The YACHT or
Yet Another Collimating Home-made Tool
by Nils Olof Carlin (updated May 12th, 2001)
After the recent discussions on the ATM list about what can be done with laser diodes (lasering at the proper current, or not lasering when fed less current) I decided to assault one of the cheap keychain laser pointers I had around, and make the collimating tool that you can see here.
This ought to be about the most accurate tool yet for adjusting the primary of a Newtonian telescope. It is a variation of the Barlowed laser that I have described elsewhere. Even if both use lasers, they work in a fundamentally different fashion from "ordinary" laser collimators!
The laser diode works as an incredibly small light source, spreading light in a somewhat asymmetric and fairly wide beam. In laser pointers, a small lens projects this into a narrow beam of "parallel" light, but this is not what I wanted here - I wanted to use a true point source instead, so I had to remove the lens. This could be the tricky part, as the lens is not easy to remove (try unscrewing it first - if that works, fine, but on the ones I have tried have not yielded so easily)- I first sawed off a piece from the outer end of the housing to reach the brass lens cap, then made a 45 deg "bevel" to it, deep enough to eventually cut through it. The lens fell off, freeing the diode (in this model, there is no window to protect the chip - in another I dismantled, there is one). You can see the tiny grey chip in the image below, between the two gold electrodes in black insulation, and on top of the little gilded oval that holds it.
WARNING: this worked for me, but if you try the same, I can't guarantee success. DO NOT try it on a laser pointer that you value highly!
I took 2 pieces of plastic tubing (electric conduit) and a roll of duct (gaffer) tape - the outer tube is 1.25" to fit the focuser, the inner some 20 mm. I taped the laser pointer to make a snug fit in the inner tube, and taped the inner tube to fit the outer one. A small screw threaded in the inner tube works the on button of the laser. Taping full turns ensures (to my satisfaction at least) that the laser diode is centered in the outer tube - the centering is the only thing that needs to be accurate (to within a few tenths of a mm).
I then cemented a faceplate to the tube - white on the outside, black on the inside and quite opaque - it should not let any laser light pass outside the approx. 5 mm hole in the center. I have made markings on it (see below), the size of the main mirror center spot - it might be easier if you make the diameter one or two millimeters larger than your center spot.
You need no perforation in the middle of the center spot (as you need with a conventional laser)! If you have one, no harm of course, and it may be helpful if you want to do a coarse collimation of the primary using a conventional laser or the lens attachment to the YACHT.
The outer tube should be long enough to put in the focuser, so that you can see the faceplate inside the tube. Ideally, the laser diode should be as far outside the focal plane as the faceplate is inside it, but I haven't bothered.
This composite image shows to the upper left the laser pointer, the inner tube, and the outer tube with the slitted piece. At the upper left is a magnified view of the laser diode, below it the lens assembly with its baffle and the cardboard faceplate, and across the bottom is the assembled YACHT. As you may see, I can use either a tube attachment with the faceplate, or I can substitute another attachment, with a small (30 mm) lens to focus a spot on the primary, to facilitate the tilting of the secondary. If you have a regular laser collimator already, you can use it instead and need not bother with the lens attachment. Either attachment is held by a piece of tubing of the same kind as the outer tubing, with a slit wide enough to squeeze fit into the outer tubing.
I put a baffle with a small hole (4-5mm) inside the lens. This gives a very sharp point to aim (with good depth-of-focus - if you use it on different telescopes of different focal lengths, no need to refocus), allowing better precision (not that it is really needed here) and also a good deal less intensity than a regular laser beam, which is good for nighttime use. Due to the good inherent centering and long distance between the diode and the lens, there is really no need to adjust the aim of it - an extra bonus!
To use it:
Use a regular laser collimator (or the YACHT with the lens attachment), tilt the secondary as usual to center the beam on the mirror center spot. Next put the YACHT in your focuser, turn it on (with the screw) and watch the light returning to the cardboard faceplate. You will see the shadow of the mirror center spot nearly centered on the faceplate - adjust the primary mirror to get it accurately centered.
If you do not know that the mirror center spot is at the true optical center, do the best star collimating you can do, when the seeing is the best you can get. If you have just done a very careful star collimation, draw the outlines of the returning shadow on the faceplate. If it is dead centered, fine, else lift off the spot or put another on where its shadow is accurately centered, and press down. This may not be easy with a closed tube, so if you can't, note the rotation of the collimator faceplate for future reference (and the rotation of the mirror, if it can rotate!) and place the shadow in the same position whenever you collimate this telescope in the future.
Is it really that accurate - and do I use it?
I'm convinced you could place the shadow with just a small fraction of a mm error - this should be good enough for the fastest mirrors, and likely (particularly with slow mirrors, where collimation isn't so critical) much more accurately than you could optically center the spot in the first place. I don't think there is anything (including the autocollimator!) to beat it, but a Cheshire device (or one of its variations) need not be much worse. My own telescopes are made to let me collimate them while looking into the focuser or Cheshire. In my small closed-tube Dob, I can't easily see the YACHT while looking inside the tube, so a Cheshire is really much easier for me to use here. However, in my13.1" truss-tube Dob, the YACHT is very easy to use - and the intensity of the red light is quite comfortable and easy on my night vision!
Always be careful with lasers!
Since the YACHT (without the lens) does not concentrate the light in a narrow beam like a regular laser pointer, the danger of eye damage is less, but do not look down the tube at the reflection of the laser diode - it may at least affect your dark adaptation.
Principle of operation:
To me the working principle is very simple - but I've had feedback from readers who do not find my description very clear. Fair enough - here is a sketch that hopefully helps explaining the operation:
In this sketch, the secondary is omitted, for clarity. The laser diode (green) emits a diverging beam of light, passing through the hole in the faceplate. It hits the mirror, and since the light source is at (or near) focus, the reflected light is parallel. It returns to the faceplate of the YACHT lighting it up, but for the shadow cast by the center spot - it is drawn a little decentered in the sketch. Since the light source is extremely small, the shadow is very sharp.