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Paracorr - how it changes the focal plane

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#1 DaveB

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Posted 12 July 2018 - 09:54 PM

I'm not sure if this is the right forum, but here goes. I'm playing with the design of a Newt for imaging only (no visual/eyepieces). I'm using Newt on the web for ray tracing. I'm trying to figure out back-focus requirements, and I will be using a Paracorr. I have a few questions regarding this drawing: http://www.televue.c...id=61&Tab=_phot

  • Am I reading this correctly that a Paracorr will move the focal plane 56mm away from where the focal plane would have been without a Paracorr?
  • Assuming the answer is yes, should I still use the ray trace without the additional 56mm when looking at vignetting? It seems like the light paths will be the same with or without the Paracorr until the light actually hits the lenses in the Paracorr.
  • Does the Paracorr cause a change in the size of the 100% illuminated field at the sensor?
  • Anything else that I should consider?

Thanks,

Dave



#2 CygnusBob

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Posted 12 July 2018 - 10:25 PM

There is a prescription for a coma corrector for Newtonian telescope's in "Telescopes Eyepieces and Astrographs" Willmann-Bell that I think must be pretty close to the Televue Paracorr 2.   Televue indicates that the focal plane is shifted outward by 47 mm by insertion of the Paracorr 2.  The design in the book shifts it out by about 38 mm.  So there is some difference.


Edited by CygnusBob, 12 July 2018 - 11:27 PM.


#3 DaveB

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Posted 12 July 2018 - 11:55 PM

You are correct, I misstated the focal plane change as 56mm but it is actually listed as 47mm on the drawing that I linked to. I mistakenly put the distance from the Paracorr flange to the new focal plane.



#4 CygnusBob

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Posted 13 July 2018 - 10:44 AM

Dave

 

Are you trying to size up the secondary mirror with regard to vignetting?  If so, I would just ray trace the Newtonian using the focal plane position without the coma corrector in place.

 

I set up an optical design model for an 8 inch f/ 4.5 primary using the prescription from the book.  The performance is pretty good.  The design model and the actual hardware using the Televue Paracorr 2 result in an f/ 5 system.


Edited by CygnusBob, 13 July 2018 - 10:47 AM.


#5 tommm

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Posted 13 July 2018 - 10:46 AM

Pretty clear from the figure here:

http://www.televue.c...id=61&Tab=_ttop

 

So the focal plane of the primary is 14mm above the P2 shoulder and the new focal plane with P2 is 60-14=46mm from the shoulder.

 

Yes, use the original prime focus plane to determine vignetting by anything in front of the P2, but use the position the focuser will be in with the P2 present.



#6 MitchAlsup

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Posted 13 July 2018 - 11:49 AM

I'm not sure if this is the right forum, but here goes. I'm playing with the design of a Newt for imaging only (no visual/eyepieces). I'm using Newt on the web for ray tracing. I'm trying to figure out back-focus requirements, and I will be using a Paracorr. I have a few questions regarding this drawing: http://www.televue.c...id=61&Tab=_phot

  • Am I reading this correctly that a Paracorr will move the focal plane 56mm away from where the focal plane would have been without a Paracorr?
  • Assuming the answer is yes, should I still use the ray trace without the additional 56mm when looking at vignetting? It seems like the light paths will be the same with or without the Paracorr until the light actually hits the lenses in the Paracorr.
  • Does the Paracorr cause a change in the size of the 100% illuminated field at the sensor?
  • Anything else that I should consider?

If you look carefully at the referenced drawing, there is an original Newtonian focus 9mm above the position of the focused paracorr 2. Because the Paracorr 2 is in the optical train, it moves the paracorr focus up by 47mm.

 

When looking at vignetting between primary and paracorr, you ray trace from the primary to the original focal plane. 

 

When looking at vignetting from last lens in paracorr to paracorr focal plane, you ray trace from the last paracorr lens to paracorr focal plane sized to 27mm in diameter at F/ratio*1.15.

 

Paracorr causes the field to be magnified by 1.15. Thus if you took an astrophoto of an object without Paracorr in the optical train, you would see it at scale X. If you took another image of that same object with paracorr in the optical train, that scale will be 1.15*x.



#7 DaveB

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Posted 13 July 2018 - 12:28 PM

Thanks everyone. I was reading it correctly, I just wanted to be doubly sure so that I didn't end up with a design that can't reach focus with my camera. The extra focus distance, coupled with the fact that the vignetting calculations are done from the original focal plane, really helps ease some of the design parameters.




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