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Equipment Discussions >> Reflectors

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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Collimation final steps.
      #5615463 - 01/10/13 12:50 PM Attachment (44 downloads)

Hi everyone,

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

Now, I have been performing collimation with my collimation cap in the last three years and thatīs the only tool I own. I will be ordering a chesire/sight tube soon, but meanwhile Iīm 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 Iīm stuck in the final step, I think itīs a rotation issue, but I cannot make the two allignments well, itīs seems that at this point I cannot separate at all the tilt-rotation movement.

Or maybe is the spider? I donīt wanīt to touch the spider any more as itīs 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.

Edited by Javier1978 (01/10/13 12:58 PM)


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5615464 - 01/10/13 12:50 PM Attachment (45 downloads)

Here is placed under the focuser.

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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5615466 - 01/10/13 12:51 PM Attachment (43 downloads)

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

Edited by Javier1978 (01/10/13 12:59 PM)


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5615473 - 01/10/13 12:53 PM Attachment (35 downloads)

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

Edited by Javier1978 (01/10/13 01:00 PM)


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5615478 - 01/10/13 12:54 PM Attachment (32 downloads)

A burned view

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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5615480 - 01/10/13 12:56 PM Attachment (34 downloads)

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

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csrlice12
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Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5615484 - 01/10/13 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?

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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: csrlice12]
      #5615495 - 01/10/13 01:03 PM

Itīs 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.

Edited by Javier1978 (01/10/13 01:04 PM)


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5615504 - 01/10/13 01:07 PM

A better resolution image:

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


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Scanning4Comets
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Reged: 12/26/04

Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5615507 - 01/10/13 01:08 PM

Yup....image compression 100%

BTW nice job!


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5615509 - 01/10/13 01:09 PM

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

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

Edited by Javier1978 (01/10/13 01:22 PM)


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Vic Menard
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Reged: 07/21/04

Loc: Bradenton, FL
Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5615613 - 01/10/13 02:10 PM

Quote:

...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.

Quote:

So, looking at the reflections I centered the secondary with the central bolt, always making iterations with the focuser axial alignment. But now Iīm stuck in the final step, I think itīs a rotation issue, but I cannot make the two allignments well, itīs 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).

Quote:

Or maybe is the spider? I donīt wanīt to touch the spider any more as itīs 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.

Quote:

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.

Quote:

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.


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Vic Menard]
      #5615745 - 01/10/13 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/

Edited by Javier1978 (01/10/13 03:41 PM)


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Vic Menard
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Reged: 07/21/04

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Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5615769 - 01/10/13 03:51 PM

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?



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.

Quote:

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.





Edited by Vic Menard (01/10/13 04:07 PM)


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Vic Menard]
      #5615841 - 01/10/13 04:31 PM

Quote:

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?




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.




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




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.?




I got my with the red circle

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


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Vic Menard
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Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5615917 - 01/10/13 05:22 PM

Quote:

...Ok, thatīs new to me. My focuser has indeed leveling screws. But, I have to say, I wouldnīt 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!

Quote:

Iīm 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.)


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Vic Menard]
      #5615942 - 01/10/13 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 donīt 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 canīt 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.

Edited by Javier1978 (01/10/13 05:46 PM)


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Vic Menard
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Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5616079 - 01/10/13 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).


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Vic Menard]
      #5616334 - 01/10/13 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 didnīt 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 donīt 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/image/73dmhrk6v/full/

Edited by Javier1978 (01/10/13 10:03 PM)


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Jason D
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Reged: 10/21/06

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Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5616623 - 01/11/13 03:05 AM Attachment (23 downloads)

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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Jason D
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Re: Stucked with the collimation final steps... new [Re: Jason D]
      #5616630 - 01/11/13 03:21 AM Attachment (13 downloads)

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



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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Jason D]
      #5616799 - 01/11/13 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.

Iīll 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.

Iīll let you know how if I can fix it though.


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Galicapernistein
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Reged: 09/24/07

Loc: Detroit Michigan
Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5617112 - 01/11/13 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?


