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Useful info about secondary mirror alignment

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#376 wh48gs

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Posted 02 December 2009 - 03:39 PM

This calls for a party.



:yay:

Vla

#377 auriga

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Posted 02 December 2009 - 11:49 PM

Funny thing is that collimation is a really simple process stuck in the middle of confusing parts.

After almost 400 posts, I think this quote from Jay Scheurie pretty much sums it up.

Seriously, it has been an arduous and passionate discussion, with some excellent contributions from the major players. Well done! :bow:

Now, I have to agree with Alexis, "Let's party!" :waytogo:



Hi folks,
I find it very disappointing that this thread has fallen short of the 400 mark. Could we have some more discussion of milk jug washers? That was the part of the thread I understood.
CS,
Bill Meyers

#378 Betelgeuze93

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Posted 24 April 2012 - 01:55 PM

Hi stargazers!

I just installed a new Orion UK 74mm secondary mirror to my 8" F4. After collimating it using a simple filmcan for positioning the mirror and a laser, the reflection of the focuser is not appearing centered in the reflection of the secondary mirror, when looking through the focuser with the eye.

Here is a picture of how it looks:


Posted Image

Is that correct?

Cheers, Simon

#379 Starman1

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Posted 24 April 2012 - 06:14 PM

Hi stargazers!

I just installed a new Orion UK 74mm secondary mirror to my 8" F4. After collimating it using a simple filmcan for positioning the mirror and a laser, the reflection of the focuser is not appearing centered in the reflection of the secondary mirror, when looking through the focuser with the eye.

Here is a picture of how it looks:


Posted Image

Is that correct?

Cheers, Simon

Your picture correctly shows the shadow of the secondary offset away from the center of the mirror, away from the focuser.
It is all the other reflections seen within that matter.

#380 Starman1

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Posted 24 April 2012 - 06:25 PM

since this ancient thread has been resurrected, it might be useful to point out that the illuminated field from the primary is not a perfect ellipse where it hits the secondary.

As has been pointed out in the long threads about Glatter's Blug and Tublug, the illuminated field that hits the secondary is fatter at the lower end of the secondary (it has a larger radius) than it is at the top of the secondary. To think of it in a highly-exaggerated form, it is shaped more like a pear.

That is the result of having a circular light image hit a 45 degree (plus or minus) surface.

However, from the perspective of a centered pupil at the focal plane of the scope, the smaller radius part of the illuminated reflection is closer to the eye and appears larger, so the pear appears round to the eye from a point on the optical axis at the focal plane.

Or not. I've been working on this thought experiment for a while and can't come to any conclusion.

#381 Jason D

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Posted 24 April 2012 - 08:06 PM

But what is important is that the 100% illumination area will be a "circle" at the eyepiece assuming the secondary mirror is positioned optimally.

Posted Image


Some might find it interesting to know that the 45 degree cross section ellipse of a cone has a slightly longer major-axis compared to the 45 degree cross section ellipse of a cylinder.


Jason

#382 CatseyeMan

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Posted 25 April 2012 - 07:25 AM

since this ancient thread has been resurrected, it might be useful to point out that the illuminated field from the primary is not a perfect ellipse where it hits the secondary.

As has been pointed out in the long threads about Glatter's Blug and Tublug, the illuminated field that hits the secondary is fatter at the lower end of the secondary (it has a larger radius) than it is at the top of the secondary. To think of it in a highly-exaggerated form, it is shaped more like a pear.

That is the result of having a circular light image hit a 45 degree (plus or minus) surface.

Don,


Your explanation using the "pear" shape generated analogy is not mathmatically consistent - any planar slice through a cone generates a "perfect" ellipse at the intersection - it is NOT pear shaped.

See: Conic Sections

What IS in play is that unless the eye is "at" the focal point (which is the only viewing location where a true circle IS projected), it is perspective illusion that causes that perfect ellipse to generate a non-circular appearance.

#383 Starman1

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Posted 25 April 2012 - 11:14 AM

Jim,
Thanks for the slap. I needed that.
Of course you are right. I attach a drawing from another thread that shows the intersection is indeed an ellipse, but the optical center is at one focus of the ellipse.
After the reflection, it is a circle, with the optical center at the center of the circle.

