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.


Noticable Differences ACF versus Edge - VISUAL

  • Please log in to reply
4 replies to this topic

#1 gaz-in


    Viking 1

  • *****
  • topic starter
  • Posts: 826
  • Joined: 17 Dec 2007

Posted 25 January 2013 - 12:30 PM

I have researched and read lots of posts about these two scopes. For visual here is what I have deduced:

1) On axis about the same as a normal SCT
2) off Axis both offer imporved visual performance over tradditional SCT

What I have not found is a comparision of the off axis performance of the ACF versus the Edge in Visual use.

Is the primary distortion present in off axis visual use of a traditional SCT Coma?

If so, does the ACF correct 90+% of the off axis distortion one sees in a traditional SCT and the Edge get up to 100% thru field flattening?

Does the extra field flattening of the Edge help with visual?

Or is the field flattening mostly a photography issue?

I would appreciate hearing you all's thoughts on this.

Would be great to hear from someone who has viewed thru both...side-by-side would be awesome..

#2 Eddgie



  • *****
  • Posts: 17229
  • Joined: 01 Feb 2006

Posted 25 January 2013 - 05:12 PM

Both the ACF and EdgeHD are coma free designs, but coma is only one of the serious off axis issues with SCTs. The other is field curvature.

What makes the comatic blur so objectionable for visual use is that with a standard SCT, due to the field curvature, a star at the edge of the field is not just comatic, but it is expanded in size because of the defocus.

If you eliminate the coma, the star only apears slightly swollen but not comatic.

And this means that as long as the defocus is smaller than about 2 or 3 arc minutes of apparent field, the blur will still look more or less point like.

However, and this is a big however, this expanded blur does become large enough to resolve if you used eyepecie designs with very large apparent fields. For example, if use a 22mm Nagler, it will be difficult to get the outside of the field and the inside of the field into focus at the same time. If you choose to focus on the inside, a keen eyed observer will actually see that stars at the edge of the field are slightly out of focus.

A 27mm Panoptic though will show the edge being much sharper when the field is in focus at the center simply because the magnificaiton is much lower in the 27mm Pan for a field that is about the same true size.

If you like 68 degree AFOV eyepieces, the ACF should give excellent results.

If on the other hand, you want to use a long focal lenght Ethos, you may have a bit of trouble keeping stars pinpointy at the edge.

Both scopes are better than the standard SCT by a long shot with modern ultra and super-ultra wide field eyepcies, but the wider AFOV eyepeices you use, the more you would benefit from the EdgeHd.

#3 Jon Isaacs

Jon Isaacs


  • *****
  • Posts: 54947
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 25 January 2013 - 05:23 PM


Here's question in a similar vein:

How does the edge performance of a standard Celestron F/6.3 corrector used with a Standard SCT compare with the Edge?


#4 GlennLeDrew


    Voyager 1

  • *****
  • Posts: 12747
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 25 January 2013 - 06:20 PM

The f/6.3 reducer is designed to correct both coma and field curvature on the 'standard' SCT. Up to the designed image circle diameter on said reducer, the visual performance off axis may be of similar quality to that delivered by the EDGE. On or near on axis, I wouldn't be surprised if the EDGE has the (ahem) edge. Also, the EDGE will offer a physically larger circle of good imagery than will the reducer-equipped SCT.

#5 Eddgie



  • *****
  • Posts: 17229
  • Joined: 01 Feb 2006

Posted 25 January 2013 - 06:34 PM

I have tried both and I personally feel like the EdgeHD gives a far better result than a focal reducer producing a similarly wide field.

I felt that I lost some brightness when using a 2" diagonal and a 2" eyepiece behind a focal reducer, but most people don't seem to be bothered by it (or don't notice it).

When using a 22mm Panoptic in a 1.25" diagonal, I did not feel like the view in a C8 was as good as with a 35mm Panoptic with no reducer (both give about the same size true field. The edge of field seemed sharper to me using just a 35mm Pan.

The EdgeHD though is in another class. On par with my 6" APO for off axis sharpness. I have never used another reflector that produced a field with such beautiful off axis performance. Stars in a 31mm Nagler are brilliantly sharp across the field.

20mm Nagler is likewise perfect to the edge (As are Panoptics in the EdgeHD).

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.

Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics