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Equipment Discussions >> Reflectors

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UmaDog
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Why do we collimate by tilting the primary?
      #5845144 - 05/07/13 09:09 AM

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.

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Thomas Karpf
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5845148 - 05/07/13 09:16 AM

If the primary was fixed and wasn't in the correct position, how would you collimate the scope?

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UmaDog
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Re: Why do we collimate by tilting the primary? new [Re: Thomas Karpf]
      #5845154 - 05/07/13 09:20 AM

As I say: by tilting the focuser. That wouldn't work?

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Thomas Karpf
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5845179 - 05/07/13 09:38 AM

Imagine a primary mirror that is HIDEOUSLY mis-collimated; the mirror is in the tube at a 45 degree angle.

You look through the focuser.

The focuser is square to the tube. The secondary is in the correct position; it is centered under the focuser.

The secondary is aimed correctly. The center of the primary mirror is centered as seen in the secondary.

The primary mirror is badly misaligned.

What you see looking through the focuser into the secondary mirror off the primary mirror is the side of the tube, not the sky.


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UmaDog
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Re: Why do we collimate by tilting the primary? new [Re: Thomas Karpf]
      #5845192 - 05/07/13 09:47 AM

I'm not sure, but I think we may be talking cross-purposes; forget the squaring of the focuser in the tube: it's unnecessary. The point is to adjust the focuser and secondary as a pair. With that in mind, shouldn't it be possible to get everything configured as you'd like?

Perhaps the sticking point is that this approach might be more likely to require translation of the focuser or secondary in addition to tilt. The current approach we employ usually only involves tilt adjustments.


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Dick Jacobson
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Re: Why do we collimate by tilting the primary? new [Re: Thomas Karpf]
      #5845204 - 05/07/13 09:54 AM

The primary purpose of collimation is to put the optical axis of the main mirror (at the center of the image plane) down the center of the focuser. If the main mirror optical axis was misaligned, you would have to laterally move both the secondary (so the axis hits the center of the secondary) and the focuser (so the beam from the secondary passes through the center of the drawtube). Much easier to just adjust the main mirror.

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Thomas Karpf
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5845219 - 05/07/13 10:00 AM

Because tilting the components is easier than translating the focuser, secondary, and/or primary mirror a couple of millimeters on some direction.

Most focusers are bolted to the tube; there's no way to easily nudge the focuser a couple of millimeters up the tube or across the surface of the tube.

Translating the secondary is slightly more possible as you can adjust the screws holding the spider in place.

Translating the primary is similarly possible.

Tilting is easier and faster. Tilting the primary mirror (by lifting one edge a millimeter) moves the axis of the mirror AT THE SECONDARY by the 1mm x focal ratio of the primary mirror.


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UmaDog
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Re: Why do we collimate by tilting the primary? new [Re: Dick Jacobson]
      #5845221 - 05/07/13 10:01 AM

Tom, I was thinking theoretically so obviously I was imagining a highly modified focuser. I wasn't envisaging something you could easily do in a stock scope. I see what you mean about the translations, though. It likely would be more confusing to have to contend with tilting the secondary and the focuser and also translating one or both of those

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Thomas Karpf
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5845240 - 05/07/13 10:13 AM

Quote:

Tom, I was thinking theoretically so obviously I was imagining a highly modified focuser. I wasn't envisaging something you could easily do in a stock scope. I see what you mean about the translations, though. It likely would be more confusing to have to contend with tilting the secondary and the focuser and also translating one or both of those




You would still need the center of the cone of light from the primary coming cleanly and squarely up the focuser to tye eyepiece. That's going to be MUCH easier to do by eye by tilting the primary than by trying to figure out where exactly the focuser needs to be and how it needs to be tilted to 'catch' the light correctly.


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careysub
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5845292 - 05/07/13 10:47 AM

Quote:

I'm not sure, but I think we may be talking cross-purposes; forget the squaring of the focuser in the tube: it's unnecessary. The point is to adjust the focuser and secondary as a pair. With that in mind, shouldn't it be possible to get everything configured as you'd like?

Perhaps the sticking point is that this approach might be more likely to require translation of the focuser or secondary in addition to tilt. The current approach we employ usually only involves tilt adjustments.




Recall that a focuser is required to support up to several pounds of optical equipment (an ES100 25mm plus a Paracorr weighs about 3.5 pounds) in a cantilever arrangement (they stick out and are supported only at one end) which is mechanically disadvantageous. This requires the focuser to be very rigidly mounted. Also, keeping the focuser plus focuser mounting board light in the UTA is an important design goal. A system that is strong and rigid, yet allows tilting and translating the focuser sounds complex and heavy.


