18 replies to this topic

[ralphjunius]

Are the Pentax XW 40mm, 30mm, 23mm and 16.5mm eyepieces all parfocal (within a few millimeters)??  If some, but not all, can you tell me which ones you know are parfocal?

 

Thanks!!

[Starman1]

Are the Pentax XW 40mm, 30mm, 23mm and 16.5mm eyepieces all parfocal (within a few millimeters)??  If some, but not all, can you tell me which ones you know are parfocal?

 

Thanks!!

Yes, they're more or less parfocal.

They all focus differently than the 1.25" Pentaxes, though.

The difference in focus between 2" and 1.25" = the height of the adapter.

With a zero height adapter, all sizes of Pentax would be parfocal.

[Ernest_SPB]

According to my measurements eyepieces in whole line Pentax XW (2" and 1.25") are nicely parfocal. Front focus is located at connection of barrel and body with tolerance 0.5 mm 

[PKDfan]

I just came in from sweeping Cygnus & Lyra with my new 40XW and can confirm pinpointy stars and finally i saw a tiny bit of field curvature from my 62mm astrograph.

I then checked parfocality with the 30XW and 16.5XW and its easily within Ernest 1/2millimeter.
Within a 1/8 of a turn of my fine focus they all came to focus. I then went in reverse 16.5XW to 40XW and it seemed closer to ideal parfocus and then to 30XW which needed slightly more refocus.

The view efficacy or how easy and alike the 3 views were i sensed equal ease and immersion and pretty equal star sizes with 30mm & 40mm focal lengths and then even smaller stars with 16.5mm.


The vast majority of stars in the fields were pinpoints with the nod of very best to 16.5XW.

Albireo and bright stars were slightly plagued with glare artifacts from the extreme pupils.

16.5XW was best because of magnification @24¼ X with a beautiful gold and royal blue.

I noticed no vignette with 40XW except for last 5degrees where very minor darkening that you got to search for to see.

Finally got my widest true field eyepiece of my dreams. PINpoint stars out to the outer 70% zone and super easy pupil.



CSS
Lance

[ralphjunius]

Dang, thanks guys!

 

I have the XW 30mm so I'm gonna have to get with the XW 16.5mm!  I really like the 30mm so I intend skipping the XW 20mm and going down to the 16.5mm in 2". 

 

Also thinking about the XW 23mm but it gets mixed reviews here and the jump to 16.5mm with an 85* field should be ok.  I do have the very good TV 26mm T6 but a lotta cranking and wobbling on the focus knob and more weight for non-tracking, light mounts. I really like parfocal.

 

BTW, the 1-1/4' XW's are parfocal with the new TAK TPL's!

 

Plus I have the Howie Glattener zero 1-1/4" adapter.

 

Wish the XW 30mm had a larger field for the C14 and Super Planetary so may go ahead and get the 40mm.  I have the 31mm Terminagler, but its just too much weight for some of my smaller mounts.  The 2" XW's are marginal at +-400grams but manageable.

 

Ralph

[Starman1]

With a 46.5mm field stop, the XW40 also has the widest true field of all the 2" eyepieces.

Edited by Starman1, 28 April 2025 - 08:37 AM.

[Starman1]

Dang, thanks guys!

 

I have the XW 30mm so I'm gonna have to get with the XW 16.5mm!  I really like the 30mm so I intend skipping the XW 20mm and going down to the 16.5mm in 2". 

 

Also thinking about the XW 23mm but it gets mixed reviews here and the jump to 16.5mm with an 85* field should be ok.  I do have the very good TV 26mm T6 but a lotta cranking and wobbling on the focus knob and more weight for non-tracking, light mounts. I really like parfocal.

 

BTW, the 1-1/4' XW's are parfocal with the new TAK TPL's!

 

Plus I have the Howie Glattener zero 1-1/4" adapter.

 

Wish the XW 30mm had a larger field for the C14 and Super Planetary so may go ahead and get the 40mm.  I have the 31mm Terminagler, but its just too much weight for some of my smaller mounts.  The 2" XW's are marginal at +-400grams but manageable.

