18 replies to this topic

[Poochpa]

Do the Pentax XWs have a safety undercut on the barrel, like the Televues? Thanks.

Mike

[CrazyPanda]

Yes, they do.

[areyoukiddingme]

But they are a little different. Japanese equipment generally seems to be built with other Japanese companies in mind. For example, the Tak prism diagonal works well with XWs, but it particularly unhappy with Explore Scientific.

 

The XW undercut is relatively narrow, and it's shaped like this [

 

The Televue undercut is wider, and also has a step down on one side. 

[Poochpa]

Thanks for the info!

Mike

[csrlice12]

However, once you have an XW in the focuser, you rarely want to remove it....

[Starman1]

Do the Pentax XWs have a safety undercut on the barrel, like the Televues? Thanks.

Mike

Check out the 2024 Eyepieces Buyer's Guide at the top of the eyepieces Forum

There is a column called "Undercuts Y or N?" that give you that information for all 1700 eyepieces.

[Poochpa]

Check out the 2024 Eyepieces Buyer's Guide at the top of the eyepieces Forum

There is a column called "Undercuts Y or N?" that give you that information for all 1700 eyepieces.

Don,

Thanks for the info!

Mike

[ausastronomer]

But they are a little different. Japanese equipment generally seems to be built with other Japanese companies in mind. For example, the Tak prism diagonal works well with XWs, but it particularly unhappy with Explore Scientific.

 

The XW undercut is relatively narrow, and it's shaped like this [

 

The Televue undercut is wider, and also has a step down on one side. 

As Scott already mentioned the Pentax XW undercut is somewhat different to most other eyepiece undercuts.  The Pentax XW undercuts are about 1/2 the width and 1/2th depth of the Televue undercuts. While most other undercuts are continually fouling and are a complete PITA, I've had all the 1.25" Pentax XW's for over 20 years and I don't ever recall fouling up on a Pentax XW undercut, either inserting it, or removing it from the focuser.

 

Cheers

Edited by ausastronomer, 18 April 2024 - 10:37 PM.

[Ernest_SPB]

Geometry of undercuts in TeleVue and Pentax eyepieces similar. Subtle difference in their parameters (depth and width) between the two less then differences between their series and even in the same series different years.    

[ausastronomer]

As Scott already mentioned the Pentax XW undercut is somewhat different to most other eyepiece undercuts.  The Pentax XW undercuts are about 1/2 the width and 1/2th depth of the Televue undercuts. While most other undercuts are continually fouling and are a complete PITA, I've had all the 1.25" Pentax XW's for over 20 years and I don't ever recall fouling up on a Pentax XW undercut, either inserting it, or removing it from the focuser.

 

Cheers

 

 

Geometry of undercuts in TeleVue and Pentax eyepieces similar. Subtle difference in their parameters (depth and width) between the two less then differences between their series and even in the same series different years.    

 

While I don't have any instrument to precisely measure the undercut depth, I can estimate it pretty well and I can measure the undercut width accurately with my outside digital calipers.

 

On all of my 1.25" Pentax XW's the undercut width is 5mm and the depth is ~.25mm.  On my 31mm Nagler, 27mm Panoptic, 13mm ETHOS (bought in 2007) and the Nagler T4's the undercut width is 12mm and the depth is ~.5mm. It's more than double the width and about twice the depth. I wouldn't call that subtle, I'd call that significant !!   All of these Televue eyepieces are 2007 vintage or older.

 

On my 6mm and 8mm TV Radians both bought in 2008, the undercut is 6mm wide and ~.5mm deep.  This is pretty close to the Pentax XW's excepting it's about 1mm wider and deeper by ~.25mm.

 

On my 6mm Delos the undercut style has changed from the older Televue eyepieces. The floor of the undercut is 6mm wide and ~.5mm deep but there is an angled "lead" of about 2.5mm tapering back to the bottom of the eyepiece barrell.  The lead is obviously designed to make extraction easier.  The full width of the undercut on the 6mm Delos is ~8.5mm.

