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Laser, laser on the wall . . . mirror

What's that? The view down the tube of the 15-inch during a laser collimation exercise.

They call it "collimation." I don't like the word. My tongue gets tangled with my gums when I try to say it.. But what it means is getting all the optical parts of a telescope in perfect alignment so the scope works at its best. The result is great images - when the atmosphere cooperates - and peace of mind even when it doesn't. So having assembled and balanced mynew, 15-inch Obsession, the next challenge was to collimatew it.

If you happen to own a traditional refractor telescope with lenses you don't worry about collimation because the optics are held in pretty rigid alignment from the start. The complex "catadioptric" scopes like my 8-inch LX90 use a combination of mirrors and lenses and while fairly rigid, things do jiggle about and get out of alignment. Collimating this kind of telescope is relatively easy. You do it by looking at a star slightly out of focus and adjusting three screws on the front of the scope until you see multiple rings that are aligned like a bullseye. I make it a habit to check the collimation on the LX90 every time I use it.

But Newtonians reflectors in general and Dobsonians in particular like the Obsession - especially if they are carted around much - get out of alignment very easily and re-aligning them used to be a rather imprecise and fussy process. Back in the good old days (c. 1970) I very rarely aligned my Newtonian reflector and when I did I was never sure if I was doing it right. Looking at mirrors that face one another results in lots of images within images within images which can drive you crazy. But then, in the "gold old days" I didn't know what I was missing. I simply did not know what a difference good alignment could make. Things looked fine to me. I was duely amazed and satisfied with just about any reasonable view. Now I'm fussier and also thankful that good alignment is just $150 away through a device sold as an extra by Obsession (and others) called a "Barlowed laser." Here's what it looks like.

The Barlowed laser is a fairly hefty package of electronics and optics which fits in the focuser where a 2-inch diameter eyepiece would normally go.

The business end includes a "Barlow" cap that unscrews. You can and do use just the laser for much of the collimation. The aluminum cap contains a tiny Barlow lens and is screwed on for the last step of the process.

Now if that word "laser" scares you a little, that's a healthy response. You need to play it safe with these things and make sure you never get the beam in your eyes, or accidentally in someone elses." Here's a few good safety rules I found posted on a Web discussion group recently:

Laser collimators use a <5mW laser. Brief direct hits into the eye are not harmful, but nonetheless one must treat the laser with respect.

Yes, there is a danger of getting hit in the eye by the beam if you're
not careful about how you use it. A few very simple safety procedures
reduce the chances of getting a beam in the eye to near zero:

1) Always make sure that the beam is turned off before removing the
collimator from the focuser tube.

2) When it's not in use, keep the beam end of the collimator pointed
at the ground.

3) Before turning on the beam, point the scope tube upwards at a steep
enough angle that the beam, if it shoots out the top of the scope,
can't possibly hit somebody.

4) Before looking down the telescope tube at the primary mirror, pass
some opaque object over the top of the tube to make sure that the beam
is hitting the secondary and isn't shooting out the top of the tube.

Back to basics

newtonian_light_path.gifThe heart of a Newtonian telescope is its two mirrors, plus an eyepiece and all three have to work together. Light from a star or planet goes down the tube and is beamed back up to a sharp focus by the parabolic primary mirror. Then a secondary mirror - a small, flat one - catches that beam and deflects it towards the side of the tube where an image is created. That image is then examined through an eyepiece which magnifies it.

The important point is the two mirrors and the eyepiece have to be in perfect optical alignment for the system to work at its best. So how do you align them? Well, some relatively crude alignments are done when the telescope is constructed, or when mirrors have been removed and then put back. I won't go into those steps here because they're not needed often. But using the laser makes these initial alignments easy as well. But why I really like it is for the routine collimation that should be done as you start each observing session.

The laser goes where the eyepiece would normally go, so what you are doing is reversing the light path. Instead of light entering the eyepiece, the small red laser beam begins there and goes first to the secondary which deflects it down to the primary. It hits the primary and bounces back up to the secondary, and then right back into the laser! Well, it does that if everything has been aligned properly.