Attached Thumbnails

  • 5189753-laserr.png


#384 starhunter50

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Posted 03 January 2013 - 01:20 PM

this is excatly what going on right now going round in circles because the mechanical parts e way off and :foreheadslap: i cant reconcile them :(

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#385 starhunter50

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Posted 03 January 2013 - 01:25 PM

another view :question:

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  • 5603000-secondary spider way off.jpg


#386 Jason D

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Posted 03 January 2013 - 02:13 PM

Are you sure the camera's lens axis intersects the OTA axis?
Looking at the photos, it seems if you raise the camera and pointed it down by a little amount then the secondary mirror will look centered.
Regardless, in this thread and many others the recommendation is to "reasonably" square your focuser and center your secondary mirror in the OTA. Are you running into issues with these two steps?
Jason

#387 starhunter50

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Posted 03 January 2013 - 02:47 PM

At first the focuser was on the secondary was way to low , so i made a 4 square measurement on the secondary center threaded rod they were all within 2-3 mm up/down/left/right.
so as close as we can say the center point is ok but the secondary mirror will not come up any higher in the focuser opening,, having had enuff of that i raised up the focuser on the bottom side by under 1/4 inch with shims , now the mirror ended up right underneath where it should be , but upon doing the laser collimation everything lined up ok , till i put the cheshire EP in WOW flipping mile out..
this after 3 days of plating with this , now i realize there may be a real chance that the mirror center on the mirror 45 degree angle holder may have been glued way off no t only in the primary direction but also in a up/down position, that being said i looked again and yes it was evident, but by how much i did not calculate, finally the mirror fell in my hands sa i was adjusting the mirror ( next time twist the metal holder not the mirror) this was help in place with MARINE heavy duty silicone...
for now i need to re think this whole secondary thing, i made my own curved spider, it worked out last year but this year since getting a new mirror , no avail, plus i have 2 mirrors 2.30 MA and 2.5-MA.
now i might go with the bigger one making it an astrograph of some sort, 10inch F4.7.
i will post my home made spider curved vane

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  • 5603155-curved1.jpg


#388 starhunter50

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Posted 03 January 2013 - 03:15 PM

ive also ran out of screw threads on the spider vanes attempting to raise the mirror up !!!it did go up but not enough.....

#389 Jason D

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Posted 04 January 2013 - 02:32 AM

another view :question:

Why don't you align your scope axially then take a photo for better assessment of the secondary mirror position. By "axially" I mean use your laser collimator to align both the secondary and primary mirrors then take the photo.
In the photo you have posted, your scope is horribly miscollimated. It is difficult to make and sort of assessment with such a badly miscollimated scope.
Jason

#390 Victor Martinez

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Posted 04 January 2013 - 09:06 AM

My 3.1" secondary mirror through a self-built cheshire. I understand that is correct enough.

Posted Image


#391 Jason D

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Posted 04 January 2013 - 11:00 AM

Your photo does not have enough info to assess the overall collimation. The only mild issue I see is the intrusion of one side of the secondary mirror as indicated in the attachment. It is not a big deal but I wanted to point it out.
Jason

Attached Thumbnails

  • 5604541-victor.jpg


#392 Victor Martinez

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Posted 04 January 2013 - 02:40 PM

Yes Jason, you're right, look through 2" Catseye Cheshire/Sight tube . Now best.

Posted Image

#393 Jason D

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Posted 04 January 2013 - 02:57 PM

Looks much better. All screw heads appear balanced.

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#394 alistairsam

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Posted 04 February 2013 - 02:18 AM

Hi,
It took me a few days, but I managed to go through all 20 pages of this thread. All the math is way above me though.
Not sure if this is the right thread, pls re-direct if required.
I've been obsessing about the focuser not being perfectly perpendicular to the optical axis until I read this thread where Jason mentions it doesn't have to be.

