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Collimation final steps.

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#1 Javier1978

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Posted 10 January 2013 - 12:50 PM

Hi everyone,

In the last time I finally understood the basics collimation steps, mostly due to Jason´s excellent posts. Thank you Jason!!

Now, I have been performing collimation with my collimation cap in the last three years and that´s the only tool I own. I will be ordering a chesire/sight tube soon, but meanwhile I´m using a complement of the basic CC to achieve the focuser axial collimation.

So, once I achieved this allignment, I started to work with the secondary placement. My focuser is fairly squared and I measured the spider vanes to be in the exact center of the tube. My secondary has a classic offset, I think.

So, looking at the reflections I centered the secondary with the central bolt, always making iterations with the focuser axial alignment. But now I´m stuck in the final step, I think it´s a rotation issue, but I cannot make the two allignments well, it´s seems that at this point I cannot separate at all the tilt-rotation movement.

Or maybe is the spider? I don´t wan´t to touch the spider any more as it´s delicate and I think this is the best position, centered and with no vanes forced to the side.

So, should I accept this secondary placement or I can do better?

Here are some pictures

This are the home made collimation cap and the aditional tool that I made.

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#2 Javier1978

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Posted 10 January 2013 - 12:50 PM

Here is placed under the focuser.

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#3 Javier1978

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Posted 10 January 2013 - 12:51 PM

Here is how the primary donut looks like through the CC + New tool.

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#4 Javier1978

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Posted 10 January 2013 - 12:53 PM

Here is the primary clipped with focuser racked out, through the CC

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#5 Javier1978

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Posted 10 January 2013 - 12:54 PM

A burned view

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#6 Javier1978

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Posted 10 January 2013 - 12:56 PM

And here is how it looks with no collimation cap. The camera position is no longer reliable though.

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#7 csrlice12

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Posted 10 January 2013 - 12:59 PM

Maybe its the camera angle, or reflection from the mirror, but your vanes look "crinkled", like the lines "offset" a little every so often. Can't see your center spot, but other then those vanes looking weird, it otherwise looks ok. Anybody know what would cause the vanes to look like that?

#8 Javier1978

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Posted 10 January 2013 - 01:03 PM

It´s the image compression, let me upload a better resolution one.

The issue is the clipped primary, I can make it fit better, but that will change my focuser axial allignment.

#9 Javier1978

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Posted 10 January 2013 - 01:07 PM

A better resolution image:

http://postimage.org...7jy8m9mzf/full/

#10 Scanning4Comets

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Posted 10 January 2013 - 01:08 PM

Yup....image compression 100%

BTW nice job!

#11 Javier1978

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Posted 10 January 2013 - 01:09 PM

And the burned one. Note that the primary is not well alligned yet.

http://postimage.org...tb1rglcef/full/

#12 Vic Menard

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Posted 10 January 2013 - 02:10 PM

...My focuser is fairly squared and I measured the spider vanes to be in the exact center of the tube. My secondary has a classic offset, I think.

More likely New Model or unidirectional offset--but not really important.

So, looking at the reflections I centered the secondary with the central bolt, always making iterations with the focuser axial alignment. But now I´m stuck in the final step, I think it´s a rotation issue, but I cannot make the two allignments well, it´s seems that at this point I cannot separate at all the tilt-rotation movement.

It is indeed a tilt/rotation error (secondary mirror is tilted below the optical axis). To correct, tilt the secondary mirror upward (tighten the lower secondary mirror tilt adjustment screw) and then recenter the primary mirror by rotating the secondary mirror. If you do this in small steps, you'll notice the alignment of the primary mirror reflection in the secondary will improve and the silhouette reflection of the secondary mirror will look more circular (it currently is somewhat elliptical and pointing downward).

Or maybe is the spider? I don´t wan´t to touch the spider any more as it´s delicate and I think this is the best position, centered and with no vanes forced to the side.

I agree--the spider is probably fine. If you need to make any further adjustments to the secondary mirror/focuser geometry, I would suggest shimming the focuser. But you need to fix the tilt/rotation error first before you can assess the final secondary mirror/focuser geometry.

So, should I accept this secondary placement or I can do better?

That's up to you. The alignment is good enough to use now, but correcting the residual tilt/rotation error isn't that difficult once you know what to do.

And the burned one. Note that the primary is not well alligned yet.

The primary appears to be well aligned (donut is centered in the reflection of the underside of the focuser drawtube). The reflection of the primary relative to the edge of the secondary mirror is misaligned, but optimal secondary mirror placement requires the alignment of three circles (the bottom edge of the focuser drawtube, the actual edge of the secondary mirror, and the reflected edge of the primary mirror).

With a sight tube, you can use the sight tube cross hairs (aligned to the primary mirror center spot) and the optimal (centered) placement of the primary mirror reflection in the secondary mirror to accomplish the same thing.

#13 Javier1978

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Posted 10 January 2013 - 03:38 PM

Thank you Vic!

My mistake about the offset!

I can correct the tilt-rotation to make the primary reflection fit better (see picture) but that affect the focuser axial collimation. What should I do? Stay with this correct visual collimation or with the previous, where the FAA is correct?

