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John

Pentax XW 30mm and 40mm Return

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Whether or not the secondary becomes an issue depends on the ratio of the secondary diameter to the primary diameter, the operating exit pupil, and the size of the eye's entrance pupil.  Let's consider a worst case where an observer's eye is closed down to about 2mm due to solar observing.  Now, suppose the telescope has a 35% obstruction by diameter and is operating with a 6mm exit pupil.  That obstruction will occupy 0.35*6mm=2.1mm of the exit pupil's dead center.  That means the observer will have to dodge their eye around the central obstruction to see anything at all.  This may have been similar to what you experienced with the 130pds.  There's nothing subtle about seeing the central obstruction.  You know it when experience it.

I don't obsess over light loss or obstruction issues when mixing and matching eyepieces to telescopes.  I've got a variety of faster and slower scopes and longer and shorter eyepieces.  There's no reason not to use a combination if you like the resultant view even if a few photons smack into your iris instead of your retina.  This is certainly more likely if you're not fully dark adapted.  This hobby is about enjoying views of the skies after  all.

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I occasionally use a 40mm 70 degree eyepiece with my F/5.3 dob, but only very occasionally. It's just not as effective under my skies as the 31mm Nagler or, even better, the 21mm Ethos. I've not worked that out mathematically but found out by trying it out under the stars.

But with my slower refractors, the 40mm / 70 is much more useful :smiley:

 

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Just recently acquired a 41mm Pan on ebay, used effectively on the moon at f7 and will be used at f6. Will give this a try with my 14" dob at f5.3, paracorr attached, for H-beta filter observing, dark skies of course. 

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7 minutes ago, scarp15 said:

Just recently acquired a 41mm Pan on ebay, used effectively on the moon at f7 and will be used at f6. Will give this a try with my 14" dob at f5.3, paracorr attached, for H-beta filter observing, dark skies of course. 

Perfect set up for California nebula! Good find on the 41 Panoptic. 

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16 hours ago, Stu said:

Any ideas why there is a difference John? Presumably they are all designed for similar scopes? The shorter focal lengths all have excellent reputations, seeming to imply negative FC is better in countering scope FC, but that must vary depending on scope type?

Eyepieces show a flat field to the eye when their field curvature matches the scope exactly.  Since manufacturers cannot know the exact degree of field curvature in the instruments in which the eyepieces will be used, flat fields are typically designed (witness how many eyepieces now have the name "flat field" or "ultra flat field").  However, many if not most scopes have negative field curvature, so naturally the 3.5mm through 10mm XWs would test better since their curvatures match the scopes better.  Eyepieces in the line from 14-40mm have positive field curvature, so are more likely to see comments about field curvature.  The 30mm and 40mm are actually quite flat, but because their field stop diameters are larger, they extend farther out into the curved fields of the scopes and people comment about the field curvature.  If you look at the curves, though, it's easy to see why the 14mm and 20mm get the most comments about field curvature.

Personally, I use a dob with a long focal length that is flattened by a Paracorr (as well as corrected for coma), and I only notice one aberration in any of these--astigmatism in the outer edges of the field.

If you look at the charts, the farther the meridional and sagittal curves separate, the more astigmatism there is (worst is the 3.5mm).

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1 hour ago, Louis D said:

Whether or not the secondary becomes an issue depends on the ratio of the secondary diameter to the primary diameter, the operating exit pupil, and the size of the eye's entrance pupil.  Let's consider a worst case where an observer's eye is closed down to about 2mm due to solar observing.  Now, suppose the telescope has a 35% obstruction by diameter and is operating with a 6mm exit pupil.  That obstruction will occupy 0.35*6mm=2.1mm of the exit pupil's dead center.  That means the observer will have to dodge their eye around the central obstruction to see anything at all.  This may have been similar to what you experienced with the 130pds.  There's nothing subtle about seeing the central obstruction.  You know it when experience it.

I don't obsess over light loss or obstruction issues when mixing and matching eyepieces to telescopes.  I've got a variety of faster and slower scopes and longer and shorter eyepieces.  There's no reason not to use a combination if you like the resultant view even if a few photons smack into your iris instead of your retina.  This is certainly more likely if you're not fully dark adapted.  This hobby is about enjoying views of the skies after  all.

It was unmistakable in the 130pds! Something ridiculous like 14x as well, ha. Just a test though. I kind of wish I'd kept the 55 plossl, it still had excellent views and lunar in particular was exceptional. 

