Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Do safety undercuts hurt collimation?

This topic has been archived. This means that you cannot reply to this topic.
18 replies to this topic

#1 stargazer193857

stargazer193857

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7331
  • Joined: 01 Dec 2013

Posted 09 July 2015 - 11:16 PM

I'm not sure if the 1/2 inch of full diameter is enough to keep the axis of the eyepiece matched to the axis of the focuser. I wonder if the under cut and the screw pushing against that region could tip the axis of the eyepiece a degree or two off the axis of the focuser. Is this an issue?

 

If so, how do you rebuild that area up?

 

Also, if our focusers point upwards, why do we need the undercuts?

 

Why not just put one screw hole in the side of it? That would hold it much tighter and not compromise the barrel.


Edited by stargazer193857, 09 July 2015 - 11:18 PM.


#2 orlyandico

orlyandico

    Cosmos

  • *****
  • Moderators
  • Posts: 9378
  • Joined: 10 Aug 2009

Posted 10 July 2015 - 12:46 AM

In my experience, yes, the safety undercut tilts the (thing) - in my case a Televue reducer - if your 2"  holder has a compression ring.  The solution is to use a setscrew not a ring.  Feathertouch focusers have a compression ring, but there's a gap and you can rotate the ring so that one of the setscrews goes through this gap (and turns the focuser into a non-compression ring focuser).



#3 stargazer193857

stargazer193857

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7331
  • Joined: 01 Dec 2013

Posted 10 July 2015 - 12:49 AM

Looks like Plossl's typically have full diameter all the way. Most eyepieces have close to 1/2" of full diameter before an undercut, and some look like only 1/4" of full diameter. Undercuts dominate the larger or more expensive eyepieces. Disturbing.



#4 FirstSight

FirstSight

    Duke of Deneb

  • *****
  • Moderators
  • Posts: 14509
  • Joined: 26 Dec 2005

Posted 10 July 2015 - 11:11 AM

I'm not sure if the 1/2 inch of full diameter is enough to keep the axis of the eyepiece matched to the axis of the focuser. I wonder if the under cut and the screw pushing against that region could tip the axis of the eyepiece a degree or two off the axis of the focuser. Is this an issue?



#5 Kipper-Feet

Kipper-Feet

    Viking 1

  • *****
  • Posts: 658
  • Joined: 22 Jan 2014

Posted 10 July 2015 - 11:28 AM

In any eyepiece barrel equipped with a safety undercut there are three zones:

 

1. The uniform section above the safety undercut.

2. The uniform (rectangular) or non-uniform (tapered) safety undercut.

3. The uniform section below the safety undercut.

 

As long the uniform sections 1. and 3. engage with the uniform bore of the focuser or adapter, how can the safety undercut influence the registration of the eyepiece?

 

There is one situation that I have encountered in which a tapered safety undercut plays havoc with the registration or collimation as per the OP.  And that is when you're using a Howie Glatter Parallizer with wider bodied 1.25" eyepieces.

 

Since the wide body engages the sides of the conical throat before the eyepiece bottoms out on the seating shelf, the upper uniform section, as per 1. above, may not engage fully over its entire depth. This is due to the unique, non-circular cross section of the bore of the Parallizer.

 

As a result the eyepiece can actually tip over when acted upon by the single set-screw on the opposite side. It does this because the narrower upper part of the tapered safety undercut can actually pass into the 'bulge' cut-out in the bore of the Parallizer.

 

I solved this by removing the flange and the conical throat of the Parallizer thus ensuring that, irrespective of how wide the eyepiece body is, the eyepiece will always bottom out completely on the seating shelf.  As such, the upper uniform section of the barrel in 1. will always engage with the bore as much as is intended.  With the upper section and the lower section fully engaged, the Parallizer is then able to do its job admirably.

 

I have no experience using a wider bodied eyepiece fitted with a barrel having a rectangular undercut in the Parallizer and so can not say if the same situation would arise.

 

Reason for Edit: Spelling, Grammar and an Omission


Edited by Richard Roseweir, 10 July 2015 - 11:32 AM.


