i've been star testing my scope lately, but when i get to the "de-focusing" step, i see circle with the reflection of my secondary in the middle okay, but i dont see any "concentric" rings at all - does this mean it's badly out of collimation? the scope is a bintel bt-252. good sharp views using low powered ep's but a bit disappointing using high powered ones...
Follow the link http://legault.club.fr/collim.html this will take you to a site entitled "The Collimation" it should help you answer some of your questions in regards to star testing.
The rings you are looking for are subtle, they don't jump out and hit you in the eye. I haven't done much star testing but a friend who has always advises to have Suiter's Star Testing book (the book from Willman-Bell) right next to you while you're learning how to do it.
Can you describe your equipment and procedure? Here is a brief description of my experiences in seeing the out-of-focus diffraction rings.
For my Vixen 4" f9 refractor, the rings become obvious when I plug in a 5mm eyepiece which is a magnification of x184.
For my Celestron C9.25 SCT, the rings become obvious when I plug in a 10mm eyepiece, which is a magnification of x235.
I can usually see rings even in bad seeing, although they are very messy and wobbly. I often find that the inside of focus rings are easier to see that the outside of focus rings. If you de-focus too much the rings can become quite faint.
