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

I Keep Forgetting

  • Please log in to reply
15 replies to this topic

#1 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7,152
  • Joined: 30 Dec 2006

Posted 11 September 2020 - 06:32 PM

...what it means in DPAC of a refractor objective when one side of focus shows a mild ((( pattern to the lines then flips to ))) on the other side of focus.

 

Please remind me.

 

Thanks!

 

Jeff


Edited by Jeff B, 11 September 2020 - 06:33 PM.


#2 Steve Dodds

Steve Dodds

    Owner - Nova Optical

  • *****
  • Vendors
  • Posts: 607
  • Joined: 02 Apr 2007
  • Loc: Utah

Posted 11 September 2020 - 07:55 PM

It's either undercorrected or over, depending on which side of focus you are on.  Both sides of focus will be opposite. 



#3 Mike Lockwood

Mike Lockwood

    Vendor, Lockwood Custom Optics

  • *****
  • Vendors
  • Posts: 1,872
  • Joined: 01 Oct 2007
  • Loc: Usually in my optical shop

Posted 11 September 2020 - 08:11 PM

I forget too.... but now I have a Michael McDonald song going through my head.....


  • Augustus and wrnchhead like this

#4 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 11 September 2020 - 08:27 PM

It's not because of missalignment or tilt of the flat relitave to the optics being tested?

A refractor should test under autocollimation the same as a reflector.

Overcorrected:
Inside (())
Outside ))((

Undercorrected:
Inside ))((
Outside (())

Correct wave front:
Inside ||||
Outside ||||

#5 Augustus

Augustus

    Fly Me To The Moon

  • *****
  • Posts: 10,043
  • Joined: 26 Dec 2015
  • Loc: Stamford, Connecticut

Posted 11 September 2020 - 08:59 PM

I forget too.... but now I have a Michael McDonald song going through my head.....

Somehow, I had the exact same song run through my head the moment I read the title.


  • wrnchhead likes this

#6 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7,152
  • Joined: 30 Dec 2006

Posted 12 September 2020 - 10:14 AM

Ok, this is what I mean.

 

In green, both inside and outside of focus.

 

Note that the spherical correction looks rather good but the lines, while parallel, bow a bit in opposite directions on each side of focus.  Also notice the rotational clocking of the lines, which is indicative of astigmatism.

 

But what does the bowing mean?  Or is it just further evidence of astigmatism.  Everything is well aligned too. 

 

It's also kinda interesting in that if I rotate the grating about 45 degrees, the bowing and stiggy basically goes away.

 

Jeff

Attached Thumbnails

  • 8 inch F9, green, inside.jpg
  • 8 inch F9, green, outside.jpg


#7 PrestonE

PrestonE

    Vanguard

  • *****
  • Posts: 2,302
  • Joined: 29 Apr 2005
  • Loc: San Miguel de Allende,Mexico

Posted 12 September 2020 - 10:19 AM

I am seeing Zoneyness at about 70% diameter and then again at about 25%...

 

In addition to what ever else there is...

 

Also, the center line should be in the Center to get an accurate assessment of 

what is actually going on.

 

Best Regards,

 

Preston



#8 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7,152
  • Joined: 30 Dec 2006

Posted 12 September 2020 - 10:31 AM

I am seeing Zoneyness at about 70% diameter and then again at about 25%...

 

In addition to what ever else there is...

 

Also, the center line should be in the Center to get an accurate assessment of 

what is actually going on.

 

Best Regards,

 

Preston

All true and thanks Preston.  Actually, the center of the center line is close to the middle. 

 

But it's that bulging that I'm curious about.  

 

Jeff



#9 ed_turco

ed_turco

    Mercury-Atlas

  • *****
  • Posts: 2,748
  • Joined: 29 Aug 2009
  • Loc: Lincoln, RI

Posted 12 September 2020 - 11:53 AM

It's not because of missalignment or tilt of the flat relitave to the optics being tested?

