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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
Postmaster
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Reged: 05/22/12

Loc: Denver, CO
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
Markus
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Reged: 12/26/04

Loc: Ontario, Canada
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
Post Laureate
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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
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
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
Post Laureate
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Reged: 07/21/04

Loc: Bradenton, FL
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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Reged: 07/21/04

Loc: Bradenton, FL
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

Loc: California
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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