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Jon Isaacs
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5845306 - 05/07/13 10:56 AM

Quote:

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.




The goal of collimation is to have the optical axes in alignment and it is desirable to have the secondary mirror properly centered/offset. I believe that if the primary were fixed, this would require adjusting not only the tilt of the secondary and focuser but also could require some lateral translation.

The focuser must not only be parallel to the optical axis but it also must be concentric, meeting both these critera means that were the position of the primary fixed, the position of the focuser or the secondary might need be moved.

Jon


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Jarad
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Re: Why do we collimate by tilting the primary? new [Re: Jon Isaacs]
      #5845313 - 05/07/13 11:06 AM

The only way I could see this working is in a truss design, where you could adjust the truss lengths in small increments to move the whole UTA to center it on the axis of the primary. In a tube it won't work because if the primary is off-center the tub itself will cut off light.

But even in the truss example, that would be a lot more complex than simply tilting the primary.

Jarad


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GaryS
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5845372 - 05/07/13 11:38 AM

Quote:

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 have to think about this a bit more, but it's not obvious to me why it wouldn't work. The advantage would be that you could make your adjustments while looking in the collimating eyepiece -- and that's a significant plus.

Regards,
Gary


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UmaDog
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Re: Why do we collimate by tilting the primary? new [Re: GaryS]
      #5845380 - 05/07/13 11:45 AM

Yes, that's how I saw it. Perhaps for tweaking the collimation, replacing primary tilt with focuser tilt might work. But for larger errors one might well have to translate the focuser and that would be awkward for the reasons mentioned above. I reckon it should be fairly easy to make a light, low-profile, and rigid tilt adjustment system at the focuser. Perhaps it would work as a viable option in combination with primary tilt adjustment. Then again, may be more trouble than it's worth. It was a just a passing thought I had...

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Thomas Karpf
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5845431 - 05/07/13 12:24 PM

Frankly, if you want to tweak primary collimation from your eyepiece, consider how you would directly tweak the primary.

Possibilities:

1) threaded rods that run down the length of the scope from the primary cell to the vicinity of the focuser so you can adjust the primary while looking through the focuser,
2) electrical controls at the focuser end that allow you to tweak remotely operated actuators at the primary end.


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Jason D
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5845631 - 05/07/13 01:35 PM


Rob,
Assuming the primary mirror optical axes is not too far off, you will need to adjust both the secondary mirror and the focuser to achieve the following two alignments:
1- Focuser axis has to intercept the optical axis
2- Above interception point has to be located on the secondary mirror surface
3- Secondary surface plane angle has to be positioned to reflect the focuser axis to the primary center and vice versa
4- Secondary mirror has to appear centered and rounded from the focuser end
Too many alignment to get things right
Below would be the proposed steps:
1- Insert a cheshire then adjust the focuser until the primary mirror looks aligned (cheshire reflection vs. center spot reflection)
2- Assess secondary mirror appearance. If not centered, then adjust the secondary mirror appropriately but that will disrupt step#1. Bear in mind these secondary adjustments will involve the center bolt – not fun.
3- Repeat steps 1 & 2 until:
a. Chesire alignment is met
b. Secondary mirror appearance looks centered and rounded
Actually, there is a unique benefit to the above system. You can shift the focal plane position with respect to the focuser by ~-10mm by pointing the focuser higher or ~+10mm by pointing it lower.
Ummm, if you use a laser collimator with a holographic attachment then the process might be easier than above but it will still be more involved than the well-established collimation steps.

Jason


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Jason D
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Re: Why do we collimate by tilting the primary? new [Re: Jason D]
      #5845648 - 05/07/13 01:44 PM

Quote:

Actually, there is a unique benefit to the above system. You can shift the focal plane position with respect to the focuser by ~-10mm by pointing the focuser higher or ~+10mm by pointing it lower.



Well, that could also cause nightmares. Imagine after you are done you find out some of your EPs can no longer come to focus.
Jason


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UmaDog
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Re: Why do we collimate by tilting the primary? new [Re: Jason D]
      #5845989 - 05/07/13 03:54 PM

Thanks for the detailed description, Jason.

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Vic Menard
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5846008 - 05/07/13 04:10 PM

Quote:

...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.



Of the three tilt elements (primary mirror, secondary mirror, and focuser) the primary mirror is unique in that even fairly significant tilt adjustments have little or no impact on the alignments of the other two elements (secondary mirror and focuser). This means one can sort out the focuser/secondary mirror geometry/alignments without consideration of the critical primary mirror tilt alignment, which can be corrected as the final step. Fixing the primary mirror alignment essentially reverses the procedure, which requires more mechanical accommodations with respect to the focuser/secondary mirror geometry/alignments and significantly complicates the alignment procedure.

