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56mm Meade Plossl vs. 40mm Pentax XW in ST-80

Eyepieces
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#1 GGK

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Posted 26 October 2021 - 12:40 PM

I recently purchased a Meade 56mm Plossl to increase the exit pupil at maximum field of view in my C9.25 SCT. My intent is to brighten the image when using an OIII filter as compared to my 40mm Pentax XW.  I won’t have my SCT set up for a few weeks, so decided to try the Plossl in my ST-80.

 

I often observe using 30mm and 40mm Pentax XW eyepieces in my SkyWatcher 80mm f/5 refractor, producing true fields of view from 5 to 6.6 degrees.  I decided to compare the 56mm Meade Plossl to the 40mm Pentax XW in the ST-80 since each gives the maximum field of view. 

 

This is not a fair comparison.  The Meade Plossl costs $85 vs. the $400 you’ll pay for the XW, and the 40mm XW obviously produces higher magnification.  In addition, at f/5, both eyepieces give a huge exit pupil that exceeds my eye pupil diameter, so any potential “brighter image” advantage the Plossl might have at, say, f/10 is gone.  And in addition to that, the Plossl is further handicapped due to the greater reduction in effective aperture based on more “wasted” light not able to get into my eye pupil.

 

However, a used 56mm Plossl is a cost effective way to reach a 46mm field stop and get the absolute largest true field of view, so with respect to that purpose, a comparison against the 40mm Pentax XW might be relevant to someone.

 

On two different mornings I observed Orion’s belt and sword (all in same view). On these mornings, the nearly full moon was about 50 degrees away. On two different evenings I observed the full / nearly full moon.  And on a final morning, I observed Pleiades and the waning 75% +/- gibbous moon. Seeing was average and transparency was very good during all sessions.

 

I was using a 2 inch Baader BBHS mirror diagonal with a 47.5mm clear aperture to assure full illumination of the eyepiece. No flattener was used. I normally wear glasses, but observe without them.

 

IMG_2624.jpg  IMG_2625.jpg

IMG_2627.jpg IMG_2626.jpg

 

Magnification

- 40mm XW = 10

- 56mm Plossl = 7

 

True Field of View
- The 40mm XW specs list a 70° AFoV and a 46.5mm field stop giving a calculated 6.66° TFoV.
- When observing Orion, the image went from above Mintake on Orion’s Belt to below the brighter stars under Hatysa at the bottom of the Sword.
- The 56mm Plossl specs indicate a 50° AFoV, and the Plossl should also show the maximum field of view for a 2-inch eyepiece. 
- In a direct comparison, the XW produced a slightly larger true field of view than the Plossl.  This makes sense because the Plossl barrel is visibly thicker than the XW barrel and I’m guessing the barrel ID is closer to 46mm than the XW's 46.5mm. In my opinion, this small TFoV difference would not be noticeable if I wasn’t looking for it.

 

Reaching focus
- The Plossl needed about 25mm more outward focuser travel to reach focus.  This was not an issue for the GSO focuser on my ST-80.

 

Eye lens diameter
- XW = 34mm hole in the adjustable eye cup and it looks like the recessed eye lens diameter is the same.
- Plossl = 25mm hole in the top surface iris with a much larger recessed eye lens.  The Plossl has a flip up rubber eye guard.

 

Eye placement
- The XW is one of the easiest eyepieces to use.  I have absolutely no issues with blackouts or with seeing the entire field stop regardless of where I set the eye cup.  My preference is to view with the eye cup nearly fully extended with my face touching the rim of the eyepiece.  This blocks side ambient light and gives me the feeling of being inside the view. 
- The Plossl was fine, but not as friendly as the XW.  It was easy enough to hold good eye position, but I could get blackouts by moving too close or too much side-to-side.  For me, the 25mm iris hole is too small and I am not quite able to see the entire field stop without moving my head slightly – even with only the 50° AFoV.  In my opinion, this is a design flaw since the recessed eye lens diameter is huge and the hole in the iris could easily be 10mm larger.  Maybe other observers with better eyes can get a little closer and see the full field stop.

 

Field curvature and image distortion at the edge of field 
- In the ST-80, both eyepieces show similar distortion when moving the full moon from the center to the edge.  The moon is round in the center of the view and goes oblong near the field stop. This is not really an issue since the TFoV is over 6.5 degrees – when moving the moon around the middle of the view, it stays nice and round.
- Field curvature is also similar for both eyepieces.  With sharp focus in the center, edge focus is a little off, but as with the distortion, this is not really noticeable when viewing at these magnifications and wide fields of view.

