Pentax XW eyepieces versus Naglers

[gregbradley]

Anyone had experience with both?

The XW10mm seems to get the most praise from my short research.

I am looking at a 3.5 or 5mm. Perhaps a 20mm later.

The Nagler 22mm Type 4 though is a classic eyepiece.

Greg.

[Camelopardalis]

It depends on the scope they’re used with. They were designed for Pentax refractors which are long out of production.

The Naglers tend to fare better in newtonians but the XW are (mostly) excellent in refractors and compound scopes. I’ve never used a 22mm Nagler but have the 20T5 and it is sharp in every scope I’ve tried it. I used to have the 13T6 (great little eyepiece) and several Ethos. When I got more into imaging I sold them and kept the XW.

I’m probably more invested than most as I have the full set. When the sky is good for my scopes, the 10 and below are great. The 14 and 20 are good in an SCT, and the 30 and 40 are my goto eyepieces in my Edge 11. They’re my keep set, but everyone’s eyes and opinions are different.

Some find they experience blackouts with some of the XWs, but I find if you wind the shaft out it eliminates it. I prefer the mechanism of the XWs compared to the Delos (for example), and find the eye relief very comfortable once you master the wind out cover. The Ethos have little eye relief by comparison, maybe by necessity.

[gregbradley]

Thanks for the very informative reply. I think I'll have to get at least one for now and see how I like it. I've been hearing about them for ages.

Greg.

[JoeBlow]

I have the 10, 7 and 3.5 XW. They are every bit as good or even slightly better than Naglers. Not only do they have excellent eye relief, but I personally find them much easier to find and hold the exit pupil, compared to Televue eyepieces, including Naglers, that can be fussy on eye placement. The main advantage Naglers have over the XWs is their big 82 degree field. But note, the 20 and 14 XW are reported to have field curvature, so I recommend a Nagler or another type in this focal range.

So if it was me, I would purchase a 5 or 3.5 XW and then the 22 T4 Nagler.

[wavelandscott]

I am a fan of both Pentax and Televue and have several of both. I use them in Reflectors and Refractors and they do well in both.

When in the mood/need of wide views, the Televue Naglers and Ethoi are hard to beat. When it comes time to pump up the magnification I like the Pentax. I have the XWs 3.5, 5, 7 and. 10 and believe them to be wonderful. Easy to look through (I like their mechanism for eye-placement) a nice flat, sharp “cool” view. Ample eye relief.

The Televue lines to me are a little pickier on eye-placement and not as much eye relief. The Ethos line has the “cool” view I like (similar to the Pentax) but with a wider field...I have the 4.7, 13 and 21 I prefer the Pentax view over the shorter Ethos but the wide field is great when looking for faint fuzzies. I also have a number of Naglers, big fan of the 31 like looking through a portal and 16 (tight eye relief) but sharp sharp sharp. The Nagler views have a warmer tone to my eye...

Can not go far wrong with either...at least in the USA the Pentax XWs are on sale at the moment.

[gregbradley]

Thanks for that advice. That's what I was thinking as well.

Greg.

[AG Hybrid]

Due to what Duncan said regarding these eyepieces being designed for telescopes that are no longer in production, they are not all effective in each type of telescopes or a broad range like Naglers are.

If I remember correctly. The 10mm XW and shorter focal lengths were rather well corrected in even sub-F5 reflectors but the 14mm and above induced heavy field curvature. I remember testing this myself a couple years back. I couldn't stand them in my 12" without a coma corrector/field flatterer.

I tested the 14, 20, 30 and 40mm and they were bloody awful without a corrector. And only passable with it. If you want a 14mm with essentially the same ergonomics, fov and image quality as a Pentax get a Delos 14mm instead.

Should be good in your refractor though.

[gregbradley]

Thanks for that write up most helpful. I'll stick to the shorter lengths.

I am also considering a Stellarvue Optimus 3.7mm which is 110 degree but large and heavy. I have had a 13mm Ethos before. It was very good. Much like a Nagler only very large and heavy plus quite expensive.

Greg.

Greg.

[Hans Tucker]

One thing I have learnt .. eyepieces are a personal choice. What maybe the ducks guts for one person may be a dud to another. I have used the XW's (5, 7, 10 and 20) I didn't really like them.

[Camelopardalis]

I used to have the plots that Pentax published, but IIRC the field curvature of the EPs 14+ differs from those <=10mm. This may explain their behaviour in refractors/SCTs vs newts.

