After working for an hour on my laser collimator I finally got the red dot on the wall to sorta stay in one point as I rotated it. I plugged it into the new Lightbridge and tried to do the first step, centering the laser beam inside the collimation dot on the mirror. What a circus. This collimator came with its own 2" adapter. I also tried the 1 1/4" adapter that came with the telescope. All the variation in adapter tightners and the screw tightners in the stock eyepiece holder made the laser dot wander all over the mirror. Turning the laser 180 degrees in the eyepiece holder made the dot wander from one side of the mirror to the other.

Add to this the crappy adjustment on the Lightbridge secondary mirror. The three screws are really hard to turn. I ended up turning the entire secondary mirror. I had to tighten up the center screw, but I am uncertain if this will change the fore and aft setting for the secondary, which has to be fairly precise with this big fast mirror. The adjustment screws on the secondary are erratic, they jump when you try and turn them, and lock up, making it necessary to back off all three and try again. They don't do what you expect. Tightening one screw should move the dot in the opposite direction from that screw, but it varied, jumping all over the place. Add all the slop in the eyepiece holder and adapters and I have no idea where the heck I am with that center dot.

 The primary adjustments went just fine. You quickly see the necessity of barlowed collimation. I tried using the white target built into the collimator. I could get the return beam nicely centered, but just touching the eyepiece holder, or tightening the adapters, threw it our of whack. Turning the laser put it way out.

Using the barlow, there is no sensitivity to the slop in the laser mounting. The shadow of the collimation dot projected back from the main mirror stays rock solid no matter what you do to the laser or eyepiece holder. Using the Blug makes it super simple, just sit at the back end of the scope and turn the knobs until the shadow of the collimation circle is dead centre.

 So what do I do about the secondary alignment? Would a cheshire eyepiece help? I have never used one, but I imagine all the slop in the adapters and eyepiece holder would still be a problem. A much better focuser is definitely on my birthday list. Any suggestions there?

Don

Very sorry to hear about your struggles, Roadgopher.  I fought with my secondary early on also, but I think most of that was my own doing.  It took several hours to get it all straightened out.  I did end up shimming my focuser to get it square with the secondary, but that was a one-time thing.  I need to collimate my laser before attempting to use it again, but that's not a big priority for my because I have had very good results with a Cheshire eyepiece.

As far as focusers are concerned Jims Mobile (JMI) makes some nice units, and the Feathertouch from Starlight Instruments are some of the best availible.

www.starlightinstruments.com

www.jimsmobile.com

There are also replacement secondary adjustment screws availible from Bob's Knobs that would make working on the adjustment easier also.  Good luck.

Roadgopher:

So what do I do about the secondary alignment? Would a cheshire eyepiece help? I have never used one, but I imagine all the slop in the adapters and eyepiece holder would still be a problem. A much better focuser is definitely on my birthday list. Any suggestions there?

Don

If your focuser is square, then the next step is aligning the secondary fore/aft to ensure it is centered under the focuser drawtube. The Cheshire eyepiece or a simple "sight tube" is an easy way to do that. You simply center the reflection of the secondary on the eyepiece cross-hairs.

You can make an eyepiece to do this by crossing two threads over the end of an eyepiece tube, or an extension tube. Another method is to use or make what's called a "collimation cap", which is simply a plastic cap for the focuser tube that is 1.25" diameter and has a hole in the center. You can make this easily from a black plastic 35mm film can (if you don't have one, ask for one at the local one-hour-photofinishing lab).

Unless you have the secondary centered under the focuser tube, then your collimation will be skewed: it may look okay visually, but when you test it with the laser or Cheshire you'll find it to be off. You can actually "make up" for this kind of misalignment using the primary and secondary screws, but your actual alignment will still be off (that is, you can get the laser to hit the center of the primary, but the reflected axis will be off.

If the collimator came with its own adapter, I'd expect the center hole in the adapter to be centered. However, I have several 2"-to-1.25" adapters that are not centered. One way to check is to insert the laser in the adapter and then do the same test with the adapter (laser On) that you did for the laser. If you see a marked difference, the adapter may be "off".