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29 minutes ago, Ships and Stars said:

Perfect set up for California nebula! Good find on the 41 Panoptic. 

Could be, although the 31T5 and 21E are each perfect for this gaining more contrast. I am hoping to draw out a bit more employing each of my three scopes with the 41mm on Barnard's Loop.

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1 hour ago, Don Pensack said:

Eyepieces show a flat field to the eye when their field curvature matches the scope exactly.  Since manufacturers cannot know the exact degree of field curvature in the instruments in which the eyepieces will be used, flat fields are typically designed (witness how many eyepieces now have the name "flat field" or "ultra flat field").  However, many if not most scopes have negative field curvature, so naturally the 3.5mm through 10mm XWs would test better since their curvatures match the scopes better.  Eyepieces in the line from 14-40mm have positive field curvature, so are more likely to see comments about field curvature.  The 30mm and 40mm are actually quite flat, but because their field stop diameters are larger, they extend farther out into the curved fields of the scopes and people comment about the field curvature.  If you look at the curves, though, it's easy to see why the 14mm and 20mm get the most comments about field curvature.

Personally, I use a dob with a long focal length that is flattened by a Paracorr (as well as corrected for coma), and I only notice one aberration in any of these--astigmatism in the outer edges of the field.

If you look at the charts, the farther the meridional and sagittal curves separate, the more astigmatism there is (worst is the 3.5mm).

Thanks very much Don, much appreciated. I guess the only remaining confusion I have is why the change from negative to positive across the range? Is it not possible to design the longer focal lengths with negative curvature or is something else at play?

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Another interesting factoid about the XWs is that they use special coatings on the cemented lens surfaces as well as the multi-coatings on the glass to air surfaces. This was considered quite innovative when the XWs first came onto the astro scene because cemented lens surfaces were not usually coated back then. Maybe this technique is used by other manufacturers now ?

 

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These might make very nice night vision eyepieces too. Key attribute is that the XW range takes TV dioptrx, which means they are compatible with the Televue night vision adapter and PVS-14 monocular.

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6 hours ago, Don Pensack said:

If you look at the charts, the farther the meridional and sagittal curves separate, the more astigmatism there is (worst is the 3.5mm).

Weirdly enough, I find the 3.5mm XW perfectly corrected right to the edge like the older 5.2mm XL, unlike the 7mm XW which mostly suffers from chromatic aberration and a bit of astigmatism.  I am extremely picky about edge astigmatism, so I don't know how to explain my experience with this wonderful eyepiece relative to Pentax diagrams.

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On 09/12/2019 at 04:17, John said:

I have a 24mm Panoptic and some shorter focal length XW's. The 2 degree difference in AFoV is not really noticable to be honest with you. I have used a Pnetax XW 30mm and compared it with a Nagler 31mm and the UWAN/Nirvana 28mm. The Pentax was a really comfortable eyepiece to use and not as heavy as the others. Here is the report I compiled back then:

31nagler30xw28nirvana11-09.pdf 448.39 kB · 14 downloads

A lot of useful information there, thanks for posting this.  I'd like to get a wider field eyepiece for my C9.25.  Here in the US the XW40 is about 15% cheaper than a Panoptic 41.  

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On 10/12/2019 at 14:24, John said:

Another interesting factoid about the XWs is that they use special coatings on the cemented lens surfaces as well as the multi-coatings on the glass to air surfaces. This was considered quite innovative when the XWs first came onto the astro scene because cemented lens surfaces were not usually coated back then. Maybe this technique is used by other manufacturers now ?

 

Yes.  TeleVue, Takahashi, etc.

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On 10/12/2019 at 13:28, Stu said:

Thanks very much Don, much appreciated. I guess the only remaining confusion I have is why the change from negative to positive across the range? Is it not possible to design the longer focal lengths with negative curvature or is something else at play?

It's quite possible they designed to be as flat as possible but that they weren't able to accomplish that and have other characteristics be where they wanted them.

