Thread: Bintel BT-252
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Old 09-08-2019, 08:29 AM
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mental4astro (Alexander)
kids+wife+scopes=happyman

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Join Date: Jun 2008
Location: sydney, australia
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Eyepieces.

Do your recall the catch phrase for Valvoline, "Oils ain't oils".

Well, same thing goes for eyepieces with scopes.

Everyone starting out in astro, including me when I started, thinks:

"This is a scope. This is an eyepiece. It will work."

Alas, things are not that simple

There's one popular misunderstanding about telescopes, that the image they produce is all focused on a single point. The WHOLE image is to a point. Well, that is true ONLY for a single star. Remember, there are stars littered all across the field of view, so now the FOCUS of a telescope is not entirely to a single point, but across a plane.

And it is not a flat plane, but a curved one, somewhat like a bowl.

AND the design of the scope determines if that bowl shape is convex or concave (what direction the bowl shape is facing) and how deep the bowl is.

Refractors, SCT's and Maks produce a convex focal plane. Newtonains (dobs included) produce a concave plane.

Eyepieces are designed to FIRST work to a particular shape of focal plane. Get the right eyepiece in the right scope, and you have an optical match, and the image is as good as it gets. Put the wrong eyepiece in the wrong scope design, and a whole host of aberrations will become apparent, and this is an optical mismatch.

As things are, it is easier and cheaper to design and manufacture eyepieces for scope that have a convex focal plane (refractors, SCT's, Maks). Designing and manufacturing eyepieces for Newts is much more difficult and expensive. There are extremely expensive eyepieces made for refractors too, and these are also much more sophisticated eyepieces.

What you will find in MOST reviews about eyepieces is see is totally dismiss an entire line of eyepieces based entirely on the use of one single focal length piece from a model line that was used in the wrong scope design. Most people do not understand optical matching, and so will see a bunch of aberrations that are all the clues to an optical mismatch, but out of ignorance will just say that the eyepiece, and that entire line of eyepieces, are rubbish...

Contemporary eyepiece design is a very complex one. As a result, from a line that is designed for a convex plane, there may be an individual focal length or two or even three that will perform just about flawlessly in a Newt! So don't make the mistake of falling for the "trash talk" about an eyepiece line. For from a line that is dismissed as "rubbish", you may find those outstanding and inexpensive gems!

What are those aberrations?

Astigmatism, field curvature, pincushion, chromatic aberration, spherical aberration, coma. Scary? No, not really. Just symptoms. This is a great site that explains these, and telescope and eyepiece optics. What's great about it is it gives a simple explanation and then leads to the much more involved mathematics behind things ONLY if you want to pursue them.

~~~~

Yeah, I know I've banged on a lot in a thread about a 10" dob and wanting to see the planets more easily. But it's as good a place as any to get you thinking a little more broadly about things.

Alex.
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