Going by the new model offset, I can understand the primary being tilted along with the secondary to correct for the offset as well as the focuser tilt, and this would achieve collimation.
But for the primary to tilt and stay aligned with the tilted secondary, it also has to shift laterally. This is apparent in Jason's diagrams as well.
But almost all primary mirror cells have spring loaded vertical bolts, so you can tilt the primary by tightening one bolt and loosening the other two, but you can't really shift it sideways?? is this correct?
I know my focuser is tilted and have corrected for it by as much as I could measure, so If I reset the primary by adjusting it to be parallel to the cell by measuring the gap, and I then adjust the secondary by moving it away from the focuser and toward the primary, would I not eventually achieve collimation?

I am using a cats eye AC and the four reflections co-incide, but in my images, star shapes always differ at edges, which is why I finally went with fixing the focuser tilt. Haven't tested yet, but the primary tilt is the bit that I don't get.
I'm also now thoroughly confused with what the reflections with the cheshire should be. I was aligning the cross hairs with the opening of the cheshire and centering that in the reflection of the cheshire, but one of the images from Jason here that is collimated, shows it off centered. what should we aim to center the cheshire cross hairs with?
Thanks

#395 Starman1

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Posted 04 February 2013 - 10:27 AM

Hi,
Going by the new model offset, I can understand the primary being tilted along with the secondary to correct for the offset as well as the focuser tilt, and this would achieve collimation.
But for the primary to tilt and stay aligned with the tilted secondary, it also has to shift laterally. This is apparent in Jason's diagrams as well.
But almost all primary mirror cells have spring loaded vertical bolts, so you can tilt the primary by tightening one bolt and loosening the other two, but you can't really shift it sideways?? is this correct?
I know my focuser is tilted and have corrected for it by as much as I could measure, so If I reset the primary by adjusting it to be parallel to the cell by measuring the gap, and I then adjust the secondary by moving it away from the focuser and toward the primary, would I not eventually achieve collimation?

If you tilt the primary, the point of tilt is below the surface of the mirror. Ergo, the top surface of the mirror does shift laterally just by tilting the mirror, albeit a very small distance.

I am using a cats eye AC and the four reflections coincide, but in my images, star shapes always differ at edges, which is why I finally went with fixing the focuser tilt. Haven't tested yet, but the primary tilt is the bit that I don't get.

What you see at the edge is coma and/or astigmatism. That is caused by the paraboloidal surface of the primary mirror and/or the eyepiece. it has nothing to do with collimation. In fact, uniform coma around the perimeter could be a good sign you've done a good job of collimation, vis-a-vis the centering of the secondary mirror under the focuser. If the star images bother you, you should save your pennies for a coma corrector like the TeleVue Paracorr.

I'm also now thoroughly confused with what the reflections with the cheshire should be. I was aligning the cross hairs with the opening of the cheshire and centering that in the reflection of the cheshire, but one of the images from Jason here that is collimated, shows it off centered. what should we aim to center the cheshire cross hairs with?

For a matter of semantics, the peep-hole/crosshairs part of your tool is called a "sight tube". The "cheshire" part of the tool is the peep-hole plus the bright ring with the dark center.
As far as secondary alignment is concerned, you adjust the secondary to place the crosshairs near your eye (not the distant reflection of the crosshairs) over the center marker's reflection from your primary mirror.
You adjust the primary tilt to center the reflection of the primary's center marker in the dark center of the bright annulus, ignoring the crosshairs.
When you're done, the image through the peep-hole will look like the picture I attach.

Attached Thumbnails

  • 5661451-Secondary & Primary Aligned - full view.jpg


#396 Jason D

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Posted 04 February 2013 - 04:09 PM

I've been obsessing about the focuser not being perfectly perpendicular to the optical axis until I read this thread where Jason mentions it doesn't have to be.


Correct. "Reasonable" sqauring is good enough.

But for the primary to tilt and stay aligned with the tilted secondary, it also has to shift laterally. This is apparent in Jason's diagrams as well.


I assume you are referring to a primary mirror shift within the OTA closer to the focuser to follow the secondary mirror position being closer to the focuser with the “new model”, correct?
If yes then you misunderstood how the new model works. The primary mirror does not shift nor is the shift necessary.

