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Equipment Discussions >> Reflectors

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Vic Menard
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Reged: 07/21/04

Loc: Bradenton, FL
Re: Why do we collimate by tilting the primary? new [Re: Thomas Karpf]
      #5848182 - 05/08/13 04:46 PM

Quote:

Quote:

I am ignoring front aperture vignetting and DSC accuracy issues.




Interesting things to ignore.



FWIW, they are both commonly ignored when using routine "offset" optical collimation procedures with a centered secondary mirror. They can usually be ignored because their respective impacts are either not visually detectable or not significant when compared to typical DSC resolution.

And while most economy focusers do not make a provision for mechanical leveling, this does not mean that they will never need to be adjusted. Before leveling (agonist/antagonist) screws became an option, I routinely shimmed focusers to optimize the focuser/secondary mirror geometry/alignment.

Again, the only reason I can think to fix the primary mirror alignment is to constrain the primary mirror axis to a position orthogonal to the mount axes for precise DSC performance. As I noted in my earlier post, this position may or may not be coincident with the OTA axis. And determining whether it is or isn't is a complicated procedure with the possibility of numerous "hit-or-miss" reiterations (both to correct the optical alignment and then test for orthogonality).

Finishing routine alignment by carefully adjusting the primary mirror tilt is simple and quick. Finishing routine alignment by adjusting the secondary mirror position and focuser axial tilt is complicated (determining the next "best" corrective step) and tedious (numerous back-and-forth, hit-or-miss repetitive adjustments).


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5848221 - 05/08/13 05:04 PM

Quote:

Tom, I think you're taking all this a bit too literally. We're just tossing ideas about. Clearly the consensus method for collimating a reflector has been arrived at because it is the most effective. Thinking about the alternatives, as Nils just did, is an interesting way of showing why. The tone of Nils' writing (e.g. the statement "and possibly even used") suggests that he doesn't consider what he describes to be worth the effort. The concern of stabilising the primary is the obvious issue, particularly at larger mirror sizes.




Rob:

I reread your first post:

"
I have a question about collimation. The way that we all do it involves tilting the primary and secondary mirrors with respect to a fixed focuser. The goal being to get the objective's image plane perpendicular to the focuser axis. However, wouldn't it be easier for the user to perform the alignment with respect to a fixed primary mirror? So adjusting only the tilt of the secondary and the tilt of the focuser? I'm sure there's some good reason for not doing this, but I don't know what it is. "

I guess some of us are still thinking about the various reasons why the conventional collimation techniques are preferred over the one you propose. When I read you original question, it seemed to be a real, practical question and not just a "tossing ideas about."

I do think that it has been amply demonstrated why a fixed primary would make for more difficult collimation...

Jon


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Jason D
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Reged: 10/21/06

Loc: California
Re: Why do we collimate by tilting the primary? new [Re: Thomas Karpf]
      #5848247 - 05/08/13 05:18 PM

Quote:

.. if you insist on not adjusting the primary ...



How can I adjust the primary if it is not an option in the hypothetical scope described by Rob (the OP)?
You seem to still be confused about this thread. No one is suggesting that new dobs be built as described by Rob. Rob is merely presenting a hypothetical scope with good questions. That is all. You keep breaching to the choir emphasizing that such a scope is not optimal. We all agree. Again this is not the point of this thread.

Quote:

1) Unless you don't care about vignetting by the front of the tube ...



I do but this is independent of the scopes we are discussing.

Quote:

2) Unless your secondary is so large that you can intercept the light cone regardless of where the mirror is pointed ...



As I stated, this is a common misconception. If the primary mirror tilts then the secondary mirror will follow the optical axis. Again, this is independent of the scope we are discussing.

Quote:

3) Unless you are not using setting circles or goto ...



Independent of the scopes we are discussing

Quote:

4) Unless coma and astigmatism are not a concern ...



I do not even know how astigmatism is related to this discussion

Quote:

Personally, I find fine movement of the primary much easier than fine motion of the secondary. Tilt-wise, small adjustments to the secondary involve much larger angles than small adjustments of the primary.



We all agree

Quote:

Moving the secondary and/or focuser to avoid adjusting the primary is an interesting choice, but not one that I would ever consider.



Nor would I.

Jason


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Why do we collimate by tilting the primary? new [Re: Nils Olof Carlin]
      #5848333 - 05/08/13 05:52 PM

Quote:

Quote:

However, wouldn't it be easier for the user to perform the alignment with respect to a fixed primary mirror? So adjusting only the tilt of the secondary and the tilt of the focuser?




One method that has been advocated (and possibly even used) is to start by removing the secondary and aligning the primary to the mounting hole in the spider - this done, the primary is left fixed. The secondary is replaced, and moved/tilted as needed to finish collimation - even with a non-adjustable focuser. This has the presumptive advantages of getting the offset right, provided it is included in the secondary mount, and having the optical axis centered at the level of the spider (not highly critical anyway).
I've never tried it, but the adjustment of the secondary sounds tedious and iterative to me, and I can't see that the result would be better.
Anyway, the critical adjustment in optical collimation is that of the primary's axis, not the focuser axis - some have even advocated doing the final centering of the primary's axis by fine-adjusting the secondary tilt (this will center the sweet spot, at the cost of de-centering the focuser axis by a much larger amount, but you may still find it acceptable in terms of image quality).

To me, starting with the focuser axis seems the easiest way, not needing iterations (at least if the miscollimation is moderate to start with).