FWIW, a similar problem would entail fixing the optical axial alignments to the mechanical OTA axis for precise DSC performance, assuming of course that the OTA axis is indeed orthogonal to the mount axes. In such a situation, if the OTA axis is the reference axis, (and the primary mirror is precisely centered in the OTA) then the primary mirror axis by definition is fixed. On the surface, this sounds to be a reasonable goal. But getting there is far more complicated than basic optical alignment...


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Nils Olof Carlin
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5847142 - 05/08/13 04:45 AM

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


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Thomas Karpf
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Re: Why do we collimate by tilting the primary? new [Re: Nils Olof Carlin]
      #5847182 - 05/08/13 06:05 AM

Quote:


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.




Okay, so how EXACTLY does one do 'FIX' the primary so it does not move? Isn't that the main job of current mirror cells (all of which require occasional collimation)?

Primary cells do not hold the mirror firmly. ABSOLUTELY preventing the mirror from moving at all under any circumstances ABSOLUTELY requires that something be touching the surface of the mirror, thus holding the mirror firmly against the mirror cell.

Until/unless this is done, there is no way to guarantee that the mirror will not move. Consequently, you will need to collimate the primary.

As the primary mirror is a huge honkin' chunk of glass which WILL move against its constraints as you're driving and hitting potholes, speed bumps and vague dips and bumps in the road, you're going to need to collimate the primary every so often.


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UmaDog
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Re: Why do we collimate by tilting the primary? new [Re: Thomas Karpf]
      #5847310 - 05/08/13 08:37 AM

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.

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Jon Isaacs
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5847497 - 05/08/13 10:34 AM

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.




I thought Tom's point was a very good one. It is all well and good to consider issues like this in their abstract forms but Tom added a dose of reality...

The primary cannot be tightly held, it can shift. It is unlikely that the focuser angle shifts, it is also difficult to adjust..

As an engineer/mechanic, ideally you adjust things that do shift, that makes for a stable mechanical system. If you are continually adjusting Y to make up for shifts in X, that can be an unstable system...

Myself, I have a hard time thinking about collimating a Truss scope after assembly without adjusting the tilt of the primary mirror.

Jon


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FirstSight
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5847520 - 05/08/13 10:44 AM

Quote:

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.




Because the primary mirror cell has collimation knobs, the focuser doesn't!


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Jason D
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Re: Why do we collimate by tilting the primary? new [Re: Jon Isaacs]
      #5847741 - 05/08/13 01:08 PM

Quote:

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.




I thought Tom's point was a very good one. It is all well and good to consider issues like this in their abstract forms but Tom added a dose of reality...

The primary cannot be tightly held, it can shift. It is unlikely that the focuser angle shifts, it is also difficult to adjust..

As an engineer/mechanic, ideally you adjust things that do shift, that makes for a stable mechanical system. If you are continually adjusting Y to make up for shifts in X, that can be an unstable system...

Myself, I have a hard time thinking about collimating a Truss scope after assembly without adjusting the tilt of the primary mirror.

Jon




But what does the primary mirror shift have to do with Rob's question? If the primary mirror shifts then it will have the same negative impact on both a typical scope and the one Rob described.

Jason


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Thomas Karpf
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Re: Why do we collimate by tilting the primary? new [Re: Jason D]
      #5847775 - 05/08/13 01:29 PM

Quote:


But what does the primary mirror shift have to do with Rob's question? If the primary mirror shifts then it will have the same negative impact on both a typical scope and the one Rob described.

Jason




To which question are you refering?

By 'shifting', we are, of course, saying that both the X-Y position of the primary mirror within the tube can change AND that the axis of rotation of the primary mirror can change (tilt), both of which would result in the primary not being aimed squarely up the tube to the secondary mirror.

Nobody makes focusers that are easy to adjust. Certainly there are no inexpensive focusers that are easy to move around once they're attached to the tube.

Assuming that the center of the secondary is on the same line as the center of the focuser, then a telescope can be collimated by adjusting the tilt of the primary and secondary mirrors.

Collimating without moving the primary could easily require you to move (not just tilt) the secondary and move and tilt the focuser.

Why on Earth would anybody WANT to build a scope where you might need to move the focuser up many millimeters (which would, of course, mean that hole in the tube for the focuser is HUGE) just so you didn't have to deal with the 'onerous' job of adjusting the angle of the primary mirror?


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Nils Olof Carlin
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Re: Why do we collimate by tilting the primary? new [Re: Thomas Karpf]
      #5847795 - 05/08/13 01:43 PM

Quote:

there is no way to guarantee that the mirror will not move. Consequently, you will need to collimate the primary.




