One thing that annoys me is that "safety undercut" or "barrel safety recess" getting snagged on the compression ring as I am withdrawing eyepieces. First - it's annoying; Second - risk of having eyepiece come out of hand; Third - likely to pull the scope (dobsonian mount) off target.
I've done a bit of research and find regular CN threads on this matter with statements such as:-
"The undercut is evil and was invented by Satan"
"Please TeleVue, give us the option of smooth barrels!"
"Yeah, what if they showed up at NEAF with a bunch of smooth-barrel eyepieces, ....... I'd vote it Equipment of the Year!"
Seems the options are:-
1. Smooth barrel
2. Tapered barrel
3. Undercut
Smooth barrel - best for brass compression rings, but eyepiece most easily removed (hence "fall out" possibility?)
Tapered barrel - best with set screw(s), can damage compression rings and/or they are not as effective on the non-flat surface
Undercut - best with set screw(s) IF they line up with the undercut. Can be a pain with compression rings, especially if the ring catches the undercut on insertion/removal.
For a reflector on dobsonian mount, the eyepiece should not ever be positions so it is "hanging", so I think I would prefer smooth barrel - I think I only have one eyepiece with a smooth barrel.
OK, who makes retrofit smooth barrels for eyepieces? Appropriately threaded for filters with non-reflective internal surface? (Hint to the machinists - there would seem to be a ready market!)
Alternatively, who has knocked their undercuts into shape? I see one report that:- "A little sanding of the brass compression ring's edges has made the undercut a non-issue for me". Anyone tried this?
I'm with you EricI am sick of the groove I bet there are a lot of people who have lost screws in the grass while changing eyepieces as they undo the retaining screw in the focuser.
As you say there is no need for the groove in a Dob
My eyepieces are undercut and never had any issues with them snagging the focuser compression ring. Maybe those people need to turn their set screws an extra revolution or two
I have often had this problem, more so with the Pentax 30 and 40 which are 2" eyepieces. Sometimes it can take quite a while to get the stupid thing out.
I had to remove the brass compression ring in my 2” Moonlite focuser body as it just, and only just, balanced precariously right on the edge of the captive groove in my 2” adapter for the SBIG ST7 ccd camera. It took me some time to chase this one down; it manifested as elongation in RA on long exposure images.
Ahem. I've been waiting for you to jump in Trevor and tell us you can machine up any barrel we need?? Perhaps I'm being optimistic?
Don't know if I would fancy watching my 30mm Pentax XW whizzing around in a chuck at 1000 RPM as I move a cutter into the side of it to remove the undercut....sends shivers down my spine thinking about it!
Do you use a focus extension tube for your EPs?
Tele Vue sell a smooth 2 inch eyepiece barrel extender for some of their EPs you can use instead of the focus extender.
See bintelshop, Telescope Accessories, page 4.
I have one 2" ep with a tapered cut and its just as annoying as those with undercuts. Both should be banned, they're a bloody nuisance.
That said, one does have to make sure the danged diagonal and the back on the scope are screwed on tight or a sudden flip will send a heavy EP to the ground.
Working for a company that specializes in making safety grooves and donut holes, I am distressed by the attacks you are all making on some of our products...
I am joking of course...we don't really make grooves or holes...
I've never had a problem with the undercut and find it hard to understand how others struggle (and they do) with them. As was pointed out by a previous poster turning the set screw an additional rotation seems to do the trick and the peace of mind knowing my eyepiece is secure is worth it to me...
That said, one does have to make sure the danged diagonal and the back on the scope are screwed on tight or a sudden flip will send a heavy EP to the ground.
Reflector on dob mount and this is not a problem. But I've had the pleasure of the diagonal suddenly deciding to "turn turtle" on my C8 - without damage so far.
Do you use a focus extension tube for your EPs?
Tele Vue sell a smooth 2 inch eyepiece barrel extender for some of their EPs you can use instead of the focus extender.
See bintelshop, Telescope Accessories, page 4.
Regards, Rob
Thanks Rob, I've had a look. As I understand it, if you insert it fully, your eyepiece would still mount on the portion where the undercut occurs.
Also I don't have any of those eyepieces.
Replacement barrels are what is required. Perhaps I could fill the undercut grove with Spakfilla?
