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

Correcting a 16" flat to correct a 16" paraboloid

  • Please log in to reply
82 replies to this topic

#76 ccaissie

ccaissie

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1767
  • Joined: 13 Sep 2010
  • Loc: Whitefield, Maine

Posted 09 November 2019 - 05:59 AM

 Figuring a 16 inch flat to where it's not going to affect the DPAC is no easy task.  IMHO it would be easier to find a location where you could test vertically and use the immersion null although that over 12 feet high.

Great video and setup.  I have just the stairway needed.  Thermals might be an issue....

 

I was at the Stewart Mirror Lab back in the 90's and saw a couple of 6 meter mirrors being tested...they were set up vertical also...didn't see any water in them...8^)


Edited by ccaissie, 09 November 2019 - 06:01 AM.


#77 BGRE

BGRE

    Vanguard

  • *****
  • Posts: 2480
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 09 November 2019 - 06:12 AM

Thermals aren't an issue with an interferometer if the air is well stirred, short exposure interferograms are taken and the computed wavefronts/surfaces averaged.



#78 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22989
  • Joined: 22 Feb 2010

Posted 09 November 2019 - 06:28 AM

Nope...see picture of ugly ronchigram...that's the unit in question...one of Meade's finest.

It’s not that ugly, and easy enough to fix better than 1/2~ before thinking about dpac, by just using ronchi at RoC for testing.



#79 ccaissie

ccaissie

    Surveyor 1

  • *****
  • topic starter
  • Posts: 1767
  • Joined: 13 Sep 2010
  • Loc: Whitefield, Maine

Posted 09 November 2019 - 08:10 AM

It’s not that ugly, and easy enough to fix better than 1/2~ before thinking about dpac, by just using ronchi at RoC for testing.

I could improve it by just taking the kink out with a 6" lap in 10 minutes, like you say, without testing...just winging it.  Since I got mirror #1 real good, I don't doubt I can correct it without the elaborate setups, but heck...there's more to ATM than just making usable mirrors....

 

Re: thermals...BGRE doesn't understand my use of a woodstove downstairs pouring heat through the aperture of my spiral staircase....I think it's a show-stopper, but intriguing to implement the Holloran water test, no?  Again it's a test that requires accurate setup, like the Waineo, the Offner, and others.  

 

I'm looking for dead-on simple and error-proof visual null testing.  Sphere-then-flat-then DPAC.  Like Foucault-on-a-sphere-simple.



#80 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22989
  • Joined: 22 Feb 2010

Posted 09 November 2019 - 09:32 AM

I said before you think about using dpac, not instead of

 

a pcx lens as a Ross null is very simple.


Edited by Pinbout, 09 November 2019 - 09:33 AM.


#81 BGRE

BGRE

    Vanguard

  • *****
  • Posts: 2480
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 09 November 2019 - 05:14 PM

I could improve it by just taking the kink out with a 6" lap in 10 minutes, like you say, without testing...just winging it.  Since I got mirror #1 real good, I don't doubt I can correct it without the elaborate setups, but heck...there's more to ATM than just making usable mirrors....

 

Re: thermals...BGRE doesn't understand my use of a woodstove downstairs pouring heat through the aperture of my spiral staircase....I think it's a show-stopper, but intriguing to implement the Holloran water test, no?  Again it's a test that requires accurate setup, like the Waineo, the Offner, and others.  

 

I'm looking for dead-on simple and error-proof visual null testing.  Sphere-then-flat-then DPAC.  Like Foucault-on-a-sphere-simple.

It shouldn't be a show stopper for interferometry if the air is sufficiently well stirred within the air column encountered by the test and reference beams and sub millisecond exposures are used. A simple plastic shrouded vertical test tunnel will not suffice. Lots of suitably disposed fans and a muslin screened vertical tunnel could be made to work. However its probably simpler easier and cheaper to use a horizontal test path with a smaller number of fans. In the presence of turbulent air a simple visual test isn't feasible. Some averaging is required. With a horizontal test path vertical stratification of the air will be problematic with any optical test.  



#82 Ed Jones

Ed Jones

    Soyuz

  • *****
  • Posts: 3866
  • Joined: 06 Apr 2004
  • Loc: Sin-sin-atti

Posted 10 November 2019 - 10:15 AM

I'd bet a plastic shroud would work given some time to stabilize.



#83 BGRE

BGRE

    Vanguard

  • *****
  • Posts: 2480
  • Joined: 21 Mar 2016
  • Loc: New Zealand

Posted 10 November 2019 - 04:47 PM

Plastic tunnels by themselves, at least the horizontal variety, tend to be significantly worse than just stirring the air well coupled with averaging.




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