Planet view obscured by overlay

[grimsay]

Hi guys,

I just picked up a 5mm TMB eyepiece for use in my 8" f6 Dob. With the resulting 240x mag I keenly sought out Jupiter last night and patiently waited for Mars and Saturn to rise.

Seeing wasn't great but the images were reasonably sharpish. However, I found with viewing all 3 planets that there was a large semi-transparent overlay around the planet (see example graphic).

Can anyone shed some light on this for me?

Thanks, Iain

[Nikolas]

Just a guess, scope is out of collimation?

[grimsay]

Quite possibly, I'm a complete newbie and don't have anything to check collimation yet... watching the classifieds.

I didn't think collimation would be so significant though... stars seem pretty good... i think, but have no experience. I've heard of collimation star test, will read up.

[raymo]

Iain, my first thought is moderate fogging up of the secondary mirror, judging by the shape of the artefact. If it happens again remove the eyepiece and have a look at the secondary mirror. Have a look down the tube at the primary mirror while you're at it. You can easily check your collimation by training your scope on a fairly bright star and defocus
it until you see a circle made up of smaller concentric circles with a black
solid circle in the middle. If everything is anywhere near concentric then
collimation is not your problem; it can be quite a long way out before it
becomes visually obvious, and your f/6 is less sensitive to collimation
than faster scopes such as f/4s and 5s.
raymo

[grimsay]

Thanks for your help guys.

It looks like you were right about the fogging Raymo. Tried again last night and I didn't notice any overlay.

A more experienced fellow also checked the collimation on a bright star for me and it is slightly out but not to much to impact things much.

Need to keep my eye on the classifieds for a collimation tool.

Cheers, Iain