Actually, I'll try the suggestion of checking if there is a sharp inward facing edge on the brass compression ring and bevel it off. That sounds very promising.
Is it possible to interchange the barrels with the cheap ones??
I have had my Televue eyepieces do this to me, but in all essence, an extra half turn or 2 helps settle the issue!
One might think that it should be possible. The problem is the variety of barrels. You might think it was simply a 1.25" and a 2" with a standard filter thread - but up at the business end, they are all so different - shapes and threads.
I remember my first wide angle - the el-cheapo 80 deg 30mm. It has a very undistinguished dull aluminium 2" barrel. No problem, sez I, I'll just take that lovely shiny steel barrel off the 2" GSO 26mm and swap. No luck - different threads.
Now, I have been able to get my eyepieces in and out and it is usually a combination of loosening the set screw further - trying to be careful (with two pairs of gloves in the cold) not to unscrew it completely - and a good wiggle of the eyepiece.
BUT that is unsatisfactory. An eyepiece should easily slide out and slide in and be nipped up firmly - is that too much to ask?
1) in a lot, if not most, of the focusers with simple setscrews the screw tightens on the lip of the undercut band and the tightening if the screw lifts that side of the eyepiece slightly out of the focuser, tipping the eyepiece. This can happen in some expensive focusers and is a negative to the "safety" groove.
2) Many "compression ring" focuser bands do not have "coined" edges on those compression bands, so the edges of the band catches on the eyepiece undercut as you try to remove it. Additionally, the "spring" in the compression band metal doesn't allow the band to spring back out of the way when the setscrew is loosened on many of the lower-priced compression rings, meaning the screw has to be over-loosened to allow easy removal of the eyepiece.
3) As I look at the eyepieces on the shelf at work, I notice the lengths and depths of the undercuts vary all over the place, as do the lengths of the 1.25" barrels. There is no standard, and so the compression ring doesn't fit into the undercut on the eyepiece that much of the time. Many times, the compression ring also tightens on the lip of the undercut but doesn't lift the eyepiece out of the focuser because there is no turning torque of the screw present where it presses on the eyepiece.
4) The term "compression ring" is a misnomer. It doesn't actually compress all around the perimeter of the eyepiece, but merely puts a strip of brass or bronze between the focuser setscrew and the eyepiece to prevent marring of the eyepiece barrel.
5) The marketplace needs to return to smooth barrels on all smaller eyepieces and make safety undercuts conical on the heavier ones. The grooved undercut needs to be abandoned as soon as possible. I can't tell you how many hundreds of times the compression ring caught on my eyepieces until I replaced the focuser drawtube with a standard setscrew without the compression ring. But after I started using a Paracorr the problem returned because the Paracorr has a compression ring. It is interesting to note that the Paracorr itself has a smooth side, though.
It is also interesting to note that the 1.25" barrels on the 6 and 8mm Ethos, which have to be used in a 1.25" adapter in the Paracorr (setting 4) have smooth sides. Yet, none of these has any tendency to fall out of the Paracorr when the thumbscrew is tightened. This just prooves how unnecessary undercut grooves on eyepieces really are.
6) Star diagonals used on SCTs that have insertion barrels that are threaded into the body of the star diagonal can, if tipped to the left with a heavy eyepiece on board, unscrew, allowing the star diagonal to instantly rotate until the eyepiece points at the ground. But, even then, the eyepiece won't fall out if the setscrew is simply tightened on the barrel. People with 2" star diagonals that are used rotated from vertical should always rotate them to the right for that reason (TeleVue, AstroPhysics, and Lumicon owners can ignore that warning--on them the body and insertion tube are machined in one piece)
7) So it's time for the industry that the safety undercut groove is a failed experiment and return to smooth barrels. Either that or make all the undercuts conical in shape.
double trouble My focusers set screw is seated pretty high which
mostly beats the undercut but throw in a differant adapter or eyepiece and it can be a pain pretty quick,, if a standard 1.25" / 2" is accepted width one way ,, why not a standard depth of undercut ?
I am sick of the groove
I bet there are a lot of people who have lost screws in the grass while changing eyepieces as they undo the retaining screw in the focuser.
Perhaps I'm being optimistic?
