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Useful info about secondary mirror alignment

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#1 Jason D

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Posted 08 April 2009 - 09:14 PM

When a laser collimator is used to achieve axial alignment, it aligns the focuser axis against the center of the primary mirror then aligns the primary axis against the center of the focuser. However, unless a holographic laser is used, the laser collimator (including barlowed laser) is not adequate to align the secondary mirror.

The secondary mirror alignment is different from the focuser axial alignment and primary axial alignment. Click and watch the following animation. In each frame, both the focuser and primary axes are aligned yet the secondary mirror alignment is all over the place.

When the focuser axis is aligned using a quality tool, you are guaranteed to see primary mirror concentric with the focuser edge (or the sight-tube edge). The primary center spot will be precisely centered in the focuser or sight-tube. This is true regardless of the primary axial alignment or the secondary alignment. A misalignment secondary will clip part of the primary mirror reflection which implies part of the light cone will also be clipped. The objective of the secondary mirror alignment is to position the secondary mirror in such a way to optimally intercept the light cone. Given a well-aligned secondary mirror, if you place your eye exactly at the center of the focuser and look down precisely along the focuser axis (a quality sight tube will help you position your eye in this manner), then you should see the whole primary mirror reflection. But that is not all, if you move your eye back and away from the focuser, you should get to a point where the primary mirror reflection edge will overlap with the secondary mirror edge. If you keep going, then the primary mirror clips will start to be clipped by the secondary mirror edge in “almost” at the same rate.

Here is a photo of a well-aligned secondary mirror.

Attached Thumbnails

  • 3033065-secondary_mirror_explained_2.JPG

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#2 Jason D

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Posted 08 April 2009 - 09:15 PM

Here is the same photo with overlaid lines and circles. Note the following:
1- Focuser and primary reflection edges are concentric and the primary center spot is centered under the focuser.
2- The secondary silhouette (secondary shadow) is pointing towards the primary mirror as indicated by the red arrow

Attached Thumbnails

  • 3033067-secondary_mirror_explained.JPG

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#3 Jason D

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Posted 08 April 2009 - 09:16 PM

We can control 3 movements for the secondary mirror.

The first movement is along the OTA/UTA axis by loosening or tightening the secondary holder center screw. That is, moving the secondary mirror closer or away from the primary mirrior.

Attached Thumbnails

  • 3033068-secondary_mirror_central_screw.gif

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#4 Jason D

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Posted 08 April 2009 - 09:16 PM

The second movement is by adjusting the appropriate spider vanes thumb knobs to move the secondary holder towards the upper wall or the lower wall of the OTA.

Attached Thumbnails

  • 3033070-secondary_mirror_spider_vanes.gif

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#5 Jason D

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Posted 08 April 2009 - 09:17 PM

The third movement is called tilt/rotate which is accomplished by loosening the secondary mirror adjustment set-screws by a tad, rotate the secondary mirror by a tad, then finally adjust the set-screws to re-align the focuser axis. This is the key movement to point the secondary silhouette offset towards the primary mirror.

Note how the silhouette offset changes direction in the attached animation

Attached Thumbnails

  • 3033072-secondary_mirror_shift_rotate.gif

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#6 Jason D

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Posted 08 April 2009 - 09:18 PM

Putting it all together:

After you finish collimation (at least the focuser axial alignment which is accomplished by adjusting the secondary mirror to place the primary center spot reflection under the cross-hairs of the sight tube), check the primary reflection and see how it is clipped. Then use the 3 movements mentioned in the prior posts until the whole primary mirror reflection is visible and the primary reflection and the secondary edges overlap as you move your head back.

IMPORTANT:
1- Do not evaluate the primary reflection clipping unless you have completed at least the focuser axial alignment step. That is, if the primary mirror center spot is not located under the cross hairs of the sight-tube, clipping visual information is false or unreliable.
2- Seeing the whole primary reflection is not enough. You need to move your head back until the primary reflection edge overlaps with the secondary mirror edge. You need to be looking through the sight tube during this evaluation
3- Do your best. Perfect secondary alignment is not necessary unless you have a relatively small secondary mirror.


