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Collimation final steps.

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#26 Jason D

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Posted 11 January 2013 - 01:56 PM

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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#27 Galicapernistein

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Posted 11 January 2013 - 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.

#28 Jason D

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Posted 11 January 2013 - 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.

#29 Galicapernistein

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Posted 11 January 2013 - 02:58 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.


Sounds good. Thanks.

#30 Javier1978

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Posted 13 January 2013 - 04:22 PM

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 I´m sure now my collimation is at least good but I can´t fix this issue. I tested the focuser once with Howie Glatter laser + tublug and this is not a focuser slop issue. I´m sure this is seriously affecting the shape of the stars. I have to say I´m quite dissapointed with this.

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#31 Jason D

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Posted 13 January 2013 - 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

#32 Jason D

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Posted 13 January 2013 - 05:05 PM

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

#33 Javier1978

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Posted 13 January 2013 - 05:55 PM

Jason, I´m very happy with my collimation now and I´ll follow your advice about stopping here.

I own a sky watcher 8" dob and I assume it won´t be a perfect mirror. But I´m 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 won´t 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.

Doesn´t make sense?

#34 Vic Menard

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Posted 13 January 2013 - 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.

#35 Javier1978

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Posted 13 January 2013 - 07:05 PM

Jason and Vic, seems like I´m 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.

I´ll try to read a bit about star testing to before doing evaluations.

Thanks to everyone!

#36 Javier1978

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Posted 28 January 2013 - 10:29 PM

Upgrade.

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 won´t 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 won´t be useful to center the secondary under the focuser because there is a huge gap between the secondary and focuser edges and I can´t 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 don´t 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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#37 Javier1978

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Posted 28 January 2013 - 10:30 PM

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

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#38 Atl

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Posted 29 January 2013 - 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.

#39 Vic Menard

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Posted 29 January 2013 - 01:23 PM

...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 won´t 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 won´t be useful to center the secondary under the focuser because there is a huge gap between the secondary and focuser edges and I can´t 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.

I also have a hard time when focusing the crosshair to perform the FAA and I don´t 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.

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?

#40 Vic Menard

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Posted 29 January 2013 - 01:47 PM

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?

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...

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.

...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?

#41 Javier1978

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Posted 29 January 2013 - 02:54 PM

[quote name="Vic Menard"]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]

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 that´s for sure.

[quote]I also have a hard time when focusing the crosshair to perform the FAA and I don´t like the reflection that produces to perform the primary axial collimation, every thing is so tiny and confusing![/quote]
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]

I did move out me eye to make focus, yet I didn´t find it accuarate because the image start to wander more if you take your eye from the peephole. As for the primary, the donut doesn´t seem to be a good figure to overlap. At some point it just hide a little bit and I don´t know any more what´s going on, although I´m quite sure I didn´t 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 I´m 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. [/quote]
Why did you cover the primary in the second image? [/quote]

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!!

#42 Starman1

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Posted 29 January 2013 - 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.

#43 Javier1978

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Posted 29 January 2013 - 08:00 PM

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 isn´t 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.

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#44 Javier1978

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Posted 29 January 2013 - 08:00 PM

And into the EP holder.

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#45 Hothersale

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Posted 31 January 2013 - 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.

#46 Javier1978

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Posted 31 January 2013 - 04:05 PM

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






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