Greg:

Thanks for the informative and detailed description of your collimation
effort. I think I'll go back and find that Christmas tree ornament I hung
in the neighbor's tree about a year ago and recheck my collimation!

Gregg L. Ruppel

Visit my astronomy site at:
http://members.aol.com/gruppel973
----- Original Message -----
Sent: Friday, June 18, 1999 8:22 AM
Subject: [M]: Collimation via an artificial star


>
> After seeing some recent discussion on SCT collimation using an
> artificial star, I decided to expend some effort to determine how best to
> accomplish this using materials and simple tools available to us all. My
> decision to pursue this was aided by my recent observations of Mars: I was
> none too impressed with the level of detail I was able to see on the
> planet and wanted to see if tweaking the collimation would improve the
> view. Since I never seemed to have seeing good enough to collimate at very
> high powers, it was likely I could improve the view.
>
> The first step was to go to Radio Shack, where I bought a high
> powered laser pointer. This was a fairly expensive item (catalog number
> 63-1053: Undeline Laser Pointer) since it is advertized to "project a
> precise point of laser light up to 400 yards away". I chose this laser
> because I was interested in setting up a target hundreds of feet away and
> wanted to have a fairly intense beam even at that distance. Reports
> indicate that using a small ball bearing, it's quite possible to set up an
> artificial star much closer than the above mentioned distance. I was
> curious to see if good collimation could be performed using a target with
> a larger radius of curvature than the previously reported (and seemingly
> ideal) 1/4" ball bearing. Since I would perforce be using a surface with
> larger radius of curvature than that, I chose to generate a suitably small
> artificial star by moving my target farther away.
>
> For a target, I found a 1 1/4" spherical metal doorknob at a hardware
> store that I decided was an object easily enough found by others as to be
> worth testing. I mounted this inside a small can whose interior was
> painted flat black. The can and target were then placed about 250' away. I
> first wanted to see if I could consistently hit the target with the laser.
> I doubted it would work, but I wanted to try the simplest scheme first: I
> mounted the laser on an angle bracket bolted to a camera tripod. With my
> wife observing with binoculars, I proceeded to try to hit the target. What
> ensued was quite amusing to both of us as stiction in the camera tripod
> mounting head prevented any precise targeting of the laser. In addition, I
> discovered what was obvious in retrospect: The laser spot was invisible
> when striking the flat black paint in the target can. (Well, what did you
> think "flat" meant, dummy? Very clever of me...) Anyway, Libby and I got a
> good chuckle out of all of this and I expended much more effort than was
> warranted trying to hit that darned target, even after it became obvious
> this would never work (which probably took about 2 minutes.) Proceed to
> Plan B...
>
> I expected from the first that I was going to have to go this route:
> The optimum way to aim the laser at the target is to mount the laser on
> the OTA itself. This was actually quite a straightforward operation.
> Obviously, what was needed was to provide suitable mounting rings exactly
> as used on a finder scope. I can center a finder in seconds, so it should
> be a piece of cake to aim the laser while looking thru the scope.
>
> My first thought was to look at the finder dovetail block to see what
> could be adapted for my requirements. It turns out that on my scope (a 10"
> f/6.3 LX-200), the dovetail block is held to the finder mounting rings via
> a pair of 8-32 cap screws. Hmm. That was convenient since I happened to
> already have an 8-32 tap. For that reason, I used 8-32 hardware
> throughout.
>
> The baseplate for the laser mount was made from a piece of scrap
> aluminum, 3 3/4" long, 3/4" wide, and 1/8" thick. I first drilled and
> tapped this so it could be mounted on the existing dovetail block from the
> LX-200 finder. The mounting rings were made from PVC pipe picked up at the
> hardware store in a size appropriate for the barrel of my laser. In my
> case, the PVC is approx 7/8" ID. In addition, I found some other plastic
> tubing in the toilet repair section of the hardware store that was a
> perfect slip fit over the barrel of the laser (which has an OD of approx
> 1/2"). I used short 5/8" sections of this tubing to protect the outer
> barrel of the laser from the adjusting screws that were to go in the
> mounting rings. To mount the rings on the baseplate, I drilled and