Nils Olof



Indeed. When I read your post, I laughed.
This was how we collimated our newtonians in the '50s and '60s. You started by aligning the primary with the centerline of the tube, then moved the secondary down and tilted it until the center of the focuser was reflected in the center of the primary.
As I recall, however, and probably because of the long f/ratios common at the time (f/8 was "fast"), we paid little or no attention to offsetting the secondary AWAY from the focuser, so the optical center of the secondary pretty much was its geometric center, and the reflected image of the primary was NOT centered in the secondary (it was offset down, toward the primary). Even illumination of the edge of the field wasn't considered important because:
--all eyepieces had narrow fields of view and were adequately illuminated.
--no one built an offset into the secondary holder and spiders were always centered on the tube so the primary alignment could be done.

The adjustment of the secondary wasn't tedious--you just lowered it until it was approximately under the focuser, centered it in the tube from side-to-side in all four directions, and rotated it until the entire primary could be seen, and adjusted its tilt until the pupil of your eye was centered on the primary center.

We've come a long way since then. I wouldn't even do it that way now on an f/12 newtonian.


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howard929
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Reged: 01/02/11

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Re: Why do we collimate by tilting the primary? new [Re: Starman1]
      #5849728 - 05/09/13 10:37 AM

Why can't the primary mirror be locked firmly in place with edge attachments resulting in collimate once and done?

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Thomas Karpf
Carpal Tunnel


Reged: 02/09/09

Loc: Newington, CT
Re: Why do we collimate by tilting the primary? new [Re: howard929]
      #5849767 - 05/09/13 10:53 AM

Quote:

Why can't the primary mirror be locked firmly in place with edge attachments resulting in collimate once and done?




Sufficient pressure on the sides to prevent the mirror from moving no matter what size pothole you drive through would most likely deform the mirror. Mirrors need to be held loosely to prevent deformation.

Consider how regularly we hear that 'the mirror in asian scope such-and-such had a great figure once I loosened the mirror clamps'.


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howard929
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Reged: 01/02/11

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Re: Why do we collimate by tilting the primary? new [Re: Thomas Karpf]
      #5849788 - 05/09/13 11:01 AM

Quote:

Quote:

Why can't the primary mirror be locked firmly in place with edge attachments resulting in collimate once and done?




Sufficient pressure on the sides to prevent the mirror from moving no matter what size pothole you drive through would most likely deform the mirror. Mirrors need to be held loosely to prevent deformation.

Consider how regularly we hear that 'the mirror in asian scope such-and-such had a great figure once I loosened the mirror clamps'.




We know that, that's pinching the mirror. Would firmly holding the mirror around its entire edge work without harm?


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Why do we collimate by tilting the primary? new [Re: howard929]
      #5849821 - 05/09/13 11:15 AM

Quote:

Quote:

Quote:

Why can't the primary mirror be locked firmly in place with edge attachments resulting in collimate once and done?




Sufficient pressure on the sides to prevent the mirror from moving no matter what size pothole you drive through would most likely deform the mirror. Mirrors need to be held loosely to prevent deformation.

Consider how regularly we hear that 'the mirror in asian scope such-and-such had a great figure once I loosened the mirror clamps'.




We know that, that's pinching the mirror. Would firmly holding the mirror around its entire edge work without harm?



Nope. Same issue--astigmatism.
In fact, there is almost an art to holding the mirror in a dob in such a way it doesn't develop astigmatism from the way it's held.
See:
http://www.cruxis.com/scope/mirroredgecalculator.htm


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tezster
scholastic sledgehammer
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Reged: 07/14/09

Loc: Missisauga, Canada
Re: Why do we collimate by tilting the primary? new [Re: Starman1]
      #5849841 - 05/09/13 11:27 AM

Related question: can significant changes in temperature cause astigmatism in a mirror glued to its cell? I once noticed this when observing below freezing during winter, but haven't noticed it when observing in milder conditions.

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Thomas Karpf
Carpal Tunnel


Reged: 02/09/09

Loc: Newington, CT
Re: Why do we collimate by tilting the primary? new [Re: tezster]
      #5849859 - 05/09/13 11:39 AM

Quote:

Related question: can significant changes in temperature cause astigmatism in a mirror glued to its cell? I once noticed this when observing below freezing during winter, but haven't noticed it when observing in milder conditions.




I wouldn't be surprised. Anything that prevents some portion of the mirror from moving while allowing another portion to move (edge of the mirror contracts/moves while glued portions can't move) can cause astigmatism.


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Nils Olof Carlin
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Reged: 07/26/04

Re: Why do we collimate by tilting the primary? new [Re: howard929]
      #5849890 - 05/09/13 11:56 AM

Quote:

We know that, that's pinching the mirror. Would firmly holding the mirror around its entire edge work without harm?




Here is one way to do just that. And if you need any sling, a steel wire can be placed accurately along the plane of the COG (most common seatbelt slings can't, but hopefully they are rarer now).
For moderately large mirrors, though, two supports at +-45 deg to the vertical(ball bearings, perhaps skateboard wheels) would be plenty (see Robert Houdart's calculator as linked by Don) and would keep the mirror in a well defined position despite not clamping it (non-touching safety clamps makes sense)

Nils Olof


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Nils Olof Carlin
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Reged: 07/26/04

Re: Why do we collimate by tilting the primary? new [Re: Starman1]
      #5849905 - 05/09/13 12:10 PM

Quote:

Indeed. When I read your post, I laughed.
This was how we collimated our newtonians in the '50s and '60s.




Don, thanks for a good history lesson. I was a late starter in this hobby, getting my first telescope (6" f/5) in 1991. Not being able to find anything better than the "classic" metod of eyeballing things concentric (described in the Norton star atlas), I had to work out the how and not least the why on my own.
But I guess the ATM list comments (in 1997 or so) on primary-first collimation were from the last of the die-hards

Nils Olof


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