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And would the Krupa collimator be the same as case C?
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Krupa

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If the focuser axial error is large then I expect racking the focuser in and out will shift the donut reflection similarly to the other mentioned tools

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If the focuser axial error is large then I expect racking the focuser in and out will shift the donut reflection similarly to the other mentioned tools

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I can't see this as much of a practical problem. You don't use any of these tools until you have aligned the secondary to center the focuser axis, anyway, and any small residual error (one, or at most a few tenths of a percent, tilt) will be far too small to allow a significant shift even if you would rack the focuser for some reason.

Lust as you may use an AC to check collimation with laser/barlow (or focused Krupa), you might use one of the latter to check the critical collimation of the primary.

Nils Olof

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Nils Olof, we are in agreement. As Alexis clarified and as I stated, only if the focuser axial error is large, a reflection shift will be discerned. I also stated that collimation caps and cheshires are not good tools to flag and correct focuser axial errors. In addition, I stated if I had an AC to precisely align the secondary then I would continue using the AC to align the primary.
The purpose of this thread is to try to clarify how different collimation tools/methods interact with the ROC -- an important source of collimation errors yet not well-understood by many reflector owners.
Some collimation tools/methods align the focuser axis with the primary mirror ROC point and some collimation tools/methods align the focuser axis with the primary mirror focal point.
Jason

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I personally prefer it for my small telescopes where the Barlowed laser is difficult to reach.

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What Jason means is that, this way, you can avoid gross focuser axis pointing errors if you have no other tool designed for doing it properly.

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What Jason means is that, this way, you can avoid gross focuser axis pointing errors if you have no other tool designed for doing it properly.

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Sixela,
if you have gross focuser error, an autocollimator won't help you the slightest - all of the reflections (BTW not including the spot on the primary itself seen via the secondary) will be far outside the view. But if everything else fails, you might step back from the focuser and try center your eye relative to the drawtube.

Nils Olof

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What about B=-2A scenario which can introduce a relatively gross focuser axial error yet provide reflections similar to a perfect collimation -- with the center spot reflection slightly off-center along with a slightly off-center foreground pupil reflection.
Jason


EDIT: Figures C and F will flag large focuser errors when racking the draw-tube in and out. However, figures B and D will not. In other words, any collimation tools/methods that align the focuser axis with the primary mirror focal point will have the ability to flag large focuser axial errors when racking the draw-tube in and out. However, any collimation tools/methods that align the focuser axis with the primary mirror ROC point will be insensitive to racking the draw-tube in and out. Cross checking between the two classes of tools/methods might lead to discrepancy even for quality scopes and tools.

Edited by Jason D (01/08/09 03:18 PM)

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Sixela,
if you have gross focuser error, an autocollimator won't help you the slightest

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Personally, I use my Orion Deluxe laser collimator along with a cheap 2X barlow to align the primary. The Orion Deluxe comes with 45 degree bullseye window which makes it a nice setup for barlowed laser.
Jason
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Jason, did you have to collimate the Orion laser, or can you use it non collimated?

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...The objective of this thread is to advise others to pause for a moment and ask the important question:
When I use collimation tool XYZ and I meet the intended alignment, what am I aligning?

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“ROC alignment” collimation tools:
Autocollimator
Laser collimator

“Focal alignment” collimation tools
Barlowed laser
Collimation cap
Cheshire
Krupa collimator

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For example, assume someone re-iterates between a quality unbarlowed laser collimator and a quality autocollimator until both tools are 100% in agreement. Specifically, reflections are stacked up via the autocollimator and the laser beam retraces itself perfectly to the laser source. That someone might be under the impression that his/her scope is perfectly collimated.

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Then he/she plugs in the cheap collimation cap and to his/her dismay, the collimation cap view suggested miscollimation. He/she dismisses the discrepancy as an incorrect reading by a cheap tool – the collimation cap. After all, both of the much more expensive tools are in agreement.

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Both the unbarlowed laser and autocollimator tell us how far off the focuser axis is from the ROC point.

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But the collimation cap tell us how far off the focuser axis is from the Focal point.

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In retrospect, the owner was better off re-iterating between either the quality laser or autocollimator versus the collimation cap. The first two are ROC alignment tools and the third is a Focal alignment tool

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So, the next time your collimation tools agree, make sure they agree against two different alignment points. The next time your collimation tools disagree, then find out what each tool is aligning so you can resolve the discrepancy.

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...re-iterating between an autocollimator (ROC alignment tool) and a cheshire (a focal alignment tool) is a well-documented procedure. Here is a different perspective to describe the same procedure:

We can start off with an ROC alignment tool – say an autocollimator – to achieve the best collimation (let us exclude CDP from the picture for now). When we are done, we only know that he focuser/AC axes intercept the ROC but we do not know if the same axes intercept the focal point.

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Next step is to use a focal alignment tool – say a Cheshire. It might look off which implies the focuser axis is not intercepting the focal point. So we adjust the primary. When we are done, we only know that the focuser/Cheshire axes intercept the focal point but we do not know if the same axes still intercept the ROC point. IMPORTANT: Only adjust the primary with a focal alignment tool. The focuser axis will rotate about the primary – think of the primary as the pivot point for the focuser axis.

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Next step is go back to the ROC alignment tool – the AC in our case. It might look off. So we adjust the secondary until it looks right. IMPORTANT: Only adjust the secondary with an ROC alignment tool. The focuser axis will rotate about the secondary – think of the secondary as the pivot point for the focuser axis.

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When both the focal alignment tool and the ROC alignment tool show good alignment then it means the focuser axis runs through the focal point (as reported by the focal alignment tool) and the focuser axis also runs through the ROC point (as reported by the ROC alignment tool). There is only one condition when this is true and that is when the focuser axis is aligned with the primary optical axis.

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