Quote:
Originally Posted by Amaranthus
Wow, I find that staggering. What criteria are you using to judge collimation accuracy?
|
I normally use high power extrafocal images (500x or more).
In this case I actually use CCD chip (QHY5L II) at focal length of over 7m.
If we think of that chip as a shortish eyepiece (chip is only 5mm across; a typical 5mm eyepiece would have itself ~5mm field stop) we are talking about 1500 times effectively to check collimation when imaging planets. And believe me it is necessary if you want a last ounce of resolution SCT can offer.
Try it yourself. I haven't really seen through too many SCTs, but of couple of dozens or so that I did, they ALL had the same problem.
If you don't believe me, check what Thierry Legault has to say about SCTs and collimation.
"People who think that a SCT does not need to be collimated very often probably do not realize the level of precision required for this type of instrument. The constraints are so high that a simple car trip always changes the alignment in small amounts, and sometimes in large amounts. Collimation can even vary according to the orientation of the optical tube (with a German mount, an interesting experiment is to point at the same star at the meridian successively from the left and the right of the mount, to observe the modification of collimation due to the reversal of the tube). This is the reason why it is advised to choose a star in the same area as the object of interest (planet or Moon). If a small misalignment can be tolerated in deep-sky observation, it is taking a big risk not checking the collimation before a planetary session. The idea is that the alignment becomes as familiar as checking the oil level or the pressure of the tires in a car before a trip !"
http://legault.perso.sfr.fr/collim.html