If the cell is sturdy, the motion is small. A well built edge support will no doubt help a lot. And what you need is redirect the primary's optical axis to the center of the focal plane, which *can* be done by adjusting the position and tilt of the secondary - you need not adjust the primary.
It is a theoretical possibility, but my point is it has been recommended on discussion lists in earnest I am sure. But not having actually tried (I can't remove the secondary from the spider in any of my telescopes, and anyway the spiders do not mark the center of the tubes), I can't imagine any worthwhile advantages (I can some disadvantages). I think this may have some bearing on the OP's question - but I have recommended starting from the focuser for the 16 years I've been on the net - and almost as long since anyone actually recommended starting from the primary as I have described here. A lot about collimation has been learned since then...

Nils Olof


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Jason D
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Re: Why do we collimate by tilting the primary? new [Re: Thomas Karpf]
      #5847826 - 05/08/13 01:55 PM

Tom, you are missing the point of this thread. No one is suggesting building a new line of scopes with adjustable focusers and fixed primary mirrors. Rob is asking an interesting hypothetical question.
Quote:

By 'shifting', we are, of course, saying that both the X-Y position of the primary mirror within the tube can change AND that the axis of rotation of the primary mirror can change (tilt), both of which would result in the primary not being aimed squarely up the tube to the secondary mirror.



And how will that have more negative impact on the hypothetical scope compared to a typical scope? A primary shift is a primary shift and will have the same negative impact on both.

Quote:

Nobody makes focusers that are easy to adjust. Certainly there are no inexpensive focusers that are easy to move around once they're attached to the tube.



This thread is not advocating building such a focuser. It is a hypothetical question about the pluses and minuses for such a system.

Quote:

Assuming that the center of the secondary is on the same line as the center of the focuser, then a telescope can be collimated by adjusting the tilt of the primary and secondary mirrors.

Collimating without moving the primary could easily require you to move (not just tilt) the secondary and move and tilt the focuser.



No need to move the focuser – just tilt it. On the other hand, the secondary mirror will have to move up and down the OTA which will add complexity -- I have already mentioned that.

Quote:

Why on Earth would anybody WANT to build a scope where you might need to move the focuser up many millimeters (which would, of course, mean that hole in the tube for the focuser is HUGE) just so you didn't have to deal with the 'onerous' job of adjusting the angle of the primary mirror?



No need to move the focuser. All that is needed is a tilt.

Jason


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Thomas Karpf
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Re: Why do we collimate by tilting the primary? new [Re: Jason D]
      #5847840 - 05/08/13 02:01 PM

Jason,

I have an f/6 reflector. If the primary is tilted 1mm (I.e. one edge is higher by 1mm), then the focal point of the scope isobars 6mm at the focuser. Not 6mm up/down the center of the focuser, but 6mm in some direction along the surface of the tube at the focuser.

How do you intend to tilt the focuser to desk with that?


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Jason D
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Re: Why do we collimate by tilting the primary? new [Re: Thomas Karpf]
      #5847867 - 05/08/13 02:14 PM

Quote:

Jason,

I have an f/6 reflector. If the primary is tilted 1mm (I.e. one edge is higher by 1mm), then the focal point of the scope isobars 6mm at the focuser. Not 6mm up/down the center of the focuser, but 6mm in some direction along the surface of the tube at the focuser.

How do you intend to tilt the focuser to desk with that?




It would be a combination of tilting the secondary mirror to reposition the focal point at the center of the focuser followed by tilting the focuser to coincide the focuser and optical axes.

There is no need to move the focuser


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Re: Why do we collimate by tilting the primary? new [Re: Jason D]
      #5847891 - 05/08/13 02:23 PM

You don't care that the secondary will no longer be intersecting the entire light cone from the primary?

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Jason D
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Re: Why do we collimate by tilting the primary? new [Re: Jason D]
      #5847900 - 05/08/13 02:28 PM Attachment (8 downloads)

See attachment
From left to right
1- A well collimated scope
2- Primay tilts and the focal point move significantly away from the focuser axis
3- I would move and tilt the secondary mirror to reposition the focal point inside the focuser
4- Then I would tilt the focuser to coincide both the focuser and optical axes. I would evaluate the centerness and roundness of the secondary mirror then make the necssary adjustments which would be iterative between secondary/focuser adjustmnets.

Left most and right most figures are optical collimated the same way; however, the focal plane moved inwards in the right most figure. I am ignoring front aperture vignetting and DSC accuracy issues.