Jason
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#7 helpwanted

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Posted 08 April 2009 - 10:24 PM

Jason... excellent thread!!!! nice animation!!!
this should be a required read for anyone buying a newt!

#8 Dean Norris

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Posted 08 April 2009 - 10:46 PM

Thank you Jason for your informative post. Your animations are great!
Dean

#9 Lamb0

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Posted 08 April 2009 - 11:19 PM

:jump: I likee! Please add a link to this thread in the "How to Collimate your Newtonian" sticky! :cool:

#10 oalithgow

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Posted 09 April 2009 - 12:56 AM

wow..!! Very nice and detailed explanation men' A++++
1000 Thanks!! put it on my favorite list.

#11 senske

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Posted 09 April 2009 - 01:02 AM

Excellent! Thank you Jason.

#12 14dobguy

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Posted 09 April 2009 - 02:04 AM

ya man thats a really good thread...the annimation is a lot easier to understand too.good job!

#13 jonstarrysky

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Posted 09 April 2009 - 03:07 AM

Well done Jason, great posts !!

Re. your third movement, called tilt/rotate. Do you mean you 1st adjust sec rotation (by physically rotating the secondary holder with all set-screws loose). And only after that, go on to adjust the secondary tilt using the set screws. Some collimation guides have these as separate stages. Precise rotation (not tilt) is a tricky one to measure in my mind and I'd be interested in tricks to make it easier. One way may be to adjust the set screws to purposefully skew the secondary tilt towards/ away from the focusser, and then rotate the secondary holder to get the laser dot to line up in the direction of the focusser. This comes from a Glatter guide, but I havent tried it yet. Apologies if I am not interpreting that guide clearly. He says someting about "tipping" the secondary to do this.

Another way would be to photograph the secondary sillhoute tru a collmation cap, rotate the secondary a bit (eg clockwise), get all the tilts correct again, then re-photograph thru the collimation cap and see if the secondary sillhoute rotation has got better or worse. Labourious way ! Can you temporarily phycially attach something to the tip of the secondary holder to act as a guide for secondary rotation ? The apparent elliptical shape of the secondary sillhoutte is quite subtle to look at.

So if rotation is correct, in the picture in your second post of this thread, the apparent elliptical shape of the secondary silhoute should appear to point away from the focusser. In your diagram the secondary silhoute to my eyes is not pointing to the red arrow, it is noticably off (to the south). But still way better than the picture in my post, where mine is miles out !! Presumably if the secondary rotation is somewhat off it will typically have very little effect on the scope performance as long as its tilt is correct.

#14 jonstarrysky

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Posted 09 April 2009 - 06:58 AM

Re. the "second movement" as listed in post #4:
With my XT10i I begun by adjusting the spider vanes thumb knobs as indicated (obviously this is the sensible way to do it). I later found that when the secondary mirror set screws are loosened there is significant play in the lateral placement of the secondary holder (not to be confused with rotation or tilt). So depending on how you are holding the secondary when the 3 set screws are tightened, you can shift the lateral position of the secondary achieving the same effects as adjusting spider vanes. I was not happy that you can do this. Might have been because to get step "movement along the OTA/UTA axis" correctly done I had to substantially move the secondary mirror towards the primary mirror. This increases the length of the central thick screw that actually holds the secondary mirror to the spider and hence its wobble. Another explanation is that this big screw is near its end of its travel and is dangling near its last thread. I hope not !

#15 Jason D

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Posted 09 April 2009 - 10:35 AM

Thank you everyone for the compliments :bow:
Jason

#16 Jason D

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Posted 09 April 2009 - 10:36 AM

Re. your third movement, called tilt/rotate. Do you mean you 1st adjust sec rotation (by physically rotating the secondary holder with all set-screws loose). And only after that, go on to adjust the secondary tilt using the set screws. Some collimation guides have these as separate stages. Precise rotation (not tilt) is a tricky one to measure in my mind and I'd be interested in tricks to make it easier. One way may be to adjust the set screws to purposefully skew the secondary tilt towards/ away from the focusser, and then rotate the secondary holder to get the laser dot to line up in the direction of the focusser. This comes from a Glatter guide, but I havent tried it yet. Apologies if I am not interpreting that guide clearly. He says someting about "tipping" the secondary to do this.


