Terry swan said:
At what point does a powermate become basically usless.
Lets say you have a 3x or 4x. Would the magnification of the 4x be some much that it is usless due to loss of resolution?
And what pieces would it be good for. I am assumint that a 4-9mm would not be to much mag. And does a powermate cut down the light coming through the eyepeice?
1.25 worse than a 2? for higher powermates (3x+)
Thanks guys for your input.
Magnification becomes useless when the exit pupil gets much smaller than half a millimeter. The exit pupil is the beam of light coming out of the eyepiece--you can see it as a round disk when you look at the eyepiece from a distance with the scope pointed at a bright object. It is equal to the focal length of the eyepiece divided by the focal ratio of your scope; so for example, a 10mm eyepiece in your f/5 reflector has a pupil of 2 mm. Now, if you put that eyepiece into a 2X barlow or powermate, it is effectively a 5 mm eyepiece, and it has an exit pupil of 1 mm. You have cut the diameter of the pupil in half, but that means you have reduced the brightness of your target (this is assuming you're looking at an extended source, like the moon) by one-quarter, because the brightness of an extended source goes with the area of the exit pupil, that is, with the square of the diameter. If you put that same 10mm eyepiece in a 4x powermate, you would have a 2.5 mm eyepiece with a 1/2 mm exit pupil: the moon would look 16 times dimmer than in the 10mm eyepiece itself.
Most eyes will open to a maximum of 7mm (and less for us older types); at the other end of the scale, 1/2 mm is the limiting exit pupil for most targets--you just run out of light and have empty magnification for anything smaller. The exception, I would say, is a really tight double: point sources like stars do not suffer diminution of light in smaller pupils as do extended subjects, and occasionally it is useful to take the scope up to "stupid power" to split a double--I've gone well into 600X or more to see a good split.
On my 10" f/5, about the highest I go on planets is with my 9mm in a 3X barlow, effectively, a 3mm eyepiece giving me 425X with an exit pupil of 0.6 mm.
Therefore, you want to arrange your eyepiece collection to proceed (in geometric progression) from about a 7mm exit pupil down to about a 1/2 mm exit pupil. At f/5, that means starting around a 35mm eyepiece and proceeding down to about a 2.5mm or 3mm eyepiece (or eyepiece + barlow combo yielding an effective 3 mm length).
As for powermates cutting down the light--as discussed above, most of the light loss is going to come from the increased power, not from transmission losses. It may be that the extra elements of the powermate cause a tiny bit more of the light to be lost, compared to a barlow, but that's probably pretty marginal. There are other reasons arguing for and against powermates vs barlows. (One is weight-powermates are heavier.) The point of the powermate is to preserve the location of the image, that is, to keep the same eye relief; a barlow tends to increase eye relief somewhat, and this can be a problem for longer fl eyepieces that already have a lot of eye relief. If you only intend to barlow shorter fl eyepieces, than the regular 1.25" Televue barlows (2x and 3x) are really excellent and you will not gain any advantage that I know of from using a powermate instead.