g'day mate
This link tells you all you need to know about the principles. They don't need to be particularly expensive but there are some very pricy ones (of course).
Thanks for the reply. I think they must be a recent addition to the market place.
Well, you see, I am a keen new boy to this topic and I am reading everything I can find. The first ad I saw was only a few weeks ago.
Those devices seem to be based on the (aptly named) 'star test' method of optical alignment- you are simply aligning the optics to obtain the best possible Point Spread Function (PSF). It's perhaps the easiest method (certainly the oldest), but is also semiquantative and requires experience to rapidly converge to optimal alignment. I use the star test regularly, whenever I need to align a microscope objective- this is trivial with the star test, but that's because the source doesn't have to be located far from the lens. With a telescope, you have to place the 'star' far enough away so that it is unresolved and within the focus range of the telescope- AFAIK, star tests are generally not used for aligning telescope mirrors.
You seem to be more or less describing how the laser collimators on sale are used. They have a laser and 45degree semi silvered mirror in place of the eyepiece and you look at the resulting ring pattern from the light coming back on the two way path. Reviews seem to suggest that some of the cheaper collimators are not reliable and need collimating before you start!
Maybe... I have used an autocollimator for alignment but didn't make the connection here. Autocollimators and associated alignment tools are rather pricy, especially when you need sub arcminute accuracy.
Hard to say- you began this thread with "Looking at real stars tells me that the collimation is not too bad but it could be better." I'm not disputing the statement, but wondering on what basis you made the claim.
On a very flimsy basis, perhaps. I was just looking at the symmetry of the pattern and comparing it with the 'bad' examples on various places I found. As the Newtonian telescope is so basic, I assumed that was enough. In fact, I couldn't thin of anything else that could be at issue. If the sub reflector is at near enough the right distance from the primary, I assumed the dominant factor would have to be symmetry. (?)
Newtonian reflectors suffer from coma- could this be what you noticed?
That's a good thought. If the star isn't near-center in the field of view then coma may cause some asymmetry.
Huh?
A well justified "Huh?", I think. I rushed round. re-reading some bits on Spheres vs paraboloids. Paraboloid eliminates Spherical aberration on axis (hence parabolic reflectors in radio reflectors) but coma is there, off axis. Slower reflector, the less coma - hardly surprising because the angles are less.
Good lord no! Parabolic mirrors have some of the worst coma you will ever come across.
Yep. Slower mirrors have less coma, for exactly the reason you mentioned.