Before you begin the process you need to know where the center of your two mirrors are and you mark them - in my case a small black paint dot on the secondary and on the primary a little black doughnut. This last is one of those sticky "reinforcements rings" you put over the holes in paper that's going in a three-ring binder. It's been painted black to make it standout on the mirror.

Frankly, I feel a bit sacrilegious putting this on a mirror, especially a 15-inch primary mirror which differs from the one you look in every day by about $2,000 of optical craftsmanship honed so fine it boggles the mind. The primary is a 2-inch thick piece of glass that is 15-inches in diameter. Machines are used to slowly grind the glass to first a spherical curve, and then a parabolic one, so that light hitting anywere on its surface is brought to a single, sharp focus point about 68-inches away. That's no mean trick - especially when you are doing this to a tolerance of a fraction of a wavelength of light. Red light has a wavelength of about 680 nanometers. A nanometer is one-billionth of a meter - which is simply too damned small for me to imagine. But you can see why I'm a little skittish about marking it up. In this case the opticians have etched a little "plus" sign into the center of the glass. Very handy, but difficult to see. So you put the little doughnut on so the "plus" is in the center. Surprisingly, you don't worry about getting fingerprints in the middle of the mirror because that section of the mirror isn't used. It's blocked by the secondary mirror - as the directions from Obsession explain.

So back to our laser beam. The first goal is to get it to hit the black dot on the secondary mirror and to do this you simply rotate the secondary mirror and move it in and out. It's on a shaft held by a wing nut, so this is fairly easy to do. (It also has three adjustment screws which you fiddle with in the next step.)

The "life saver" is your second aiming point. You try to get the laser beam to strike it dead center. You do that by making fine adjustments on three screws that are behind the secondary mirror and tilt it slightly one way or another. Bingo! Now you have a beam that travels from the eyepiece to the secondary mirror to the main mirror.

Confusing? It is for me and I took the picture! You're looking at the main mirror from the top - and one side - of the tube and what it is doing is reflecting back part of the inside of the tube. But the important point here is that little black ring where the red light is hitting. That's the center of the mirror, although it may not look like it from this nagle.

Just one step left. You have to go to the back of the telescope - bottom - and adjust the primary. It too has three screw behind it with large knurled knobs that are easy to find in the dark. (And it will frequently be dark when you do this.)

Now what you are trying to do is see where the red dot is being projected and make it return to the secondary. When you first do this it may be off by quite bit, so taping a large piece of paper over the top of the tube opening might help.(Or you could simply hold a piece of cardboard up there - or with the Obsession, the mirror cover comes in handy.) That will show you where the beam is hitting and you can "walk" it over to the secondary by turning the screws behind the primary mirror. Fun, huh? It is , really. But it's the last part I like best. (This last step is obviously easier to do with two people. With one you do a bit of running back and forth from one end of the telescope to the other - but hey, that's about the most exercise you get while observing!)

At first I found it kind of difficult to tell when the return beam was hitting the secondary in the center. Then I added the "Barlow" to the laser and that made all the difference. This is an aluminum disc with lens that goes on the end of the laser. It results in is a doughnut-shaped shadow that forms right on the silver disc of the Barlow. All you have to do is get that nice round shadow right around the hole that the laser emerges from. Cooool! Here's what a "miss" looks like:

And here's a direct hit!

And that's it. I've done this twice now, so I'm an expert ;-) Yes, it's a tad confusing to do the first time. But once you've done it and seen it, repeating the process is just a matter of a minute or two. As I understand it, in most cases you'll just be fine tuning, doing the last step or two. And when you're done you know your optical system is going to perform at its best. Yes, images will not be perfect. Earth's artmospehere presents a constantly changing challenge. But at least you now know that it really is the atmosphere - and not your telescope's optics - that is raising havoc with the image. And there are those times when the atmosphere settles down and "seeing" is at its best and those are the nights every amateur astronomer looks forward to and it's good to know your scope is ready to do its best when those special nights arrive.

Posted by Greg Stone at May 24, 2005 04:44 PM Comments? Please email me:

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