 

Ralph

Caution: the 16.5mm is 700g, and the 23mm is 740g.

[ralphjunius]

Oops, Don, I am confused!  Thanks! 

 

All the 2" XW's are around 700g!  The 31mm Terminagler is 1000g but i have found the 300g difference with the 30mm XW tolerable.   Its the 1-1/4's that are around 400g.  I obviously have no feel for grams!

[PKDfan]

Oops, Don, I am confused! Thanks!

All the 2" XW's are around 700g! The 31mm Terminagler is 1000g but i have found the 300g difference with the 30mm XW tolerable. Its the 1-1/4's that are around 400g. I obviously have no feel for grams!

I'll weigh them for you right now Ralph !

40XW is 700 grams
30XW is 685 grams
16.5XW is 735grams

Plus or minus 1 gram and with caps on.


I'm still having vivid flashbacks from the insanely small stars in the new 10.1X 40XW view.

Surprisingly i didn't notice the huge disparity in AFOV sizes between the old XW's and the new 85degree 16.5XW.
I must be blind !

Perhaps it was the immersion depth that fooled my eyes ? Not sure how that wasn't super obvious but what i am is that the the investment for these big XW glass was very wise.

The parfocality between the three was nearly exact and thats also a big plus in my book.

The design between the new 23XW and 16.5XW is dramatic too with a much more complex Smyth component and simple lenses after the prefocal component in the 16.5XW.

It almost equals Als masterpiece Type2 20mm Nagler for immersion and field excellence.

A massive accomplishment in my book.

I wonder if Ricoh will design another focal length or two ?

Beat Tele Vue or at least keep pace with them with the new T7's ?

Ricoh/Pentax still has waterproofing and SMC coatinga and in my experience the twist up eyeguard beats Delos adjustment although its OK too.
Personal preference i guess.


CSS
Lance

[ralphjunius]

Thanks for the weights!  Looks like my XW 30mm is the lightest, with the XW 16.5mm about 2 oz heavier, which i can live with.

 

Funny, Ricoh put the 16.5 in a 2" barrel?  The field stop figures to be around 24.5mm which fits in a 1-1/4 barrel.  A lotta "extra glass"?  Maybe Ricoh is just using the best part of the large diameter lenses in the main body??  Regardless I like being able to get down to 16.5mm parfocal, similar weight and without a 1-1/4" adapter.

 

I have always preferred the screw down cap on all the XW and XL Pentax's for the very precise and no BS eye relief adjustments!  And even more I like the less blackout than just about any other wide angle eyepiece.  

 

I did have a full set of XL's that I thought had a little less blackout, but they were stolen out of my locked trailer for the 20" dob at a motel returning from TSP.  

 

I often get into an object visually for well over an hour and the combo of exact eye placement and reduced blackout lets you do it very comfortably.  I believe comfort and parfocal are both underrated.  

 

And did i say the XW and XL's compete overall very well visually with everything I've tested them against, ha!  Ok except for the lone 8mm TMB monocentric I have.

 

Didn't know about Al's type 2 20mm masterpiece?  It came out raising 5 kids before I could afford anything better than a Koenig (that I did enjoy).

 

Ok, I went ahead and ordered the XW 40mm and 16.5mm to complement the 30mm, all being parfocal.

Edited by ralphjunius, 29 April 2025 - 06:57 AM.

[Starman1]

Thanks for the weights!  Looks like my XW 30mm is the lightest, with the XW 16.5mm about 2 oz heavier, which i can live with.

 

Funny, Ricoh put the 16.5 in a 2" barrel?  The field stop figures to be around 24.5mm which fits in a 1-1/4 barrel.  A lotta "extra glass"?  Maybe Ricoh is just using the best part of the large diameter lenses in the main body??  Regardless I like being able to get down to 16.5mm parfocal, similar weight and without a 1-1/4" adapter.

 

I have always preferred the screw down cap on all the XW and XL Pentax's for the very precise and no BS eye relief adjustments!  And even more I like the less blackout than just about any other wide angle eyepiece.  

 

I did have a full set of XL's that I thought had a little less blackout, but they were stolen out of my locked trailer for the 20" dob at a motel returning from TSP.  

