly locate   them on your triangles by drilling small holes in the triangle corners at your PLOP locations.

 

I have a bunch of these pads, would be happy to send a set down to you. If you want to try them, drop me a PM.

 

CS

Bob

 

cell mirror pads coplanar.jpg


Edited by Bob4BVM, 11 March 2020 - 03:30 PM.


#27 Steve Dodds

Steve Dodds

    Owner - Nova Optical

  • *****
  • Vendors
  • Posts: 654
  • Joined: 02 Apr 2007
  • Loc: Utah

Posted 11 March 2020 - 03:53 PM

I use 1" furniture glides, adhesive backed, pack of 6 about $3


  • Don H, Pinbout, sopticals and 1 other like this

#28 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 25,097
  • Joined: 22 Feb 2010

Posted 11 March 2020 - 07:26 PM

 

I would not use the nylon acorn nuts unless the back has been ground flat,

Who said anything bout acorn nuts?


  • Augustus likes this

#29 Bob4BVM

Bob4BVM

    Vanguard

  • *****
  • Posts: 2,476
  • Joined: 23 Mar 2015
  • Loc: W. Oregon

Posted 11 March 2020 - 10:51 PM

Who said anything bout acorn nuts?

no one.

When I read cap screws my mind saw acorn nuts.

My bad, sorry !


  • Pinbout likes this

#30 Tom Duncan

Tom Duncan

    Surveyor 1

  • *****
  • Vendors
  • topic starter
  • Posts: 1,552
  • Joined: 11 Nov 2008
  • Loc: Chico, CA

Posted 14 March 2020 - 11:54 AM

Dennis at DobStuff is sending me the silicone and small plastic dots he has used successfully for years. Not sure why it didn't work back when this scope was made, perhaps too large of dabs or an odd batch of silicone. 

 

I'll report back when I get the job done and can get the scope back out under the stars. 

 

Tom



#31 Bob4BVM

Bob4BVM

    Vanguard

  • *****
  • Posts: 2,476
  • Joined: 23 Mar 2015
  • Loc: W. Oregon

Posted 14 March 2020 - 02:50 PM

Something that escapes me is how one would locate any type of stick-on pads at the exact PLOP locations. We go to great lengths to get an ideal mirror-support schematic from PLOP, good to several decimal places, then work hard to build a cell to those tolerances. All that seems lost if we do not exactly locate the COE of the pads at their exact positions on the triangles. To my thinking that is nearly impossible short of having drilled PLOP-location holes to receive pinned pads, nylon screw-heads, or similar item with some sort of shank to center pads in said holes. 

 

What is the method to locate stick-on pads ? Do you draw an outline of the pad on the triangle, centered on the PLOP point, or what ?

 

CS

Bob

 

cell mirror pads w bushgs.jpg


Edited by Bob4BVM, 14 March 2020 - 02:52 PM.

  • Greg Campbell likes this

#32 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 25,097
  • Joined: 22 Feb 2010

Posted 14 March 2020 - 04:22 PM

 

What is the method to locate stick-on pads ? Do you draw an outline of the pad on the triangle, centered on the PLOP point, or what ?

I layout everything in AutoCAD 

 

I don’t see your problem.


  • dave brock likes this

#33 Bob4BVM

Bob4BVM

    Vanguard

  • *****
  • Posts: 2,476
  • Joined: 23 Mar 2015
  • Loc: W. Oregon

Posted 14 March 2020 - 07:56 PM

I layout everything in AutoCAD 

 

I don’t see your problem.

 I get that

Guess I didn't make myself very clear.

PLOP does the same thing, as far as providing a dimensioned layout of the support points

My question is about how you go about the physical assembly, ie the mechanical location of the pads

Does autocad assemble the physical parts for you,  ?


  • Retsub likes this

#34 ccaissie

ccaissie

    Vanguard

  • *****
  • Posts: 2,040
  • Joined: 13 Sep 2010
  • Loc: Whitefield, Maine

Posted 15 March 2020 - 07:41 AM

 I get that

Guess I didn't make myself very clear.

PLOP does the same thing, as far as providing a dimensioned layout of the support points

My question is about how you go about the physical assembly, ie the mechanical location of the pads

Does autocad assemble the physical parts for you,  ?

