Eyepiece undercut.

[Rusted]

I routinely drill and tap for more thumbscrews until there are three. It still isn't enough for the extenders on the long filter stack of a PST mod.
The Baader Twist-lock mechanism is very secure for holding heavy loads like the Lacerta 2" solar prism + binoviewers.
It also has the advantage that you don't need to reach tiny thumbscrews tight up against its large, flat surface.

For the worst, ever socket design and construction I offer the PST eyepiece receptacle.  Or just the entire PST itself, if you like.
Until it is turned into something much more useful.   No, not a desktop pen and pencil holder!

[johninderby]

With compression rings like anything else good quaity ones such as Baader do.work the best. Number of thumbscews and position of the band and thickness of the band do matter. Also more important that the ring on a focuser holds well than that on the diagonal which is usually just holding an eyepiece. 

Hate the old fashioned plain screw method. Even if there are three screws they all need to be tightened equaly to hold an eyepiece squarely. 

[Merlin66]

The holding screws have to be slightly off set to each other, then they work well.

 

[F15Rules]

I hate undercuts. Period.

Dave😃

[Saganite]

6 minutes ago, F15Rules said:

I hate undercuts. Period.

Dave😃

Must agree !

[mikeDnight]

9 hours ago, Merlin66 said:

Tried many different solutions - metal tape, epoxy, etc etc etc none of them worked really successfully - just canvass the suppliers to drop the  (un)"safety recess".

 

I think the best way to resolve the problems caused by undercuts, is not to have them in the first place. Televue's undercuts are the worst as they are rediculously deep, but they must be aware their undercuts are hated as CN is littered with complaints about them - so it must be that they simply don't care! One TV spokesman commented that its down to "marketing". That being the case, I nolonger have a single TV eyepiece in my collection, and the wonderful thing is, I don't miss them one bit.

 

[johninderby]

Here’s a suggestion for Televue. Offer an optional barrel without any undercut. Of cpurse knowing Televue it would be a £99.99 option. 😁  

[F15Rules]

26 minutes ago, mikeDnight said:

One TV spokesman commented that its down to "marketing"

If that is true then what price the concept of "meeting customer needs / preferences"? - a concept that drove the marketing strategy of all of the major companies I worked for in my sales & marketing career - and the same for my competitors!

I'm not in the game of knocking Tele Vue..they make excellent products and have been very innovative over many years. But eyepieces undercuts and their refusal to get rid of them are (along with sky high prices), the reason that I haven't owned a TV eyepiece for c 10 years. I just vote with my feet.

I should also say that I don't like Explore Scientific tapered barrels either, although not as aggravating as the undercuts.

Can't we just get back to nice smooth barrels,? No undercuts mean one less manufacturing step needed, so maybe even a small cost saving to the customer?.. now that WOULD be innovative!😁

Dave

 

[Second Time Around]

Does anyone remember the old RAS screw threads on eyepieces?  They were the standard when I first took up astronomy. When and why did they disappear?

[John]

Actually I find the tapered design of eyepiece barrel (eg: Explore Scientific) more of an issue than the under cut when it comes to potentially being out of alignment when using a compression ring fitting. I can live with the shallow undercuts that most manufacturers use these days. At least Tele Vue put a small chamfer on the lower edge of their under cuts now, which helps when removing the eyepiece I think.

People have been moaning about under cuts on an off on forums like this since I've been a member of them, which is over a decade now with little effect it seems  

 

[johninderby]

Part of the problem is there is no standardisation on the postion of the compression rings. If all compression rings were a certain distance down from the top then then they could avoid a lot of the problems with undercuts.  

Edited January 20, 2020 by johninderby

[Rusted]

2 hours ago, Second Time Around said:

Does anyone remember the old RAS screw threads on eyepieces? 
They were the standard when I first took up astronomy. When and why did they disappear?

I still have two "simple" [one lens] eyepieces in turned brass, with RAS threads.
Which I bought in my early teens back in about 1960.
I still have the heavy, brass, push-pull focuser to match.
Which I used with a 60mm, 1/2 diopter, simple objective [spectacle lens] of 2m focus.
Used as an aerial telescope hanging from the washing line post.
To see a huge and colourful Saturn low in the sky.
All by the same company but the name now escapes me. Irving?
Probably listed in Exchange & Mart back then.