Another source of error is the adapter's clamping mechanism. If it has a single screw and no compression ring, then it will be forcing the laser off to the side opposite the screw. You could shim up that side (use tape, for example) to see if that takes up the slack.

I'm assuming you're using a 1.25" laser colliimator. If you're using a 2" collimator then it's a different story ...

Wouldn't the accuracy of the crosshairs on your film container cap be important? If I screwed it up and had the vertical crosshair offset to the left, would that affect my collimation of the secondary? Sorry, I've been observing since I was 15 but this is the first time I have ever been able to afford a "real" telescope. Kind of an old school beginner.

Yes, my collimator is a 1 1/4" so I am using adapters, and I think that is part of the problem. The 2" adapter that came with the collimater has a simple set screw. The 1 1/4" adapter that came with the Lightbridge has a tightening ring. Both show a lot of variation when the tightening screws are moved the least little bit.

Tempting to send Howie Glatter the big bucks for a real 2" laser collimator and remove the variable caused by the adapters. I am amazed how a simple 45 degree rotation of the laser in the eyepiece holder makes such a huge difference in the secondary collimation, but I guess that is why lasers make good collimaters. The slightest variation is immediately visible. \

So how do I know where to set the collimator to give a "true" reading on the secondary?

Don

 

Roadgopher:

Wouldn't the accuracy of the crosshairs on your film container cap be important? If I screwed it up and had the vertical crosshair offset to the left, would that affect my collimation of the secondary?

Yes, but remember that all we're trying to do here is "center" things. Even if you use a rather coarse thread, it will be relatively easy to center the thread on the barrel. Look at www.scopestuff.com for options. I think their inexpensive Cheshire eyepieces are about $30. 

...Yes, my collimator is a 1 1/4" so I am using adapters, and I think that is part of the problem. The 2" adapter that came with the collimater has a simple set screw. The 1 1/4" adapter that came with the Lightbridge has a tightening ring. Both show a lot of variation when the tightening screws are moved the least little bit.

Tempting to send Howie Glatter the big bucks for a real 2" laser collimator and remove the variable caused by the adapters...

OK, but remember that you're going to be using the adapter with the eyepieces later. Having the eyepieces off-center is as bad as having the collimator off-center.

Here's a tip I've used with a couple of out-of-round adapters: First collimate the laser, using the trick of laying it in a "wedge" on a tabletop and rotating it to see whether its beam forms a circle on the wall. I think you've done that already. Once you know the laser is okay, then put it in the adapter and repeat the procedure. If the adapter is out-of-round, or the hole is off-center, it will show up as a circle on the wall again. It may also be that both the focuser drawtube AND the adapter are off. If so, you might find a rotation of the adapter that is very close to being centered. If so, you can scribe a mark on the adapter and the focuser tube while the adapter is locked at the "best" position. After that, any time you collimate you first make sure that the adapter is in this same position.

So how do I know where to set the collimator to give a "true" reading on the secondary?

There is good news/bad news here:

• The good news is that the LightBridge is an open-truss design, so it's easy to get a look at the face of the secondary. When the scope is collimated, both the incident and reflected beam should hit the center of the secondary (which is not marked). Do this with the laser NOT barlowed while you adjust the secondary. If you have trouble seeing it, make a "collimation target" that exactly fits the face of the secondary (hold it on with a rubber band if you like). Then adjust the secondary fore/aft until the laser beam hits the center of that target.

Tip: If you have PowerPoint or a similar drawing program on your computer, use the drawing tools to make the target. Once you get it like you like it, you can save the file and print it out whenever you need a new one.

• The bad news here is that the LightBridge is an open-truss design, so from one setup to the next the tension and adjustment in the struts, and between the struts and both the upper cage and the lower bucket, will vary, throwing your collimation off.