I'm sure the tolerances for locating the actual support points could be determined, but whatever it is, I'm sure it's within the tolerances available to a careful craftsman and hand tools....+/- 1/16" ?


  • Pinbout and Matthew Paul like this

#35 Bob4BVM

Bob4BVM

    Vanguard

  • *****
  • Posts: 2,476
  • Joined: 23 Mar 2015
  • Loc: W. Oregon

Posted 15 March 2020 - 02:24 PM

I'm sure the tolerances for locating the actual support points could be determined, but whatever it is, I'm sure it's within the tolerances available to a careful craftsman and hand tools....+/- 1/16" ?

I am terrible at expressing my ideas. My issue is that with a pinless (stick on) pad, you are covering up the center mark on the triangle, so are somewhat blind in sticking it on the triangle.

I thought of the solution below, you just need to measure the pad diameter and draw yourself an outline with a compass, using a punched center mark.

Hope this may be of some help to folks using stick-on pads...

CS

Bob

Stick-On Mirror Pads-Locating.png

 


Edited by Bob4BVM, 15 March 2020 - 02:29 PM.


#36 Tom Duncan

Tom Duncan

    Surveyor 1

  • *****
  • Vendors
  • topic starter
  • Posts: 1,552
  • Joined: 11 Nov 2008
  • Loc: Chico, CA

Posted 12 June 2020 - 11:34 AM

UPDATE: 6/12/2020

 

Remounted the mirror on the cell using the same technique and advice/materials from Dennis Steele (DobStuff). While the edges no longer look like a walnut there is obvious astigmatism when the star gets near to pinpoint. 

 

I think the next step is to test the mirror and secondary to find out where it's coming from. 

 

Will keep this thread updated as new info surfaces. 

 

Tom


  • paul, PrestonE and Pinbout like this

#37 Don W

Don W

    Founding Member

  • *****
  • Posts: 24,563
  • Joined: 19 May 2003
  • Loc: Wisconsin, USA

Posted 12 June 2020 - 11:47 AM

I bought a used Oddysey 1 in the late 80s. Huge, cumbersome and images were awful. I tossed everything but the primary and built a very nice dob around it. Images were no better. The mirror had severe astigmatism. Sold the the mirror to a friend who had it refigured into something usable. I took the dob mount and using longer truss tubes I put in a Galaxy 18" f/5.5 mirror into it. The images were amazing, some of the best views I've ever had in a dob.


  • Jon Isaacs likes this

#38 Greg Campbell

Greg Campbell

    Vostok 1

  • *****
  • Posts: 141
  • Joined: 27 Apr 2010
  • Loc: Southern Aridzona

Posted 12 June 2020 - 12:03 PM

UPDATE: 6/12/2020

 

Remounted the mirror on the cell using the same technique and advice/materials from Dennis Steele (DobStuff). While the edges no longer look like a walnut there is obvious astigmatism when the star gets near to pinpoint. 

 

I think the next step is to test the mirror and secondary to find out where it's coming from. 

 

Will keep this thread updated as new info surfaces. 

 

Tom

If not too difficult, try rotating the secondary and see of the astigmatism axis follows.



#39 pierce

pierce

    Viking 1

  • -----
  • Posts: 851
  • Joined: 18 Sep 2007
  • Loc: Santa Cruz, CA

Posted 12 June 2020 - 01:06 PM

I bought a used Oddysey 1 in the late 80s. Huge, cumbersome and images were awful. I tossed everything but the primary and built a very nice dob around it. Images were no better. The mirror had severe astigmatism. Sold the the mirror to a friend who had it refigured into something usable. I took the dob mount and using longer truss tubes I put in a Galaxy 18" f/5.5 mirror into it. The images were amazing, some of the best views I've ever had in a dob.

My Obsession Classic 20 f/5 has a Galaxy mirror, from the early/mid 90s, and is superb.


  • Don W likes this

#40 Tom Duncan

Tom Duncan

    Surveyor 1

  • *****
  • Vendors
  • topic starter
  • Posts: 1,552
  • Joined: 11 Nov 2008
  • Loc: Chico, CA

Posted 12 June 2020 - 01:34 PM

If not too difficult, try rotating the secondary and see of the astigmatism axis follows.

