Paul Corfield
Well-known member
Paul,
I wonder if you have considered using a barlow instead of a teleconverter in that astro scope?
Because a telephoto already has negative focus group in the rear of the lens a teleconverter is made to correct for this group. On an astro scope at prime focus no such group is present.
Because of this you might want to try to use a barlow in place of the teleconverter. It MIGHT, and I'm not sure, give better performance in an astro scope than a teleconverter which is made to correct for a conventional telephoto lens.
In any case a barlow is cheap and wouldn't cost a lot to try one out on your setup.
SF
A few shots taken with an 80mm Astro-Tech APO triplet with a 2x barlow.
Sout Fork, you are the man. :t:
Initially I had tried a 2X barlow but I could reach focus on anything throughout the range. I needed like a couple of extra inches of travel. What you said today got me thinking and with a little swapping around of optics I go it to work. What I did was unscrew the chrome barrel containing the optics from the barlow lens and I screwed that into the t-mount. The t-mount originally had a standard 1.25" chrome barrel. I've now screwed that onto the barlow. To get it to work I have the camera+t-mount with barlow+old barlow housing with old t-mount chrome barrel. Mounting them in that order gives me the extra couple of inches I needed on the focus.
Here's two photos, just cropped/resized and nothing else done, no sharpening etc. One on the left taken with the barlow configuration and the one on the right with the 2X teleconverter. There's very little in it but the barlow does have the edge. Took another one with the barlow combined with the teleconverter (approx 2880mm equiv). The sun had gone in and it was a struggle but on a bright sunny day it would be usable if you needed the extra long range.
Cheers Sout Fork.
Paul.
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