 

So it would appear that Televue have changed their undercut style at least 3 times.  My 2007 and earlier Televues all have the same undercut, regardless of series. The 2008 vintage Televue eyepieces have the flat undercut but much narrower than the earlier ones; and my 6mm Delos purchased ~2014  has the undercut with the bevel or lead.

 

I'll post some pictures later tonite

 

Cheers

Edited by ausastronomer, 19 April 2024 - 03:20 AM.

[CrazyPanda]

So it would appear that Televue have changed their undercut style at least 3 times.

They definitely have. Different vintages and different lines all have different undercuts. I have at least 3 different ones in my case.

EDIT: images of some of my Tele Vues:

 

DeLite

 

Also checked my TV Plossls, newer 24 Panoptic, and images of my now sold Delos, and this undercut size and style with tapered lower lip seems to be representative of all TV's newer 1.25" eyepieces.

 

The tapered lower lip snags less when removing it from accessories with compression rings.

 

Older 19mm and 35mm Panoptics (circa 1998)

 

Shallower undercut, more abrupt bottom lip. I can tell you from experience these love to snag more when removing them from accessories with compression rings. I almost dropped the 35 Panoptic because one time my grip slipped when removing it one time.

 

Looked at images of my now sold 12mm Nagler Type 2 and 7mm Nagler Type 1 and they both have similar undercuts.

 

6, 8, 10 Ethos

 

This image is of the 10 Ethos but is representative of the 6 and 8 I have as well.

 

3.7 & 4.7 Ethos

 

Note the shallower undercut than the 6, 8, and 10 Ethoi. Still has a mild taper on the lower lip.

 

21 Ethos

 

Similar to the 6, 8, and 10 Ethos. Seems to be representative of Tele Vue's newer 2" eyepieces.

 

31 Nagler

 

Same style undercut, but shallower than the 21 Ethos. I suspect this is because the field stop is so wide in the 31N there wouldn't have been enough barrel left if they used their usual undercut depth here.

 

And some bonus pictures:

 

• 22 Nagler Type 4 with older style hard undercut: https://www.astronom.../3/3/3300_1.jpg
• 22 Nagler Type 4 with newer style tapered lower lip: https://www.bhphotov...22mm_329428.jpg

 

Pentax XW for comparison:

 

Pentax XW undercut depth is about the same as the older Tele Vue undercut depth (a hair shallower), but it's quite narrow. I suspect that not all accessories grip into this undercut, and a good portion grab on the full part of the barrel, defeating the purpose.

 

For instance, in trying out the 10 XW in my Paracorr's 1.25" adapter, the undercut is not only narrower than the brass compression ring, it's offset by a couple millimeters. As such, the brass compression ring does not actually seat into the undercut and only a tiny twist of the thumb screw is needed to release the eyepiece.

 

However, the lower lip of the 10 XW undercut loves to catch on the upper lip of the 1.25" adapter's compression ring notch if it's pulled out at a bit of an angle. This means the 10 XW has the worst of both worlds - the undercut does not prevent it from slipping out if the thumb screw gets loose, but it does still get caught extracting it from the adapter.

 

The more I think about this and deliberately test these eyepieces, the more I'm with Jon Isaacs on this - the problem isn't eyepiece undercuts, the problem is compression rings. Yes, technically you can snag the eyepiece on the upper lip of an undercut when inserting it into a smooth adapter, but the snagging on the way in can also happen from the bottom of the eyepiece barrel on the lower lip of a compression ring notch. And it's the upper lip of the compression ring notch that ultimately catches the lower lip of the undercut in many eyepieces. What's more is Don has mentioned that fully tapered undercuts can twist the compression ring and cause the eyepiece to get lodged in some cases, or just bend it out of shape.

 

Compressing rings and their corresponding notches in accessories are responsible for most eyepiece snagging. Even if you remove the compression ring, you're still left with the notch, and it's probably even worse because there's nothing filling it and it's more likely to catch the eyepiece.