#6 BillP

BillP

    Hubble

  • *****
  • Posts: 18594
  • Joined: 26 Nov 2006

Posted 10 July 2015 - 02:00 PM

Undercuts definitely affects the precise orthogonal setup of your scope.  And the issue compounds as it is not just the eyepiece, but when the 2" to 1.25" adapter has an undercut that can throw things off even more...and then if the diagonal has an undercut on the tube going into the focuser then yet again more impact.  When I did my comparison of diagonals I evaluated all this and was surprised how they impacted alignments.  Now when it is off, how much that impacts the sharpness of a view is another issue.  Not sure on that.  But in the end, it certainly cannot help the view that is for sure. 

 

Undercuts, in all their forms and incarnations, introduce a host of problems and issues -- keeping an orthogonal setup, sticking eyepieces, and ability to merge images when in a binoviewer.  They are a royal p.i.t.a. for binoviewing.  In the end, the undercut and its variants introduces more issues that it purportedly solves...why I call it the undercut defect.    ;)


Edited by BillP, 10 July 2015 - 02:00 PM.


#7 nevy

nevy

    Apollo

  • *****
  • Posts: 1470
  • Joined: 07 Feb 2012

Posted 10 July 2015 - 03:40 PM

I would of thought that it doesn't effect collimation that much  because it's said that having the focuser not perpendicular doesn't matter to much , I could be wrong though  & im sure someone will put me right if I am. 

Its a good job laser collimators don't have undercuts. 


Edited by nevy, 10 July 2015 - 03:40 PM.


#8 rowdy388

rowdy388

    Gemini

  • *****
  • Posts: 3400
  • Joined: 09 Apr 2013

Posted 10 July 2015 - 04:07 PM

I would of thought that it doesn't effect collimation that much  because it's said that having the focuser not perpendicular doesn't matter to much , I could be wrong though  & im sure someone will put me right if I am. 

Its a good job laser collimators don't have undercuts. 

As long as the focuser, primary and secondary are all aligned then the focuser being perpendicular is just gravy.

The eyepiece not aligned with the focuser caused by undercuts is a problem though.  The double tapered eyepiece

barrels which try to fix some of the problems are no better IMO and create their own problems because at least

the ones with undercuts have that little section at the top that is uniform with the bottom.  Best solution seems to be

a smooth barrel.

Dave Y



#9 FirstSight

FirstSight

    Duke of Deneb

  • *****
  • Moderators
  • Posts: 14509
  • Joined: 26 Dec 2005

Posted 10 July 2015 - 06:45 PM

I just finished carefully inspecting the 2" diagonal (and 1.25" adapter as well) on my NP-101 refractor (both of which use a compression ring to clamp eyepieces in place) - and two Televue EPs I frequently use with that scope, both of which have the undercut feature on their respective barrels (the 1.25" 13T6 Nagler and the 2" 26T5 Nagler).

1) The level at which the focuser/adapter compression ring operates precisely matches the level of the undercut area on both respective eyepiece barrels when seated in the focuser or adapter.  Of course, mine is an ideal case of a Televue eyepiece mating with a Televue focuser/scope.

2) However, irrespective of whether the compression ring clamps the undercut area or a non-undercut area, the respective machining on the focuser, 1.25" adapter, and both eyepiece barrels is so precise that there is no extra play in the eyepiece barrel-focuser fit for the compression ring to cause enough misalignment of eyepiece axis with focuser/optical axis to cause any sort of visible miscollimation or other induced aberration.  YMMV of course if you're using a less precisely machined adapter with a bit of potential play.

3) An issue I have seen with my other scope (William Optics Megrez 90) is with the rotatable focuser feature of that scope - I found that whenever the set screws governing the rotatable focuser feature were set loose enough to make that feature operable at all, this also caused the focuser body to be loose enough to slightly sag out of alignment with the lenses' optical axis - inducing a slight but quite noticeable amount of miscollimation and astigmatism.  I solved that by clamping the focuser orientation down tight in one position, and the problem went away (I never found the rotatable focuser feature to be at all useful to me anyway).  However, I suspect the effect you all are alleging can happen when compression rings and undercuts meet is probably similar to what I saw with the rotatable focuser flaw in my M90 - different factual cause, but similar underlying mechanism.

