Laser Cross Newtonian Collimator experiments

[paul]

I picked up a pair of cross lasers,

for about £11 through Amazon. My idea was to use them for quick alignment of a tripod to marks on a wall or fence. I still think that is worth checking.  With continued bad weather I was curious to see how useful they were for newtonian collimation. I decided to sacrifice and adapt an old collimator:

For the obsessive tweaker there are benefits to using a cross:

1. Seeing that the laser is central to the primary is much more obvious for secondary alignment...

A sort of barlowed collimation is also possible, the doughnut ring shadow is visible and the returned cross can be seen to intersect the hole in the laser barrel, so primary adjustment will be more accurate than a standard laser dot.

(If the collimator white disk had been at 45 degrees like most then this would be easier to view and image - its one of the reasons I sacrificed this collimator!)

2. Primary Mechanical Tube alignment and clipping. It is possible to align optically but be very off alignment mechanically, particularly if you have removed the mirrors for re-silvering as I did recently).

Does the  tube edge clip the primary mirror's view of the sky?

Even though the gap between rim and mirror reflection varies around the tube the simple test shows no clipping and I guess this leads to reduced cone error too. (Without this test you would have to waste one of our precious clear nights to image a defocused star and check it is round and start tweaking.)

3. You can get more confidence the focuser and secondary alignment is square.

Without the secondary in place you can align the focuser and spider to be square on the tube using lined paper.

Fitting the secondary and after say initial collimation you can do a confidence check on how central and square the secondary is mechanically:

Knowing the point of origin of the laser, the width of the secondary, measuring, doing some maths and voila you can know the size of the shadow expected. You can mark up some paper and check your results with a ruler. This shadow is further away from the laser than the secondary so the width is exaggerated, it should be easier to get optimal positioning.

It would even be possible to plot the shadow expected and use that plot for initial secondary alignment!

4. Secondary illumination?? 

When adjusting the collimating screws of the primary you are moving it towards or away from the secondary. I do this blind as I look at the laser spot on the collimator.  I wonder if over time the primary has moved too far up or down the tube (the adjustment screws have about 15mm of travel). 

Can the vignetting of my images be improved by getting the distance between secondary and primary spot on?

By positioning the laser emitting point at the expected primary focus the cross will illuminate the full width of the secondary and the cone of laser light from the secondary to the primary should match the primary mirror's light cone.

If all the planets align that should cause the primary to be illuminated edge to edge. If it spills over the edge of the primary then the primary needs to move up the tube to intersect it. If the laser doesn't get to the the edge its preferable - you are getting good illumination of the secondary but may be losing some of the capability of the primary if it goes too far. 

 

Given that these cross lasers are so cheap I am surprised I haven't seen them in collimators from China already.

The thing about bad weather is you can spend time, hours and days tweaking to little or no benefit - but it is fun.

Now if anyone knows where I can get a cheap concentric circle laser to play with then things can step up another notch? !

[paul]

(Untested) way of finding the approximate focus point of the primary: Place a sheet of paper over the opening to the scope. The laser cross will illuminate through it. When the cross on the paper is the same size as the primary (assuming the primary is fully illuminated) you know you have the laser at the primary focus point.

[paul]

Dusting of my large scope to test the mount controller retrofit I thought to check collimation with a cheshire eyepiece. It was very sobering to find it really is possible to laser align your scope with mirrors at strange angles. Basically, laser alignment is for little tweaks once things are setup. I am still finding the cross laser the easiest laser way to get accurate collimation:

Secondary on to the primary

 

Primary reflection back to the laser (note the shadow of the doughnut ring binder)

Long exposure images have proven this to result in symmetric and sharp spikes on bright stars. It gives me confidence that further collimation steps are not needed.