Pentax XW vs Vixen LVW

[Kunama]

Just a little update to this thread, I finally got a chance to try out the Pentax XF 8.5 and 12mm eyepieces this evening and as expected they are incredibly sharp. I compared them to the LVW 8 and 13mm in both the FS60CB and the TSA120.

I found the XFs are very sharp on axis and more neutral in tone. The LVWs have a warmer tone especially at the edge of field but the difference was minimal. On the trap stars in M42 the XF8.5 was better than the 8LVW.

The 8.5mm is the better of the 2 XFs being sharp across the full view, the 12 shows noticeable curvature in the FS60CB, less so in the TSA120. The LVWs are like the XF8.5, sharp from edge to edge.

I certainly would not rush into selling any of my LVWs but if there were more of these compact little XFs about I would love to build a set for the FS60.

[casstony]

The curvature evident in the XF12, XW14 and XW20 is a known issue with many scopes.

[janoskiss (Steve H)]

The XF-12 does indeed exhibit more field curvature than the XF-8.5 but it is nowhere near as much as the XW-14 or the XW-20. The XF-8.5 is a gem and the XF-12 is also a very usable and pleasing EP in pretty much any scope (to my eyes). Cannot say the same for the XW-14 or XW-20. The two XFs are essentially the same eyepiece with a more powerful barlow group in the XF-8.5. In a long barlow (Orion ultrascopic) the XF-12 is flawless in any scope from f/4 up. I tend to use it barlowed on planets and it's just gorgeous.

The XF-12 is also ideal for deep sky observing in f/6 scopes (which for most people means 8" dobs).

[mental4astro (Alexander)]

Like I mentioned earlier, the XW's for being a "premium" line are VERY scope design sensitive. What works in a Newtonian isn't the best in an SCT or Mak, & vise versa. Very few lines do well with all scope designs. By the sounds of it so too does the XF line struggle.

The LVW's are one that do well in all scopes, with the exception being the 30mm. And Vixen themselves acknowledge this by the different appearance of the 30mm.

[Allan]

Anyone looking at buying into this class of eyepiece would be well advised to consider the Delos. While I can't comment on the Vixens, the Delos are a little better optically than the Pentax, at least in the shorter focal lengths I have used. The Delos have performed well in any telescope I have put them in.

[brunono2 (Brian)]

I don't agree. Pentax are one of the best eyepieces for planetary viewing on short focal length refractors- I having been using them for many years and they are superior to most others including Televue eyepieces

regards

Bruno

[mental4astro (Alexander)]

Bruno, no one is disputing their image quality. The only thing that is being said is they are better in some scopes than in others. When there is an optical match, bugger all touches them.

There have been a couple of Pentax lines mentioned. Which line are you referring to, and which focal lengths do you have? It would be good to know which particular Pentax EPs are best in a fast refractor. I to have a fast refractor, an achromat, not best for high power, but would be good to know what works best with one.

[Profiler (Profiler)]

I have done side by side comparisons with a number of XWs with Delos on a few refractors and optically couldn't discern any difference between them.

I also took some measurements and was amazed to see that while the housings were slightly different the optical elements appeared to be virtually identical.

However, I did find that the comparative size and weight of the XW to be smaller, lighter and "very subjectively" felt far more sturdier. I "personally" thought the adjustable eye-cup design on the Delos as something that would be prone to break if twisted the wrong way/over twisted and/or after prolonged use. I also found it quite inferior in terms of ease of use in comparison to the very simple twist top design on the XW.

Finally, before this triggers any howls of protest and indignation from Delos fans I do have quite a few of the very big ticket Televue eyepieces which are great as well but from "my" experiences and preferences I personally think the XWs are a better eyepiece to the Delos (only) for a variety of reasons in the scopes I have tested them both upon. Thus, when I want 100' fov then Ethos has no rival and a low power flat field the Terminagler still reigns supreme in my view.

So, as many have commented before there is no one holy grail in eyepiece lines that is perfect in all circumstances and they all have strengths and weakness in terms of what some can do better than others and most importantly there is a personal taste element to all of them.

[brunono2 (Brian)]

As far as Pentax eyepieces are concerned which I use for short focal length refractors -I have various - XL, Zoom XL + Zoom XF. When seeing is excellent I use a 2.25 times baader barlow which screws into the bottom of the eyepiece -last week I pushed my 4 inch F5 and 5 inch F5 scopes up to 80 per inch on Jupiter - using the baader as well- the views were fantastic on the Pentax eyepieces I preferred the views to on these versus other eyepieces -Tak + Televue radians which I have

To be fair I don't have eg Ethos eyepieces so I cannot compare to eyepieces such as these but I am extremely satisfied with Pentax

[Camelopardalis (Dunk)]

The Baader 2.25x Barlow is excellent IMO I've used it with my Panoptic and Nagler to great effect in my travel scope.

I find the XW10 tricky in my C8 but when I can manage to keep my head still it's a wonderful view. Haven't tried an LVW yet, but sounds like I should

[AG Hybrid (Adrian)]

I've been looking at this thread on and off for what seems like forever. I'd like to contribute but I'm not really sure what to say. I have a 10XW. The views are brilliant in my 4", my 12" and every other telescope I've looked through with it.
Every LVW I've looked through gave brilliant views as well. Both are very comfortable to view through, absurdly sharp, excellent light scatter control and have a nice neutral tone. It has been well document that the >14mm XW's don't perform well in fast newtonians due to their lens design and they introduce extra field curvature in the image. A paracorr will fix that up I'm told.
No really issues like noted in the Vixen LVW range, except maybe the 30mm.
I'd recommend people just buy whatever they can afford and whatever is in stock with suppliers.