 

I often get into an object visually for well over an hour and the combo of exact eye placement and reduced blackout lets you do it very comfortably.  I believe comfort and parfocal are both underrated.  

 

And did i say the XW and XL's compete overall very well visually with everything I've tested them against, ha!  Ok except for the lone 8mm TMB monocentric I have.

 

Didn't know about Al's type 2 20mm masterpiece?  It came out raising 5 kids before I could afford anything better than a Koenig (that I did enjoy).

 

Ok, I went ahead and ordered the XW 40mm and 16.5mm to complement the 30mm, all being parfocal.

In a negative/positive design, the light expands between the bottom lens and the next set of lenses in the barrel.  As a result, you cannot see a maximum field stop in those eyepieces without severe vignetting.

And the wider the apparent field, the larger the eyepiece needs to be.

So an 82° eyepiece at 30-31mm is as large a field stop as can be accomplished, at ~42-42.5mm.  Shrink the apparent field to 68°, and you can get a 46mm field stop.

The 16.5mm Pentax is an 85° eyepiece.  It required a 2" barrel.  Shrink that to 68°, and it can be 1.25".

It's why the 13mm Ethos, with a 22.5mm field stop, can be a 1.25" eyepiece, but the 14mm 100° ES, with a 25mm field stop, had to be a 2" eyepiece.

[Jon Isaacs]

Another example:

 

16 mm Type 5 Nagler: 22.1 mm Field Stop  1.25 inch

 

17 mm Type 4 Nagler: 24.3 mm Field Stop. 2 inch

 

Jon

[PKDfan]

Another example:

16 mm Type 5 Nagler: 22.1 mm Field Stop 1.25 inch

17 mm Type 4 Nagler: 24.3 mm Field Stop. 2 inch

Jon

And another, T2 20mm Nagler has a 27.4mm FS so to avoid vignette at ~82° AFOV a 2" barrel was needed.


CSS
Lance

[PKDfan]

Thanks for the weights! Looks like my XW 30mm is the lightest, with the XW 16.5mm about 2 oz heavier, which i can live with.

Funny, Ricoh put the 16.5 in a 2" barrel? The field stop figures to be around 24.5mm which fits in a 1-1/4 barrel. A lotta "extra glass"? Maybe Ricoh is just using the best part of the large diameter lenses in the main body?? Regardless I like being able to get down to 16.5mm parfocal, similar weight and without a 1-1/4" adapter.

I have always preferred the screw down cap on all the XW and XL Pentax's for the very precise and no BS eye relief adjustments! And even more I like the less blackout than just about any other wide angle eyepiece.

I did have a full set of XL's that I thought had a little less blackout, but they were stolen out of my locked trailer for the 20" dob at a motel returning from TSP.

I often get into an object visually for well over an hour and the combo of exact eye placement and reduced blackout lets you do it very comfortably. I believe comfort and parfocal are both underrated.

And did i say the XW and XL's compete overall very well visually with everything I've tested them against, ha! Ok except for the lone 8mm TMB monocentric I have.

Didn't know about Al's type 2 20mm masterpiece? It came out raising 5 kids before I could afford anything better than a Koenig (that I did enjoy).

Ok, I went ahead and ordered the XW 40mm and 16.5mm to complement the 30mm, all being parfocal.

You Will love them Ralph !

I'm Still reeling from the view of my new 40XW.

I can see why when Pentax stopped production of the 30mm & 40mm focal lengths their prices went through the roof.

I'm kinda flabbergasted that the 10.1X and 13.1X views were so different.

Writing about it makes me want to pull out my 100ED and give it a go there. My 62mm 400mm length scope views were of the proverbial write home and gush embarrasingly type.

One really weird effect i noticed was the 40XW's field without bright stars seemed to focus slightly differently than with a bright star in the field.
And it seemed flatter as well.

A very strange situation that i'll explore further.

I felt a tight kinship with it at the very first moment of crisp focus so the relief was palpable that my search for a 'searcher' was complete.


