3 replies to this topic

[saemark30]

Does it flatten the field also for imaging?

I have the original model and it has an unusual in focus requirement. Anyone know the exact amount for a 8" F/5 mirror?

[MitchAlsup]

Paracorr does flatten the field a bit.

 

About a dozen years ago I had a conversation with Al and he indicated that P1 corrected the field curvature of a Newtonian with a FL of about 80 inches. I don't know about P2 or P3.

 

With an 8" F/5 (FL=40 inches) P1 will get rid of about 1/2 of the field curvature.

[gdd]

Fortunately a Newtonian has 1/3 the field curvature of a refractor of the same focal length, so it is not so much of a problem.

 

Gale

[Starman1]

The flattening of the field alone would reduce the defocus of stars at the edge of the field of a low power eyepiece.

Removing coma goes the extra mile.

 

A story:

 

I first got a 12.5" f/5 scope (it was actually f/4.96) in 2004, along with a 35mm Panoptic as a low power eyepiece.

 

I was horrified.  The field of view looked like the stars from the flight deck of the Millenium Falcon when it went to light speed.

I thought, "Coma is horrifyingly bad!!!", so the next day I got a Paracorr (I).

 

Well, it eliminated coma and flattened the field slightly, but not enough!

The stars at the edge of the field were now round, but they were still out of focus.

 

The appearance of coma was made worse by the field curvature of the eyepiece added to the field curvature of the scope.

So what I was seeing was a vast number of de-focused comatic stars.  

Want to make coma appear worse than it is at f/5?  Put the edge of field stars out of focus. 

 

I replaced the 35mm Panoptic with a 31mm Nagler, and though the Nagler had a wider true field (42mm versus 38.7mm field stop), the stars at the edge were in much tighter focus.  I kept that eyepiece for years until I noticed my own eyes' astigmatism encroaching on the edge of the field (in an eyepiece without it).  Then I obtained a 21mm Ethos, which has a wide (though not as wide as the 31 Nagler) true field.  The smaller exit pupil eliminated my eyes' natural astigmatism, and the eyepiece had a "wide-enough" true field for a low power, so off went the 31mm Nagler to a new home.

 

There are many things, I've learned, that influence the visibility of coma in a newtonian, but field curvature in the eyepiece is definitely one of them.  

 

As for the Paracorr II in imaging?  Everything you need to know is here:

http://televue.com/e...ot#.VPIT2l0TkoU

If you have a typical DSLR, the correct distance will be nearly achieved with a Paracorr T-Ring adapter (TRG-1072), and a T-ring on the camera.  If your chip is not as far inside the camera as most are, a small spacer may be required.  For CCD cameras, which often have their chips closer to the input aperture, spacers are usually necessary.

Edited by Starman1, 28 February 2015 - 02:20 PM.