 

The moon - Both eyepieces produced a sharp image with fine detail and contrast at low magnification that gave the impression that the moon was floating out in space.  The higher magnification of the XW clearly wins because of the larger image size and being able to see more minor detail.

 

Orion – The moon was about 50 degrees away.  Sky background darkness level appeared equal in both eyepieces, but the XW clearly showed more stars overall and with the wider AFoV displayed a more impressive view.  This was very evident when I put NGC1981 in the center of the field.  Fainter stars easily visible with the XW fully disappeared in the Plossl.

 

Pleiades – Pleaides at a 6.5° field of view is amazing in the 40mm XW with 70° AFoV, but lost some of it’s luster in the Plossl at 50° AFoV.  And like in Orion, many stars easily visible in the XW were not seen in the Plossl.  I focused on the asterism Ally’s braid dropping from Alcyone.  The 40mm XW showed all 7 stars in nice contrast, but in the 56mm Plossl, I had to use averted vision to know that the 4th, 5th, and 6th stars were there.  In addition, there is a semicircle (sort of) of stars nearing 2 degrees out from Atlas that sort of hug Pleiades in a 6° field of view.  These stars were easily seen in the XW, but quite dim and almost unnoticeable in the Plossl.  

 

 

Overall, my clear preference in the ST80 is the 40mm Pentax XW at 70° AFoV.  It was more comfortable to use and provided more impressive views with more visible stars compared to the 56mm Meade Plossl.

 

Thinking about why the 56mm Plossl did not show as many stars -- I believe this is more related to the reduced effective aperture from the extra large exit pupil that's well beyond my eye pupil diameter. If I use 6mm for my eye pupil size, and if I did the math right, the effective telescope aperture drops to 60mm when using the 40mm XW, and all the way down to 43mm when using the 56mm Plossl. 

 

 

My conclusion is that although the 56mm Meade Plossl can be used to reach the widest possible field of view in the ST-80, I would not recommend buying it for that purpose.  I know of no lower-priced eyepieces with a 46mm field stop, so although I very much like the 6.5 degree views, if I was starting out, I would consider a lower cost eyepiece with a ~35mm field stop diameter for a nice 5 degree field of view instead.

 

 

When I get the SCT set up, I'll compare the Plossl and XW again.

 

Gary


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#2 GGK

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Posted 26 October 2021 - 04:05 PM

If anyone has compared other 55 / 56mm Plossls to the Meade, I would be interested in hearing your comments.  The Meade Plossl I purchased is the only one I've ever used, and I don't know how it compares to Celestron or TeleVue or other brand. Thanks.



#3 Mike Mc

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Posted 26 October 2021 - 05:26 PM

Here’s an old review comparing Meade 4-element and 5-element vs Televue: https://www.cloudyni...mm-5-elem-r2198. A lot of discussion of this over the years. 
 

hope this helps. 
 

cs


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#4 sanbai

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Posted 26 October 2021 - 05:32 PM

For maximum true field I have both a TV 55 Plössl and a TV 31T5.

For *observing*, the only advantage of the Plössl is when used for large, dim nebulae coupled with a narrowband filter *and* dark skies (Bortle 5 or better). That's in my current f/10 SCT and f/7.5 refractor. For whatever else, the 31T5 is the best option, no doubt. (An exception could be for aesthetics with specific open clusters; I don't know any example, though).

However, I'm using these days the Plössl as a convenient finder eyepiece in both scopes. Example of this week is for quick observations of Jupiter, no other finder and even using the Evolution mount in fully manual. I don't even bring it to focus, Jupiter and Saturn are bright enough (I have a barrel extender in the Plössl).

I recently split my eyepieces in two cases, and the Plössl went with the lighter set. Grab and go even for home use. As a finder it's way more convenient than the heavy and bulky 31T5.
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#5 GGK

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Posted 26 October 2021 - 05:41 PM

Here’s an old review comparing Meade 4-element and 5-element vs Televue: https://www.cloudyni...mm-5-elem-r2198. A lot of discussion of this over the years. 
 

hope this helps. 
 

cs

Very interesting, thanks. If I find that I like this focal length in the SCT for OIII filter use on large nebula,  the removable eye extension of the Televue might be better for me in terms of seeing the full field to the field stop. Of course, the Televue eye lens opening might be larger than the one on my Meade Plossl, which would also resolve that problem. 
 