As for the primary, I ment it was not "dead center" under the collimation cap, I can do a little better in the final alignment of the primary.

Thanks for the sight tube tip.

Here is the correct visual collimation:

http://postimage.org/image/kwo581763/

#14 Vic Menard

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Posted 10 January 2013 - 03:51 PM

...I can correct the tilt-rotation to make the primary reflection fit better (see picture) but that affect the focuser axial collimation. What should I do? Stay with this correct visual collimation or with the previous, where the FAA is correct?

I suggest staying with the correct visual collimation and adjusting the mechanical focuser leveling to correct the focuser axial alignment (focuser is pointing a little too high). If your focuser doesn't have a leveling adjustment, you can try loosening the lower mounting screws and shimming. I've had good luck with thin plastic washers placed between the focuser base and the OTA and secured with the mounting screws.

That said, the red circle surrounding the secondary mirror silhouette appears to be "hiding" a small residual elliptical shape (pointing around 4 o'clock), and the donut center spot isn't quite aligned vertically. These small, residual errors can affect the read of the secondary mirror/focuser geometry--so they should be sorted out first.

As for the primary, I ment it was not "dead center" under the collimation cap, I can do a little better in the final alignment of the primary.

:waytogo:

#15 Javier1978

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Posted 10 January 2013 - 04:31 PM

[quote name="Vic Menard"][quote]...I can correct the tilt-rotation to make the primary reflection fit better (see picture) but that affect the focuser axial collimation. What should I do? Stay with this correct visual collimation or with the previous, where the FAA is correct?[/quote]

I suggest staying with the correct visual collimation and adjusting the mechanical focuser leveling to correct the focuser axial alignment (focuser is pointing a little too high). If your focuser doesn't have a leveling adjustment, you can try loosening the lower mounting screws and shimming. I've had good luck with thin plastic washers placed between the focuser base and the OTA and secured with the mounting screws. [/quote]

Ok, that´s new to me. My focuser has indeed leveling screws. But, I have to say, I wouldn´t like to start again to deal with the focuser. I think I can live with the FAA a little off, at least for now.

[/quote]That said, the red circle surrounding the secondary mirror silhouette appears to be "hiding" a small residual elliptical shape (pointing around 4 o'clock), and the donut center spot isn't quite aligned vertically. These small, residual errors can affect the read of the secondary mirror/focuser geometry--so they should be sorted out first.?[/quote]

I got my with the red circle :roflmao:

I´m not sure what you mean with the donut center spot vertical alignment. Thank you for the patience!

#16 Vic Menard

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Posted 10 January 2013 - 05:22 PM

...Ok, that´s new to me. My focuser has indeed leveling screws. But, I have to say, I wouldn´t like to start again to deal with the focuser. I think I can live with the FAA a little off, at least for now.

The secondary mirror placement has no prescribed tolerance--but the focuser axis does. So I suggest you get the best possible secondary mirror placement, and then align the focuser axis by tweaking the secondary mirror tilt. Always finish with fine alignment of the primary mirror.

When you get a sight tube, I think you'll find it's quite easy to make the necessary adjustments to the focuser leveling screws. It's much easier than trying to adjust the spider vanes to correct this misalignment!

I´m not sure what you mean with the donut center spot vertical alignment. Thank you for the patience!

With offset, it's normal for the donut center spot to be decentered right to left. In your image (after I adjusted the exposure), the donut also appears to be decentered top to bottom, and it shouldn't be. The amount is quite small, but if I can see it, I usually correct it...

(Of course, it could also be due to a residual tilt/rotation error. Adjusting the exposure revealed the donut, not the underside of the focuser draw tube.)

#17 Javier1978

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Posted 10 January 2013 - 05:43 PM

Ok, I will do my best with the placement, adjust the focuser axis and then the primary.

I know about the importance of the focuser axis alignment. It sounds stupid but I don´t fell comfortable clipping the primary light cone and I "think" I have a better collimation when everything looks better through the collimation cap. Of course I can´t check the focuser alignment with it, so there is vital information missing there.

I get the donut thing know. In the previous images where the focuser axis is OK it looks much better.

#18 Vic Menard

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Posted 10 January 2013 - 07:12 PM

Not sure if you still have the 8-inch f/6, but assuming you do, for high magnification performance:
The focuser axial tolerance is about +/-6mm (about 3-percent of your primary mirror diameter), and
the primary mirror axial tolerance is about +/-1.2mm (less than 1-percent of your primary mirror diameter).
If you know the diameter of your donut center spot, you can get a good estimate of these tolerances using the tools you have.

Your collimation cap magnifies any primary mirror axial error 2X, so as long as the donut and the collimation cap pupil are aligned to +/-2.4mm, you're good to go.

An incorrectly placed secondary mirror normally only affects balanced fov illumination and is usually not an issue visually. A severely misaligned secondary mirror can cause image flare and other artifacts. As long as the secondary mirror appears more or less "circular" and most/all of the primary mirror reflection is visible when using the collimation cap, the placement will be usable. Secondary mirror coverage is checked near the focal plane (not racked all the way out).