You do not have to loosen all 3 set-screws. Loosen few just enough to allow you to rotate the secondary. The amount of tilt show be small – and I mean small.
If the silhouette is pointing downward, the I rotate the secondary in such a way to bring the upper edge of the secondary closer to me. If the silhouette is pointing upward, the I rotate the secondary in such a way to bring the lower edge of the secondary closer to me.

Should you tilt first then rotate or vice versa, I do not think it matters. Regardless, the final fine tuning will have to be done by adjusting the tilt.


Another way would be to photograph the secondary sillhoute tru a collmation cap, rotate the secondary a bit (eg clockwise), get all the tilts correct again, then re-photograph thru the collimation cap and see if the secondary sillhoute rotation has got better or worse. Labourious way ! Can you temporarily phycially attach something to the tip of the secondary holder to act as a guide for secondary rotation ? The apparent elliptical shape of the secondary sillhoutte is quite subtle to look at.


I do not believe you need a camera. Use you best judgment and accept the final result. If the silhouette direction looks good enough, then forget about it and focus on getting the primary mirror reflection edge to overlap as much as possible with the secondary mirror edge. Again, do your best and accept the final result.

So if rotation is correct, in the picture in your second post of this thread, the apparent elliptical shape of the secondary silhoute should appear to point away from the focusser. In your diagram the secondary silhoute to my eyes is not pointing to the red arrow, it is noticably off (to the south). But still way better than the picture in my post, where mine is miles out !! Presumably if the secondary rotation is somewhat off it will typically have very little effect on the scope performance as long as its tilt is correct.


It is an eye illusion. The silhouette in my photo is circular (as it should be) and is actually pointing towards the primary mirror. See attachment.

Jason

Attached Thumbnails

  • 3033975-secondary_mirror_explained_3.JPG


#17 Jason D

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Posted 09 April 2009 - 10:46 AM

Re. the "second movement" as listed in post #4:
With my XT10i I begun by adjusting the spider vanes thumb knobs as indicated (obviously this is the sensible way to do it). I later found that when the secondary mirror set screws are loosened there is significant play in the lateral placement of the secondary holder (not to be confused with rotation or tilt).


Before you start adjusting the lateral position of the secondary, ensure the secondary silhouette is pointing in the right direction, first. When you start adjusting the lateral position, avoid rotating the secondary and adjust using the set-screws. That is, do not loosen the set-screws when adjusting the spider vanes thumb knobs. Always keep in mind: Secondary position evaluation should be done ONLY when the focuser axes is aligned – getting the primary center spot reflection under the cross-hairs of the sight-tube or getting the laser beam dead centered in the center spot. So, do not lateral shift and evaluate simultaneously. Shift then align focuser axis then evaluate.

Jason

#18 Dennis Sakva

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Posted 10 April 2009 - 03:23 AM

Jason, thanks for very useful thread. I find this secondary mirror adjustment the most confusing :silly: of all collimation process.
Could you please tell which error leads to which results. Like tilted focal plane, uneven vignetting, etc.
Thanks!
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#19 Jason D

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Posted 10 April 2009 - 10:32 AM

Jason, thanks for very useful thread. I find this secondary mirror adjustment the most confusing :silly: of all collimation process.
Could you please tell which error leads to which results. Like tilted focal plane, uneven vignetting, etc.
Thanks!



Let us review collimation fundamentals. When we collimate, we strive to meet 3 different and independent alignments:

1- Focuser axial alignment calls for positioning the secondary mirror surface to reflect the focuser axis to the dead center of the primary mirror. A relatively sizable focuser axial error (FAE) will cause the primary focal plane to be tilted with respect to the eyepiece focal plane which prevents bringing the whole FOV to focus.
2- Primary axial alignment calls for positioning the primary mirror axis to center the primary focal plane in the focuser. A relatively sizable primary axial error (PAE) will introduce coma on-axis (in the middle of the FOV).
3- Secondary mirror alignment calls for positioning the secondary mirror to center the 100% illumination area in the center of the focuser to optimize field edge illumination. A relatively sizable secondary mirror alignment error will unevenly distribute and reduce the overall illumination across the FOV.