[Alan White]

1 hour ago, johninderby said:

Part of the problem is there is no standardisation on the postion of the compression rings. If all compression rings were a certain distance down from the top then then they could avoid a lot of the problems with undercuts.  

I think that is one of the route causes John,
well other than the darned undercut themselves!

[SimM]

On 20/01/2020 at 07:26, johninderby said:

Even if there are three screws they all need to be tightened equaly to hold an eyepiece squarely. 

If the eyepiece is in a diagonal with both screws "undone", then it sits squarely mounted and shouldn't have much side-to-side movement. Obviously it cannot be restrained without screws e.g. when slewing to a new position.

If there are two screws, that are offset by 90 degrees, as the screws are tightened they align the eyepiece with the sides of the mount in two planes.

Having three screws would make the alignment more like the alignment of a Finder Scope which varies the direction to deliberately align the FOV with the main optics. Depending on the order/degree in which the screws are tightened, it would realign the eyepiece so that it's no longer parallel with the sides.

[ollypenrice]

On 19/01/2020 at 18:35, Alan White said:

An undercut is a royal pain in my opinion, but sadly something I live with as I like TeleVue kit.

The images from John do explain visually rather well.
Supposedly the undercut ensures the item does not slip from the telescope.

I disagree as the undercut catches when taking items on off the scope and move the scope position. 
The only time something fell from my scope was an undercut that caught as I removed an EP.
But each to our own favours of course, some love them, many loathe them. 

YES!!!  If there were an industry standard then one could make a case for the undercut but, since there isn't, and since we are also cursed with an assortment of conflicting clamping methods, they present more danger than security. A momentary snag of the kind you mention resulted in my 35mm Panoptic hitting the ground from the dizzy heights of a 20 inch Dob near the zenith.

Olly

[Rusted]

1 hour ago, SimM said:

If the eyepiece is in a diagonal with both screws "undone", then it sits squarely mounted and shouldn't have much side-to-side movement. Obviously it cannot be restrained without screws e.g. when slewing to a new position.

If there are two screws, that are offset by 90 degrees, as the screws are tightened they align the eyepiece with the sides of the mount in two planes.

Having three screws would make the alignment more like the alignment of a Finder Scope which varies the direction to deliberately align the FOV with the main optics. Depending on the order/degree in which the screws are tightened, it would realign the eyepiece so that it's no longer parallel with the sides.

Disagree. Both the "plug" and "socket" are well defined, parallel cylinders with very close tolerances.
Parallelism is assured with the first screw to be tightened. The further two screws merely secure the item more firmly in the socket.
The secured item would remain safely in place regardless of load and equatorial mounting orientation causing rotation of the receptacle provided three thumbscrews are used.

No taper, nor variation in seating diameters is likely, given mass production using massive CNC machines with very stiff boring bars and external bit holders.
The side loads involved during cutting are absolutely minuscule with modern, ceramic, or other hard, cutting bits in free cutting aluminium using free flowing, cutting fluid.
Both items remain chucked for both internal and external turning. There is no removal midway as might occur with amateur hand turning.

Optical Concentricity is a function of the differences in diameter of the two cylinders involved and can never be worse than one screw.

Given the minute errors involved one would be very unlikely to see, nor detect, any optical defect in the image regardless of the number of thumbscrews.

A simple test would involve mounting both items, once tightened together, in a precision, self centering lathe chuck or, even better a precision collet.
A lever, dial indicator will cheerfully monitor concentricity and parallelism while turning the chuck, or collet holder, by hand.

A finder has wide tolerances over widely spaced constraints along the axis. It is designed as such to allow completely independent adjustment to allow alignment.

Or words to that effect.

[John]

Back in the days when I used to play around the the Chromacor CA / SA correctors and large achromat refractors, the installation of the Chromacor (which screwed onto the end of a 2 inch diagonal barrel) was preceeded by drilling and tapping a 3rd set screw hole in the 2 inch adaptor on the end of the drawtube. The Chromacor required very precise centering in the drawtube and the 3-screw arrangement was said to be the best way to achieve that. Compression ring adapters were not advocated.

 

[Piero]

When an eyepiece undercut is a problem, I use some black tape (the type that is used by electricians) to make the barrel flat. That's it.

[Merlin66]

Piero,

Didn't work for me....

