Parfocal rings and undercut barrels

[Paz]

I'm thinking of getting parfocal rings for my eyepiece set ups but was wondering what happens if the spot you need to lock a parfocal ring is where a barrel is undercut and the ring won't sit flush against the barrel. Do parfocal rings work in this situation?

[Doc]

I don't know I've never even considered it.

I suppose if you have a lathe you could undercut the parfocal rings to match.

[Ricochet]

Baader do a ring with a compression clamp for 2" eyepieces which should work better than a normal ring that only has screws. 

https://www.firstlightoptics.com/baader-planetarium/baader-hyperion-2-inch-fine-tuning-stop-ring.html

With my my 28mm Nirvana I found that the tapered style undercut meant that the ring could slip up the barrel. Alan at skies the limit had some spacer rings made (in 1.25" and 2") so that I could pad out the distance between the ring and the underside of the eyepiece. Of course this requires having enough space above the ring to fit a spacer (or two as in the photo below). 

 

[Ruud]

I think the compression ring might be under a lot of stress if tightened half-over half-outside an undercut.

Filling up the undercut would help if you end up with this problem.

Maybe you could test before you buy. First focus with the eyepiece that needs the most in-travel, then focus the other eyepieces not with the focuser but by sliding them out a bit and noting how far out they need to go.

I don't mind focusing myself so I've never considered parfocal eyepieces or rings, and I don't speak from personal experience, but I believe that even with a parfocal set of eyepieces you still to fine focus when you change eyepieces. Is coarse focusing your problem or fine focusing?

[Louis D]

I ran into this very problem parfocalizing my 12mm Nagler T4 which focuses about 20mm below its shoulder.  I stacked five 4mm thick 50mm inner diameter O-rings up against the underside of its shoulder and added a 20mm 2" extension to the 2" skirt.  Now it focuses within a millimeter or two of the rest of my eyepieces.  They have enough stretch to stay put even in the undercut.  You could probably find O-rings to fit 1.25" barrels as well on ebay.  The best part is they don't mar the chrome on the barrel like a pinpoint set screw (grub screw).  The only downside is the rubber smell.

[Paz]

19 hours ago, Ruud said:

Is coarse focusing your problem or fine focusing?

My interest has arisen only  from looking for small improvements in set up.

Parfocalising will save a bit of time focusing, and would mean I can leave my paracorr on one setting when using a paracorr.

Another problem parfocalising  may help me with is sometimes on faint targets when you change eyepieces and have to refocus you can't do it on the faint target because if it's too far out of focus it's invisible or its so faint there isn't enough "information" to be able to focus with so I have to find a bright enough target  to refocus on and then refund the target. Being bang on or very close to focus between eyepieces would avoid this complication.

[Ruud]

Just now, Paz said:

Parfocalising will save a bit of time focusing, and would mean I can leave my paracorr on one setting when using a paracorr.

Another problem parfocalising  may help me with is sometimes on faint targets when you change eyepieces and have to refocus you can't do it on the faint target because if it's too far out of focus it's invisible or its so faint there isn't enough "information" to be able to focus with so I have to find a bright enough target  to refocus on and then refund the target. Being bang on or very close to focus between eyepieces would avoid this complication.

I see. I hope it all works out!

[NGC 1502]

 

I’m very late to this thread, but agree with earlier mention of filling the undercut if you wish to fit a parfocal ring in that position on the EP barrel. Best is a hard tape like some Dymo label tape I used to have, but much softer regular black tape works reasonably well, cut to the width of the undercut, several layers as needed.  That’s if the undercut is a uniform depth, tapered undercuts would be a bit of a challenge to fill.

Some parfocal rings have grub screws with a sharp end where it contacts the barrel, that’s not a problem if tightening onto tape, but could marr the chrome barrel, especially if tightened hard to lock it in place. It’s VERY fiddly, but I’ve removed grub screws to file the end smooth......don’t drop it on the shed floor, you probably won’t be able to find it........no prizes for guessing how I know that ?

Ed.

[Don Pensack]

On 1/23/2018 at 13:49, Paz said:

I'm thinking of getting parfocal rings for my eyepiece set ups but was wondering what happens if the spot you need to lock a parfocal ring is where a barrel is undercut and the ring won't sit flush against the barrel. Do parfocal rings work in this situation?

Yes, but you need to have 3-screw parfocalizing rings.