|
Steve,
One of the best tests for sharpness of a high power eyepiece is to count the craterlets in Plato (on the moon) and compare which craterlets you can and can't see in the respective eyepieces and how easy they are to see. Unfortunately this doesn't help with the longer focal length eyepieces you are dealing with here.
One option is as Geoff mentioned to focus on a star and see how tight they focus down but there are a few things to be aware of here.
When you are working with lunar/planetary features, to compare eyepiece sharpness, the analysis generally takes long enough to even out the effects of variable seeing as you will get enough moments of "good seeing" with each eyepiece to enable you to judge the eyepieces performance when the air was still. It's unlikely that you will get this same evening out effect on a single star or starfield, as their simply isn't enough to concentrate on with "a few stars". You need to be able to evaluate what is attributable to "seeing" and what is or isn't attributable to the eyepieces themselves and sometimes you need more than "a little" experience to figure this out. As Geoff mentions if your going to do this using a starfield make sure its a night of good stable seeing to eliminate as much as possible atmospheric variations. I also recommend that you try to use stars of approximately Mag 7 to Mag 10 (if you use this method) as stars in this range will not be as affected by diffraction effects eminating from the scope itself (eg. spider, secondary, mirror clips etc).
Another option is to pick the eyepiece "you think" is going to be the least sharpest of the bunch. Study the moon and select a lunar feature that is on the verge of visibilty and resolution and then see how it looks with the other eyepieces and see how much better the other eyepieces resolve the same features. Then work in the reverse order on a different set of features starting with the eyepiece you think will be the sharpest.
CS-John B
|