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About secondary mirror rotation and tilt

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#1 Jason D

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Posted 30 November 2013 - 01:44 PM

First, the following two posts are not intended for everyone. Feel free to ignore this thread if you wish.

Secondary mirror rotation and tilt is somewhat a confusing subject. Many have noticed after collimating their scopes with laser collimators by adjusting the secondary mirror to redirect the laser beam to the primary center then adjust the primary mirror to have the laser retrace its path back to the source, they can rotate the secondary mirror slightly and repeat the collimation steps successfully. Does this make sense? How can we introduce a rotation “error” then still get the laser beam to hit the primary center and retrace its path? Are we eliminating the introduced rotation “error” by “tilt” adjustments or are we hiding the rotation error by the “tilt” adjustments.

Well, let us talk about the term “tilt”. When we tilt the secondary mirror using one of the 3 (or 4) adjustment set screws, we are actually performing a rotation and displacement – mathematically speaking. That is, there is a “rotation” element when we tilt the secondary mirror. The axis of this rotation is perpendicular to the plane that includes both the central secondary bolt and the set screw being adjusted. That means, each set screw adjustment will rotate the secondary mirror slightly about a unique axis. Really, there are 4 rotation axes when we adjust the secondary mirror. The first is the rotation around the central bolt – the obvious rotation axis. The remaining three (assuming your secondary mirror has 3 set screws) are at 120 degrees apart and “perpendicular” to tube axis.

Back to the first paragraph, when we introduce a slight rotation error around the central bolt after having completed a successful collimation, we can negate and eliminate that error by only adjusting the set screws. The inherent rotation elements introduced by the secondary mirror set screws will negate the original rotation error. The end result is a scope with a perfect axially aligned – just like it was before the introduction of the rotation error. So, what is the difference between both setups? The appearance of the secondary mirror will be different – though it might not be noticeable.

Let us consider the following hypothetical experiment. Imagine a square mirror placed at a 45 degree angle as shown in figure “A”. Assume we have the ability to rotate the mirror about all 3 axes (X, Y, and Z) . A laser beam racing down the Y axis will end up reflecting off the mirror surface in the direction of the -Z axis as shown in figure “B”. Figure “C” is the same setup but shown from the Y axis perspective. Rotation around the Z axis is similar to rotating the secondary mirror round the central bolt. Rotations around the X and Y axes are similar to set screw adjustments.

Now, if we rotate the square mirror around the Z axis counter-clockwise by 45 degrees, we will introduce a rotation error and the reflected laser beam will significantly off-target. Can we eliminate the Z rotation error by introducing new rotations only via the X and Y axes? The answer is YES. By rotating the mirror around the Y axis by 35.26 degree clockwise followed by a 15 degree clockwise rotation around the X axis, we will end up succeeding in redirecting the laser beam down the -Z axis again as shown in figure “D”. But the mirror appearance is different.

How can this be? Well, the final location of the mirror in our example is equivalent to rotating the mirror by 58.64 degrees around its axis – an axis that is perpendicular to the mirror surface and runs through its center. The final mirror surface planar position is exactly the same and the laser beam will see the exact same thing in figures “C” and “D”. Bear in mind that straight laser beams do not interact with the secondary mirror edge and therefore are insensitive to the appearance/shape of the secondary mirror. Only the planar position of the secondary mirror in 3D will impact the laser beam reflection – not its shape.

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#2 Jason D

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Posted 30 November 2013 - 01:46 PM

I will relate the info provided in the previous post to reflectors. Sometimes we will complete collimation by laser then look down an empty focuser and see the secondary mirror not positioned correctly and only part of the primary mirror reflection is visible.
Posted Image
We can “tilt” the secondary mirror to bring most of the primary mirror reflection into view and compensate for the “tilt” with a rotation around the central bolt. The end result is bringing the primary mirror reflection into view even though the final appearance of the secondary mirror might slightly look oval.
Posted Image
There is NOTHING sacred about the appearance of the secondary mirror. We would like it to be as round as possible but if it is little oval, that is OK. Having a slightly oval mirror has NOTHING to do with coma.
Here is an animation based on real photos. In both frames, the laser collimator will indicate perfect axial alignment. The tilt in one frame was compensated for by a rotation around the central bolt.
Posted Image
Above two frames were taken from the following animation. Again, each frame represents a perfectly aligned setup by laser.
Posted Image

Jason

#3 GlennLeDrew

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Posted 30 November 2013 - 04:45 PM

Excellent information! Those animated images are brilliant.

