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Laser Collimation

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#1 Z28500

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Posted 01 December 2012 - 03:53 AM

I just built a really nice refractor for my son, now I need to collimate it and have never done it before. Therefore I am thinking about buying a High Point 1-1/4" laser collimator. That one will do the trick right? I think all I need to do is get a 1-1/4" adapter for the 2" focuser.
Here's a picture of it without the dew shield and tripod. It's a Jaegers 103mm F/15, Alum 0.050" wall tube, turned on a lathe, with a 2 speed (able to collimate) Crawmach focuser, and collimatable Crawmach lens cell.
Soon it will even be in Astronomy Technology Today Magazine, I've already written the article and spoken to the editor. Just have to finish up a couple of little things on the tripod which has over 300 hand made parts!
Sure is a lot of fun as most of you guys already know.
Any collimation advice would be greatly appreciated.
Thanks,
Z

P.S. Do y'all's spell checkers dislike refractor, focuser, and collimate?

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#2 Mirzam

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Posted 01 December 2012 - 08:10 AM

All you really need is a cheshire eyepiece, although I would use a laser to check the squaring of the focuser wrt the objective lens.

http://spacealberta....r/collimate.htm

JimC

#3 Pinbout

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Posted 01 December 2012 - 08:18 AM

you have to collimate that laser. I have one and it needed to be collimated.

http://www.stark-lab.../llcc/llcc.html

#4 DAVIDG

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Posted 01 December 2012 - 11:20 AM

It's real easy to do. First make sure that your laser is collimated. Then you place it in the focuser and be sure there is little or no play. Rotate the laser in the focuser and look at the spot on the lens, it shouldn't move. If it does then shim the laser with tape or paper until it doesn't. Do worry that the laser spot is not centered on the lens, that is next, just that it doesn't move when you rotate the collimator. Your laser collimator is now centered in the focuser tube. Now adjust the tip and tilt of the focuser so the laser spot on the surface of the lens, is in the exact center. I use a paper mask with the center marked, to position it. Now the focuser is squared. Next look down into the lens, be careful not to look directly at the laser and you'll see a number of reflected spots hitting somewhere at the bottom of the tube and near the end of the laser. These spots are from the laser reflecting off the different surfaces of the elements in the objective. Adjust the objective tip/tilt of the cell so all the spots fall back on the hole in the laser collimator were the laser is shiny out from. Your lens is now perfectly collimated.

- Dave

#5 Mirzam

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Posted 01 December 2012 - 11:25 AM

Great explanation Dave! Never occurred to me to do it that way.

JimC

#6 Z28500

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Posted 01 December 2012 - 03:51 PM

Thanks for the advice. But what do you mean collimate the laser? Will it come with instructions perhaps?
Z

#7 Mirzam

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Posted 01 December 2012 - 04:41 PM

The beam of the laser may not be parallel to the body of the laser. High quality lasers, such as a Glatter laser, will work right out of the box. Lessor quality lasers may need adjustment. Typically, you firmly hold the laser in the crook of a piece of metal angle, point it towards a wall ~10 feet away, rotate the laser, and note whether the spot remains fixed or rotates in a circle. Use adjustment screws to tweak the beam holder until the spot does not move when the laser is rotated.

JimC

#8 Howie Glatter

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Posted 01 December 2012 - 06:11 PM

" . . Adjust the objective tip/tilt of the cell so all the spots fall back on the hole in the laser collimator were the laser is shiny out from. Your lens is now perfectly collimated."

Theoretically, this is true, but in practice the adjustment is problematical, and subject to errors exactly analogous to those of non-Barlowed laser collimation of a Newtonian primary. If the drawtube alignment with the objective has any error at all, it will be compounded when the objective is adjusted. Also, if the objective glass has any free lateral motion in its cell it will be impossible to get a consistent reflected beam impact.
On top of that, any centering or tip and tilt errors of the objective elements with respect to each other will show up as multiple spots reflected back even when the incident beam is perfectly on axis. If it is not, then even a perfect objective may reflect back multiple spots.

#9 Z28500

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Posted 02 December 2012 - 10:24 AM

Thanks everyone, next step is to order it.
Z

#10 m. allan noah

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Posted 02 December 2012 - 06:05 PM

Then you place it in the focuser and be sure there is little or no play. Rotate the laser in the focuser and look at the spot on the lens, it shouldn't move.


I'm not sure that's a valid test. If the laser axis is offset and also slightly out of parallel from the axis of its holder, you can have the laser appear to be collimated at a certain distance. At other distances, the spot will move.

allan

#11 Howie Glatter

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Posted 02 December 2012 - 06:23 PM

"If the laser axis is offset and also slightly out of parallel from the axis of its holder, you can have the laser appear to be collimated at a certain distance."

Thank you for pointing that out. It is an important point that is almost never considered in discussions here.
I pay close attention to it.

#12 Vic Menard

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Posted 02 December 2012 - 06:45 PM

"If the laser axis is offset and also slightly out of parallel from the axis of its holder, you can have the laser appear to be collimated at a certain distance."

Thank you for pointing that out. It is an important point that is almost never considered in discussions here.
I pay close attention to it.

Could you clarify this for me?
I'm aware of the possibility of precession when the laser is freely rotated in a cylindrical holder (focuser drawtube). But if the laser is secured each time before reading the alignment, wouldn't that minimize effects caused by both offset and precession? I understand that the conditions are not the same as those with your parallizer accessory, but I would expect the possibility of observing a consistent read due to equal but opposite offset and tilt errors canceling each other out unlikely if the laser is carefully secured in the drawtube before each read. Am I missing something?

