Quote:
Originally Posted by spaceboy0
Okay have done an experiment. I focused the 8 inch Dob on a tree on a mountain about 2 miles away.
I used an 18 mm eyepiece in both situations.
I marked with a felt pen the point on the focuser tube where the image came to focus with the 2 X screwed directly into the 18 mm eyepiece.
Then repeated with the 2 X lens inside the 2 X adapter and 18 mm eyepiece inside that combo.
In the latter, with the 18 mm eyepiece inside the 2 X adapter and 2 X lens inside that combo, the focuser was 14 mm further in than with the 2 X lens screwed directly into the 18 mm eyepiece.
I got a sharp focus in both situations.
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The latter configuration will give you the full 2X multiplication and the best optical quality. The former will give you less power and might introduce some spherical aberration that could degrade your images.
You have given the settings for visual use. If you rack the focuser out slightly, the telescope can be made to project a focused image onto film several inches behind the eyepiece. The trouble is that if you put the eyepiece/Barlow combination into the camera adapter tube, it will be at least an inch and a quarter, maybe more, farther out from the scope, and the focuser may not be able to go in enough farther to achieve such a focus. There is no way the eyepiece barrel can fit into the small end of the camera adapter from the inside, because both tubes have an outside diameter of 1.25 inch.
You would need a custom-made tube that would slide into the front end of the camera adapter, with a suitable lens centered in it. A 5x microscope objective would be a good choice, because it is corrected for just that optical geometry. You would need a machinist with a lathe to make the adapter.
As has been pointed out before, all of this is a moot point unless the telescope is mounted on a guided equatorial mount.