FOCUS problems

[jezza]

Am using a 20mm 2'' meade eyepiece on a 12'' meade lightbridge. The focuser has to be pulled all the way out to achieve any focus at all. The middle of the viewed image is OKish but the view towards the edges goes progressively blurred. Kind of like a Star Wars hyperdrive look. When I place a Hyperion 20mm (21mm?) the image is perfect. Just wondering if the primary mirror could be too close to the secondary? PS have used a laser collimator to achieve primary mirror centre point. (forgot to check return point alignment though)
Any ideas out there?

[Zuts]

Originally Posted by jezza

Am using a 20mm 2'' meade eyepiece on a 12'' meade lightbridge. The focuser has to be pulled all the way out to achieve any focus at all. The middle of the viewed image is OKish but the view towards the edges goes progressively blurred. Kind of like a Star Wars hyperdrive look. When I place a Hyperion 20mm (21mm?) the image is perfect. Just wondering if the primary mirror could be too close to the secondary? PS have used a laser collimator to achieve primary mirror centre point. (forgot to check return point alignment though)
Any ideas out there?

Hi,

If the Meade is focussed then it's focussed. Are you saying that it's not focussed in any portion of the field of view?

I think this says more about the quality of the meade as opposed to the quality of the Hyperion. Since you say the Hyperion gives you good sharp contrasty views then your collimation is probably OK.

Paul

[Jeff]

Hi, I don't know of a 20mm 2" Meade eyepiece - are you sure it's not the 26mm Meade QX that may have come with the LightBridge?

If so, then you can have focus at the edges or in the middle, but not perfect for both. It has a nice bright & wide fields of view, but tends to show aberrations for the outer 30-40% of the view for fairly fast reflectors (f/5 or below).

And yes, with my LightBridge I also needed to have the focusser all (or almost all) of the way out to bring this eyepiece into focus. Make sure your focusser is travelling all of the way out (sometimes the tension and/or locking knobs on the Crayford focusser need adjusting). Another tip ... you can also affix your Meade EP slightly higher up in the focusser barrel .. see what that does.

My solution was to get a few Televue EPs ... cost some dollars, but they hold their value and might be worth a try ... especially if you use the scope a lot.

Cheers,
Jeff

[jezza]

Apologies first ... it is a 26mmQX. Hyperion is 21mm.
Thanks for the info on fast refractors
Are the aberrations you mention 'motion blurred' items (like I seem to have) OR coma-like problems as when photographing the moon (IE: false colour around edges?)? The problem I have is more like 30% OK in middle and rest progressively worse. I thought (probably naively) that the eyepiece provided with the scope would be ideal. Focus is achieved only at maximum focuser extension (centre of image only). Moving the eyepiece up the barrel doesn't help clarity.

Still wondering if Moving the mirrors to maximum distance apart would reduce the focus point further down the barrel? Anyway will keep experimenting.

Thanks very much for your replies
Jezza
...mmmmmm Televue....