But almost all primary mirror cells have spring loaded vertical bolts, so you can tilt the primary by tightening one bolt and loosening the other two, but you can't really shift it sideways?? is this correct?


No, sideways shift is needed for the new model.

I know my focuser is tilted and have corrected for it by as much as I could measure, so If I reset the primary by adjusting it to be parallel to the cell by measuring the gap, and I then adjust the secondary by moving it away from the focuser and toward the primary, would I not eventually achieve collimation?


I am getting the impression your understanding of collimation is incorrect. Collimation is not about squaring the focuser, offsetting the secondary away from the focuser, and centering the primary in the OTA. Collimation is about aligning the optics – not about aligning the mechanics – though the latter will facilitate collimation.

I am using a cats eye AC and the four reflections co-incide, but in my images, star shapes always differ at edges, which is why I finally went with fixing the focuser tilt. Haven't tested yet, but the primary tilt is the bit that I don't get.


I am getting the impression you are only aligning reflections P and 1 which will cause reflections 2 and 3 to disappear, hence, giving the illusion all four reflections are aligned. Did you use the CDP method?

I'm also now thoroughly confused with what the reflections with the cheshire should be. I was aligning the cross hairs with the opening of the cheshire and centering that in the reflection of the cheshire, but one of the images from Jason here that is collimated, shows it off centered. what should we aim to center the cheshire cross hairs with?


It sounds you are aligning the cross-hairs with the spider vanes reflections, is that what you are trying to do?

Sorry, I do not mean to add to your frustration…

Jason

#397 Jason D

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Posted 26 February 2013 - 02:20 AM

I am resurrecting this thread to add more useful information about secondary mirror alignment.

Many beginners purchase a laser collimator and follow the proper steps to find out they can’t see all of the primary mirror clips via the secondary mirror. By proper steps I meant aligning the secondary mirror by redirecting the laser beam to the primary center then aligning the primary mirror by redirecting the laser beam back to its source. When beginners run into this issue they wonder if the problem is with the quality of their laser collimators. Then they realign the secondary mirror using the collimation cap to bring all of the primary clips to view. Now the laser beam no longer hits the primary mirror center. At this point frustration builds and no matter what they do, they just can’t reconcile between the collimation cap and the laser collimator.

In the above scenario, the straight laser beam only interacts with a tiny portion of the secondary mirror surface. It does not interact with the secondary mirror edge; therefore, how can a laser collimator that does not interact with the secondary mirror edge is expected to align the secondary mirror under the focuser!!!

Here is the catch. There are two alignments for the secondary mirror. Let me repeat this important statement: There are two different and independent alignments for the secondary mirror. The first is somewhat “coarse” which positions the secondary mirror under the focuser. The second is more “fine” which redirects the focuser axis (laser beam) to the primary mirror center. The first alignment is responsible for optimizing illumination whereas the second is meant to eliminate focal plane tilt. In layman’s terms, the first is responsible for optimally positioning the secondary mirror with respect to the star light cone reflection off the primary mirror and the second ensures the eyepiece lens and the primary mirror focal plane are parallel. Both alignments are independent. That is, you can align one without the other.

A straight laser beam can be used for the second alignment – not the first. This is the main source of confusion. Following the laser collimator proper steps will ensure the eyepiece lens and the primary mirror focal plane are parallel but it will not ensure the secondary mirror centered under the focuser which means it will not guarantee all primary mirror clips can be seen.

Here is how you can reconcile between a good laser collimator and a collimation cap:

1- Complete the alignment of both the secondary and the primary mirrors using the laser collimator following the proper steps. Completing this step will ensure the primary mirror reflection is well-centered under the focuser. Let me repeat: Completing this step will ensure the primary mirror reflection is well-centered under the focuser. Now you can use the primary mirror reflection as your reference to optimally position the secondary mirror under the focuser. You do not need to reference the focuser edge.

2- Insert the collimation cap and ensure the collimation cap pupil’s reflection is aligned with the primary mirror center spot. It should. If it is not then you have a more fundamental problem. Now your eye axis is aligned with the focuser axis. Maintain your eye position and evaluate how well the primary mirror reflection is aligned with the secondary mirror edge. More specifically, which part of the primary mirror reflection you can't see.

