V-Blocks and Secondary Spots

I appreciate all the help given regarding laser collimators. I have a couple more questions that I'd rather start over on again.

On the subject of V-blocks and laser collimators. I understand a V-block is a simple machining jig? You lay the collimator in the V and rotate it and see if the beam moves in a circular pattern on a target instead of remaining a dot as it would if the laser is perfectly aligned with the structure of the device?

My collimator is adjustable via three setscrews around the circumference.

If so, what is a source for v-blocks, and how do you use them? My collimator has a tube only about 1" long where it slides into the focuser, then a large knurled ring, then a tube about 3" long with a setscrew you screw in to turn on the laser. How can this device be laid in a v-shaped jig and rotated with the knurled ring in the center of it? I could see if it was a cylinder with the same circumference along its entire length, but what about the larger knurled knob in the middle of everything? What kind of "target" do you use and how far away should it be?

Is there a good website that will explain this to me? I've looked with not much success so far. I hope somebody is saying, "Hell, yes, just go here and read so I don't have to try to explain to this moron."

Also, is it that critical to center-spot a secondary mirror for collimation? It's easy to make a paper template and center-spot a primary, but what about for an elliptical secondary? Never done that. Is there some technique for doing so no matter what the size of the mirror?

And, if I you have a sonotube dob instead of an open tube, how can you even see if the laser is hitting both dots or circle on both mirrors at the same time by looking in the end of the tube? You can't look down the focuser tube and see it because the laser device is there. Do people really spot their secondary?


Eric

>>If so, what is a source for v-blocks,

Go get a small piece of wood, as thick as you can find, and a saw, and cut the wood in the shape of a 90 degree V.

>>and how do you use them?

If you have a vice, stick the wood in the vice, with the open ends of the "v" pointing up. If you don't have a vice, just put the block of wood on a counter or table top (with the open end of the "v" up).

It would be nice if the laser were cylindrical the whole length, but you know it has to be round at the very bottom where it inserts into the focuser tube. Put this round part, so it nestles in the v, turn the laser on, and look at the wall a few feet (or more if possible) away.

Now rotate the collimator, and see if the spot moves. If it stays still, the collimator is collimated. If not, not.

If you need to adjust, turn the adjusting screws some, and see how it affects the beam. Adjust, and repeat until the laser point on the wall does not move as your rotate.

>>What kind of "target" do you use

The target is a blank wall. You could put a post-it note on the wall with a pencil mark to show where the laser is hitting. But it isn't necessary.

>>and how far away should it be?

I don't know that it matters much, I would try for at least three or four feet. THe further away, the more movement you get from any error (which is good), on the other hand, that movement is further away for you to see.

>>Also, is it that critical to center-spot a secondary mirror for collimation?

I would say that if you are using a laser collimator it is part of the procedure to center spot the primary, but not needed for the secondary.

Of course this assumes you have already centered the secondary under the focuser (or properly offset from the center if you really want to be right on. This can be done with a standard non-laser collimator, or a film can, or by eyeball.

>> It's easy to make a paper template and center-spot a primary, but what about for an elliptical secondary?

I've seen the secondary template it in some ATM book. (Kriege's Big Dob book if I am not mistaken). But I don't think you have to center spot the secondary.

>>And, if I you have a sonotube dob instead of an open tube, how can you even see if the laser is hitting both dots or circle on both mirrors at the same time by looking in the end of the tube?

Yes, remember that you can see the reflection of the secondary in the primary, so just look at the primary to see where the laser is hitting the secondary.

>>Do people really spot their secondary?

Yes, some people do. I don't.

By the way, lasers shine right through a spot. However, my primary usually has a donut instead of a black spot to maintain maximum reflectivity.

Alex






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Hi Eric
I made my V block from a piece of 2x4 and cut a 90* V into it and some plywood. The V does not have to be exactly 90* just close so the round portion that inserts into the focuser is able to rest in the V and be rotated. It is helpful to have the V cut square to the back but not critical. Then mount it someplace like a vise that is sturdy or I screwed and glued it to a piece of plywood and c-clamped it to a heavy workbench.
I set mine up so I had the laser dot hitting a piece of paper about 15 feet from the laser and carefully rotated the laser while gently holding it down in the v-block and flush against the back. In your case with the short 1" portion that you are squaring the lase to, it might be easier to have someone help you mark the laser dot while you hold and rotate the laser. Rotate every 90* and mark the center of the dot and repeat this until you start to see a pattern (usually a circle or if you are lucky just a dot). Make adjustments to the laser and note which way the laser dot moves in relation to which adjustment screw you moved, and repeat the procedure until you get nothing but a dot. Remember to make very small adjustments and keep this methodical so you get repeatability in the laser dot location. I rotated 90* then 2 light taps in the laser to make it rest then walk down and mark the dot. This will take some time and it seems like I walked 5 miles 15 feet at a time to get mine down to a dot.
Hope this helps
Buck

Thanks for the wealth of information, guys. For now, I'm going to pass on spotting my secondary (I've read both sides online and both seem pretty adamant about either doing it or not) but I am going to construct a v-block device to make sure my laser's square. Thanks again!

Eric

http://www.astromart.com/articles/article.asp?article_id=96
Please read this article I wrote and it may help you.
A V-block is a piece of 90° angle iron or aluminum or any material you have.
When you have a laser that has a fat ring in the center, then make a cradle of a V-block with a section cut out for the fat ring.
When using a target place it at least 15' away. I supply a printable target in the article.
Only move the screws a tiny bit one at a time when collimating.
Don't expect to get perfection, at 15' if you can get it within a 1/8" circle, you should be OK. Try to get it close as you can. The more distance the target is from the laser the more accurate the collimation achieved.
Do not center spot the secondary as the center is not where the laser dot lands, it is the optical center and not the physical center. Plus the secondary should be free of spots because it is all in use, unlike th primary that has a center that is shaded by the secondary.
Jim Fly makes a great reusable acetate template for center spotting a primary ( 4-15" or 4-30"). He also makes a great triangle to use for this spot. It is available with a hole in the center of 5/32, 3/16, & 1/4". Or you can cut a circle out of paper
fold it twice and cut of the point of the pie shape and use that for a center template.
You can use a small mechanics mirror to look at the face of the secondary and see the return dot as it overlaps the dot on the secondary.

I agree with Herbert and Floyd on the issue of the V=blocks and alignment for a laser collimator. I think it is better to use 2 V=blocks instead of one like Herbert says, and I made mine that way. I also feel that the farther away you aim the laser to adjust it the better. However most of you have missed the biggest point. Get a good laser to begin with and you won't have to mess with it. Howie Glatter uses a plastic block and beats the hell out of his lasers on it, then checks them again to make sure they are still in alignment. If they hold there alignment they pass and he ships them to you. These are precision tools so get a good one. These aren't eyepieces so you only need one.

I've eyeballed my collimator (standard "deluxe" version sold by Orion, Owl Services, etc.).

It basically consists of three pieces of different dimensions; the barrel that slides into the focuser, a knurled holding ring, then a cylinder in the rear that houses the laser.

The two barrel components that would rest in a v-block are of a different diameter. The front barrel is a little under 1.25" and the back barrel housing the laser is exactly 1.25"

If I used one v-block on each barrel, would this still work in theory, except the laser would be pointed down a little since the "rear" v-block would be higher than the front? Even if the unit were pointed "nose down," wouldn't it still rotate evenly and achieve the same effect?

Eric