CSS
Lance

[ralphjunius]

Don, please clarify and/or further explain from your previous post:  "In a negative/positive design, the light expands between the bottom lens and the next set of lenses in the barrel.  As a result, you cannot see a maximum field stop in those eyepieces without severe vignetting."

 

I presume the "bottom lens" is the one that goes into the focuser??  I also presume you are saying "you cannot see a maximum filed stop  in those eyepieces ?with a 1-1/4"? barrel that goes into the focuser?  without severe vignetting.  If so wouldn't that all depend upon the location of the "bottom lens" in the 1-1/4" barrel that goes into the focuser.   

 

But I also presume with the very large diameter "top" of all the Pentax XW eyepieces that there is plenty of room to put lenses much larger than 1-1/4".

 

So I don't understand where your conclusions come from regarding the 85* and the 68* field stops unless you have all the exact measurements for the eyepieces?

 

I have heard of this many years ago but have never really understood it.  I appreciate your help.

Edited by ralphjunius, 30 April 2025 - 08:38 AM.

[Ernest_SPB]

The 16.5mm Pentax is an 85° eyepiece.  It required a 2" barrel.

Do you have any proofs of the statement?

 

Diameter of effective field stop for the EP is 24.5 mm - quite enough for 1.25", especially for slow (F10) scope. It will cause certain level of vignetting for a fast (F4-F5), but not critical level. 

 

E.G. We had even a sample of 19mm 82-deg. UWA in 1.25" barrel

[Starman1]

Don, please clarify and/or further explain from your previous post:  "In a negative/positive design, the light expands between the bottom lens and the next set of lenses in the barrel.  As a result, you cannot see a maximum field stop in those eyepieces without severe vignetting."

 

I presume the "bottom lens" is the one that goes into the focuser??  I also presume you are saying "you cannot see a maximum filed stop  in those eyepieces ?with a 1-1/4"? barrel that goes into the focuser?  without severe vignetting.  If so wouldn't that all depend upon the location of the "bottom lens" in the 1-1/4" barrel that goes into the focuser.   

 

But I also presume with the very large diameter "top" of all the Pentax XW eyepieces that there is plenty of room to put lenses much larger than 1-1/4".

 

So I don't understand where your conclusions come from regarding the 85* and the 68* field stops unless you have all the exact measurements for the eyepieces?

 

I have heard of this many years ago but have never really understood it.  I appreciate your help.

A negative lens in the lower barrel expands the light cone into a larger lens above it in the positive stack.  The actual iris is above the negative lens but the field stop we see is above that, near the positive stack.

As a result of this wider virtual field stop, there is a limit to how wide the field stop can be and fit inside the barrel.  

What that means is that a field stop the size of the inside of the lower barrel will not fit in a 1.25" eyepiece without the top of the 1.25" barrel seriously cutting into the light entering the upper lens assembly.

So the maximum field stop in a negative positive eyepiece will not be as large as in an all-positive design, where the field stop is below the field lens and only limited by the I.D. of the lower barrel.

 

The transition from 1.25" to 2" in a 68° field occurs at the 24-25mm focal length transition.  For 82°, it occurs at the 16-17mm focal length transition.  There have been eyepieces longer than 16mm in 1.25", but they had quite noticeable vignetting.

Meade made a 18mm 82° in 1.25" for a while, but the second production was changed to 2" due to vignetting.

 

I will find an illustration that explains it.

In this illustration, they have accidentally reversed the "physical field stop" and "Effective Field Stop" descriptions.

The physical field stop is above the negative lens, while the effective field stop (also called the virtual field stop) is higher in the barrel, nearer the upper lenses, and is where the eyepiece actually focuses.

Attached Thumbnails

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[Starman1]

Do you have any proofs of the statement?

 

Diameter of effective field stop for the EP is 24.5 mm - quite enough for 1.25", especially for slow (F10) scope. It will cause certain level of vignetting for a fast (F4-F5), but not critical level. 

 

E.G. We had even a sample of 19mm 82-deg. UWA in 1.25" barrel

Yes.  The 85° field of view, in a negative/positive eyepiece design, with unnoticeable vignetting, will occur in the 15-16mm range.

