Hartmann mask based collimation Is there a detailed description for a Hartmann mask based collimation process?
I've been told by a friend that is possible to collimate an SCT by placing a three round holes Hartmann mask at the secondary and then covering one hole at a time. Unfortunately the guy did not clearly describe the full procedure and what I should try to obtain looking in the eyepiece (or looking at an attached webcam video).
 A: The described procedure is suitable for an SCT telescope.
See bottom for reference URLs of images/blog posts/instructions to build the mask.
See bottom also for an easier method (without mask) if you have a webcam adapted for telescope usage and a pc connected to the SCT mount.
"Apply the Hartmann mask to the telescope and align the three holes at 120 degrees with three adjustment screws on the secondary mirror. Point a star bright enough (eg. magnitude 1 or 2), using a high magnification (we may also need a night with good seeing, but that's another story ...) and you focus . 
If the telescope is not collimated, not get to see a single spot of light move and symmetrical, but you'll probably see a patch of distorted and elongated. It does not matter for now.
During the procedure, adjust the position of the telescope so that the star that you are using is always the center of the visual field.
Referring to the figures, we will call A , B and C (clockwise) the three collimation screws and the holes on the mask.
Temporarily cover the hole A with a piece of paper, cloth, or what you will, so that no light can enter through this hole (I use a post-it). Focus, bringing the two spots of light to match each other.
Remove the cover from the hole A and attach it to hole B . You should see 2 points of light, or an oblong spot. Do not touch the focus , but act on the collimation screw A until the two points of light unite .
Focus again, then move the cover from the hole B to hole C .  Act on the screw B until the two points of light do come together again.
Focus again, then move the cover from the hole C to hole A. Act on screw C , until the two points of light do come together again.
Repeat the above steps until the point star will appear in all conditions. In the end, free all the holes and check the symmetry of the image you see. You can even fine-tune the collimation repeating the procedure with a higher magnification.
Confirmation that you have worked well will come observing the image of a star with no mask and at the highest magnification (2-3 times the diameter of the telescope in mm): If the diffraction pattern is perfectly symmetrical, then the telescope is perfectly collimated (bad seeing may prevent us from making this check)."

Original instructions can be found here: http://gerlos.altervista.org/collimazione-telescopio-sc-maschera-hartmann   . In Italian but you can easily get it translated by Google.
Here are the same instructions in French: http://colmic.free.fr/collim/collimat.htm
And here are some good instructions to build the mask: http://www.iceinspace.com.au/63-187-0-0-1-0.html
A nice and free software that allows you to collimate using a webcam and a star. Seems very fast and even more precise: http://www.astrogeeks.com/Bliss/MetaGuide/ 
A: I'm the original author of the Italian instructions pointed out by Mario (tahnks for your translation in English!). Feel free to ask questions in English if you don't understand something or if something doesn't work.
One important point to add is that it's better to use a mask with triangular holes instead of circular ones: this way, when you are close to focus you will see 6 spikes of light from the star, and pseudo-Airy disk in diffraction pattern will be smaller. You can also judge your collimation looking at the symmetry of that pattern. If some spikes are brighter than others, or not equally spaced, you need a better collimation.
You can judge the quality your collimation also when focusing with a Bahtinov mask (the definitive focus aid, imho). Looking at the symmetry of the pattern generated by the mask and rotating the mask you can see if the telescope need recollimation or not.
For some images of symmetrical patterns with various focus aids you can check this page (it's in Italian, but the shots should speak for themselves):
http://gerlos.altervista.org/maschera-bahtinov-messa-fuoco-super-precisa