[gregbradley]

Was it you wanted a wider field of view or something else?

Greg.

[Hans Tucker]

I chose the XW's because I wanted to try widefield eyepieces and they received good reviews. I have tried a couple of the TeleVue Ethos (10 & 13) and I am looking at trying the Nikon NAV series and Tak UW series. I found that I didn't sell my XW 7mm so if you want to loan it to try it out it wont be a problem.

[Don Pensack]

The 10mm, 7mm, 5mm, and 3.5mm all have negative field curvature.
The 14mm, 20mm, 30mm, and 40mm all have positive field curvature.
It depends whether that matches your scope as to whether you see a flat field or not.
Here are the astigmatism/FC curve graphs:
https://web.archive.org/web/20150522023653/http://pentaxplus.jp/archives/tech/xo-xw/64.html
If the two curves (sagittal and meridional) deviate, that indicates astigmatism. The center of the field is at the bottom and the edge is at the top. The scale is in diopters (the eye can usually accommodate 1, but 2 would be seen as curved).
Ideally, the two curves would not deviate and they would be vertical.

The 30mm and 40mm have been out of production a long time, but are coming back soon--perhaps by the end of the year.

[ausastronomer]

I own all of the 1.25" Pentax XW's and 19 different Televue eyepieces including all of the Nagler T4's. I also own the 31mm T5, and several ETHOS, DELOS, Radians and Panoptics.

In focal lengths 10mm and less my preference is for the Pentax XW's. The 3.5mm, 5mm, 7mm and 10mm Pentax XW's are exceptional eyepieces and provide excellent flat field views in ALL TELESCOPE TYPES, including Refractors and Newtonians. I prefer the eye guard adjustment over the various eye guard systems used by Televue, although I will say the eye guard system on the DELOS is a monumental improvement over the instadrop system on the Nagler T4's and the Radians.

In focal lengths over 10mm I like the DELOS and the Nagler T4's. Although in a fast Newtonian the Nagler T4's really need a paracorr. If you don't need long eye relief the Nagler T6's are excellent. Above 20mm I like the 26mm and 31mm Nagler T5's and the 27mm and 35mm Televue Panoptics.

If money is no object the 17mm and 12.5mm Nikon NAV HW are currently the best eyepieces money can buy. They have performed exceptionally in every telescope I have used them in. The 12.5mm Doctor is also excellent, but again a lot of money.

Cheers
John B

[gregbradley]

wow, some fantastic responses here. Some very knowledgeable and experienced visual astronomers know their stuff.

Thanks.

Greg.

[bigjoe]

Greg late addition here but..I have Tak Abbes ans Delites which I love by far..might want to read this compared to Pentax xw or Deloi which I also own/owned and use.
https://astronomyconnect.com/forums/articles/tele-vue-delite-eyepieces-first-light-review.1/

Bigjoe

[gregbradley]

Most helpful, thanks BigJoe.

Greg.

[GrahamL]

I have had and do own a couple of XW s 10 /5 mm might get a 3.5 one day , as mentioned above extending the eyeguard to the sweet spot that suites you makes these eyepieces extremely comfortable to use .


I also had the 14mm for quite a while and found it really good despite the bad press it gets .


The other longer lengths I dont have experiance with , I do recall the 30 and 40 mm being not overly popular on some performance issues ? and ( they were pricey) then you you couldn't buy them anymore and people forget .

[wavelandscott]

In the US the Pentax XW are on sale at $269...I believe this to be a really good price for an excellent eyepiece line (3.5mm to 10mm).

I do not know how long the sale lasts but if I did not already own them I would be buying.

[Stonius]

Its a pity all manufacturers dont publish these. *Do any other manufacturers publish this data?

It's also interesting to note, that
1) if it is indeed true that different types of telescopes have different focal plane curvature, that...
2) must be matched to the curvature of the eyepiece used, then...
3) given that the XW's were originally designed for the pentax spotting scopes,
4) half of the range must have been not fit for purpose, by design.

Which doesnt seem right to me. Given the luxury of custom designing high quality eyepieces to suit a particular telescope, why would they design half their range 'wrong' for that purpose? It doesnt make sense.

Markus

[Wavytone]

Markus if they did publish the data it would be game over for most.

Keeping it as a guessing game means healthy sales continue as we are forced to try pot luck, buy them more or less at random & sell the ones we don’t like.

As you know the Japanese market is dominated by small refractors hence the Japanese eyepieces suit these, and are also ok in SCTs and maks.

