University Lowbrow Astronomers

It’s Cool to be Square!

by Doug Scobel
Printed in Reflections: April, 2003.

When it comes to collimation of your Newtonian reflector, anyway.  When all your optical elements are square to each other, or in other words, are collimated, then your telescope is producing images as good as it can deliver, and life is good.  But if they are misaligned, even a little, then your scope isn’t living up to its capability.

But this is easier said than done.  Having just finished a little refurbishment on my 13” homemade Dobsonian, including replacement of the focuser (requiring re-alignment of everything), I thought I’d pass on a few tips I discovered during the process.

First, you have to start with the focuser.  If the focuser is not square to the tube, then you will never be able to fully collimate the optical path.  The easiest way to do this is with a laser collimator.  First, remove the secondary mirror, sometimes referred to as the diagonal mirror.  Insert the laser collimator, and turn it on.  Measure the distance between the projected beam on the opposite side of the tube and the end of the tube.  Adjust the tilt of the focuser until that distance matches the distance from the end of the tube to the center of the focuser.  Some focusers have adjusting push-pull screws for adjusting the tilt.  If yours does not then you’ll have to use shims of some sort.

Next, ensure that your spider is exactly centered in the tube.  If you are offsetting the diagonal (which is usually done only in fast focal ratio, large aperture Newtonians), then offset it away from the focuser by the offset amount.  Whether your diagonal is going to be offset or not, it must be exactly centered laterally, or side-to-side.  While you are at it, make sure that you have adequate tension in the spider vanes.  Just like spokes in a bicycle wheel, the vanes need to have tension in them to maintain the spider’s position, and to provide sufficient rigidity to support the secondary mirror.

Now clamp up a threaded rod in the central hole in the spider.  The beam from the laser collimator should strike it exactly along the center line of the threaded rod.  If it does not, then adjust the tilt of the focuser up and down until it does.  Making this adjustment may upset the longitudinal adjustment you made earlier.  You may have to go back and forth between the two adjustments until they are both correct.

Now that your focuser is exactly square to the tube, it’s on to the secondary mirror.  This is where things get tricky.  Should you offset the diagonal, or should you not?  Should you mark the center of the primary, or should you not?  What about marking the center of the diagonal?  In my scope, I indeed have offset the diagonal, and marked the center of the primary mirror, and marked the offset center of the secondary.  Being a 13.1” f/4.5, I felt that offsetting the diagonal was worth the trouble.  Actually, I think that in a fast system, offsetting the secondary mirror makes its alignment easier, rather than more difficult.  Unfortunately, there’s too much for me to discuss here regarding diagonal offset, marking your mirrors, etc., (maybe a future article?), so I’ll simply describe the procedure with a scope set up like mine.

First, ensure that your primary mirror is installed, and that it is exactly centered in the tube.  Place a Cheshire eyepiece in the focuser.  When sighting through the peephole, adjust the secondary until the dot on the secondary is lined up with the center mark on the primary.  Now adjust the tilt of the secondary until the primary mirror is centered in the outline of the secondary.  If the center dot on the secondary no longer lines up with the center mark on the primary, then you will probably need to adjust the position of the secondary longitudinally in the tube.  If it is off laterally, then either your spider is not centered in the tube, or your focuser is not square, or both.  Again, you’ll probably have to go back and forth between the two adjustments a few times until you get it all just right.

Once you are satisfied with the alignment of the focuser and the secondary mirror, you are most of the way home.  All that remains now is adjusting the tilt of the primary mirror.  Simply adjust its tilt until the reflection of the peephole is exactly centered on the center mark on the primary.  That’s all there is to it!

Now this is about as good as you can do it on the bench.  To get it “perfect,” you’ll need to get it out under the stars.  Point your scope at a star, and put in a high power eyepiece.  I like to use Polaris, because it’s always in the sky (for those of us here in North America, anyway), and it won’t move on you, even at high power.  Center the star in the field, and then purposely de-focus it.  The image should expand to a disk, and the shadow of the secondary mirror should be visible in the center.  If everything is aligned correctly, then the secondary’s shadow should be offset away from the direction of the focuser just a tad (it should be exactly centered with a non-offset secondary).  If it is not, then tweak the primary until it is.  You should only have to make minor adjustments at this point.  If it is a significant amount off, then you have something misaligned further upstream, and you’ll have to go back and double check your other adjustments.

While you’re at it, you should now align your finder(s) so that Polaris is centered in them, too.  This will make finding things a lot easier.

There!  You’re collimated!  And you thought that it wasn’t cool to be square!


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