 

I'm not sure how undercuts + compression rings can cause any significant miscollimation problem with focusers/adapters that are machined to precise tolerances are mated with eyepiece barrels (even undercut ones) which are also well-machined to precise tolerances.  My Nagler eyepieces just barely, but cleanly slide into and out of the focuser - the undercuts can occasionally cause a slight annoyance by momentarily hanging up if you try to lift the eyepiece out in any direction but precisely straight up, but what seems a big annoyance to many of you is but a momentary blip to me. YMMV quite obviously for all the howling about undercuts.



#10 FirstSight

FirstSight

    Duke of Deneb

  • *****
  • Moderators
  • Posts: 14509
  • Joined: 26 Dec 2005

Posted 10 July 2015 - 08:23 PM

I did think of one case where adjusting the tension on the clamping screw of a scope's focuser will visibly affect the collimation, even with a very precisely machined focuser. In the Moonlite CR-2 focuser I have on my 12" f/4.9 reflector, changing the tension even a modest bit on the clamping screw is enough to cause an apparently perfect "stack" of center spot images to come quite obviously undone.  However, the autocollimator's view of alignment is a highly amplified one, and the nature and amount of the variation I described is inherent in ANY clamping system - and is below any threshold where it produces any visible degree of miscollimation or aberration in the views through any eyepieces.  Eliminating undercuts would do nothing to wipe out this irreducible residue of the compression from any sort of retaining clamp, whether it used a compression ring or just a nylon set screw.



#11 Brian Carter

Brian Carter

    Fly Me to the Moon

  • *****
  • Posts: 5044
  • Joined: 24 Nov 2004

Posted 10 July 2015 - 10:20 PM

I just want to take this opportunity to say that I hate undercuts. But I hate the tapered barrel on ES eyepieces even more. Took a bad idea and made it worse.

I know it's off topic. But to answer your question, I've never had a collimation problem with undercuts. I do a star test after I collimate and things look good. With wide field eyepieces, I can only take a rough guess at where the absolute center really is anyway. And on a dob, things drift anyway... And I'm too cheap to get a paracorr at F/5.

#12 bgi

bgi

    Apollo

  • *****
  • Posts: 1383
  • Joined: 01 Dec 2005

Posted 11 July 2015 - 07:04 AM

In any eyepiece barrel equipped with a safety undercut there are three zones:

 

1. The uniform section above the safety undercut.

2. The uniform (rectangular) or non-uniform (tapered) safety undercut.

3. The uniform section below the safety undercut.

 

As long the uniform sections 1. and 3. engage with the uniform bore of the focuser or adapter, how can the safety undercut influence the registration of the eyepiece?

 

It can't.  Perhaps there are eyepieces without section 1.  If so, that's a design blunder.  As long as sections 1 and 3 engage the fixed portion focuser or adapter (not the compression ring) then there's no potential for the eyepiece to be off-axis.  Although this is a possibility, I've not yet seen an example of this causing an issue.



#13 rowdy388

rowdy388

    Gemini

  • *****
  • Posts: 3400
  • Joined: 09 Apr 2013

Posted 11 July 2015 - 09:38 AM

 

In any eyepiece barrel equipped with a safety undercut there are three zones:

 

1. The uniform section above the safety undercut.

2. The uniform (rectangular) or non-uniform (tapered) safety undercut.

3. The uniform section below the safety undercut.

 

As long the uniform sections 1. and 3. engage with the uniform bore of the focuser or adapter, how can the safety undercut influence the registration of the eyepiece?

 

It can't.  Perhaps there are eyepieces without section 1.  If so, that's a design blunder.  As long as sections 1 and 3 engage the fixed portion focuser or adapter (not the compression ring) then there's no potential for the eyepiece to be off-axis.  Although this is a possibility, I've not yet seen an example of this causing an issue.

 

Richard gave an example using the Glatter Parallizer.  Some wide 1.25 eyepieces such as the XW's do not seat

fully into the tapered top and section 1 therefore does not engage.  I had the same issue.

Dave Y



#14 stargazer193857

stargazer193857

    Fly Me to the Moon

  • *****
  • topic starter
  • Posts: 7331
  • Joined: 01 Dec 2013

Posted 11 July 2015 - 12:28 PM

I took a closer look at the TMB. It does have a third region and is meant to be all the way in. No collimation issues for telescopes, unless they are ATM pvc manual twist focusers.