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Jason D
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Re: Stucked with the collimation final steps... new [Re: Galicapernistein]
      #5617239 - 01/11/13 12:57 PM

Quote:

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


Quote:

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


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Galicapernistein
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Reged: 09/24/07

Loc: Detroit Michigan
Re: Stucked with the collimation final steps... new [Re: Jason D]
      #5617257 - 01/11/13 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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Jason D
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Re: Stucked with the collimation final steps... new [Re: Galicapernistein]
      #5617323 - 01/11/13 01:56 PM

Quote:

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?



Rotating and tilting the secondary mirror are NOT orthogonal movements. Tilting the secondary mirror does have a “rotation” component to the movement. Therefore, it is possible to rotate the mirror by a small amount them negate that rotation with a tilt. I know this is somewhat unintuitive.
In the attachment, both frames represent mirror positions that meet axial alignment requirements. That is, in each frame the laser beam will hit the primary mirror center then retrace its path to the source. One frame involves a rotate and tilt adjustment.



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Galicapernistein
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Reged: 09/24/07

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Re: Stucked with the collimation final steps... new [Re: Jason D]
      #5617346 - 01/11/13 02:12 PM

So a sight tube should be used to properly place the secondary, and then the laser collimator used afterward to fine tune the alignment, correct? And any fine tuning should only require the smallest of tweaks I would think.

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Jason D
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Re: Stucked with the collimation final steps... new [Re: Galicapernistein]
      #5617365 - 01/11/13 02:27 PM

It is an iterative process. Starting with positioning the secondary mirror is a good idea. If you have a sight-tube with cross hairs then you can continue with the sight-tube/cheshire combo tool. Of course, you can also use a laser collimator with a plain sight tube.

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Galicapernistein
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Reged: 09/24/07

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Re: Stucked with the collimation final steps... new [Re: Jason D]
      #5617412 - 01/11/13 02:58 PM

Quote:

It is an iterative process. Starting with positioning the secondary mirror is a good idea. If you have a sight-tube with cross hairs then you can continue with the sight-tube/cheshire combo tool. Of course, you can also use a laser collimator with a plain sight tube.






Sounds good. Thanks.


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Galicapernistein]
      #5621150 - 01/13/13 04:22 PM Attachment (19 downloads)

Ok, this is my final result. Still some error in the placemente of the secondary. I think my axial alignments are whithin the tolerance limits, but I get a nice start test in intra focus that shows a very good collimation and an ugly collimation in extra focus. I have always had this issue with my reflectors, but I thought it was because I had a bad collimation. But Iīm sure now my collimation is at least good but I canīt fix this issue. I tested the focuser once with Howie Glatter laser + tublug and this is not a focuser slop issue. Iīm sure this is seriously affecting the shape of the stars. I have to say Iīm quite dissapointed with this.

Edited by Javier1978 (01/13/13 04:27 PM)


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Jason D
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Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5621235 - 01/13/13 05:00 PM

As far as collimation, I would advise you to stop. Your collimation looks good.

With respect to the star test, what you have described seems to be consistent with a spherical aberration – undercorrection to be exact. I do not know what type of reflector you have but it is common for mass produced primary mirrors to have some degree of spherical aberration. Having spherical aberration does NOT necessarily translate to bad images at the eyepiece.

Bear in mind if your intra/extra tests look perfect and identical then you mirror is in the upper 90s% strehl. That is a premium range.

Jason


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Jason D
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Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5621241 - 01/13/13 05:05 PM

Quote:

an ugly collimation in extra focus



I assumed by "ugly" you meant a defocused star image with poorly defined rings (poor contrast) -- yet the unfocused star is circular and the rings are concentric, correct?
Jason


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Jason D]
      #5621334 - 01/13/13 05:55 PM

Jason, Iīm very happy with my collimation now and Iīll follow your advice about stopping here.

I own a sky watcher 8" dob and I assume it wonīt be a perfect mirror. But Iīm pretty concerned about this issue since I asume I could get somehow nicer stars.

When I defocus in extra focus a star, it will remain round but the shadow of the secondary will move toward an edge and the rings wonīt remain concentrics. It looks good in intra focus though. Aside from this, seems like a very nice optic and it have very nice contrast. I have tested its resolution with the moon (Plato craterlets or Catena Davy) and some close multiples systems like Nu Scorpi or Hn40 in trifid and it worked great. Yet the stars shape is somehow unaesthetic.