#13 Mirzam

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Posted 02 December 2012 - 07:09 PM

Dave did say "first make sure the laser is collimated".

JimC

#14 m. allan noah

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Posted 02 December 2012 - 07:14 PM

Dave did say "first make sure the laser is collimated".


Of course. But how many people using a 1.25 inch laser check its collimation in their 2 inch adapter?

allan

#15 Vic Menard

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Posted 02 December 2012 - 07:44 PM

Dave did say "first make sure the laser is collimated".


Of course. But how many people using a 1.25 inch laser check its collimation in their 2 inch adapter?

If the user's high-magnification eyepieces will require the 2-inch adapter, I would hope the user at least verifies that the adapter is capable of maintaining the alignment tolerances suitable for the observing/imaging application. If, OTOH, the user's eyepieces/imaging equipment are all 2-inch, or a 2-inch Paracorr/coma corrector is always used for observing/imaging--then the user should absolutely verify the alignment of the adapter/laser in the drawtube--and if a significant alignment error can't be easily resolved, perhaps upgrading the adapter and/or laser would be an sensible solution.

#16 Howie Glatter

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Posted 02 December 2012 - 08:20 PM

Hi Vic,

If the laser beam exits the collimator body off-center, the beam can still be adjusted to stay motionless at some particular distance, as the collimator is rotated on axis. The beam will be tracing two narrow tip-to-tip cones in the air.
This is usually caused by the maker not paying attention to centering the laser module aperture within the collimator body. If you are testing or adjusting a collimator by rotating it, it is good to check laser centering in the collimator body by observing the beam impact on a piece of paper held just in front of the aperture.


#17 Vic Menard

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Posted 02 December 2012 - 08:46 PM

Hi Howie,

Are we talking about a 1.25-inch laser in a sloppy adapter, or a 2-inch laser that's poorly made? If it's a 1.25-inch laser with an internal tilt error, it seems to me that the error should become evident with various rotations of the laser in the adapter and the adapter in the drawtube. If it's a 2-inch laser with such an error (and the error is significant enough to impact optical performance), the problem should become evident when the laser is Barlowed or at extreme distances of focuser travel with an unBarlowed laser.

It seems to me that such unusual circumstances could be much more easily resolved using a redundant axial tool (or two)?

#18 Howie Glatter

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Posted 02 December 2012 - 09:20 PM

Hi Vic,

I was responding to M. Allan Noah, where he said "If the laser axis is offset and also slightly out of parallel from the axis of its holder, you can have the laser appear to be collimated at a certain distance. At other distances, the spot will move."

I took him to mean collimator body by "holder", so I thought he was referring to laser module decentering within the collimator body, and I went from there. Maybe I am like a blind guy with a stick trying to describe the thing in the middle of the room.

#19 Vic Menard

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Posted 02 December 2012 - 09:34 PM

Howie,
I suspect I'm the blind one. Since I use a Glatter laser, I've come to expect a certain precision...

While I understand it would be possible (in fact, probable) for someone to inadvertently misalign the internal laser tilt to accommodate an internal laser offset--I also routinely suggest redundant axial verification.

FWIW, if a user is going to rely on the veracity of a laser--and the tolerances are very small--that user should either invest in a precision tool, or be ready to validate the precision himself.

#20 Z28500

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Posted 02 December 2012 - 11:02 PM

I think some very good points have been brought up. Certainly common sense would have close tolerances between the 1-1/2" to 2" adapter. Kinda makes me want to fabricate my own adapter.(Turn my own adapter) I hadn't really thought of that before. It's probably a good idea.
Z

#21 m. allan noah

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Posted 03 December 2012 - 09:10 AM

I was intentionally vague about the sources of this particular misalignment, specifically because there are multiple sources, including some I've likely not thought of :)

In any case, my only point was that the use of a laser (like any collimation tool) requires careful consideration of the geometries involved. I daresay everyone involved in this conversation takes that as a given, so please consider the comment to have been directed at our silent audience ;)

allan

#22 Vic Menard

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Posted 03 December 2012 - 02:23 PM

...In any case, my only point was that the use of a laser (like any collimation tool) requires careful consideration of the geometries involved. I daresay everyone involved in this conversation takes that as a given, so please consider the comment to have been directed at our silent audience ;)

I'm still pondering the offset laser scenario.

It seems to me that such a laser, intentionally collimated with an internal tilt error to compensate for the offset, might appear to be collimated relative to the focuser axial signature (laser beam is centered in/on the primary mirror center spot) through rotation in the focuser, but will fail the primary mirror axial signature (the unBarlowed return beam coincides with the outgoing beam and falls inside the laser aperture) when rotated.

If the laser aperture is offset relative to the laser body by more than a hundredth of an inch or so, and the Barlowed accessory is centered on the laser aperture, I would expect the offset to still be relatively obvious. If, OTOH, the Barlow target is centered on an accessory (like the Blug or on an actual Barlow), I suspect the offset error will be significantly reduced.

Now, if the laser is offset and the 2- to 1.25-inch adapter is also offset, and the two parts are allowed to rotate independently of each other in the focuser drawtube, I can't imagine a simple tilt error solution that will deliver consistent alignment of the outgoing beam or the return beam. A redundant axial tool would seem to be the best solution (and a replacement adapter!)

FTR, the OP's 4-inch f/15 refractor is likely to be quite forgiving when dealing with axial collimation. I usually collimate refractors with a windowed Cheshire eyepiece, verifying the stacked reflections through the focuser travel (I use a handheld fluorescent light to illuminate the Cheshire). I've always found this relatively simple alignment procedure to deliver excellent performance.






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