#15 BillP

BillP

    Hubble

  • *****
  • Posts: 18594
  • Joined: 26 Nov 2006

Posted 11 July 2015 - 12:51 PM

I'm not sure how undercuts + compression rings can cause any significant miscollimation problem with focusers/adapters that are machined to precise tolerances are mated with eyepiece barrels (even undercut ones) which are also well-machined to precise tolerances.  

 

Part of the beauty of staying within one brand for everything, then the complete system typically comes together better and everything designed to mate properly and effectively.  Downside is of course it limits you to those wares.  So the problem of alignment being on or off starts occurring when you mix and match components, regardless of branding.  As example in my diagonal review, the TV diagonals were having an issue staying square in my Tak focuser...or you can flip it and say the Tak focuser was having an issue with TV diagonals.  Bottom line is if I had all Tak sure it would all have zero issues, just as if I had all TV or all AP, etc.



#16 bgi

bgi

    Apollo

  • *****
  • Posts: 1383
  • Joined: 01 Dec 2005

Posted 11 July 2015 - 01:19 PM

 

 

In any eyepiece barrel equipped with a safety undercut there are three zones:

 

1. The uniform section above the safety undercut.

2. The uniform (rectangular) or non-uniform (tapered) safety undercut.

3. The uniform section below the safety undercut.

 

As long the uniform sections 1. and 3. engage with the uniform bore of the focuser or adapter, how can the safety undercut influence the registration of the eyepiece?

 

It can't.  Perhaps there are eyepieces without section 1.  If so, that's a design blunder.  As long as sections 1 and 3 engage the fixed portion focuser or adapter (not the compression ring) then there's no potential for the eyepiece to be off-axis.  Although this is a possibility, I've not yet seen an example of this causing an issue.

 

Richard gave an example using the Glatter Parallizer.  Some wide 1.25 eyepieces such as the XW's do not seat

fully into the tapered top and section 1 therefore does not engage.  I had the same issue.

Dave Y

 

 

Fair enough.  Call it a design blunder or incompatible designs if the top full-sized shoulder doesn't engage the top of the receiver.  I agree that it's an issue for a lot of people.  So far, I haven't run into it (knock on wood).  The worst is the case of eyepieces getting stuck in binoviewers.  That must be quite a mess.  I'm sure I'll complain if I ever experience the problem!

 

To be clear, the "It can't" claim had conditions which don't apply in the examples quoted.  We're in agreement!  :)

 

best regards


Edited by bgi, 11 July 2015 - 01:24 PM.


#17 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 77754
  • Joined: 16 Jun 2004

Posted 12 July 2015 - 07:09 PM

Something to think about:

 

The primary effect of the misaligned undercut is to cause a tilt to focal eyepieces focal plane. The importance of the tilt can be evaluated experimentally, in the field... 

 

In my experience, it takes a fair amount of tilt to be noticeable..

 

Jon



#18 FirstSight

FirstSight

    Duke of Deneb

  • *****
  • Moderators
  • Posts: 14509
  • Joined: 26 Dec 2005

Posted 12 July 2015 - 09:31 PM

 

I'm not sure how undercuts + compression rings can cause any significant miscollimation problem with focusers/adapters that are machined to precise tolerances are mated with eyepiece barrels (even undercut ones) which are also well-machined to precise tolerances.

 
Part of the beauty of staying within one brand for everything, then the complete system typically comes together better and everything designed to mate properly and effectively.  Downside is of course it limits you to those wares.

 


Gosh, Bill you make it sound like I'm confined to the Televue penal colony!   :grin:  Must be one of those country-club prisons...



#19 David E

David E

    Fly Me to the Moon

  • *****
  • Posts: 5377
  • Joined: 25 May 2006

Posted 13 July 2015 - 02:33 PM

With 10+ years of viewing, I've never noticed a problem with undercuts regarding this issue when mono-viewing. But with binoviewers, yes undercuts can be a problem with eye-to-eye collimation. Undercuts really are a solution without a problem, as most accessories now use the compression ring (rather than set screws) making undercuts irrelevant.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics