Pentax 3.5 XW

[Louis D]

7 hours ago, Timebandit said:

Woow , getting on for 4 years now . where does time go ?

 

I know the feeling.  I run into adults with little children in stores and they stop me to say hi.  I finally have to ask them who they are, and they explain they were little kids when I knew them last in the scout troops my kids were in!  The feeling of being old comes crashing down on me in those moments. 😱

[Louis D]

7 hours ago, John said:

the XW 14 and 20m although I've alse read that many folks don't have a problem with those.

I think it has to do with the user's focus accommodation which is tied to age.  I had no problems using the Pentax XL 14mm until my mid-40s when I started having to wear bifocals.  It was at that point I could see its field curvature that many folks had mentioned over the years that I could never see.

[F15Rules]

I compared my then 14mm XW to my first Morpheus 14mm, and my 20mm XW to my Vixen LVW 22mm. I found to my eyes that the Morpheus and Vixen gave me much more pleasing views, with less field curvature in the Morpheus, with a wider field of view, and almost no FC in the Vixen and a lot in the XW 20mm.

But the 5, 7 and 10mm XWs I owned were simply superb, and I'm quite sure the 3.5mm XW is every bit as good as its shorter focal length siblings..Enjoy! 👍🙂.

Dave

[mikeDnight]

The 3.5mm XW was the first XW I bought. It still remains one of my all time favourite eyepieces, even though I nolonger own any.

I've nothing but happy memories of the XW's.

[John]

2 hours ago, F15Rules said:

I compared my then 14mm XW to my first Morpheus 14mm, and my 20mm XW to my Vixen LVW 22mm. I found to my eyes that the Morpheus and Vixen gave me much more pleasing views, with less field curvature in the Morpheus, with a wider field of view, and almost no FC in the Vixen and a lot in the XW 20mm.

But the 5, 7 and 10mm XWs I owned were simply superb, and I'm quite sure the 3.5mm XW is every bit as good as its shorter focal length siblings..Enjoy! 👍🙂.

Dave

I've really got to try a Morpheus sometime

[markse68]

3 hours ago, F15Rules said:

I compared my then 14mm XW to my first Morpheus 14mm, and my 20mm XW to my Vixen LVW 22mm. I found to my eyes that the Morpheus and Vixen gave me much more pleasing views, with less field curvature in the Morpheus, with a wider field of view, and almost no FC in the Vixen and a lot in the XW 20mm.

But the 5, 7 and 10mm XWs I owned were simply superb, and I'm quite sure the 3.5mm XW is every bit as good as its shorter focal length siblings..Enjoy! 👍🙂.

Dave

Must be scope dependent I guess - I'm not seeing any detectable FC on the moon at all with either the 14 or 20 in my f8 dob. Assuming FC would manifest as loss of focus at the edges?

Mark

[John]

The interesting thing about the 14mm and 20mm Pentax XW's is that their FC is the opposite of the 10, 7, 5 and 3.5mm focal lengths. In some scope designs (medium-fast and fast refractors ?) they apparently add to the FC that the scope already produces making it more obvious to those who find it bothersome.

Maybe they suit a newtonian better ?

 

[Geoff Barnes]

2 hours ago, John said:

I've really got to try a Morpheus sometime

I'd be happy to send you my 6.5mm Morpheus for a trial @John although postage might be an issue.

Surely someone on SGL nearer home could oblige?

In my 12 inch SW Dob it is simply stunning!  

[markse68]

1 hour ago, John said:

Maybe they suit a newtonian better ?

