Skewd diffaction patterns clues ?

[mr.sneezy (Martin)]

I have made a combination collimation eyepiece and setup my reflector scope as best as possible for the first go at it. I get a nice round star diffraction pattern in the center of the view now (it was rubbish when I first unpacked and tried out the scope).
What I have noticed now that I bought a better eye piece, is that the pattern skews right as I shift the object to the left of center, but it still looks round when off center to the right.
Would that imply the focuser tube is not square to the main tube and main mirror ? Or can it be something else too ?
Martin

[Astro78]

hmmm well all eyepieces exhibit this towards the edge, mostly due to astigmatism. But your experiencing this to one side, so we'll rule that out.

Unless your focuser is skewed noticeably by the naked eye, your correcting for this naturally through collimation. So would say this aint the problem. Even it were a problem- this would only cause vignetting, no other effects to the image.

Sounds like your suffering off axis coma from the primary needing to be better collimated .

A laser would be a worthy investment provided you take the time to collimate the laser itself, which is easy but a little time consuming.

A cheshire or use of one will be handy to get your secondary set (usually done once) but this is mostly for getting a max. illuminated image - and to ensure the light path is hitting the centre of the secondary where the surface is flatest/most accurate.

Would say keep going with the star test and keep adjusting that primary - but this wont be easy unless your pretty close to collimation in the first place. Enter the combination chesire or better the laser.

Add: also check that your mirror isn't being pinched hard on one side by any of the 3 mirror holders. This could be a factor making the star test frustrating

[mr.sneezy (Martin)]

I also notice now that if I lean on the focuser tube a little while viewing off center to the left that it gets better. But the other side view gets worse. I think I was pushing the focuser tube away from the distorted side...

[Astro78]

well sounds like it's your focuser throwing the collimation out. It can be skewed slightly, as long as it doesn't move after you star test/collimate. But if it's moving around then you're going to have problems.

I currently use a cheap rack and pinion style and had similar problems until pulling it apart, lining the tube with new teflon lining, for a snug fit, and then carefully tightened the 2 allen screws and the main nob until the laser was steady.

Until I got the laser I had no idea just how much the focuser was moving around and throwing off the collimation by just focusing. It looked steady as to the naked eye, that was for sure.

[mr.sneezy (Martin)]

Can I borrow your laser ;-)

[Astro78]

Adelaide huh....nope...sorry pal