Doesnīt make sense?

Edited by Javier1978 (01/13/13 06:41 PM)


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Vic Menard
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Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5621395 - 01/13/13 06:29 PM

If the shadow of the secondary mirror is visible, it's possible you're seeing the secondary mirror offset, clipping caused by the primary mirror axis offset and/or clipping caused by the secondary mirror (the latter two would be more obvious outside of the focal plane).

As long as the infocus star shape and diffraction pattern is good, I wouldn't worry too much about the out of focus star shape.

Also remember that a cooling mirror tends to pull toward over correction (soft outer diffraction ring inside focus, hard outer diffraction ring outside focus). In both cases, the test star should show a few diffraction rings and the silhouette of the secondary mirror and spider should not be visible.

Nu Scorpi AB at 1.5 arcseconds shouldn't be too tough from your southerly locale (IIRC, CD is 2.3). But it is setting pretty early this time of year, so your optics can't be too bad.

That said... you still look like you have a small, but persistent, tilt/rotation error. But it's so small it is certainly insignificant and should not have any impact on your in focus image performance.


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Vic Menard]
      #5621460 - 01/13/13 07:05 PM

Jason and Vic, seems like Iīm not doing things right with the star test, maybe not enough power or going to far out of focus and starting to see the secondary shadow and the spider vanes at some point.

Iīll try to read a bit about star testing to before doing evaluations.

Thanks to everyone!

Edited by Javier1978 (01/13/13 07:05 PM)


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5649397 - 01/28/13 10:29 PM Attachment (11 downloads)

Upgrade.

I received my chesire/sightube a few days ago, and I certainly donīt like it. My main concern is that is long and heavy and wonīt go deep into the focuser. I put some tape to prevent play, but its weight produces flexure, I can tell. This make it quite inaccurate. Besides, it wonīt be useful to center the secondary under the focuser because there is a huge gap between the secondary and focuser edges and I canīt see the primary clips. I guess this is the wrong lenght for my scope.

I also have a hard time when focusing the crosshair to perform the FAA and I donīt like the reflection that produces to perform the primary axial collimation, every thing is so tiny and confusing!

Anyway, while waiting it to arrive I made some last tweeks to the mechanicalls alignments and I improved my collimation cap with a shiny surface. Now is much easier to collimate the primary.

This is how my final collimation looks.


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5649399 - 01/28/13 10:30 PM Attachment (12 downloads)

And this is how it looks the secondary after performing axial collimations. I quite happy with the home made tools now!

Edited by Javier1978 (01/28/13 10:41 PM)


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Atl
professor emeritus
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Reged: 04/13/12

Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5650081 - 01/29/13 11:20 AM

I have the same issue with a Celestron cheshire sight tube. I cannot see the primary clips no matter where the tube is positioned...in ...out...it only shows a small area around the center spot of the primary and not the whole reflection. I think this tool is designed for small 4 to 6" telescopes as it is an 1 1/4" size...not 2" in diameter. It also wiggles a lot in the focuser when you pull it out. The instructions don't even show what a fully collimated telescope should look like through the tool...they just show what it looks like uncollimated...everyone already knows what that looks like...lol. It works alright with my 6" Meade reflector, but with my 12.5" it is a wash. It does show when the secondary is basically under the focuser, but for actual collimation it is a poor tool for the job. I have an Astrosystems autocollimator. I have taken to using that for the bulk of the work. I know this is not optimal, but it is what I have. I think next week I will buy an Astrosystems 2" sight tube. That should help.

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Vic Menard
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Reged: 07/21/04

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Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5650318 - 01/29/13 01:23 PM

Quote:

...I received my chesire/sightube a few days ago, and I certainly don't like it. My main concern is that is long and heavy and wonīt go deep into the focuser. I put some tape to prevent play, but its weight produces flexure, I can tell. This make it quite inaccurate. Besides, it wonīt be useful to center the secondary under the focuser because there is a huge gap between the secondary and focuser edges and I canīt see the primary clips. I guess this is the wrong lenght for my scope.