#19 Javier1978

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Posted 10 January 2013 - 09:45 PM

Vic,

I thought about your comment about focuser pointing to high. When I tried to square the focuser, I adjusted exactly those srews. When racked in, the shadow of the draw tube didn´t seem to be well alligned with the spider, but i trusted my squaring method.

After reading your comment, I loosened all the leveling screws of the focuser and I started again, but now with the visual reference that you suggested. Things went much better and I only did very little adjusments.

I don´t know what you think, but this is how the the reflections look like (through collimation cap almost all focuser racked out) after performing a very precise (ok, precise for my home made tools) focuser axial alignment. I know some further work might be needed, but now I fell very comfortable with this secondary placement, I can clearly see all the primary with its clips, this is enought for me at this time.

BTW, I still use a 8" f6, and the given information is very useful.

Thank you very much, I have found your help very, very valuable!

Picture:

http://postimage.org...73dmhrk6v/full/

#20 Jason D

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Posted 11 January 2013 - 03:05 AM

Hello Javier,
You still have a rotate/tilt error. To fix it, rotate your secondary mirror clockwise while looking down the OTA then recollimate as usual starting with the focuser axis then the primary mirror. The clockwise rotation needs to be by a small amount.
See attachment. The secondary mirror silhouette needs to be circular and shifted towards the primary mirror. In your case, it is elliptical.
But you really do not have to make the above adjustment. You can keep it as is. It is up to you.
By the way, you do seem to have a classic secondary mount. I can tell because you vertical spider vanes reflection are not aligned with the secondary silhouette – not aligned with the vertical yellow line. But as Vic mentioned this has no impact on your collimation.
Jason

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#21 Jason D

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Posted 11 January 2013 - 03:21 AM

Here is a hint:
Start with aligning the focuser axis then the primary axis using the same collimation cap method you have outlined. Doing so will place the primary mirror reflection at the center of the focuser opening regardless of the secondary mirror positioning and regardless of any rotate/tilt errors.
Then and only then assess the positioning of your secondary mirror under the focuser. You can decide on your next secondary mirror move by examining the relation between the primary mirror reflection against the secondary mirror. In addition, examine the shape and direction of possible elongation of the secondary silhouette.
Adjust the secondary mirror. Ignore the primary mirror reflection during this adjustment.
When done, repeat the whole steps again until everything looks aligned.
Refer to the attached animation for further clarification.
Jason

Posted Image

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#22 Javier1978

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Posted 11 January 2013 - 08:00 AM

Thank you Jason!

I thought I had the classical offset as my secondary is not glued in its center to the holder.

I see what you are talking about and I read your post about the secondary placement a couple of times and saw the possible error drawings.

I´ll try tomorrow to do this final alignment. I think all is going to be easier with the sight tube. Now I have to switch between the CC only to check reflections and the CC + the tiny diaphragm in the focuser to check the FA and do this completely blind to the reflections.

I´ll let you know how if I can fix it though.

#23 Galicapernistein

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Posted 11 January 2013 - 11:41 AM

"An incorrectly placed secondary mirror normally only affects balanced fov illumination and is usually not an issue visually."

Question - Does a laser collimator always indicate that the secondary is properly angled toward the eyepiece, regardless of whether the secondary is properly placed for a fully illuminated field? In other words, if the laser is shining in the primary donut, and the collimator shows that the laser is aligned, is the only question whether the secondary should be raised or lowered? Doesn't an aligned laser mean that the secondary doesn't have to be twisted from side to side?

#24 Jason D

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Posted 11 January 2013 - 12:57 PM

Question - Does a laser collimator always indicate that the secondary is properly angled toward the eyepiece, regardless of whether the secondary is properly placed for a fully illuminated field?

Yes


In other words, if the laser is shining in the primary donut, and the collimator shows that the laser is aligned, is the only question whether the secondary should be raised or lowered? Doesn't an aligned laser mean that the secondary doesn't have to be twisted from side to side?


No. Even when the laser beam hits the primary center, to optimally position the secondary mirror under the focuser one or multiple of the following movements might be necessary:
1- Raise/lower the secondary mirror
2- Twist and tilt the secondary mirror
3- Adjust the spider vanes
4- Adjust the focuser
When you achieve axial alignment with unbarlowed quality laser, you are ensuring the primary and eyepiece focal planes are parallel and optical axes for both are coincident. Regardless of twisting and tilting, axial alignment will be achieved as long as the laser hits the primary center and re-traces its path to the emitter. Bear in mind that you can twist the secondary mirror then compensate for the twist with a tilt to get the laser to hit the primary center and trace its path to the source.
Jason

#25 Galicapernistein

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Posted 11 January 2013 - 01:09 PM

"Bear in mind that you can twist the secondary mirror then compensate for the twist with a tilt to get the laser to hit the primary center and trace its path to the source."

Assuming that the secondary is centered, and the focuser is properly aligned, wouldn't tilting the secondary alter the angle at which the secondary faces the focuser?






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