The 23mm Pentax 85° has noticeable and serious vignetting at the edge of the field because of how low in the barrel the negative lens sits.

I think it should have been a 2.4" eyepiece, but, of course, that is not a common size for commercial telescopes.

 

Several years ago, there was an 84° 20mm eyepiece in 1.25" made in Japan (It was called "Wide Scan", if I recall).  It had serious and quite noticeable vignetting.

I owned one for a short while but sold it.

Several years later, Meade sold an 18mm 82° in 1.25", but it only lasted until a second production, where it became 2".

 

If the negative lens had been positioned high in the lower barrel, it would have been quite possible for the 16.5mm Pentax to be a 1.25" eyepiece.

But, 1) it would have required a huge amount of in focus, and 2) it would have had all its weight above the lower barrel, which would be precarious in a telescope.

[ralphjunius]

A negative lens in the lower barrel expands the light cone into a larger lens above it in the positive stack.  The actual iris is above the negative lens but the field stop we see is above that, near the positive stack.

As a result of this wider virtual field stop, there is a limit to how wide the field stop can be and fit inside the barrel.  

What that means is that a field stop the size of the inside of the lower barrel will not fit in a 1.25" eyepiece without the top of the 1.25" barrel seriously cutting into the light entering the upper lens assembly.

So the maximum field stop in a negative positive eyepiece will not be as large as in an all-positive design, where the field stop is below the field lens and only limited by the I.D. of the lower barrel.

 

The transition from 1.25" to 2" in a 68° field occurs at the 24-25mm focal length transition.  For 82°, it occurs at the 16-17mm focal length transition.  There have been eyepieces longer than 16mm in 1.25", but they had quite noticeable vignetting.

Meade made a 18mm 82° in 1.25" for a while, but the second production was changed to 2" due to vignetting.

 

I will find an illustration that explains it.

In this illustration, they have accidentally reversed the "physical field stop" and "Effective Field Stop" descriptions.

The physical field stop is above the negative lens, while the effective field stop (also called the virtual field stop) is higher in the barrel, nearer the upper lenses, and is where the eyepiece actually focusesI think I've got it!

 Don, I've been reading your post for two days now, and I think I've finally got it.  I got into real trouble for a while because I failed to read near the end of your post that "they have accidentally reversed the "physical field stop" and "Effective Field Stop" locations in the diagram.  HA!  Thought maybe I was in The Twilight Zone.

 

This is what I now understand:

 

The negative lens is in the left part of the diagram in the Pentax XW16.5's 2" barrel that inserts into the focuser.  The negative lens magnifies the image from where it enters the negative lens at the left  of the diagram and continues to the right. The image needs to get out of the 2" barrel as it expands before it gets vignetted by the 2" barrel's inside diameter of about 46mm.  The expanding image coming from the negative lens of the Pentax XW6.5 quickly exceeds the 27mm inside diameter of a 1-14" barrel. So the 1-1/4' barrel is too small and would vignette the image going to the positive lens in the big diameter part of the eyepiece on the right side of the diagram.  So a 1-1/4" eyepiece barrel  doesn't  doesn't work with the XW 16.5! This is because the design and configuration of the XW16.5 is such that the above happens.

 

I finally understand why the Tele Vue Specifications table specifically notes that the filed stop dimensions are "effective" for the wide field eyepieces because as it says they have "internal focal planes". 

 

BTW, that TV Spec table is by far the best and most useful eyepiece specification I have ever seen.

 

Also, worth noting is that many years ago Al Nagler did manage to design and configure lenses in a16mm  1-1/4" eyepiece with an 82 degree apparent field of view: the Nagler 16mm Type 5. Though many Tele Vue eyepieces are absolute favorites of mine, and all of mine are at least very good, I spent  a night many years ago with a friend's 16mm T5 and decided not to buy one.  I'll be real interested to see how the XW16.5 performs in my scopes if the weather ever clears up.

 

Thanks a million Don for unveiling one of my long standing, foggy eyepiece mysteries!!!  Of course let me know if I got any of this wrong.

Edited by ralphjunius, 02 May 2025 - 07:21 PM.