> carefully tapped the 5/8" long sections of PVC for 8-32 truss head screws.
> I then cut off the screws (using the cut-off wheel in my Dremel tool) to
> the length needed to be flush on the inside of the PVC rings when they
> were mounted on the aluminum baseplate. Each PVC ring was then carefully
> drilled and tapped for three 8-32 cap screws spaced equally around the
> periphery. I used cap screws of 1" length and jammed knurled nuts against
> the bottom of the cap to provide a handy surface for easy finger
> adjustment.
>
> That was it. A piece of cake to fabricate and the mounting only cost
> a few dollars in parts. I used a miraculuous, high-tech, super-deluxe,
> spring-loaded clamp to hold shut the momentarily-closed switch on the
> laser: Most would call it a clothes pin but I hate to disillusion readers
> with such a mundane designation for this piece of precision equipment.
>
> After my previous experience, I was a little more clever in my target
> choice. (The only way to go was up...) This time, I mounted the doorknob
> on a stake with a piece of corrugated cardboard, roughly 18"x24" as a
> background. I mounted the laser on the OTA and placed the target approx
> 150' away. All of my viewing was done *without* a diagonal in place. I
> spent a few seconds trying to hit the piece of cardboard with the laser
> while viewing with my 28 mm EP. I wasn't going to waste much time at that:
> When I didn't see it very quickly, I moved to a target in the garden much
> closer to get a preliminary alignment of the laser. That was easy on a
> target that was maybe 40' away. When I moved back to the real target, it
> was very easy to get the laser beam precisely centered on the target and
> centered in the FOV of the EP. I quickly ran up thru higher and higher
> powered EPs 'til I was maxed out with my available optics: A 6.4 mm Meade
> Series 4000 Super Plossl and the Meade 126 barlow (nominally 500x). At
> each change of EP, I recentered the generated artificial star using the
> adjustments on the laser mounting rings to make sure I was centered on the
> target and moving the scope to precisely center the image in the FOV.
> (Sounds complicated but is really quite trivial to do.) For my scope, even
> at the nominal 500x, there was only a very slight asymmetry to the very
> slightly out-of-focus diffraction rings. Having never collimated at such
> high magnification before, I very quickly discovered how incredibly
> sensitive the image is to tweaks of the collimation adjustment screws.
> After a little fussing around overshooting and making things worse, I had
> the diffraction rings perfectly symmetric at this magnification. Boy, that
> sure was easy.
>
> BTW, I was doing this procedure in the late afternoon to early
> evening when the atmosphere is typically pretty ugly. It didn't really
> matter much to the diffraction rings. (I was working over a grassy
> surface, of course.) There just wasn't enough of the turbulent atmosphere
> along my baseline to cause problems at the maximum magnification I was
> using. I could actually see some distortion caused by the winds (fairly
> gusty when I was doing this) but it was easy to work around.
>
> So... Was it worth buying the laser for collimation? Nope. Turns out
> that at every step, it was actually easier to use the reflection of the
> sun on the target. The diffraction rings were much brighter and more
> clearly defined. For that matter, the solar reflection was actually almost
> too bright to use. Movement of the sun-generated artificial star (which
> might be a problem when you use, say, a car bumper for a target) was
> negligible because of the small target.
>
> Final recommendation: Unless you have special circumstances where it
> might be desireable to collimate often under conditions when the sun is
> not visible (maybe you live in Seattle), save the $$$ and set up a target
> in the sun. Collimation at the highest magnification you have available is
> really easy this way. For those that have never done it at high
> magnification before, keep in mind that the image is *very* sensitive to
> small tweaks of the adjustment screws on the secondary!
>
> Addendum: I've acquired ball bearings that are 1/2" in diameter and
> I'm going to experiment with them, too, as soon as I figure a way to get
> them out of their cage. I don't expect any change in the conclusions
> reached here, though. The target I used, even though 1 1/4" in diameter,
> clearly produced an artificial star at 150' that was adequate for
> collimation.
>
> Greg Hartke
> Sykesville, MD
>
> BTW, it didn't do a thing for my viewing of Mars. Bad seeing is bad
> seeing. I rather expected that but, hey, I've gotta find some good seeing
> eventually...
>

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