Panoptics 19 mm opinion and suggestion on set.

[Fedele]

Hi to all. 

I have a TSA120, a Mewlon 180c, a FS60CB
I have a whole series of eyepieces that I would like to replace. I currently have:
- n 2 Taka Abbe Ortho 32mm
- n 2 TV Ploss 25 mm
- n 2 taka Ortho 18 mm (my favorite eyepieces)
- n 2 FF 12mm (which I want to replace)
- n 2 Tala LE 7.5
- n 1 TOE 4 mm that I would like to double
other and various single EPs that I do not mention
In general, I love the large field even in high resolution. I bought with high hopes, two 19 mm Panoptics TVs thinking it was the right focal length to take into consideration. To my surprise on all telescopes they revealed a strong field curvature that I did not imagine at these levels. I like these eyepieces for their quality and compactness, but this problem bothers me when I look at the Moon or a cluster that takes up the whole field. On M42 they are perfect.
I was told that Pans don't introduce curvature, they simply capture more curved field of the telescope. this is true? i don't think.
would I have done better with 17mm Naglers?

2) do you recommend a double complete setup that can be used in the turret on all three instruments?
I thought:
n 1 ES 2 "68 ° 28 mm for the greatest possible field on Mewlon and usable on the other two in monoocular
n 2 Pan 19 mm precisely that I would now like to replace with Nagler 17 or ES to save money and which have less circulation

n 2 ES 11 mm 82 ° 31.8

add a 4 mm TOE
n 2 TOE 2,5 mm or you can suggest some other cheaper wide field eyepiece.

thanks

[JeremyS]

The Panoptic 24 is well known for its rectilinear distortion. It makes the moon look egg shaped when it is near the edge of the field. I understand the Pan 19 exhibits similar. Might this be what you see, rather than field curvature?

[Fedele]

Yes. Is another aberration?

[JeremyS]

4 minutes ago, Fedele said:

Yes. Is another aberration?

Yes. designing a wide field eyepiece has many trade offs that need to be balanced. And some people are more accepting of some trade offs than others. I’ve been looking for an alternative to the Pan 24 for a year. E.g. the APM 24 UFF and Tak Erfle 28 are lovely eyepieces with less RD, but they have other cons. Which is why I still have my Pan 24.

Life is full of compromises 😊

[Don Pensack]

The wider the field, the more distortion an eyepiece has at the edge of the field.

 

Rectilinear distortion, RD, which either stretches (pincushion) or compresses (barrel) radial lines is found in just about all eyepieces.

Usually pincushion is chosen to mitigate other aberrations (like chromatic aberration).

Pincushion distortion makes straight lines look like this as they cross the field  )  |  (

Barrel distortion makes straight lines look like this as they cross the field  (  |  )

Our eyes see up to 7% pincushion distortion as undistorted, so this is the form of RD chosen in 98% of all eyepieces.

Its primary noticeability is in panning across star fields.  Barrel distortion is especially noticeable this way because it makes the field look as if it is passing over a ball behind the field (aka rolling ball distortion).

 

The presence of rectilinear distortion reduces the effects of angular magnification distortion, AMD, which changes the magnification of the eyepiece from center to edge.

This type of distortion is worse, since objects tend to shrink or grow as they near the edge of the field.

If the magnification is lower at the edge of the field, the background sky might be brighter, which is also a negative.

 

As RD goes up, AMD goes down, and vice versa.  Some designers opt for some of both.  I see that in a lot of binoculars.  It isn't the best solution for astronomy, however.

 

Field curvature, where focus at the edge of the field is a different place than the focus at the center, is only a problem if your eye cannot accommodate the change in focus.

Older people, whose eyes accommodate less, have more problem with this than younger people.

Many people are unaware that refractors have inherent field curvature and may interface with eyepieces to yield more visible field curvature than reflectors.

Old eyes and refractors may not go well together, especially a refractor as small as 60mm.

Field curvature might be completely unnoticeable if the curvatures of eyepiece and scope match.  In essence, ) + ) = |

In can be atrocious if the curves are opposite, like ) + ( which puts the edges of the field very far out of focus when the centers match.

You will see this in reviews where someone comments on the field curvature in an eyepiece, when it is really the mismatch between eyepiece and scope.

) in the scope + | in the eyepiece (flat) = ) to the eye.

 

Eyepieces can be designed to have nearly zero noticeable distortion at the edge of the field.  The absence of distortion, or orthoscopy, will confine the eyepiece to very narrow apparent fields, however,

like the 42° fields in most Abbe orthoscopics (Abbe was an optical designer).  Wider fields of view cannot be orthoscopic.

 

Then there is astigmatism in eyepieces, which is a worse problem, but that is a discussion for another post.

 

 

 

 

[Louis D]

You really need to verify which issue you're seeing.  If you're truly seeing field curvature, you can focus it out by refocusing for the edge.  If you're simply seeing field distortion as others above describe, it is not called field curvature.  Calling it that simply misguides us in trying to help you out.

If you are truly seeing field curvature, a TSFLAT2 used ahead the diagonal can generally flatten the field.  I use them on both my 72ED and 90mm APO refractors to good effect with my wide field eyepieces.

Edited March 21, 2022 by Louis D

[Highburymark]

Here’s the difference between a 25mm orthoscopic microscope eyepiece and the Panoptic 24 looking at a squared target through a 60ED refractor. The Panoptic has a huge amount of pincushion distortion. Yet it’s a sparking eyepiece if used in the right scopes on the right objects.

[Voyager 3]

9 hours ago, Louis D said:

You really need to verify which issue you're seeing.  If you're truly seeing field curvature, you can focus it out by refocusing for the edge.  If you're simply seeing field distortion as others above describe, it is not called field curvature.  Calling it that simply misguides us in trying help you out.

If you are truly seeing field curvature, a TSFLAT2 used ahead the diagonal can generally flatten the field.  I use them on both my 72ED and 90mm APO refractors to good effect with my wide field eyepieces.

What diagonal are you using with the FF Louis? 

Edited March 19, 2022 by Voyager 3

[Louis D]

I'm using GSO diagonals (which are sold under a variety of brands) with either an SCT to M48 thread converter on the front with a 15mm M48 extension ring or an SCT male thread to 2" nosepiece adapter as a single piece solution.  Simply screwing the TSFLAT2 onto the front of the diagonal nosepiece provides too much separation (about an extra 15mm, IIRC) and overcorrects the field curvature.

I've read that the Long Perng diagonals (the ones with the convex eyepiece receiver barrel sold as WO, Orion, StellaMira, etc.) natively have M48 threads on the nose barrel thread instead of SCT threads, so that might be an easier approach to mounting a TSFLAT2.  Perhaps someone on here who has one can confirm their nosepiece thread size.

I've got a bunch more M48 spacer rings of various lengths now, so I'll see if I can fine tune the spacing for both the 72ED and 90mm APO in the future.  Technically, the 90mm should require 12mm less separation having a 50% longer focal length, but I've found 15mm of extension to still provide better field flattening over no extension, subject to the aforementioned fine tuning.