Televues are aimed at the US market which is dominated by big dobs, and SCTs in which pretty much anything is ok.

The odd ones out imho are flat-field refractors (ie have a flattener, or APOs with 4...6 elements) ... there aren’t many truly flat-field eyepieces !

[Stonius]

But doesn't it seem strange that Pentax had the luxury of designing *any eyepiece they liked* to match their scopes, but settled on an 'inferior design'?


It implies they either didn't know, (the eyepieces are not cheap - they could have used a different design) or that the design is not actually as problematic as people say it is.


I have to say, I'm not as experienced as some, but I bought my set based on A/B Nagler comparisons and I found the Pentax to be a shade more contrasty, though it was splitting hairs.


But field curvature has never been an issue for me. Perhaps because the eye is an imperfect instrument. Curvature in a photographic setting is terrible, but in a visual observing scenario where you're trying to keep your eye centred on a <5mm exit pupil... field curvature has to be quite extreme before it's even noticable. It's far less noticable than other abberations, IMO.


All I know is that I don't go from a 20mm to a 5mm and suddenly go 'yuck!'.



-Markus

[ausastronomer]

Hi Markus,

It comes from the fact that despite what a couple of people on this forum will try to tell you, eyepiece designers rarely design eyepieces to work in certain telescope types. They design them to correct for certain aberrations to satisfy the design parameters and intended end use of the eyepiece.

The 7mm, 10mm, 14mm and 20mm Pentax XW's were designed for Spotting Scope use and that's the way they were marketed by Pentax for well over 15 years. On the Pentax website these 4 eyepieces were listed under "Spotting Scope Eyepieces" and all of the other focal lengths in the XW series were listed under "Astronomical Eyepieces"

Being intended for Spotting Scope use these 4 eyepieces are corrected for "Rectilinear Distortion" (Barrel and Pincussion) where you need straight lines to appear straight and not bent. Most Astronomical eyepieces are corrected for "Angular Magnification Distortion" and not for rectilinear distortion. Despite modern computer programs, glass types and advanced eyepiece design, it is not possible to correct for both at the same time. I spoke at length with Al Nagler on this very topic about 12 or more years ago. He advised that his eyepieces were generally corrected for "Angular Magnification Distortion" and not rectilinear distortion, with the Nagler T4's being the exception, as these were corrected for rectilinear distortion and suitable for daytime use in refractors.

Cheers
John B

[Don Pensack]

John,
Try the T4's in the daytime-- (look at any straight line) they have substantial rectilinear distortion, in the form of pincushion. Maybe they don't have as much AMD as the apparent fields would imply, but they are not corrected for rectilinear distortion.

[Don Pensack]

Markus,
The ability to accommodate field curvature diminishes with age. Older observers may not be able to accommodate the FC of certain focal lengths of the XW.

[Don Pensack]

Even Pentax no longer publishes this data. My link used the Wayback Machine to find a link several years old and long deleted from the web.

As for field curvature and sign--shorter focal length eyepieces cover smaller portions of a curved focal plane in a telescope. The Earth is curved, but 1m of ground can be quite flat. So it is more important to match the field curvature of the average scope with long focal length eyepieces than it is with short focal length eyepieces because the long ones see a larger portion of the scope's focal plane, and see more curvature.

Note that the longer focal lengths of XWs also have fewer elements than the short ones, so there is a greater freedom of design in the shorter eyepieces.

I can't speak for compound catadioptric scopes, but other forms of commercial amateur scopes have positive field curvature at the focal planes, so one would normally have expected the longer focal lengths to fare well in those scopes. Note, though, that short focal length refractors have a lot of field curvature, with a radius of curvature about the same as 1/3 the focal length, whereas newtonians have considerably flatter fields, both because of the longer focal lengths, and because their radii of curvature matches the focal length. An eyepiece that appears to have a flat field in one might not appear flat in the other, even if the field curvature in the eyepiece matches the scope's sign.

[ausastronomer]

I have all 3 of the T4's but I've never tried them in the daytime because I don't own a refractor, other than finderscopes, binoculars and rifle scopes.

I am only repeating what Al Nagler told me in about 2007, when I asked him some questions about them. I was only drinking coffee at the time so pretty sure I interpreted him correctly.

Cheers
John B

[Don Pensack]

The T4s do have, overall, low distortion.

[Stonius]

That's interesting - I wonder why that is?

