Telescope Reviews: Favorite DS mag?

Oh no, that rule doesn't apply to stars or star clusters! The "rule" I stated earlier only applies to object that you cannot resolve stars in- galaxies, nebulae, very faint globular clusters. My bad- I ALWAYS forget to caveat that rule with the fact it only applies to unresolved objects...

The reason it doesn't apply to resolvable objects is because with a resolvable object like a globular, the ACTUAL detail you are looking for is tiny- a arcsecond across with good seeing. You're not looking to see the entire object as a whole, you're looking for individual stars. Secondly, individual stars do not have an angular size, so you can't really say what an optimal magnifcation is for them. There is probably a rule for individual stars, but I don't know it, and it's certainly very dependent on seeing.

Also note in that rule I stated, I said "an object or detail". This means, for example, if you want to see the spiral arms of M51, the optimal magnifcation for M51 ISN'T the lowest magnifcation with an exit pupil of 7mm or less at which it spans at least 5 degrees. Instead, the optimal magnifcation would be the magnification at which the width of the spiral arms spans 5 degrees. If you want to see individual star forming regions within the spiral arms, then the optimal magnification would be even higher. As I already said, since what your looking for in globular clusters is individual stars, which are tiny objects, then the optimal magnifation would be pretty high. More than that, the rule I stated previously cannot help you, because once again, stars are not extended objects.

As for bright globulars, The higher the magnfication you use, the better you will resolve a glob, up to a point of course, that point dicated by seeing, optics, optical alignment, etc. That point is probably somewhere around the tradditional rule for the highest usable power, the "60X per inch of aperture" rule. However, a more asthetically pleasing view may occur somewhat below the lowest power for maximum resolution. I'd say that I like to use anywhere between 10-20X per inch of aperture on globs- that's around 2mm exit pupil, plus or minus a half or so. So like 250X-500X at least in my 25". Of course, it also depends on the globular, so that rule for like 250X-500X is really just for ones like M13, M15, M3, etc.

Come to think of it, at least with really big scopes, the most athestically pleasing view of a globular may come at the highest magnification that frames the entire object, and wouldn't be dependent on exit pupil.

Edited by tatarjj (07/21/08 02:22 PM)