Telescope Reviews: Seven magnifications with 2 EPs and 1 Barlow

Last night I discovered the answer to the riddle: How can one get seven magnifications with two non-zoom eyepieces and one Barlow? (Actually, one can get eight if one chooses the eyepieces right.)

I was using a cheap Owl Astronomy 2X Barlow, a 27mm Kellner, and a cheap 10mm Owl Plossl. It's easy to get four magnifications:
27mm -> 33X (on my F/4.5 8" scope)
10mm -> 90X
27mm + Barlow -> 67X
10mm + Barlow -> 180X

Two more are pretty easy. Unscrew the lens from the Barlow and screw it into the front of the eyepiece:

27mm + Barlow-lens-only -> 50X
10mm + Barlow-lens-only -> 135X (This is actually my favorite combination--I typically store the 10mm eyepiece with the Barlow lens on it)

What was trickier was to get the seventh magnification from my collection of optics. It turns out that the Barlow tube can be unscrewed into two pieces. One of these pieces is about 2 inches long, and has a filter thread on one end, and a somewhat larger thread on the other. The 10mm Plossl has an outside barrel that is screwed into the eyepiece with a filter thread. I remove the Plossl's barrel, and replace it by the tube from the Barlow. I screw the Barlow lens into the bottom of that tube (that's it's proper place, after all). Now, I put the Plossl, without its outside barrel, into the tube. It has about 1mm wiggle-room, as the tube's ID is wider than the OD of the Plossl's inner barrel. But I attach the Plossl with masking tape (or I could just leave it to sit there by gravity--the top part is wider than the tube, so it won't fall in).

Ploss + masking tape + 2 inches of tube + Barlow-lens -> 155X

(Jupiter looked almost as well at 155X as in my 135X combination. The pretty center of M25 looked better at 155X than at 135X.)

It's a bit of a nuisance to do all this shuffling of lenses, and one is likely to get them dirty.

I haven't tried the same trick with the 27mm, but I don't think it disassembles the way the 10mm does. If it did, I'd also be able to add 58X to my armory. But I don't need 58X if I have 50 and 67X. The 155X, though, was nice, because I wanted something a bit more than 135X, and 180X was too much.

I had more fun shuffling eyepiece pieces than observing last night. But at least I had fun.

It wasn't entirely a pointless exercise. I was thinking of buying a 6mm or 7mm EP, and wanted to see if 6mm would be too much magnification. (Answer: it's fine.)

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Coulter Odyssey 8"
Skymaster 15x70
BPTs4 8x30
25(?)mm Rini, 27mm Kellner, Owl 10mm Plossl, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer

Edited by arpruss (09/22/08 07:35 PM)