Hmm, not sure how I'd rotate the secondary but as there are three threaded posts for the cell to sit in the rocker box I could rotate the primary. 

 

I'll give that a try if I don't have the primary checked before I have a chance to do it.

 

Tom 



#41 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 25,097
  • Joined: 22 Feb 2010

Posted 12 June 2020 - 01:39 PM

Hmm, not sure how I'd rotate the secondary but as there are three threaded posts for the cell to sit in the rocker box I could rotate the primary. 

 

I'll give that a try if I don't have the primary checked before I have a chance to do it.

 

Tom 

ke or ronchi test thru focus on a star... see if there's clocking.



#42 pierce

pierce

    Viking 1

  • -----
  • Posts: 851
  • Joined: 18 Sep 2007
  • Loc: Santa Cruz, CA

Posted 12 June 2020 - 01:50 PM

If not too difficult, try rotating the secondary and see of the astigmatism axis follows.

 

how do you rotate an elliptical secondary on a newtonian??


  • Don W likes this

#43 mark cowan

mark cowan

    Vendor (Veritas Optics)

  • *****
  • Vendors
  • Posts: 9,712
  • Joined: 03 Jun 2005
  • Loc: salem, OR

Posted 12 June 2020 - 03:54 PM

I think he meant rotate the primary. :lol:



#44 tomb18

tomb18

    Mariner 2

  • -----
  • Posts: 261
  • Joined: 13 Oct 2015

Posted 13 June 2020 - 12:07 AM

When I was a young teen, I drooled over the ads for the coulter mirrors.  I finally saved up for one and it took another year to save for a mount.  By was I disappointed.  Never could see much on Jupiter for example. 35 years later I ground my first mirror.  A 10" f 6.6. I tested it at 1/11th wave on the wave front.  Won first prize small optics at Stellaphane. I decided to test the Coulter after this.  1 wave.  Garbage.  I think it cost me decades of not bothering with astronomy.

Tom


  • LarsMalmgren and Pinbout like this

#45 pastorgalactico

pastorgalactico

    Vostok 1

  • -----
  • Posts: 181
  • Joined: 02 Oct 2010
  • Loc: SPAIN

Posted 13 June 2020 - 10:03 AM

Hello;
What works best and is easiest to put is to put nylon screws with cap nuts or cap nuts at each vertex of the triangle as support points.
That is fantastic.
Never glue the mirror to the triangles.
Regards.

#46 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 25,097
  • Joined: 22 Feb 2010

Posted 13 June 2020 - 10:34 AM

Hello;
What works best and is easiest to put is to put nylon screws with cap nuts or cap nuts at each vertex of the triangle as support points.
That is fantastic.
Never glue the mirror to the triangles.
Regards.

You mean nylon cap screw heads, nutz on the bottom heads on the top 

 

lol.gif lol.gif lol.gif


Edited by Pinbout, 13 June 2020 - 10:38 AM.


#47 pastorgalactico

pastorgalactico

    Vostok 1

  • -----
  • Posts: 181
  • Joined: 02 Oct 2010
  • Loc: SPAIN

Posted 13 June 2020 - 11:46 AM

Hello;
I mean the walnut-shaped blind nuts.
Pardon my English translated from Spanish.
Regards.

#48 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 25,097
  • Joined: 22 Feb 2010

Posted 13 June 2020 - 12:09 PM

Hello;
I mean the walnut-shaped blind nuts.
Pardon my English translated from Spanish.
Regards.

pictures are worth more than a 1,000  languages



#49 pastorgalactico

pastorgalactico

    Vostok 1

  • -----
  • Posts: 181
  • Joined: 02 Oct 2010
  • Loc: SPAIN

Posted 13 June 2020 - 12:16 PM

Screenshot-20200613-191152.png


  • LarsMalmgren likes this

#50 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 25,097
  • Joined: 22 Feb 2010

Posted 13 June 2020 - 12:20 PM

Screenshot-20200613-191152.png

acorn nutz...  not walnut but acorn... lol.gif lol.gif lol.gif


  • stubeeef likes this


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