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House of Optics (remember them?), very kindly sent me the entire range of XW's bar for the 3.5, which I'd already bought from FLO back in the late 2000's. HoH said I could try them out at my leisure and return any I didn't want. I bought them all with the exception of the 30mm & 40mm. At the time I had Naglers galore, but the XW's had a clarity about them that just appealed to me and that I prefered over the Naglers. I've never really been won over by any 40mm eyepiece as of yet. The sky background has always been too bright for my liking. When it came to the 30mm XW, well this was a very close call. Comparing the 30mm XW with my 31mm Nagler over several nights, I finally decided the 31mm Nagler gave the better over all view of star fields. The 30mm XW just missed being as sharp towards the edge of the field. I really would have preferred the 30XW as it would have satisfied my OCD,  but the difference in image was the deciding factor. However, if it was down to cost and if I hadn't already owned the 31 Nagler, I'd have been extremely happy with the 30mm XW. I wouldnt buy the 31Nagler at its new price over the 30mm XW, but there was a difference to my eye. 

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I will wait and see the reports of others, before ordering the XW 40, as the LVW 42 get very limited use (the Nagler 31T5 usually gives the better views, with a darker background). Buying the XW 40 would get in the way of more "urgent" upgrades (like the 6" solar scope project). Choices, choices. ;)

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3 hours ago, michael.h.f.wilkinson said:

I will wait and see the reports of others, before ordering the XW 40, as the LVW 42 get very limited use (the Nagler 31T5 usually gives the better views, with a darker background). Buying the XW 40 would get in the way of more "urgent" upgrades (like the 6" solar scope project). Choices, choices. ;)

I don't often use a 40mm eyepiece either so the Aero ED 40 that I have does just fine for the £50 it cost me. Its actually a very nice eyepiece :icon_biggrin:

 

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1 hour ago, John said:

I don't often use a 40mm eyepiece either so the Aero ED 40 that I have does just fine for the £50 it cost me. Its actually a very nice eyepiece :icon_biggrin:

 

That is apparently a clone of the TMB Paragon I used to have. I foolishly sold it, regretted it and got the Vixen LVW 42. That is also comfortable, and certainly very sharp, but it has quite a bit of distortion, and is a bit more prone to kidney-beaning and blackouts than the Paragon.

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Who will get one of the new Pentax XW 30mm and 40mm?

Do these two eyepieces have a place in today's eyepiece market?

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They do have a place and I'm sure are excellent eyepieces.

I wont be getting them. I have used a 30mm XW in the past and liked it but I liked my 31mm Nagler a little more. As I said in my earlier post, I don't use the 40mm focal length enough to justtify spending £380 when my Aero ED is pretty good in that niche for 1/7th of the cost (used).

For other folks I can see that these 2 inch XW's might be just what they are looking for :smiley:

 

 

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51 minutes ago, Stardust1 said:

Who will get one of the new Pentax XW 30mm and 40mm?

Do these two eyepieces have a place in today's eyepiece market?

I'm good with my 30mm APM UFF at 30mm.  I've found at least one comparison that liked it better than the XW except in comfort.  Since I find the UFF comfortable enough, I'll stick with it.

I'm also good with my decloaked 40mm Meade SWA 5000 at 40mm.  I picked it up for $125 back during the big Meade blowout sale when they were bought by Sunny.  I can't imagine that the XW will be three times better to justify the expense for an eyepiece I use just to get my scopes roughly on target and to occasionally scan star fields at low power.

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1 hour ago, Stardust1 said:

Who will get one of the new Pentax XW 30mm and 40mm?

Do these two eyepieces have a place in today's eyepiece market?

If I was really rich I think I’d probably maybe buy the full set of XWs and be happy ever after 

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15 minutes ago, markse68 said:

If I was really rich I think I’d probably maybe buy the full set of XWs and be happy ever after 

They really are beautiful though, and you might not notice it so much if you buy one at a time. e.g. Add to basket, add to basket, add to basket.......etc, etc.:icon_biggrin:

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36 minutes ago, mikeDnight said:

They really are beautiful though, and you might not notice it so much if you buy one at a time. e.g. Add to basket, add to basket, add to basket.......etc, etc.:icon_biggrin:

Adding them to the basket is easy. Paying for the basket full is the painfull bit !

 

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I had the 40mm XW from new for some years, used it with a 14" f/6 dob and a paracorr. It was seriously brilliant viewing for the Virgo cluster and other deep sky stuff. In the end I sold it for financial reasons for what I paid for it....a few months later s/h prices went through the roof after they were dropped from the range. 

One minor minus...I have short sight and this meant the eyepiece worked with the focal plane almost on the front surface of the field lens...any dust particles were all too obvious if it was used on the moon, as occasionally happened with a f/15 scope. A total non-issue on deep-sky though..or if I used specs. 

I'd buy another for the US prices quoted above..

RL

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