The optimal length of the sight tube approximates the focal ratio of the primary mirror. You can find more here (scroll down about half way). When the sight tube is too long, you encounter this scenario:
1.) When the sight tube is pushed in to far, you can't see the edge of the primary mirror
2.) When the sight tube is pulled out to see the edge of the primary mirror, the apparent size of the secondary mirror is reduced effectively occulting the edge of the primary mirror.
The solution is a shorter sight tube (perhaps a bit less than the primary mirror focal ratio).
FYI, the sight tube focal ratio is equal to the distance from the pupil (top) to the bottom of the sight tube divided by the aperture at the bottom of the sight tube.

That said, the registration problem (sloppy fit) you've described indicates a problem with the sight tube, 2- to 1.25-inch adapter, and/or the focuser drawtube.

Quote:

I also have a hard time when focusing the crosshair to perform the FAA and I donīt like the reflection that produces to perform the primary axial collimation, every thing is so tiny and confusing!



Some people find that if they pull their eye away from the sight tube pupil, the sight tube cross hairs become more distinct relative to the primary mirror center spot. With regards to "tiny and confusing"--everything's the same size as it is in your collimation cap, except the field of view is narrower. There's certainly more going on with the sight tube cross hairs overlapping the field of view, but the fact that you can "see" errors that aren't obvious in a collimation cap is the reason why sight tube collimation is important.

Quote:

Anyway, while waiting it to arrive I made some last tweeks to the mechanicalls alignments and I improved my collimation cap with a shiny surface. Now is much easier to collimate the primary.

This is how my final collimation looks.



Why did you cover the primary in the second image?


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Vic Menard
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Re: Stucked with the collimation final steps... new [Re: Atl]
      #5650362 - 01/29/13 01:47 PM

Quote:

I have the same issue with a Celestron cheshire sight tube. I cannot see the primary clips no matter where the tube is positioned...in ...out...it only shows a small area around the center spot of the primary and not the whole reflection.



What's the focal ratio of the sight tube compared to your primary mirror?

Quote:

I think this tool is designed for small 4 to 6" telescopes as it is an 1 1/4" size...not 2" in diameter.



It's the focal ratio (length to aperture) that defines usability. The fact that most smaller aperture Newtonians have long focal ratios could make your assessment true, depending on the sight tube focal ratio...

Quote:

It also wiggles a lot in the focuser when you pull it out.



When possible, you should fully insert the sight tube and then rack the focuser out to minimize registration errors. If you still can't see the outer edge of the primary mirror (and the primary mirror clips), you can still align the focuser axis by adjusting the secondary mirror to bring the sight tube cross hairs into alignment with the primary mirror center spot. When that alignment is correct, even if you can't see them, the primary mirror clips will be aligned to the sight tube. If you can see the real edge of the secondary mirror (light from the primary mirror should be flooding the entire face of the secondary mirror if you can't see any mirror clips) at the same time and it appears concentric with the bottom edge of the sight tube, then your secondary mirror placement and focuser axial alignment are both good.

Quote:

...I have an Astrosystems autocollimator. I have taken to using that for the bulk of the work. I know this is not optimal, but it is what I have. I think next week I will buy an Astrosystems 2" sight tube. That should help.



Well, 2-inch tools do eliminate the need for a 2- to 1.25-inch adapter, which is a common source of registration issues.
Do you have the 2-inch AstroSystems autocollimator?
Have you looked at the CatsEye TeleCat XLS combo tool?


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Vic Menard]
      #5650487 - 01/29/13 02:54 PM

Quote:

That said, the registration problem (sloppy fit) you've described indicates a problem with the sight tube, 2- to 1.25-inch adapter, and/or the focuser drawtube.




My focuser and 1,25 adapter are rock solid, no slope at all. As said before, I tested it with the tublug reflection racking in and out and it has no slope at all. The problem is that this is a heavy and long device and even with tapes to prevent play, if put it and pusch up a little bit the reflection will move. But if you are pointing as a problem that the hole of the 1,25 adapter is oversized thatīs for sure.