A refractor should test under autocollimation the same as a reflector.

Overcorrected:
Inside (())
Outside ))((


 

True.  But, if one uses a Foucault test, the shadows are reversed. When it appears that an objective shows the normal shadows for overcorrection as in a mirror,one needs to "deepen" the center to make it disappear..  I got caught in that trap my very first time



#10 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 12 September 2020 - 01:20 PM

Am I wrong that this would likely be coma do to misalignment in one or both of the optics? By rotating the OTA, you can determine if it is due to misalignment in the telescope, or in the test fixture. Spherical aberration, Overcorrection/undercorrection being the primary flaw would show symmetrical bowing of the ronchi lines vertically through the optical axis. There is astigmatism indicated by the tilting of the lines left to right, but the asymmetrical curvature of the lines inside and outside, to me, indicate misalignment/coma. Could Wedge be the issue? I'll look through my books when I get home. 

Have you tested other optics against this flat? What does the telescope show with a star test? 
is this a commercially made scope or one that you made yourself?


Edited by Matthew Paul, 12 September 2020 - 01:20 PM.


#11 Jeff B

Jeff B

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7,152
  • Joined: 30 Dec 2006

Posted 12 September 2020 - 03:16 PM

Ok, so I rotated the screen by 90 degrees and got this, keystoning, which IME means some sort of element alignment issue.

 

So next I'll try to take the lens apart and try some stuff.  Sometimes it's just a simple as rotating the elements and gently tapping the lens cell to center the elements as I snug down the retaining ring a bit.

 

Attached is a before and after group of that with a 6" F6.5 objective.

 

 

Attached Thumbnails

  • 8 inch F9, Green,Rotated, Outside.jpg
  • Before and After.jpg

  • Matthew Paul likes this

#12 Matthew Paul

Matthew Paul

    Mariner 2

  • -----
  • Posts: 218
  • Joined: 06 Jun 2016
  • Loc: Orange County, New York

Posted 12 September 2020 - 03:24 PM

Very good. Not the issue but good that you are working to figure it out. I assume that it is a doublet? Is it a scope that you fabricated or purchased?



#13 Steve Dodds

Steve Dodds

    Owner - Nova Optical

  • *****
  • Vendors
  • Posts: 607
  • Joined: 02 Apr 2007
  • Loc: Utah

Posted 12 September 2020 - 05:49 PM

Ok, so I rotated the screen by 90 degrees and got this, keystoning, which IME means some sort of element alignment issue.

 

So next I'll try to take the lens apart and try some stuff.  Sometimes it's just a simple as rotating the elements and gently tapping the lens cell to center the elements as I snug down the retaining ring a bit.

 

Attached is a before and after group of that with a 6" F6.5 objective.

It;s real hard to judge alignment using DPAC because any misalignment in the lens cam be corrected by misaligning the flat  so everything looks perfect.



#14 ed_turco

ed_turco

    Mercury-Atlas

  • *****
  • Posts: 2,748
  • Joined: 29 Aug 2009
  • Loc: Lincoln, RI

Posted 20 September 2020 - 11:38 AM

What was the topic again please?


  • Don H and Garyth64 like this

#15 davidc135

davidc135

    Apollo

  • -----
  • Posts: 1,205
  • Joined: 28 May 2014
  • Loc: Wales, UK

Posted 20 September 2020 - 12:44 PM

Hopefully, you will find that there is some wedge in the airspace between the lenses and that resolving that solves coma and astigmatism, although I don't know. It should be easy to align the cell by placing it against the flat which I guess is what you are doing.  David



#16 monoceros

monoceros

    Lift Off

  • -----
  • Posts: 13
  • Joined: 04 Nov 2015
  • Loc: Middletown, CT

Posted 09 October 2020 - 11:45 AM

What was the topic again please?

funnypost.gifrofl2.gif




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