Low Power Wide Angle Inexpensive Eyepiece for my Mak? Ha! I should be so lucky!

[John]

13 minutes ago, Mr Spock said:

No, 72° 

It is also physically measured at 72°, it's just not 42mm... See the images for comparison - all taken with the same iPhone. You see how much larger the fov is over the 22mm 65° LVW.

I've seen some with 65 degrees marked on them. Maybe Vixen aren't sure themselves !

[Louis D]

I guess it comes down to, do you mark them with the apparent field of view (AFOV) as seen by the eye or the effective apparent field (eAFOV) of view that allows the classic true field of view calculation to work.  Vixen used the first one initially until astro forum people loudly complained that it couldn't possibly have a 72° eAFOV (without even measuring the AFOV via projection before complaining), and then Vixen switched to the second one to shut them up.

I've seen a third method used to advertise AFOV, the comparative method.  The 35mm Baader Scopos Extreme is marked 70° despite having a 65°/66° AFOV and a 68° eAFOV due to its 41.3mm field stop diameter.  I guess they figured that since it has a slightly larger field stop than the 35mm Panoptic (38.7mm) and the same focal length, it must have a slightly wider AFOV (70° vs 68°), ignoring that it is the Panoptic's distortion that gets it to 68°.  The BSE 35mm is very sharp in the central 60%, has a slight blurring for some reason at midfield for maybe 10% that goes away if you tilt your head just right, and then sharpens up again toward the edge.  It really feels like a Pentax XL with that ~65° AFOV.  It feels a bit constrictive after using 70° to 78° eyepieces like the Pentax XW, Delos, and Morpheus.  On the other hand, distortion across the field is very moderate compared to a Panoptic.  It is a huge and heavy eyepiece, so there probably was just no saving it from discontinuation.

The 26mm (really 25mm) Meade MWA takes a similar approach.  Since it has the exact same 41mm field stop diameter as the 25mm ES-100, and the same actual focal length, it must also have a 100° AFOV by Meade's reasoning, and so they advertise it as such.  In reality, it has an 83° AFOV and a 90° eAFOV due to distortion of the opposite sign from the ES-100.  If it didn't have so much SAEP, it would actually have been a very good eyepiece.  It's pretty much sharp to the edge at f/6, but you can't see it unless you push in tight (10mm of eye relief) and lose the midfield part of the view.  If you hang back at 18mm of eye relief, you get a 79.4° AFOV and a very usable 37.7mm field stop diameter with only very moderate SAEP shadows.  Thus, it could be considered a good way to complete a Morpheus set with a 25mm version, but that's not how they marketed it.

Edited January 7, 2022 by Louis D

[John]

I was quite impressed by the Aero ED 40mm clone that I had for a while. It was relatively lightweight for it's size, well corrected across the field of view even in my 12 inch F/5.3 dobsonian (although the exit pupil with that scope would not be the most efficient) and seemed to provide an AFoV which was the equivalent of the Pentax XW's (70 degrees).

 

[Louis D]

1 hour ago, vlaiv said:

Image consists out of full moon in the center of the field and surrounding sky up to field stop (sky is bright enough against field stop so that you can easily distinguish it in each eye).

If focal lengths are the same - then moon will overlap perfectly from both eyes - there will be no double image of it.

If AFOV is the same - then field stops will align perfectly - there will be no double image.

You can have the same moon image and different surrounding sky images and you can have different moon sizes for same sky/field stop image. There are third and fourth cases - when both are the same and both are different.

This shows that AFOV and magnification are not effectively the same thing.

(you don't need the moon to compare AFOVs of two different eyepieces - you just need blank well lit wall - hold two eyepieces - each against one eye and let brain try to merge the image - if you have trouble and can't align field stops - that means AFOVs are different - and in fact, you can judge which one is larger by favoring each eye in turn).

You're assuming that distortion/magnification is the same across the field for the two eyepieces.  Let's say that they are both n millimeters focal length in the inner 10% of the field, then craters there will nicely merge in a binoviewer.  However, if distortion/magnification varies differently across the field, then craters further out won't merge because they are of different sizes at different positions in each field.  AFOV can be the same, but more or less of the moon might be visible due to differing distortion/magnification across the field.

I understand that there are two major types of distortion.  One where objects are the right size but positioned wrong across the field and one where objects are positioned correctly but vary in size across the field.  I'm asserting that at least the second one qualifies as magnification or focal length changing across the field.  I can't recall ever having an eyepiece that shows objects as the same size across the field, but moving them at differing speeds across the field during panning which the first type of distortion implies.  They always get bigger as they speed up toward the edge or slow down as they shrink toward the edge, but they never remain the exact same size from edge to edge as they speed up or slow down.

[Louis D]

10 minutes ago, John said:

I was quite impressed by the Aero ED 40mm clone that I had for a while. It was relatively lightweight for it's size, well corrected across the field of view even in my 12 inch F/5.3 dobsonian (although the exit pupil with that scope would not be the most efficient) and seemed to provide an AFoV which was the equivalent of the Pentax XW's (70 degrees).

Dang it, now you're making me want to hunt one down on the secondary market just to compare it to the 40mm Meade SWA 5000 and 40mm Pentax XW. 😉

[Don Pensack]

On 07/01/2022 at 08:40, vlaiv said:

I don't think they quite are.

Imagine following scenario, you have two eyepieces.

You look thru one eyepiece with your left eye and thru other eyepiece with your right eye and let your brain try to merge image.

Image consists out of full moon in the center of the field and surrounding sky up to field stop (sky is bright enough against field stop so that you can easily distinguish it in each eye).

If focal lengths are the same - then moon will overlap perfectly from both eyes - there will be no double image of it.

If AFOV is the same - then field stops will align perfectly - there will be no double image.

You can have the same moon image and different surrounding sky images and you can have different moon sizes for same sky/field stop image. There are third and fourth cases - when both are the same and both are different.

This shows that AFOV and magnification are not effectively the same thing.

(you don't need the moon to compare AFOVs of two different eyepieces - you just need blank well lit wall - hold two eyepieces - each against one eye and let brain try to merge the image - if you have trouble and can't align field stops - that means AFOVs are different - and in fact, you can judge which one is larger by favoring each eye in turn).

This is the reason I asked in the first place about percent of distortion. Televue has same formulae about relationship of AFOV, field stop and focal length on one of their pages:

https://www.televue.com/engine/TV3b_page.asp?id=113

Now, as you mentioned:

If that eyepiece has zero AMD, then we need to use

and if we use that formula on 24mm FL and 27mm field stop we get:

beta = 27 / 24 = 1.125 radians = 64.46°

But if we do something else - and assume that AFOV is indeed ~68° then we can do following:

68° = 27 / actual_fl => actual_fl = 27mm / 68° = ~22.75

So maybe Panoptic 24mm is actually 22.75mm FL eyepiece.

Or maybe there is some middle ground and FL is something like 23.4mm and AFOV is from there 66° and for marketing purposes it is declared as 24mm 68° to be in line with rest of Panoptic EPs.

In the end - maybe field stop is not precisely 27mm but a bit more?

 

I agree,  perhaps the focal length is not exactly 24mm, or the field stop is not exactly 27mm, or the apparent field is not 68°.

However, those calculations seem to be assuming orthoscopy, and distortion can alter the figures.

I just measured a new one, and the field stop is 27.15mm +/-0.02mm