Daytime Telescope Optics Alignment Procedure

(By Michael W. Conte)

 

Disclaimer : Though I developed this method through sheer blind luck and trial and error, I take no credit for "inventing" it. I’m sure other people have other methods that are similar or even better. Some of the illustrations were stolen outright from Thierry LEGAULT's home page (http://perso.club-internet.fr/legault/index.html : EXCELLENT SITE!)

Okay, now that the "legal" stuff is done....

 

Overview: The method that I use to align my telescopes during the daytime is to provide a substitute star utilizing the reflection of the Sun off a convex object. While this does not provide a "pure" star point image (due to imperfections in the reflecting medium), it has proven adequate enough to enable me to achieve an alignment accurate enough so that all that is needed (at most) is a very small fine tuning of the telescope prior to use. Since my primary instrument (10" LX200) is permanently mounted, I find that I only need to touch up the alignment once or twice a year. However, for use with portable telescopes, this method works well. If you have a Newtonian telescope, the laser collimator systems works very well, but this method will too! It’s biggest advantage I’ve found is that I don’t have to contend with poor seeing conditions and trying to chase a damn star all over the sky because I’ve "adjusted it" out of my field of view!

Some observers contend that it’s best to align the optics with whatever accessories you’ll have in the optical path during normal use. This primarily means a star diagonal. This is up to you. I normally align my scope straight through, no focal reducer, nada.

Materials needed : One (1) ball bearing, metallic, shiny and ¼" in diameter or larger.

Wooden shim or a piece of wood about 1" wide, 12" long and ¼" thick.

In an emergency, I have used any available item that is convex and reflective i.e., Christmas tree ornaments, Baby Moon Hubcaps, the reflective insulators on telephone poles, etc.

Finger nail polish (your choice of colors).

Construction Details : Drill hole slightly smaller in diameter then the ball bearing in the wood. Stick bearing in hole (that was simple).

 

Alignment Procedure :

See Fig.1 for the details of construction of the sophisticated, precision Optical Alignment Device.

Shim.jpg (24727 bytes)

 

In Fig. 2 are the detailed instructions for configuring the device in actual use.

Principle.jpg (11321 bytes)

 

Seriously, all I do is stick the ball bearing shim in the ground about 50 to 100 ft. away from the telescope (make sure it’s in the sunlight), aim the telescope at it, load up a high power eyepiece (250X or better) and focus. What you won’t see is something similar to Fig. 3, a nice textbook image of stellar diffraction rings.

Diffraction rings.jpg (7530 bytes)

Because the ball bearing (or what ever you’re using) is not an optical quality surface, you won’t get optical quality results. What I normally see is a "spiky" image, with the central point source and irregular rings. DON’T BE CONCERNED! It doesn’t mean your optics suck! I know, the optics in my 10" are very good (for SCT’s). What you’re more concerned with here is if the central point source is actually in the center of the rings.

Adjusting.jpg (9225 bytes)

If not, if your image looks something like Fig. 4 (and this isn’t bad, I’ve seen MUCH worse), your optics aren’t aligned.

Now, to the actual collimating magic....

In SCT’s, you need to adjust the secondary collimating screws on the front of the corrector plate. DO THIS IN VERY SMALL STEPS! Because of the amplification of the secondary mirror, a little twist goes a long way. The general rule of thumb for Newtonians is that the reflection will move away from the screw being tightened. I haven’t tested this theory out on SCT’s yet, but what the hell, you’ll figure it out pretty quick. What this means is that if you look at Fig. 4, you’ll see that the telescope is mis-aligned with the central dot to far to the right. It needs to come left. How do we do this? There are several combinations of tightening/loosing which will accomplish this. I’d start by tightening screw 3 about ¼ turn. This should move the reflection up and to the left. Then loosing screw 2, this should bring it down and more to the left. You get the drift. Keep adjusting until the central spot is dead center.

Now, go to the highest power you can muster and fine tune your adjustment.

After you’ve completed your adjustments, you can either wait until the stars come out to verify the accuracy of your alignment, or do what I do, take a little of the finger nail polish and dab some on the alignment screws (SCT’s only fellows). This will keep incidental vibration from loosing the screws.

There you have it. I hope this method works as well for you as it has for me. Feel free to write and let me know how it works. If you come up with some thing better definitely let me know! Good luck, have fun!

 

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