The goal here is to get the upper cage dialed in. After that you get the primary dialed in. After that, any time you set up, use the same trusses in the same places (mark them) and try to keep the tension the same. That will allow you to tweak collimation quickly since the difference should be small between one setup and the next. If you vary the order of the trusses and/or the tension, then the difference can be larger.

Another way to approach truing up the secondary is to use a very long 1.25" or 2" aluminum or plastic tube, with a crosshair glued to one end, and insert it all the way into the focuser so the crosshair end "hovers" just above the secondary itself. If the physical distance between the end of the "sight tube" and the face of the secondary is very small, then it will be easier to ensure you have the secondary centered.

Another tip you might try is to do the truing of the secondary while the primary is covered. The cover for the primary is white, so you might want to cover it with a dark cloth while you're doing this. That is, put the standard cover on the mirror box, then put a dark cloth over that. This will help you see the secondary mirror (alone) through the focuser draw tube. You should be able to get pretty close with this method without using the laser at all.

 My recommendation is get or make a sight tube - or better yet - buy a Cheshire collimation tube and use it to adjust your secondary. That way you'll get the correct offset. Centering the laser on the secondary will get you close but the tube will get you dead-on.

Also, lateral shift in the focuser as I understand it is a rather insensitive misalignment and being off center 20 or 30 thousandths wouldn't wreck the train. Angular shift is worse and is why the barlowed laser is such a good idea - it ignores angular shift in the focuser; 

Marking the 1 1/4 adapter is a good idea, but how do you determine if the eyepiece holder is part of the problem? With no real reference it is hard to imagine what you compare to what. I have used a barlowed laser (Blug) to set up the primary, so I assume that I can check and see if the non-barlowed return laser spot hits the center of the secondary. This is another difficult concept since the secondary is elliptical and the optical center of the secondary is not the physical center of the ellipse. Because of the angle of the secondary, it looks like a circle as you view it through the eyepiece and the center of that virtual circle is the optical center of the secondary. I don't know how you would determine this on the elliptical paper target.

 Another issue, while I was turning the adjustment screws on the secondary, trying to center the laser dot on the main mirror, the entire secondary rotated around the mounting bolt. I turned it back, but now how do I know the secondary is square to the focuser tube? I guess I could measure the distance to the secondary from the top edge of the focuser. Is there a better way?

Don

 

Ever notice how close the word Meade is to *** ( French for brown and stinky ). I have used both in reference to this scope. I guess I should buy a Cheshire to use as a sight tube. I will check all the reflections out. Part of the problem is my collimator. It is an Olivon, same as the Zhumell. I had to dig out rubber sealant to get at the adjustment screws, not made to be user friendly for adjustment. As delivered it described an inch wide circle when rotated. The 3 adjustment screws are not equally spaced around the tube. One is by itself and the other two are close together. The solitary screw doesn't seem to do much, and the other two move in random directions each time I adjust them. I managed to get the circle on the wall reasonably tight, but by no means spot on.

With all the variablity in the laser itself, the 1 1/4 adapter, the focuser tube, and the secondary mirror itself spinning like a top, I am at a loss of where to even start with all this. Too many sources of error. I think I had better work on the laser until it is dead accurate at 6 feet. At least then I can have one rock to stand on. I just don't know what to do to get it there.

(Weird, the forum software won't let me display the French word ***. Is homeland security still mad at France. I thought we were all eating French fries again. )

So after 2 years experience with a Lightbridge 16 I am wondering what everybody else's experience has been. The collimation problems were largely solved with Bob's knobs and a Howie Glatter collimator and blog. But I still am not happy with the focus on this scope. I pay careful attention to cooling the mirror over as long a period as possible and collimate accurately for each observing session but I just can't get a sharp limb on Jupiter. It is a very soft focus, which looks fine on deep sky objects. Nebula and galaxies are bright as you would expect with this aperture. But compared to many homebuilts, Obsessions and Porta-balls that I have looked through at star parties, this scope does not have the depth and contrast they do, and I just can't get a decent focus no matter how I try. So, I guess it is the mirror. I am very disappointed with this scope. What is the experience with these Meade dobs out there?