Gary



#6 Mike Mc

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Posted 26 October 2021 - 07:19 PM

I’ll be very interested to read your comparison of the big plossl vs the Pentax xw40 in your C9.25. I’m seriously considering the xw40 as well. I have the old 5 element Meade plossl. I love the plossl’s clarity, sharp field stop, crispness to the edge. But I struggle some with the extra long eye relief. I fashioned a home made eye cup from a spray paint lid, and it helps a lot. I do enjoy it under dark skies with large targets, Pleiades, Andromeda galaxy, double cluster, etc.  But I find myself using the ES30-82 a lot more on the wide fields due to the higher mag. But the ES just doesn’t have enough exit pupil with a lot of extended nebula under a filter. so I’m hoping a good 40mm will hit a sweet spot of magnification and image brightness especially on emission nebula targets. 

 

It’s sure easy to talk myself into one more eyepiece :)


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#7 CHASLX200

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Posted 26 October 2021 - 07:40 PM

The 55mm Televue and Meade 56mm Plossl are nice in a SCT.


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#8 Jon Isaacs

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Posted 26 October 2021 - 07:48 PM

Overall, my clear preference in the ST80 is the 40mm Pentax XW at 70° AFoV.  It was more comfortable to use and provided more impressive views with more visible stars compared to the 56mm Meade Plossl.

 

Thinking about why the 56mm Plossl did not show as many stars -- I believe this is more related to the reduced effective aperture from the extra large exit pupil that's well beyond my eye pupil diameter. If I use 6mm for my eye pupil size, and if I did the math right, the effective telescope aperture drops to 60mm when using the 40mm XW, and all the way down to 43mm when using the 56mm Plossl.

 

 

Gary:  

 

First let me say that I think you did :goodjob: in your comparison.  

 

I checked your calculations and they seem to be right on the money.  And I believe your are also right on when it comes to the reason you did not see as many stars with the 56mm Plossl. This is something that is not often mentioned when discussing oversized exit pupils, the surface brightness of the object, of the sky, are not diminished but the stars, since their brightness depends on aperture, are dimmed.  

 

I have an ST-80 with a 2 inch focuser, I actually have a field flattener for it as well.  I mostly use the 31mm Nagler which provides a 6.0 degree field with a 6.2 mm exit pupil at 13x.  It's really a great view but it's a very expensive eyepiece.  

 

The 30mm APM 70 degree UFF would provide a 5.2 degree field at a little over 13x with a 6mm exit pupil, that's a very nice view.  In terms of an inexpensive eyepiece, the 32mm 70 degree eyepieces like the Orion Q-70 would be a good choice.  Quite a bit of off-axis astigmatism to go with the refractors field curvature but its livable and that would be a 5.7 degree field at 12.5 x with a 6.4 mm exit pupil.  

 

Nice work, nice observations.. :waytogo:

 

Jon


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#9 GGK

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Posted 29 October 2021 - 10:14 AM

I viewed Pleaides again Wednesday night with no moon,  comparing the 40mm Pentax XW against the 56mm Meade Plossl under much darker skies than the first session.  Seeing was only fair with the weather front approaching, but transparency was very good.  I wanted to know how much the moon’s light pollution was impacting the results of the first comparison.

 

Images in both the 56mm Plossl and the 40mm XW were very nice and improved as expected, but the XW continued to show more faint stars than the Plossl. 

 

I believe this supports the comments above:   Reduced effective aperture caused by the 56mm Plossl oversized exit pupil at f/5 is the likely reason that fainter stars clearly shown by the 40mm eyepiece are not visible in the 56mm eyepiece.

 

Gary


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#10 Jon Isaacs

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Posted 29 October 2021 - 10:29 AM

I viewed Pleaides again Wednesday night with no moon,  comparing the 40mm Pentax XW against the 56mm Meade Plossl under much darker skies than the first session.  Seeing was only fair with the weather front approaching, but transparency was very good.  I wanted to know how much the moon’s light pollution was impacting the results of the first comparison.

 

Images in both the 56mm Plossl and the 40mm XW were very nice and improved as expected, but the XW continued to show more faint stars than the Plossl. 

 

I believe this supports the comments above:   Reduced effective aperture caused by the 56mm Plossl oversized exit pupil at f/5 is the likely reason that fainter stars clearly shown by the 40mm eyepiece are not visible in the 56mm eyepiece.

 

Gary

 

Gary:

 

I think there's little doubt that the reduced aperture is the reason you are seeing less stars.  The sky brightness with both eyepieces is identical but stars are are twice as bright in the 40mm.