The holding screws quickly chewed it up.......

[Louis D]

19 hours ago, Merlin66 said:

Piero,

Didn't work for me....

The holding screws quickly chewed it up.......

Have you tried pinstriping tape from the automotive supply store?  This CN poster has had good luck with it.

[Louis D]

22 hours ago, Rusted said:

Disagree. Both the "plug" and "socket" are well defined, parallel cylinders with very close tolerances.
Parallelism is assured with the first screw to be tightened. The further two screws merely secure the item more firmly in the socket.
The secured item would remain safely in place regardless of load and equatorial mounting orientation causing rotation of the receptacle provided three thumbscrews are used.

No taper, nor variation in seating diameters is likely, given mass production using massive CNC machines with very stiff boring bars and external bit holders.
The side loads involved during cutting are absolutely minuscule with modern, ceramic, or other hard, cutting bits in free cutting aluminium using free flowing, cutting fluid.
Both items remain chucked for both internal and external turning. There is no removal midway as might occur with amateur hand turning.

Optical Concentricity is a function of the differences in diameter of the two cylinders involved and can never be worse than one screw.

Given the minute errors involved one would be very unlikely to see, nor detect, any optical defect in the image regardless of the number of thumbscrews.

A simple test would involve mounting both items, once tightened together, in a precision, self centering lathe chuck or, even better a precision collet.
A lever, dial indicator will cheerfully monitor concentricity and parallelism while turning the chuck, or collet holder, by hand.

A finder has wide tolerances over widely spaced constraints along the axis. It is designed as such to allow completely independent adjustment to allow alignment.

Or words to that effect.

Well, a lot of folks had issues merging binoviewer images when the eyepieces were held by three screws instead of a single collet (not sure if undercuts were involved since I never owned one of these binoviewers).  You rarely see binoviewers these days being sold with three screws per eyepiece holder.  However, the collet-style holders still have issues with the undercuts causing eyepieces to tip.  I prefer eyepieces with no undercut for this reason when using binoviewers.

I'll agree that I've not noticed any difference in concentricity of a single light path when using 1, 2, or even 3 screws to tighten a diagonal or eyepiece into place.

[Alan White]

Having now pondered the replies in this thread and folks bodges to fill the undercuts,
a question for you all...

Has anyone had the undercut eyepiece refitted with a non undercut 1.25" or 2" fitting?
I was pondering this, probably not practical, but thought I would ask.

[johninderby]

That brings up another standards, or lack of, issue. Why does it seem that every eyiepiece  manufacturer uses a different size / thread on the eyepiece barrels so you usually can't swap them? 🤔

Edited January 23, 2020 by johninderby

[Louis D]

1 hour ago, Alan White said:

Having now pondered the replies in this thread and folks bodges to fill the undercuts,
a question for you all...

Has anyone had the undercut eyepiece refitted with a non undercut 1.25" or 2" fitting?
I was pondering this, probably not practical, but thought I would ask.

Not really practical for eyepieces with a Smyth lens in the lower barrel, either.

[Don Pensack]

On 19/01/2020 at 12:17, JamesF said:

I don't have a problem with beveled undercuts, to be honest.  They seem like a very reasonable compromise.  I've even machined them into various bits and pieces I've made myself.

James

The conically tapered undercut was designed to work with a thumbscrew.  When the screw tightens on the eyepiece, it actually pulls the eyepiece down into flat contact with the focuser.  If the screw is a bit loose, it still catches the eyepiece.

An excellent idea with thumbscrews.

However, the taper can permanently distort a brass split ring binding (mistakenly called compression ring), and result in damage to the collet in a twist-lock or click-lock binding.  The split ring and twist-lock binders were designed for smooth barrels.  So what we have now is focusers, barlows and coma correctors designed for smooth barrels, but we're using eyepieces designed for thumbscrews.  A great state of affairs, and one seemingly without resolution.  The eyepiece manufacturers aren't going to abandon the undercuts, and focusers and other accessories aren't going to abandon whatever binds the eyepiece without a mark (I blame the used market for that).  Tele Vue, at least, has beveled the lip of the undercut so it won't catch on the lip as the eyepiece is removed.

Personally, I went back to a smooth focuser with a large nylon 1/4" x 20 tpi thumbscrew--no mark and compatible with all eyepiece barrels and undercut shapes.