Is there a difference between a newt and a frac in terms of native fc John? Or could it be just focal ratio? Mind you @F15Rules must have been using a slower scope than my f8

it’s a different matter with my xl40- that does suffer quite noticeable loss of focus at edge but I wonder if it’s just an erfle - it seems to have a lot less glass than my other pentaxes- seems a simpler design but I only really use it for finding things and hopefully the veil at some point- it’s wide enough as to not really be an issue generally

mark

[F15Rules]

Hi Mark,

This article might help you..

https://www.cloudynights.com/topic/537212-pentax-xw-20-any-field-curvature-in-a-4-f74-apo/

I think you would have to try it for yourself, in your own scopes. For what it's worth, my own experience was that:

1. I wanted to build a complete set of Pentax XWs, (up to and including the 20mm, as the 30mm and 40mm were discontinued and went for silly money on the used market - ironically, both have been reintroduced by Ricoh in the past year or so) based on their excellence in the short lengths. But when I acquired the 14mm I could clearly see that focus which was sharp in the main part of the 70 degree fov changed to being out of focus the nearer I moved towards the edges of the field. Also, in daylight viewing of say a TV aerial, the straight lines of the aerial become progressively more curved in appearance the closer I moved the aerial towards the edge of the field.

2. I found this effect to be if anything worse in the 20mm XW.

3. I bought a new Baader Morpheus 14mm when they were first introduced and on offer, and found it to be more pleasing in every way than the XW14mm. And the 76deg fov is noticeably wider than the XW, if not sharp totally to the edge..the sharp portion exceeded the whole 70 Deg fov of the XW.

4. I found the same with my Vixen LVW 22mm, apart from the LVW fov, which is 65 Deg Vs the 70 Deg if the XW. Both I found sharp right to the edge. I now tend to look for individual eyepieces I really like, and which work for me, whatever the brand, rather than feeling I must have a full set of any one brand.

5. Field Curvature DOES seem more obvious to some observers than to others, and it appears, according to the article from CN that I linked to above, that that some scopes have their own FC which can increase or correct for, the FC in eyepieces.

6. Finally, the article suggests that, for some observers the LVW does not have as good light transmission as the XW.. I can only say that I looked carefully for this and could not find it, they both matched each other to my eyes, perhaps the LVW even edging it over the Pentax.

I made these observations in an excellent Vixen ED103s apo doublet of known excellent quality, operating at F7.7.

I should also say that although my right eye has deteriorated in recent years (I have now trained my left to be my main Cyclops viewing eye), this was not the case at the time of these comparisons some 4 years ago.

I hope this helps you Mark, as I say we all perceive things differently, so you and others may come to differing conclusions..🙂

Dave

[Ricochet]

6 hours ago, markse68 said:

Is there a difference between a newt and a frac in terms of native fc

The radius of curvature for a Newtonian is equal to its focal length. 

The radius of curvature for a doublet refractor is 1/3 of its focal length.

The higher the radius of curvature, the flatter the image. 

9 hours ago, markse68 said:

Must be scope dependent I guess - I'm not seeing any detectable FC on the moon at all with either the 14 or 20 in my f8 dob. Assuming FC would manifest as loss of focus at the edges?

You will see it most obviously with stars. Focus the centre of the FoV exactly and then look at the stars at the edge. Refocus to see the difference. I can see it in my 14mm and 8" f6 (fl 1218mm), but it isn't really an issue. Focusing slightly off centre keeps everything quite tidy across the field for my eyes. If your scope has an even longer focal length than mine then you will see even less of this effect. In fact, if you look for it, you might notice that the predominant issue is actually astigmatism at the edge. This is also shown in the diagram below, where the amount that the two lines pull away from the centre is field curvature, and the difference between the two lines is astigmatism.

17 minutes ago, F15Rules said:

I made these observations in an excellent Vixen ED103s apo doublet of known excellent quality, operating at F7.7.

I can see why you would have had trouble with FC while using that scope. 

[markse68]

2 hours ago, Ricochet said:

The radius of curvature for a Newtonian is equal to its focal length. 

The radius of curvature for a doublet refractor is 1/3 of its focal length.

The higher the radius of curvature, the flatter the image. 

You will see it most obviously with stars. Focus the centre of the FoV exactly and then look at the stars at the edge. Refocus to see the difference. I can see it in my 14mm and 8" f6 (fl 1218mm), but it isn't really an issue. Focusing slightly off centre keeps everything quite tidy across the field for my eyes. If your scope has an even longer focal length than mine then you will see even less of this effect. In fact, if you look for it, you might notice that the predominant issue is actually astigmatism at the edge. This is also shown in the diagram below, where the amount that the two lines pull away from the centre is field curvature, and the difference between the two lines is astigmatism.