Edited by CrazyPanda, 19 April 2024 - 07:29 AM.

[Poochpa]

Informative discussion. I posted my question bc of my dislike for the undercut on TV eyepieces. I tried a friend's older Pentax 40XL in a 2" diagonal with compression band, and I didn't notice the EP catching when I removed it, unlike my Panoptics and Naglers.

Mike

Edited by Poochpa, 19 April 2024 - 10:15 AM.

[Starman1]

Yes, the undercut, whether cylindrical or conical taper, is designed to save the eyepiece when a slightly too loose thumbscrew is tightened against the eyepiece.  The undercuts aren't really much of a problem if inserted into a smooth bore focuser or adapter and tightened down with a thumbscrew.  And a brass split ring binder isn't much of a problem if tightened against a smooth barreled eyepiece, because it does what it is supposed to do--prevent the scratching of the barrel from the thumbscrew.

The assumption that a brass split ring binder was intended to fit into the undercut on the barrel is a poor assumption, as this rarely happens because every eyepiece brand and focuser and adapter brand has different ideas of the dimensions for the undercut and brass split ring width and position.  Much of the time, the brass split ring only partially grabs the eyepiece.

 

I solved the adapter problem by using a Twist-Lock adapter with a 15mm long collet to grab the eyepiece both above and below the undercut.  Eyepieces slide in and out with no snags now.

I solved the focuser problem by using a smooth bore drawtube with a Delrin 1/4x20 thumbscrew.

And I've mostly solved the Paracorr's Tunable top problem by removing the split ring, using Delrin thumbscrews, and using an adapter with smooth sides.  I can use 11 of my 13 eyepieces as 1.25" eyepieces in the Paracorr, and one of my 2"-only eyepieces has a smooth barrel, making for easy removal, and the other has its undercut filled with copper tape.

 

I shouldn't have had to go to such lengths to solve the problems created by undercuts, but that is the fault of the manufacturers who haven't developed a standard for manufacturing of these things.

And it's the fault of the paranoid who view an eyepiece with marks on the outside as less valuable than one without marks, regardless of the condition of the optics in the eyepiece.

And the people who can't seem to remember to tighten their eyepieces down when inserted.

I had telescopes and eyepieces back when thumbscrews tightened down on smooth-barreled eyepieces that were inserted in smooth-bored focusers.  A 1/16 turn of a thumbscrew allowed easy removal of the eyepiece, and nothing bumped or snagged on insertion or removal.  I don't think the systems we have now are an improvement on that.

Edited by Starman1, 19 April 2024 - 10:24 AM.

[vrodriguez2324]

I was going to jump in to say that the Pentax XW do not have undercuts =)

 

This thread made me go back and check my 5mm XW and indeed it does, lol. I thought they didn't because, like others have said, it is smaller and shallower. I can't remember having to wiggle it out of a focuser, something that I have to do with most of my other eyepieces that have the undercut. 

[25585]

Thumbscrews have their own issues. Thinking you have unscrewed one enough, but have not, so its end catches in the undercut rut is BAD. Its more an issue with chunkier eyepieces that might get pulled out at an angle, especially in the haste of changing them.

[ckwastro]

I now fill all of the undercuts on my eyepieces with aluminum tape. (Thanks Don P)! Copper tape will work too. Eliminated any hang-up issues I used to have with the undercuts and brass clamp rings, and binoviewer collets. 

Edited by ckwastro, 20 April 2024 - 12:15 PM.

[areyoukiddingme]

I've never messed with filling in the undercuts with tape, but would think the XW would be easier than the Televue, as there's no tapered edge.

[Poochpa]

I've never messed with filling in the undercuts with tape, but would think the XW would be easier than the Televue, as there's no tapered edge.

Yes, at least the Pentax XL that I tried was, and assume the same is true of the XWs, but wanted to post to make sure.

 

Mike

[ckwastro]

I've never messed with filling in the undercuts with tape, but would think the XW would be easier than the Televue, as there's no tapered edge.

They were all very easy to fill. Both TV and Pentax included.