Can always sell them if you don't like it.

Personally though, I prefer the Delos line myself.

[Wavytone]

Originally Posted by brunono2

one of the best eyepieces for planetary viewing on short focal length refractors

Bruno there's a good reason why - the curvature of the focal plane in refractors (and SCT's) is usually convex towards the eyepiece. In newtonians it's concave. Then there's coma, which fast newtonians have. This makes a huge difference as to how eyepieces perform in different scopes.

There is no such thing as an eyepiece that is excellent in all scopes - for that to be true, all scopes would have to produce effectively no aberrations, which simply isn't the case.

Some eyepieces work as well as they do in fast (f/4) Newtonians because to some extent they have a modest amount of negative coma and a curved field to match. This appears to be the case with the Explore Scientific eyepieces, the TV Naglers, Delos and Ethos.

I've only had one occasion to briefly try a Pentax XW20mm and 10mm, which at the time seemed excellent edge-to-edge but I wasn't looking at them critically. Unfortunately I don't know anyone with a set to allow a bake-off with my LVW set.

My only gripe would be the "safety groove", which IMHO is utterly annoying, as one who has never dropped an eyepiece in 40 years.

[Camelopardalis (Dunk)]

Originally Posted by Wavytone

I've only had one occasion to briefly try a Pentax XW20mm and 10mm, which at the time seemed excellent edge-to-edge but I wasn't looking at them critically. Unfortunately I don't know anyone with a set to allow a bake-off with my LVW set.

Remind me the next time you're heading for the mountains...I've got the 7 and upwards but I don't always carry them.

[gb_astro]

Originally Posted by Wavytone

Some eyepieces work as well as they do in fast (f/4) Newtonians because to some extent they have a modest amount of negative coma and a curved field to match. This appears to be the case with the Explore Scientific eyepieces, the TV Naglers, Delos and Ethos.

Anything is possible of course but I think Televue would be more concerned with optimising their eyepieces for their own line of refractors.
I really can't see them reversing curvature and adding coma to match third party newtonians.

gb.

[Wavytone]

Actually I'm sure they do, but it's complicated and only evident if you do some ray tracing of a Newtonian or refractor, and the eyepiece. The usual criterion used is the diameter of the "circle of confusion" containing 90% of the incoming energy.

Basically, while the curvature of a refractor is of the opposite sign, most serious refractors are f/7-f/9. Most dobs are around f/4-f/5. What this means is that an eyepiece design with a curved focal plane with a curvature (1/R) half that of the dob focal plane produces about the same size circle of confusion in both a dob and a refractor - the net aberration is about the same in magnitude, but of opposite sign.

Similarly, building in some negative coma in the design produces an eyepiece that performs exceptionally well in a fast Newtonian, yet is still quite acceptable in refractors, maks and SCT's.

You could design an eyepiece fully corrected for coma and curvature, resulting in an even smaller circle of confusion over a wide field for a fast Newtonian - as someone did (the Klee "Pretoria" eyepieces) with really superb results, however if used in a refractor or SCT the result will be very poor indeed.

This unfortunately was the undoing of the Pretoria eyepiece as many buyers at the time simply didn't understand it was totally unsuited to their Celestrons and achro refractors. The result was it got a bad name.

Another eyepiece that performed very badly in sone types of scope was the Koenig - though I never figured out what sort of scope it was supposed to match. Field curvature was always a huge problem with these ep's.

[Astromelb]

Dear I In S'ers

Great thread guys, on two wonderful series of Ep's.

Having a full set of both options, I find the performance is dependent on which scope they are being used in.

The Pentax's are wonderful, but sadly the recently new owners of Pentax, Hoya Corp of Japan decided in their wisdom to delete the astronomy gear from the Pentax range, very sad.

The Vixen LVW are excellent eyepieces, and as they are current production and available from the Oz agent for Vixen - My Astro Shop in Queensland, so they are a terrific buy.

If you are prepared to do work on the web pages globally - such as CN & Astronomy Buy Sell UK you can find Pentax XW's fairly 2nd hand reasonably regularly, but usually need to be very very quick to secure them.

The large 2 inch Pentax XW's (30mm and 40mm) are very very hard to get and you need to move fast as they sell very very fast.

All this said I do find that different telescopes perform differently with different eyepieces, so it is always a matter when I get a new telescope to test which eyepieces suit that particular telescope the best, which is always a test and trial process.

Eyepieces are absolutely critical, as I find so many people with a telescope that is most capable, but get eyepieces that are simply not good enough. The eyepiece is located at the focal point, this is the location where the image is produced, making the eyepiece the most important part of the system

Clear skies.

[janoskiss (Steve H)]

Wow, this thread just refuses to die! No more XWs for me. Also had my XFs stolen (in two separate incidents). I repurchased them last year after a few years of making do with plossls and a no-name 30mm UWA. That's how much I love those XFs. But I'm getting too old and lazy for the heavy and bulky XWs. No more awkward questions: 'Is that a Pentax in your pocket ...?'

[bytor666]

I've used a lot of XW's in a lot of scopes from Newts, to 'Fractors to SCT's and they are all excellent in all scopes, except for a slight bit of FC coming from the 14mm and 20mm XW's in Newtonians.

None of the FC can be seen in the SCT's from the 14mm or 20mm XW's.

Cheers!