#4 klim

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Posted 01 December 2013 - 10:35 AM

Will the image at the eyepiece suffer if the secondary is laser aligned but not centered?

#5 GlennLeDrew

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Posted 01 December 2013 - 10:42 AM

klim,
The image will not suffer if collimated. The offset secondary merely results in the field illumination becomes asymmetric. A significant enough offset will result in on-axis clipping of the light cone, this being more likely the more 'tightly' sized the secondary (as for planetary Newts designed to have the minimal obstruction.)

#6 ThreeD

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Posted 01 December 2013 - 11:58 AM

So what I take from this is that it is possible to compensate for a rotated secondary by using the standard secondary collimation screws and in fact this will happen through the standard secondary collimation process.

I do believe, however, the amount of adjustment possible in typical scope will still require that the initial rotational error be minimal so in reality it is not possible to compensate for gross errors. For example, I know my secondary collimation screws aren't capable of inducing anything close to a 10° rotation of the secondary mirror and thus they aren't capable of compensating for such a gross error.

In the end one should still try to get the rotation as close as possible when installing the secondary but one does not need to obsess about it being perfect as standard collimation procedure will correct any minor error.

Good stuff!

#7 Vic Menard

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Posted 01 December 2013 - 12:05 PM

I will relate the info provided in the previous post to reflectors.

These graphics illustrate the "best" secondary mirror fit when the mechanical focuser axis is misaligned (pointing above or below the OTA axis in the graphic). In each graphic, if the focuser is racked further in, the entire reflection of the primary mirror will be visible. In the first graphic, the primary mirror reflection will be decentered relative to the "low" secondary mirror, while in the second graphic, the primary mirror reflection will appear centered in the secondary mirror which appears as an ellipse with major axis from 1:30 to 7:30. Another interesting anomalous presentation of the secondary mirror is a "perfectly" mechanically squared focuser with a "perfectly" mechanically centered secondary mirror (and a similarly centered primary mirror)--and a non-centered (offset) secondary mirror alignment.

There are, of course, many other mechanical issues that, if not sorted out, will deliver similar "skewed" secondary mirror presentations (non-"squared" spiders or UTAs, non-coaxial secondary mirror support stalks or mounting bolts/studs, non-centered primary mirrors, etc.). And then there are various combined mechanical errors that can serendipitously deliver fully corrected axial alignment and what appears to be a reasonably aligned, correctly offset and rounded secondary mirror (which I suspect is more common than the reader might think).

As Jason opines, secondary mirror placement is (thankfully) not mission critical to optical performance. Ideally, the whole reflection of the primary mirror should be visible from the center of the focal plane with a little room to spare to minimize the potential negative contributions from the edges of the secondary mirror at high magnification. If the Newtonian has been configured for planetary observing (smallest possible secondary mirror obstruction), the secondary mirror surface accuracy should be very good (including the edge) and the placement/presentation should be as close to "textbook" as possible (to minimize in-focus clipping). Generally speaking, these "planetary" Newtonian configurations include longer focal ratio primary mirrors, although current trends, even with this specialized application, are continuously pushing focal ratios faster.

Finally, if the mechanical focuser alignment includes "leveling" screws, almost any anomalous secondary mirror presentation in a simple Newtonian configuration can be visually corrected (this is because the focuser axis is the reference axis). Mechanically correcting these anomalous alignments is a much more complicated procedure because the reference axis is (usually) the OTA axis. This becomes an issue when a front aperture corrector is part of the optical train (SNTs and MNTs) or when DSC performance is critical.

#8 Vic Menard

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Posted 01 December 2013 - 12:26 PM

So what I take from this is that it is possible to compensate for a rotated secondary by using the standard secondary collimation screws and in fact this will happen through the standard secondary collimation process.

It's possible to compensate to achieve a correct focuser axial alignment, although not necessarily the optimal secondary mirror presentation/placement. If, for instance, the secondary mirror placement has already been corrected, and an autocollimator reveals a residual focuser axial error, it's common to make the (small) correction via the secondary mirror tilt adjustment.

