Telescope Reviews: Do I have what I need for my 16" Dob for DSO's?

mantrain,

LPs? HPs? Low Power? High Power? I've never heard those acronyms before. At first I thought you meant Light Pollution and Hewlett-Packard.

One basic concept to keep in mind when choosing eyepieces is the exit pupil they would produce when used in your telescope. Exit pupil = focal length of eyepiece / f-number of telescope. With this knowledge you can set your LP and HP and go on from there.

In general, the widest exit pupil should not exceed the widest dilation of the pupil of your eye, which is no more than 7mm in most people (probably less for older folks). This will set the limit for the longest focal length eyepiece. (Eyepiece focal length = exit pupil * f-number). Also, for telescopes which have a secondary mirror, the exit pupil should definitely not exceed about 8mm, or you will probably start seeing the shadow of the secondary as a big blob in the center of the field of view. This will be much more noticeable during the day and when observing bright extended objects at night, but it will still degrade the image. If your telescope is f 4 and the pupil of your eye dilates to 7mm, the longest focal length eyepiece should be no longer than about 7mm * 4 = 28mm.

IMO & IME, the narrowest exit pupil - which will set the limit for your shortest focal length eyepiece - should not be less than about 0.5mm, and 0.8mm would probably be better for all but the steadiest nights. Assuming your telescope is f 4, that would give you as your shortest focal length about 0.8mm * 4 = 3.2mm, or 0.5mm * 4 = 2mm. For a 16" f/4, a 3.2mm eyepiece will yield 508x, while a 2mm eyepiece will give you 813x! This will be further limited by seeing conditions and whether or not your mount tracks. You'd probably be safer with a 4mm eyepiece for about 407x and a comfortable 1mm exit pupil. IME, for DSO, only planetary nebulae would get much use out of powers that high.

Of course, if your telescope's f-number is not 4, you'll have to adjust the figures for your specific f-number.

Assuming you're at f/4, a 50mm eyepiece would be much too long a focal length for your telescope. 50mm / 4 = 12.5mm. I doubt if anyone has a 12.5mm eye pupil! Also, that is way beyond the 8mm Big Black Blob limit for reflectors.

Paracorrs are nice, but they are not a necessity. It'd be better to gain some experience with how things look without one. Then maybe borrow another observer's Paracorr to try it out before you buy. Or, if the coma is very obvious to you and you just can't stand it, go get yourself one.

Clear Skies,
Mike