The first two are critical. The third (which is the subject of this thread) is not as critical for visual observation but could be critical for astrophotography.

The following illustrates PAE and FAE

Posted Image

The attachment illustrates the secondary mirror alignment error

Vic Menard’s addendum website has more interesting information about secondary mirror alignment.

Attached Thumbnails

  • 3035954-illumination_fov.PNG


#20 Starman1

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Posted 10 April 2009 - 02:32 PM

Jason,
One of the curious things about using a sight tube to gauge concentricity of secondary outline and focuser that I've run into with the telescoping sight tube from Catseye is that I have to adjust the length of the sight tube considerably shorter than the f/ratio template suggests in order to see the outline of the mirror.
If I set the sight tube using the template to the f/5 of my scope, I have to pull the sight tube several inches out of my focuser in order to see the entire secondary. On the other hand, if I adjust the sight tube to be a couple inches shorter (say, around f/3.5), I can fully insert the sight tube and still see the entire outline of the secondary.
That makes sense, since the secondary-to-focal plane distance will vary substantially, even at the same f/ratio.
The sight tube length should be related to the distance from secondary-to-focal plane (often referred to as the intercept distance) so that the perspective from the peephole to the front lip of the sight tube to the outer edge of the secondary all line up.
A few simple diagrams show this quite well.
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#21 Shawn H

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Posted 10 April 2009 - 03:30 PM

Jason Khadder the man!
You never cease to amaze me! :rainbow: Thanks bud! :cool: Shawn

#22 Jason D

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Posted 10 April 2009 - 04:23 PM

Don,
I understand what you are describing.
Optimal placement of the secondary mirror requires more than getting the bottom edge of the sight-tube, the secondary mirror edge, and the primary reflection all concentric. For optimal secondary mirror placement, you need to overlap the secondary edge with the primary reflection edge even if it means pulling out the sight-tube.
Refer to the attachment (which is highly exaggerated to illustrate the point).

The diagram to the left is optimal. The secondary and primary reflection edges overlap. As a result, the 100% illumination circular area is centered in the FOV.

The diagram to the right is semi-optimal. The secondary and primary reflections are concentric but do not overlap. The primary reflection is inside the secondary and you can see a good portion of the area surrounding the primary mirror. This setup will actually skew the 100% illumination circular area towards the lower edge of the secondary mirror.

To determines the 100% illumination area graphically, draw a line from the primary edge to the secondary edge and finally to the focal plane. Do the same at the opposite edges of the primary and secondary mirrors. The area between the two focal plane intersection points delimits the 100% illumination area.

Jason

Attached Thumbnails

  • 3036616-illumination_fov2.PNG


#23 Jason D

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Posted 10 April 2009 - 04:29 PM

Jason Khadder the man!
You never cease to amaze me! :rainbow: Thanks bud! :cool: Shawn


Shawn, you are just too kind :waytogo:

#24 hudson_yak

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Posted 10 April 2009 - 09:31 PM

Seems to me what Don is describing is influenced by how oversized the secondary is. A secondary that provides 100% at only the very center of the field would use a sight tube matching the focal ratio, with the peephole positioned at the focal plane.

As the secondary is sized for more than just the very center of the field being 100% illuminated, usually, the sight tube F-ratio and position of the peephole will be different. From what I've read before the F-ratio should be a bit longer (not shorter) and the peephole located further out to ensure the 100% illuminated area is centered.

Mike

#25 Jason D

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Posted 10 April 2009 - 10:15 PM

Stacking the secondary mirror edge against the primary mirror reflection edge will yield the most optimal secondary placement regardless of the sight tube length and the F-ratio.

Here is what I would do with an adjustable sight tube:
1- Set the shortest length.
2- Rack out the drawtube until the primary mirror reflection apparent size is as large as the secondary mirror apparent size – but not larger
3- Extend the sight tube length until its lower opening apparent size is a tad larger than the secondary mirror apparent size

Then take it from there. No need to account for the secondary mirror relative size to the primary mirror size, no need to account for the distance between the secondary mirror and sight-tube pupil location, and no need to account for the F-ratio.

Jason


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