[Don Pensack]

The lens of the eye not only yellows then turns brown as we age (UV damage), but also hardens so the muscles in the eye gradually grow less able to change the lens shape to accommodate for distance.
It starts with near vision first. At around 40 we need reading glasses. Then, 50 requires bi-focals and 60 trifocals and the near prescription strengthens.
Alas, the lenses don't "freeze" at infinity focus, but somewhere closer because we live indoors, so older individuals need a distance prescription too.

If UV damage gets severe enough,small opacities become apparent in spots in the lenses, which gradually progress toward a cloudy translucency to the lens. This is called cataracts and the mitigation of the effects requires a lens replacement--a common surgery.
Human trials start this year for a steroid-like substance which can dissolve cataracts. It worked in 98% of all mice so there is hope that lens replacement surgery may be a thing of the past in a few years.

[Stonius]

I had heard that, but I thought that was to do with the eye's ability to focus *in general*.


I thought field curvature was more about a focal plane that is curved. Achieving sharp focus for the middle makes the outside go out of focus, and vice versa. So I'm thinking that as long as your eye can achieve focus at all, curvature should be just as much of a problem at any age, shouldn't it? Unless the shape of your retina (ie; the focal plane of the eye itself) changes with age?


Cheers


Markus

[Stonius]

So by looking at the graphs it would be true to say that;


The 14mm (and perhaps 5mm) have the least astigmatism, even though it has negative field curvature.


The 30 & 40mm have the least field curvature, even though it does have some slight astigmatism.


I'll test this next time I'm out, but I don't know how well I'll go, as it's easier to compare different EP's of the same focal length than different EP's in the same range of different focal length.


Cheers mate :-)

[Don Pensack]

What, though, do you mean by "out of focus"? Even if the edge requires a slightly different focus than the center, if you can easily focus your eye on each, would you notice the field curvature? Probably not.
Even at 68, with some eyepieces that have a curved field, if I focus the eyepiece on a star halfway to the edge, I can see the entire field in focus, whereas if I focus on a star in the center, the edge appears to be out of focus. My eye still accommodates a curve of half the diopter change, but not the complete one.
A young person can easily handle a 2 diopter change and the eye will still see the field in focus.
Here is a scholarly article on the effects of aging and accommodation in the eye:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2908311/

[Stonius]

I'm trying to understand why *field curvature* would be related to age.


I get that eyesight deteriorates with age as does the ability of the eye to focus, and I get what you're saying about focusing halfway to the edge of the field to keep the inner and outer stars within the circle of confusion.


But correct me if I'm wrong; a curved focal plane is not fixed by shifting the focal point of the eye; instead you would have to be able to change the shape of your retina to accomodate to a curved focal plane, wouldn't you? I don't think people (young or old) are able to do this are they?


Cheers.

Markus

[Don Pensack]

Field curvature in an eyepiece is a fixed value. But can you accommodate the focus change in your own eye necessary to focus on both the center and the edge? That is what changes with age.
When you focus on the TV across the room and then change your gaze to a book in your lap, your eye is changing focus by changing the shape of the lens in the eye.
It is this ability that changes with age.

So if you have a sightly curved focal plane and you are young, when you change your gaze from the center to the edge, your eye automatically adjusts its own focus and the image is in focus both places.
There is a limit to this, and even young eyes cannot accommodate too much focus change. Older eyes can accommodate even less, so where the younger eye may see the whole field in focus, the older eye may not.

The point is that a slightly curved focal plane in an eyepiece may not be a problem for younger observers, but may be a problem for older observers.
And, because they have more strongly curved focal planes, this issue may arise more with refractors than with reflectors.

[GrahamL]

Great something else to go wrong :lol: we should improve with age .


I hadn't noticed pentax have stopped production of the 3.5 and 5 it seems .

[Don Pensack]

Ricoh's website is dated.
The 3.5mm and 5mm are still available and the 30mm and 40mm are coming back before the end of the year.

[ab1963]

Hi Don
Do you have any idea how much the XW40 will retail at.....

[Camelopardalis]

A decade ago when I bought mine, the cheapest I could find was roughly the equivalent of $1000.

The 1.25” versions have held their US$ price pretty well, but then they have been available all along.

It may be that over the passage of time people have lost their enthusiasm for them...

[Don Pensack]

When the 30mm and 40mm were available new last time, they were US$579.
This time, I'm pretty sure they'll be less than US$500.

[ab1963]

Thanks to both of you for the input always wanted the 40mm but any i have seen on the used market the price has been horrendous so if they are around the $500 USD mark i will have one......