 

Jon


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#11 Voyager 3

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Posted 29 October 2021 - 10:48 AM

And reduced magnification too .. (same as reduced exit pupil)
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#12 GGK

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Posted 29 October 2021 - 11:06 AM

Gary:

 

I think there's little doubt that the reduced aperture is the reason you are seeing less stars.  The sky brightness with both eyepieces is identical but stars are are twice as bright in the 40mm.

 

Jon

Yea, I agree.  This is something I knew on paper, but was surprised to see it demonstrated so obviously in this comparison. 

 

I grew up in the f/10 world always wishing I had a bigger exit pupil.  More things to think about at f/5.


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#13 sanbai

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Posted 29 October 2021 - 11:41 AM

Yea, I agree.  This is something I knew on paper, but was surprised to see it demonstrated so obviously in this comparison. 

 

I grew up in the f/10 world always wishing I had a bigger exit pupil.  More things to think about at f/5.

A large exit pupil is nice to have, but it shows a lot the light pollution. That, and the need of (my mild) astigmatism correction (I have Dioptrx) will probably impact the use of the 31T5 in my incoming telescope (f/5.15 effective) in favor of the Ethos 17 (no correction needed). In my usual (not-so-)dark site will be just OK with the 31mm, but from home, forget about.


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#14 Dave Mitsky

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Posted 29 October 2021 - 01:02 PM

The Astronomical Society of Harrisburg owns an old 55mm University Optics Plössl and a 56mm Meade Series 4000 Super Plössl.  I've never compared the two but will do so at the next opportunity.

I've always preferred the views that a 38mm Agena SWA and an old 40mm University Optics MK-70 produce in the club's f/10 SCTs and 17" f/15 classical Cassegrain.


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#15 faackanders2

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Posted 30 October 2021 - 09:18 PM

The Astronomical Society of Harrisburg owns an old 55mm University Optics Plössl and a 56mm Meade Series 4000 Super Plössl.  I've never compared the two but will do so at the next opportunity.

I've always preferred the views that a 38mm Agena SWA and an old 40mm University Optics MK-70 produce in the club's f/10 SCTs and 17" f/15 classical Cassegrain.

I have UO 40mm 70AFOV MK-70 Koenig and love it for max TFOV in a 2" eyepiece!


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#16 pweiler

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Posted 01 November 2021 - 09:11 AM

I just sold a less than 1-year Meade Super Plossl 56mm. It was a great eyepiece in a cc8 f/12 scope!, but under heavily light polluted Bortle 8 skies, where I view most of the time, a 40mm or smaller works slightly better. Under the very few trips to better skies, the 56mm’s big exit pupil and wide and flat field of view really starts to be appreciated. The 56/55mm eyepiece is at the more extreme end of eyepieces sizes. I recommend checking with other owners/users of your scope to determine if it is going to work on your scope first. Some scopes might need adjustments to reach focus and reflectors might show a secondary shadow, coma, or other aberrations especially in fast scopes. On paper this EP shows impressive numbers for exit pupil, fov, low power, but some scopes are just not designed to handle a 56/55mm out of the box. My 2-cents.
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#17 sanbai

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Posted 01 November 2021 - 12:00 PM

An update on my previous post.

On Saturday I went to our club's "dark site" (It would say we had a good Bortle 5, and it was a good night). The Helix planetary nebula (w/ C8"edgeHD f/10 + OIII), and the California nebula (w/ 80mmED f/7.5 + H-beta) were two targets where the the Plössl could shine. However, I did like a bit more the result with the 31T5. The contrast should not be different (because these are extended objects), but the deeper black of the sky offered by the 31T5 did probably help. Again, the difference was small.

 

Again, this is mostly an issue of exit pupil and pollution, not optical quality (although a bit of that too). I think those sky conditions are still suboptimal for large exit pupils (to my despite). The 55 still continues to be a favorite for finder use though. Form factor and size are important, even if the effort of grabbing something  like a 31T5 is apparently minimal (and that is which makes me think about completing a set of Naglers T6, which in addition are parfocal... no surprise that Jon keeps them)


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#18 faackanders2

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Posted 01 November 2021 - 08:22 PM

An update on my previous post.

On Saturday I went to our club's "dark site" (It would say we had a good Bortle 5, and it was a good night). The Helix planetary nebula (w/ C8"edgeHD f/10 + OIII), and the California nebula (w/ 80mmED f/7.5 + H-beta) were two targets where the the Plössl could shine. However, I did like a bit more the result with the 31T5. The contrast should not be different (because these are extended objects), but the deeper black of the sky offered by the 31T5 did probably help. Again, the difference was small.