...I do believe, however, the amount of adjustment possible in typical scope will still require that the initial rotational error be minimal so in reality it is not possible to compensate for gross errors. For example, I know my secondary collimation screws aren't capable of inducing anything close to a 10° rotation of the secondary mirror and thus they aren't capable of compensating for such a gross error.


I had Jim Fly use POVRay to model a 5-degree rotation error with accommodating tilt correction for New Perspectives because it's so common. And I've seen significantly pronounced combination errors (almost certainly more than 10-degrees) caused by simple thin beam laser alignment without any subsequent visual assessment--the secondary mounting was AstroSystems (or a clone).

In the end one should still try to get the rotation as close as possible when installing the secondary but one does not need to obsess about it being perfect as standard collimation procedure will correct any minor error.

In this case, the "standard collimation procedure" caused the error (and is the reason Rick Singmaster calls two of the four secondary mirror tilt alignment screws, "the forbidden screws").

But, in general, I agree with your commentary, provided the secondary mirror adjustments include subsequent visual assessment (and correction) as needed.

#9 ThreeD

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Posted 01 December 2013 - 06:28 PM

Yes, if the secondary is centered under the focuser but the rotation of the secondary is off a bit then using the secondary collimation screws to correct for the rotation is going to throw the centering out of whack. As has been mentioned, the centering being off isn't a major problem but I think it is fair to say that we all strive to have things as perfect as can be in order to realize the full potential of our equipment. That is why we obsess over what amounts to very minor issues that we probably won't even notice at the eyepiece.

#10 Vic Menard

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Posted 01 December 2013 - 06:51 PM

...That is why we obsess over what amounts to very minor issues that we probably won't even notice at the eyepiece.

I try to keep my obsessing to issues that are visible in the eyepiece. After obsessing over perfect alignment during my early years in this hobby, I've since been schooled to keep the alignments in tolerance.

FWIW, with the very fast focal ratios in common use today, maintaining tolerances provides plenty to obsess over.

#11 Jason D

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Posted 02 December 2013 - 02:25 AM

I would like to clarify the points I was trying to convey in my first two posts:

1- It is possible to correct a small secondary rotational error with the tilt setscrews and vice versa; therefore, centering the spider vanes and squaring the focuser do not have to be done with high precision.

2- The correction of the rotational error with the setscrews is a 100% accurate correction for the focuser axial alignment.

3- The appearance of the secondary mirror might not be as round after correction but that does not mean perfect collimation has not been achieved. Perfect collimation is met when:
A) Axial alignment is achieved
B) Good field illumination is achieved

A slightly oval secondary mirror that was tilted to correct for a small rotational error can still meet the above two requirements and achieve perfect collimation.

None of the above points are new. Vic has been preaching them for a long time.

Jason

#12 Steve OK

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Posted 02 December 2013 - 07:46 AM

While the experts are on board here, here is a question about secondary rotation...

When using a laser to set the secondary rotation, the laser spot follows an arc across the primary as the secondary is rotated. It seems to me that the correct rotational position of the secondary would be where the laser spot is directly above (on the focuser side) the center spot. On my diagram, that is where the dashed line and the red arc intersect. Any comment?

Steve

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#13 Vic Menard

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Posted 02 December 2013 - 08:33 AM

...Any comment?

That's an excellent "in the box" observation.

But let's take it "out of the box". Given all "perfect" mechanicals, if there's a residual tilt error in the secondary mirror adjustment, making your correction will induce a combined tilt/rotation error. A simple thin beam laser does not differentiate these errors.

Let's go the rest of the way "out of the box". If the mechanical focuser alignment is not correct/optimized for the secondary mirror geometry, or any other mechanical error exists that could "skew" the optimal secondary mirror placement, using a laser to adjust rotation to the midpoint of the arc you've described is relatively meaningless without additional visual assessment. A simple thin beam laser is at its best when assessing/correcting the focuser axial alignment. The optimal secondary mirror placement is most easily visualized by making three circular edges concentric. Axial alignment (center of the fov--image performance) and edge alignment (perimeter of the fov--image illumination) work together to deliver the best view in the eyepiece.