I can see why you would have had trouble with FC while using that scope. 

Thanks @Ricochet that’s interesting to know about different scope types differing fc. I may take a more critical look in an open cluster or something but tbh as it’s not bothering me or detectable now I’m slightly loath to go searching for something that might then upset me

Interesting to look at those graphs too as it looks like the shorter focal lengths should suffer as badly or worse from any astigmatism but as I’m not noticing that in any of my Pentax eps I’m guessing that this could just be a case of looking too closely at the details and like someone in the CN thread F15rules linked to said it was brave and perhaps unwise of Pentax to release those test results when not many other ep manufacturers do... I think Pentax must have really improved on the longer fl eps with the xws as the 30 and 40 graphs look the best of the lot- my 40xl as mention is far from flat fielded and suffers astigmatism too. It’s very compact and light though on the plus side (I think half the weight of the xw40!) 😉

@F15Rules it’s strange that one eyepiece can behave so differently for different people and scopes Dave! but yes I guess the shorter fl xws cancel out curvature and improve field flatness whereas the 14 and 20 can add to and make curvature more obvious. In my scope I’ve not noticed an issue. I do have other eps where I do- many plossls, the BCO 10mm which I think suffers because they opened the stop wider than is normal for that type of Abbe ortho design, erfles etc but not these. I like these xws very much 

Mark

[John]

FWIW I find the 10 - 3.5  XW's free of astigmatism in my F/5.3 12 inch dobsonian. I don't like astigmatism so if it's there, it bothers me.

Eyepieces are very personal choices though. We say this again and again and it is true. What one person raves about, another may well find does not suit them at all

[Barry-W-Fenner]

On 03/06/2020 at 11:48, markse68 said:

Thanks @Ricochet that’s interesting to know about different scope types differing fc. I may take a more critical look in an open cluster or something but tbh as it’s not bothering me or detectable now I’m slightly loath to go searching for something that might then upset me

Interesting to look at those graphs too as it looks like the shorter focal lengths should suffer as badly or worse from any astigmatism but as I’m not noticing that in any of my Pentax eps I’m guessing that this could just be a case of looking too closely at the details and like someone in the CN thread F15rules linked to said it was brave and perhaps unwise of Pentax to release those test results when not many other ep manufacturers do... I think Pentax must have really improved on the longer fl eps with the xws as the 30 and 40 graphs look the best of the lot- my 40xl as mention is far from flat fielded and suffers astigmatism too. It’s very compact and light though on the plus side (I think half the weight of the xw40!) 😉

@F15Rules it’s strange that one eyepiece can behave so differently for different people and scopes Dave! but yes I guess the shorter fl xws cancel out curvature and improve field flatness whereas the 14 and 20 can add to and make curvature more obvious. In my scope I’ve not noticed an issue. I do have other eps where I do- many plossls, the BCO 10mm which I think suffers because they opened the stop wider than is normal for that type of Abbe ortho design, erfles etc but not these. I like these xws very much 

Mark

 

[Don Pensack]

On 02/06/2020 at 14:07, John said:

I've really got to try a Morpheus sometime

John, try this:

https://astronomyconnect.com/forums/articles/baader-morpheus-17-5mm-eyepiece-review.71/

[Don Pensack]

Field curvature of the eyepiece has to match the scope to be seen as a flat field.

Scope + Eyepiece = result

| + | = |

( +( = |

) + ) = |

) + ( = (

) + | = )

Most telescopes have positive field curvature, which is why the 10mm, 7mm, 5mm, and 3.5mm Pentax XWs have been so highly reviewed,

and why the 40mm, 30mm, 20mm and 14mm have gotten so many comments about field curvature.

Since manufacturers do not publish the FC details about their eyepieces, the safest route is to look for flat field eyepieces.

Essentially, newtonians with focal lengths >1 meter have flat fields.

Flat field refractors have flat fields.

Refractors in general do not, and that includes triplets as well as doublets.

That makes eyepiece selection a bit of trial-and-error.