Jason

Edited by Jason D (05/08/13 03:20 PM)


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Jason D
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Re: Why do we collimate by tilting the primary? new [Re: Thomas Karpf]
      #5847904 - 05/08/13 02:30 PM Attachment (5 downloads)

Quote:

You don't care that the secondary will no longer be intersecting the entire light cone from the primary?



It will because you would move the secondary mirror. You do not need to have the optical axis coincident with the OTA axis to intercept the whole light cone. This is a common misconception.
Jason


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Jarad
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Re: Why do we collimate by tilting the primary? new [Re: Jason D]
      #5847965 - 05/08/13 03:03 PM

Very nice illustration.

That's why I said the only way I could see it being practical is for a truss dob, where you achieve the movement and tilt of the secondary and focuser by adjusting the trusses (essentially moving the whole UTA to be where the primary is pointing to).

But it will generally be far easier to just tilt the primary. You can do that with only 3 adjustment points vs. 6 to 8 if you are adjusting the trusses.

Of course, you can pull the old relativity frame of reference trick and say that from the point of view of the mirror, the mirror is already sitting still and the whole tube moves relative to it when you adjust the collimation bolts...

Jarad


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csrlice12
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Re: Why do we collimate by tilting the primary? new [Re: Jarad]
      #5848071 - 05/08/13 03:56 PM

John Dobson just used to kick the end of his dob to collimate it...how far we've come....

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Thomas Karpf
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Re: Why do we collimate by tilting the primary? new [Re: Jason D]
      #5848086 - 05/08/13 04:03 PM

Quote:

I am ignoring front aperture vignetting and DSC accuracy issues.





Interesting things to ignore.


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Jason D
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Re: Why do we collimate by tilting the primary? new [Re: Thomas Karpf]
      #5848116 - 05/08/13 04:17 PM

Your point was if the primary tilts then the focuser has to move. I replied with illustrations clarifying that focuser movement is not needed. Only a secondary movement/tilt and a focuser tilt needed.

Quote:

Quote:

I am ignoring front aperture vignetting and DSC accuracy issues.



Interesting things to ignore.




I am surprised that was the only statement from my reply that got your attention.

If the primary mirror tilts that much due to cell play then collimation is doomed regardless of whether hypothetical or the typical scopes are used.

A primary mirror with that much tilt will introduce front-end aperture vignetting regardless of the scope type.

Jason


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UmaDog
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Re: Why do we collimate by tilting the primary? new [Re: Thomas Karpf]
      #5848130 - 05/08/13 04:23 PM

I still kick mine! But only as a prelude to the main event. A little jolt settles the primary in the sling, you see.

Jason, thanks for taking the time to make the diagrams. Until I saw them, I thought the focuser would need translating. Now I see that you would need to translate only the secondary. At least I think that's what I'm getting from your drawings This is all rather interesting: helps clear out my misunderstandings.


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Jason D
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Re: Why do we collimate by tilting the primary? new [Re: UmaDog]
      #5848161 - 05/08/13 04:38 PM

Quote:

Until I saw them, I thought the focuser would need translating. Now I see that you would need to translate only the secondary. At least I think that's what I'm getting from your drawings



Correct. The secondary mirror will need to move up/down via the central bolt, to tilt via the 3 set screws, and to move sideways via the spider vanes thumb knobs. The latter will worsen the diffraction spikes.
But if I were to design such a scope, I would redesign the secondary mirror hub. I would leave the spider vanes alone and come up with a mechanism to slide the secondary mirror sideways with reasonable ease.
Jason


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Thomas Karpf
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Re: Why do we collimate by tilting the primary? new [Re: Jason D]
      #5848175 - 05/08/13 04:44 PM

Quote:

Your point was if the primary tilts then the focuser has to move. I replied with illustrations clarifying that focuser movement is not needed. Only a secondary movement/tilt and a focuser tilt needed.





If the primary tilts then you do need to move the secondary and possibly the focuser if you insist on not adjusting the primary ...

1) Unless you don't care about vignetting by the front of the tube ...
2) Unless your secondary is so large that you can intercept the light cone regardless of where the mirror is pointed ...
3) Unless you are not using setting circles or goto ...
4) Unless coma and astigmatism are not a concern ...

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.

As long as you can casually translate your secondary across the tube, then adjusting the secondary instead of the primary is possible, if silly. By doing so, you make it more difficult to collimate by eye because the secondary is no longer in the middle of the tube. Of course, if the adjustment requires you to move the secondary at right angles to the focuser, then you can no longer aim the light cone squarely down the focuser, but perhaps that's not a concern for you as well.

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


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Vic Menard
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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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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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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
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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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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
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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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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
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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
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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
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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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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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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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