#14 Jason D

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Posted 02 December 2013 - 09:41 AM

While the experts are on board here, here is a question about secondary rotation...

When using a laser to set the secondary rotation, the laser spot follows an arc across the primary as the secondary is rotated. It seems to me that the correct rotational position of the secondary would be where the laser spot is directly above (on the focuser side) the center spot. On my diagram, that is where the dashed line and the red arc intersect. Any comment?

Steve


Steve, when I looked at your diagram I thought it was mine :)

http://www.cloudynig...ber/5704527/...

Posted Image

I use the arc method; however, the method might not work if one or more of the setscrews are almost tightened.

Sometimes someone will run into the dilemma of not being able to move the laser beam to the primary center because the setscrew that was moving the beam towards the center becomes too tightened. The solution is to loosen the same set screw and rotate the secondary to get the laser little pass the center. Now you can use the opposite setscrew(s) which should have more adjustment room to bring the laser back to the center.

Jason

#15 Jason D

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Posted 02 December 2013 - 09:51 AM

I should have added that the "arc" method helps only in the axial alignment. Placing the secondary mirror optimally under the focuser is a completely different alignment.
Jason

#16 Steve OK

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Posted 02 December 2013 - 10:32 AM

Jason, Vic, thanks for the replies. Jason, in all likelihood that is your diagram! I probably read that thread, tried in on my scopes, then forgot where I learned it. I know a lot of folks out here really appreciate the work the two of you have done to help educate us on the fine points of collimating a reflector, and the patience you have shown. Thanks!

Steve

#17 howard929

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Posted 02 December 2013 - 10:54 AM

Vic,

I imagine that a mechanical engineer could aptly describe all of the variables involved with driving a nail with a hammer but with that full knowledge it doesn't mean that engineer could do so without hitting his/her thumb.

And much like learning how a tunneling diode performs a quantum mechanical task it's not necessary to know, just that they're in there and the device works as intended because of it is usually more then enough for most people.

So, pardon me Please. Is there a simple grounded take away from this fine discussion? Do this to the secondary mirror, but no, don't do that?

#18 Vic Menard

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Posted 02 December 2013 - 01:11 PM

...So, pardon me Please. Is there a simple grounded take away from this fine discussion? Do this to the secondary mirror, but no, don't do that?

Of course.
First, the axial alignments have tolerances--so when all is said and done, it's important that those tolerances are maintained. And the most critical axial alignment is the primary mirror axial alignment.

And second, with regard to secondary mirror placement, there is no tolerance. As long as the secondary mirror looks reasonably "round" and the full primary mirror reflection is visible on the face of the secondary mirror, that's likely to be good enough (as long as the axial alignments are within tolerance).

In my experience, the mechanical "twists" surrounding what should be a simple three-part alignment strategy (two axes and secondary mirror placement) are precisely the areas where most novice Newtonian enthusiasts end up crashing and burning. Consider, if you will, "squaring" the focuser. Most novices (and some not-so novice) are fixated with "squaring" the focuser. I spend more time in these discussions explaining the focuser/secondary mirror geometry than on any other collimation question. And if the user expects a "textbook" secondary mirror presentation...the discussion can easily become tangled in the myriad mechanical tasks which, as you say, are "not necessary to know", but are still complicated to resolve.

FWIW, "this fine discussion" was started with the opening salvo, "First, the following two posts are not intended for everyone. Feel free to ignore this thread if you wish." I believe Jason knew this discussion (even with his excellent graphics and animations) could become mired in the details and he felt compelled (justly IMO) to start with that cautionary opening.

#19 Jason D

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Posted 02 December 2013 - 01:27 PM

FWIW, "this fine discussion" was started with the opening salvo, "First, the following two posts are not intended for everyone. Feel free to ignore this thread if you wish." I believe Jason knew this discussion (even with his excellent graphics and animations) could become mired in the details and he felt compelled (justly IMO) to start with that cautionary opening.


:waytogo:

#20 Steve OK

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Posted 02 December 2013 - 01:59 PM

And there are certainly those of us who want to read and ponder the finer points of collimation here.

Steve

#21 backwoody

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Posted 03 December 2013 - 11:19 PM

Interesting and informative. Thanks all.

#22 SAL

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Posted 04 December 2013 - 08:57 AM

Excellent information!






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