 

Again, this is mostly an issue of exit pupil and pollution, not optical quality (although a bit of that too). I think those sky conditions are still suboptimal for large exit pupils (to my despite). The 55 still continues to be a favorite for finder use though. Form factor and size are important, even if the effort of grabbing something  like a 31T5 is apparently minimal (and that is which makes me think about completing a set of Naglers T6, which in addition are parfocal... no surprise that Jon keeps them)

I would think a 41mm 68AFOV Panoptic or 40mm 70AFOV UO MK70 Koenig would perform better than 55mm Possil in any scope, since they would have same TFOV and higher power and hence more contrast, but the panoptic would cost more and they are probably heavier also.


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#19 Dave Mitsky

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Posted 02 November 2021 - 12:13 AM

If anyone has compared other 55 / 56mm Plossls to the Meade, I would be interested in hearing your comments.  The Meade Plossl I purchased is the only one I've ever used, and I don't know how it compares to Celestron or TeleVue or other brand. Thanks.

 

The Astronomical Society of Harrisburg owns an old 55mm University Optics Plössl and a 56mm Meade Series 4000 Super Plössl.  I've never compared the two but will do so at the next opportunity.

I've always preferred the views that a 38mm Agena SWA and an old 40mm University Optics MK-70 produce in the club's f/10 SCTs and 17" f/15 classical Cassegrain.

I briefly compared the 55mm University Optics Plössl and the 56mm Meade Series 4000 Super Plössl last night using the club's 17" classical Cassegrain. 

 

I tried both eyepieces on Jupiter.  The difference in magnification between the two was only 2x.  The 55mm seemed to have a more clearly defined field stop but was harder to maintain proper eye placement with, despite the lengthy eye guard.  The 56mm was more comfortable to use.  Neither eyepiece seemed to exhibit any lateral color when Jupiter was placed at the field edge.

I was going to do further comparisons a bit later but clouds rolled in before I had expected.

Attached Thumbnails

  • 55 and 56mm Plossls 10-31-21 IMG_0691 Reprocessed Resized 900.jpg

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#20 Jon Isaacs

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Posted 02 November 2021 - 12:52 AM

An update on my previous post.

On Saturday I went to our club's "dark site" (It would say we had a good Bortle 5, and it was a good night). The Helix planetary nebula (w/ C8"edgeHD f/10 + OIII), and the California nebula (w/ 80mmED f/7.5 + H-beta) were two targets where the the Plössl could shine. However, I did like a bit more the result with the 31T5. The contrast should not be different (because these are extended objects), but the deeper black of the sky offered by the 31T5 did probably help. Again, the difference was small.

 

Again, this is mostly an issue of exit pupil and pollution, not optical quality (although a bit of that too). I think those sky conditions are still suboptimal for large exit pupils (to my despite). The 55 still continues to be a favorite for finder use though. Form factor and size are important, even if the effort of grabbing something  like a 31T5 is apparently minimal (and that is which makes me think about completing a set of Naglers T6, which in addition are parfocal... no surprise that Jon keeps them)

 

Since this thread is primarily about the 56mm Plossl versus the 40mm Pentax in an ST-80, it took me a while to realize you weren't talking about an F/5 ST-80 but rather about an 80mm F/7.5 and an 8 inch F/10.  In these scopes, the 55mm Plossl would provide very reasonable 7.3mm and 5.5mm exit pupils and I could see preferring it as a finder.  I wouldn't consider it with the ST-80 as even for someone with a 7mm exit pupil, the effect aperture would be 50mm.

 

And, a 40mm SWA could be a good balance, widest field of view with more magnification.. I use the 41mm Panoptic in my 120mm F/7.5 as my low power eyepiece.

 

Jon


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#21 GGK

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Posted 02 November 2021 - 06:10 AM

I would think a 41mm 68AFOV Panoptic or 40mm 70AFOV UO MK70 Koenig would perform better than 55mm Possil in any scope, since they would have same TFOV and higher power and hence more contrast, but the panoptic would cost more and they are probably heavier also.

In the ST-80, the 40mm Pentax XW was clearly the better choice over the 56mm Plossl for the reasons discussed in the earlier posts. 

 

Although I did the first comparison in the ST-80, I purchased the 56mm Plossl to use in my C9.25 SCT at f/10.  My goal is to get the brightest image at max field of view with the SCT for better images using an OIII filter.  The exit pupils in the SCT will be 5.5mm for the Plossl and 4.0mm for the 40mm Pentax XW.  Up to this point, the 40mm XW has been my longest focal length eyepiece.  I hope to get that comparison done in a couple of weeks - I changed tripods and am waiting on an adapter plate which is just about done.

 

Gary




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