Greg,
Thanks for the tips. Very well written and documented. I live in NYC and star collimation is pretty close to impossible and I have been looking for an accurate treatment of the "artificial star" procedure. This should be printed in Sky & Telescope. BTW...if the ½" ball bearings are in a bearing raceway, you can use an awl or an icepick to first puncture and then pry off the raceway shield and then tap out the inner raceway collar with a hammer and drift. Secure the bearing assembly in a vise before proceding. Thanks again.

Frank


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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