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Equipment Discussions >> Reflectors

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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: 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.

Ill 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.

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


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Galicapernistein
super member


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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Reged: 10/21/06

Loc: California
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
super member


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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Reged: 10/21/06

Loc: California
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
super member


Reged: 09/24/07

Loc: Detroit Michigan
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
Postmaster
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Reged: 10/21/06

Loc: California
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
super member


Reged: 09/24/07

Loc: Detroit Michigan
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 Im sure now my collimation is at least good but I cant fix this issue. I tested the focuser once with Howie Glatter laser + tublug and this is not a focuser slop issue. Im sure this is seriously affecting the shape of the stars. I have to say Im quite dissapointed with this.

Edited by Javier1978 (01/13/13 04:27 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]
      #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
Postmaster
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Reged: 10/21/06

Loc: California
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, Im very happy with my collimation now and Ill follow your advice about stopping here.

I own a sky watcher 8" dob and I assume it wont be a perfect mirror. But Im 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 wont 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.

Doesnt make sense?

Edited by Javier1978 (01/13/13 06:41 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]
      #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 Im 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.

Ill 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 dont like it. My main concern is that is long and heavy and wont 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 wont be useful to center the secondary under the focuser because there is a huge gap between the secondary and focuser edges and I cant 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 dont 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

Loc: Bradenton, FL
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 wont 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 wont be useful to center the secondary under the focuser because there is a huge gap between the secondary and focuser edges and I cant 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 dont 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
Post Laureate
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Reged: 07/21/04

Loc: Bradenton, FL
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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