3- Think of the secondary mirror as a window to the primary mirror reflection. As I stated above, the primary mirror reflection is well-centered under the focuser after completing the proper laser collimation steps. Now figure out how best to move the window “secondary mirror” to bring the whole primary mirror reflection to view. Check the attached animations. In each frame, both secondary/primary mirrors were aligned with a laser collimator following proper steps. See how the primary mirror reflection remains still and centered regardless of the secondary movement.

4- To move the secondary mirror, you can use any combination of the following movements:

a. Use the center bolt to move the secondary up or down the OTA
b. You can rotate the secondary mirror
c. You can use the spider vanes thumb knobs to move the secondary across the focuser. If you decide to make this movement, ensure that opposite spider vanes remain parallel as much as possible to minimize diffraction spikes

5- IMPORTANT: As you make the movements as outlined in step 4, ignore the primary mirror reflection. For example, if you decide to move the window “secondary mirror” closer to the primary mirror via the center bolt, make the movement without looking at any reflectiond. When you are done, go back to step 1. Do not evaluate the secondary position until you have completed the laser collimation steps since only completing the laser collimation steps will ensure the primary mirror reflection is centered under the focuser.

6- Repeat steps 1 to 5 until both the laser collimator and the collimation cap reconcile.

Posted Image


Posted Image

#398 Tim L

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Posted 26 February 2013 - 01:48 PM

Excellent information as always, Jason! :)

I recently discovered I had a different issue with not seeing all three mirror clips simultaneously on my Z10. The clips are mounted at 12 o'clock, 4 o'clock, and 8 o'clock around the mirror. Gravity pulls the mirror down toward the 6 o'clock position, so the clip up at the top (12 o'clock) doesn't visibly protrude into the mirror edge as much as the other two.

Figuring that out saved me some alignment grief. :p

Clear skies :rainbow:

Edit--hey, 2000 posts!

#399 Vic Menard

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Posted 26 February 2013 - 03:13 PM

Tim,
Generally speaking, when you "square" the focuser to the already "squared" secondary (spider), you create an alignment plane defined by the mechanical focuser axis and the long axis that passes through the center of the spider (perpendicular to the spider vanes).

If the mirror center falls "below" this plane (as you've described), the optimal secondary mirror alignment will be "skewed" (the secondary mirror presentation will not appear precisely rounded). The fix, of course, is to either correct the primary mirror placement, change the mechanical alignments to accommodate the primary mirror placement, or just go ahead and use the optimal secondary mirror placement. As Jason has already noted, the critical axial alignments can be fully corrected for any secondary mirror placement. This particular misalignment is similar to the optimal secondary mirror alignment when the mechanical focuser axis is misaligned above or below this alignment plane.

Then again, as Jason mentioned in the earlier post at the top of this page (20), the relationship of the long axis to the short axis of the secondary mirror ellipse is usually a 45-degree cut on a cylinder (not a cone). As long as the secondary mirror shape is truly elliptical, there should be an optimal placement (offset) that allows the secondary mirror presentation to appear rounded and centered as viewed from the focuser.

This "front end" geometry (especially when coupled with some user's fixations on mechanically squaring various components) is the cause for many more collimation headaches than the simpler, routine axial alignments.

(Rereading your post, with a gravitationally sagging primary mirror, I would expect the mirror clip at 12 o'clock to be more visible in an otherwise "squared" front end geometry--but hey, it's mirrors!)

#400 Vic Menard

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Posted 26 February 2013 - 03:31 PM

...Now figure out how best to move the window “secondary mirror” to bring the whole primary mirror reflection to view.

I believe this is the part that confuses most beginners (and quite a few experts). Fore and aft (offset) is pretty much linear. Tilt and rotation (or a combination of the two) often leads to other issues residing outside of the alignment plane...

This procedure is further complicated when the reference (primary mirror reflection) is suddenly removed from the center of the fov when any secondary mirror "adjustment" is applied (especially when the collimation cap pupil is located near the apex). Advantage here goes to the sight tube, with its "fixed" bottom edge defining the alignment cone.


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