DonM

 

 

i dont use anything- strictly an eyeball alignment gets everything in near collimation- make sure the primary and secondary are physically centered, then start from there, youll adjust the secondary offset, then align the primary- i finish up with an arti-star (which costs about $1)- i see no need for all that other crap

Could be the telescope, I can't say for sure without looking through it myself. However, There are other factors that will effect the focus and have nothing to do with the telescope. Sky-glow, astronomical Seeing, and Transparency are just a few of the more common culprits.
Secondly yet more importantly are the eyepieces, that alone can make a world of difference. Are you doing a side by side comparison at the star parties using the same eyepiece under the same sky conditions?
Are you using a light shroud on the telescope or do you just leave it wide open?

We have tried various eyeball techniques, and a cheshire eyepiece. After all, people aligned scopes before lasers. Sometimes we are on and  mostly not. The big problem is the portability. Tearing down a scope and bouncing it around in the trunk for hours really knocks our the collimation. I suspect you could get away with eyeball methods for an f 8 scope, but when you get down to f 5 and below, the collimation gets way more touchy. The laser setup is a joy, fast, and accurate.

 DonM

We have traded eyepieces around with all the scopes. One of my observing buddies has a Nagler 14mm and we use that as the standard of comparison. He won't let me have it on long term loan, the rotter. My scope just doesn't seem to give the kind of deep gorgeous views that I have seen with some homebuilt scopes. I admit that this is very subjective. And I suspect that these fast scopes are much more affected by seeing conditions. Saturn wasn't bad last spring, but Jupiter is very poor right now. However, we are in the north and Jupiter is very  low in the south. I have black pipe insulations around the trusses and also use a shroud. We are going to paint all the white edges black as well. I guess we will do all the tuneups we can and then what I get is what I get.

DonM

Roadgopher:

The laser setup is a joy, fast, and accurate.

 DonM

Only if the laser itself is accurately collimated.

Roadgopher,

The very first step you should be performing, is to see if the focuser is square to the OTA axis.

Most mass produced scopes usually need to have the focuser shimmed because they are not mounted parallel to the OTA, My friends light bridge needed shimming, so did both of my Dob's, his and my diagonal was not properly centered, and this came right from the factory misaligned to begin with. 

I don't usually worry about the set screws shifting the barlow or Cheshire, IF you observe with the set screws on the focuser tightened, then you should check for squareness and collimate the same way.  My thinking is that when you tighten your set screw on the eyepiece you will be off again anyways,

Unless you can properly shim the inside of the focuser, and then check to make sure that the focuser is still square, You may be better off with a premium focuser upgrade in the future.

Dennis

tkerr:

Only if the laser itself is accurately collimated

Absolutely true. My first lasers were terrible. Doing the standard test of rotating them in a V groove metal casting resulted in the laser dot describing a circle on the wall opposite. They were impossible to adjust. The Howie Glatter laser has been dead on after a year of constant use.

 Don M

 

Star Dragon:

The very first step you should be performing, is to see if the focuser is square to the OTA axis.

This is something that does concern me. The only test that I have done is to rack the focuser all the way in and all the way out to see if the laser dot moves at all. It does not. My laser has the holographic screen and I have used it to assess the focuser, but I don't know enough about that kind of test. It seems to be ok. Still, I agree that focuser squareness is one of the important variables. I checked into the price of a feather touch and had to grab my heart pills when I got the answer. I will have to wait for a lucky lottery ticket for that.

 DonM

 

 The only method I use in the field that I have found to be accurate and repeatable on my f4.5 is using the Cheshire sight tube to adjust the secondary and the barlowed laser to adjust the primary. The barlowed laser is very forgiving to a misalgned laser or any slop in the focuser. (Note: the barlowed laser requires the center of the primary to be marked)

If I intend to kick up the magnification for some planetary viewing I'll tweak the secondary with a 2" Catseye collimator .

At this f ratio, a naked-eye visual collimation merely confirms all the optical components of my scope are present and accounted for.