Quote:

I also have a hard time when focusing the crosshair to perform the FAA and I donīt like the reflection that produces to perform the primary axial collimation, every thing is so tiny and confusing!



Some people find that if they pull their eye away from the sight tube pupil, the sight tube cross hairs become more distinct relative to the primary mirror center spot. With regards to "tiny and confusing"--everything's the same size as it is in your collimation cap, except the field of view is narrower. There's certainly more going on with the sight tube cross hairs overlapping the field of view, but the fact that you can "see" errors that aren't obvious in a collimation cap is the reason why sight tube collimation is important.




I did move out me eye to make focus, yet I didnīt find it accuarate because the image start to wander more if you take your eye from the peephole. As for the primary, the donut doesnīt seem to be a good figure to overlap. At some point it just hide a little bit and I donīt know any more whatīs going on, although Iīm quite sure I didnīt arrive to an accurate alignment. You are right about the size though. With my improved collimation cap I can see the hole into the donut, even I can see (its seen in the image) a tiny liquid paper point that shows the actual center of the mirror. And Iīm quite sure there is no possible slope with it.


Quote:

Anyway, while waiting it to arrive I made some last tweeks to the mechanicalls alignments and I improved my collimation cap with a shiny surface. Now is much easier to collimate the primary.

This is how my final collimation looks.



Why did you cover the primary in the second image?




I was trying to see the final shape of the secondary without any (to me at least) distracting reflection.

Thank you for the information, I will check your website later!!


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5650938 - 01/29/13 06:54 PM

If you pull your eye back from the peephole to focus on the crosshairs, use your hand to place your thumb on your face and your little finger on the tool. Your head will not bob back and forth as much.

As for star images, once you have achieved collimation, you have only taken care of one of the three "C"s: collimation.
The other two are : Cooling and Conditions.

Cooling. Never examine the quality of star images if the mirror hasn't cooled down. If you don't use a fan, the mirror should have been outside at least 3 hours. If you do use a fan, one hour. Because heat in front of the mirror will warp the star images and keep them from beina as sharp as the optics may allow.

Conditions: Seeing varies from night to night and hour to hour. Most places, seeing conditions are better in the midnight-to-dawn hours because turbulence in the atmosphere has settled down.
If the jet stream is overhead, if the weather map shows the pressure isobars close together, if a front is coming through, or if you live on the lee side of a mountain, you will not see good seeing and your star images will be blobs. Also, never examine a star that is only a few degrees above a roof because the heat rising from the roof will create poor seeing.

When you see good seeing and star images are tiny little pinpoints, THEN you can evaluate the mirror and see the level of optical quality there.

P.S. A 2" combination tool has a much larger field of view and avoids the adapter fitment problem.


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Starman1]
      #5651054 - 01/29/13 08:00 PM Attachment (14 downloads)

Thank you Don for the information about star testing and the hand tip.

Regarding the fit of the chesire eyepiece, maybe some images will clarify my thoughts. The 1,25 adpater is OK, I think the problem is that the portion of the chesire that goes into the eyepiece is too short compared to the other portion. This is, in my opinion, what cause a poor fitting.

Vic pointed the focal lenght of the eyepiece, but isnīt also important how much it goes into the eyepiece holder? I think if the proportion where the other way round (the mayor part of the chesire going into the eyepiece holder) I would be able to cover my secondary mirror.

Edited by Javier1978 (01/29/13 08:02 PM)


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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5651055 - 01/29/13 08:00 PM Attachment (14 downloads)

And into the EP holder.

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Hothersale
sage


Reged: 10/13/09

Loc: Victoria, BC
Re: Stucked with the collimation final steps... new [Re: Javier1978]
      #5654471 - 01/31/13 02:56 PM

You are supposed to insert it farther than that. Insert it all the way up to the shoulder of the tube. Basically as far as it will go.

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Javier1978
scholastic sledgehammer


Reged: 02/12/09

Re: Stucked with the collimation final steps... new [Re: Hothersale]
      #5654603 - 01/31/13 04:05 PM

This is as far as it will go, I would be happy to insert it farhter.

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