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Strix,
Because the primary is such a short focal length sphere, it generates horrible amounts of spherical aberration, as you've seen. Any corrector will have to compensate by introducing the opposite form of spherical aberration. This is what the full-aperture corrector plate did. Even though it was but a single element, its location *before* the primary results in a net zero power. This keeps chromatic aberration to a minimum.

Obtaining this to a good enough degree with even a large sample of sub-aperture lenses to choose from will most probably not work. A small corrector made from spherical lenses will have to have significant optical power, and so chromatic aberrations will rear their ugly heads and must in their turn be dealt with. So even if by accident the spherical aberration was reasonably well negated by a cobbled-up sub-aperture corrector, one or more aberrations will be prominent, such as longitudinal chromatic aberration, astigmatism and lateral color, to name a few.

If nothing else, you'll begin to obtain some understanding of optics as you experiment! And there's nothing like going hands-on to accelerate the learning process.

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Without the corrector an SCT is equivalent to a sphere of comparable focal length. Since this is 4" f/12 it should be OK.

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Glenn, thanks for your comments. You’re right, experimenting is crucial for a sound learning process. But as I’ve posted before, the views aren’t so terrible. The views of planets (Jupiter) and moon, and to a less degree stars, aren’t really bad. Some banding and craters can be seen. The problem is a fuzzy whitish halo around objects, no deformations at all. For short tubes SCT’s as the B&L, if I’m right a f/2 primary and a secondary with a negative focal ratio of f/6 yields a system focal ratio of f/12.0. Actually, if the system is f/12, and as Danny pointed out it should be OK (I’ve builted a f/8 newtonian using a spherical primary and views are OK), but looking at some ray tracing on a fast mirror it’s clear enough that peripheral rays never get into focus thus spherical aberration arises. But, if the system is f/12 means that the secondary mirror does its work. BTW, I’ve never seen a B&L corrector plate but I’ve seen some shots of the corrector plates of this same telescope and it look really flat (search for Bausch on CN classifieds and you can see one or two examples showing that the front of the corrector plate is straight flat, judging for the reflection). Being a really fast primary mirror, Danny’s observation about the lack of a noticeable curve of the corrector confuses me, because I’m expecting (fruits of my experiments) that the corrector’s figure must be, if not strong, noticeable negative. Worst, I’ve read that a Schmidt corrector is mounted with the flat side toward the mirror!!! So, does B&L 4000 criterion have, at all, a real Schmidt corrector plate or “it’s just an off-the-shelf plate glass optical window” as Danny pointed out? This could explain the “raving” comments of Uncle Rod about this little scope…

Perhaps Thomas is right, getting the corrector plate from other small telescope and perform some experiments could be nice… Also, Tim’s suggestion about the “adoption” of other small SCT to use the mount is feasible, but being at the end of the world I fear that both options are not practicable… Although our custom officers are really honest (nothing is lost) they ask for their share and don’t show any mercy. Only gifts escape (sometimes) of being burdened with taxes. Anyway, I’ve have another question to you all, dear “Cat” lovers… As I’m thinking to build a thin four vane spider to continue my experiments, it is really critical to set the secondary mirror mount in the same original place were the corrector is supposed to be??? Currently, as you can observe in the attached image, the secondary mirror is well buried into the tube, near the primary baffle. I’m suspecting there’s some vignetting or it dissapear while focusing?

Thanks in advance. A really disconcerted SCT Optics 101 student gives you all his best regards….

Strix

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Hi folks,

I know there have been telescopes that REALLY have just a window in front of the tube and NO schmidt plate. Here is one of those:

http://www.vega-sky-center.com/pages/VAO_Jason_Comet-Chaser_114mm_Catadioptric.html

but that has nothing to do with the B&L 4000 which really should be a Schmidt cassegrain design.

Of course it's possible to do away with the Schmidt corrector plate by making the two mirrors hyperbolic instead of spheric. Then the scope would become a Ritchey Chretien design and not need any corrector any more. But I guess that it is far away from trivial to make such a kind of mirror.

It is then much easier to make your own Schmidt corrector plate.

There is a German forum entry where a guy discusses that he has made a Schmidt plate on his own. The discussion does not go that much into details, but basically he states that it is not more complicated than making a standard telescope mirror (what many amateurs already have done):

http://www.astro-foren.de/showthread.php?t=9519

BTW I find it interesting that the secondary mirror is not only attached via the Schmidt plate. With all SCTs that I know, the secondary is just mounted in the midst of the Schmidt plate, with no additonal spider or the like.

kind regards,
Thomas

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Hi folks,

I know there have been telescopes that REALLY have just a window in front of the tube and NO schmidt plate. Here is one of those:

http://www.vega-sky-center.com/pages/VAO_Jason_Comet-Chaser_114mm_Catadioptric.html

but that has nothing to do with the B&L 4000 which really should be a Schmidt cassegrain design.

Of course it's possible to do away with the Schmidt corrector plate by making the two mirrors hyperbolic instead of spheric. Then the scope would become a Ritchey Chretien design and not need any corrector any more. But I guess that it is far away from trivial to make such a kind of mirror.

It is then much easier to make your own Schmidt corrector plate.

There is a German forum entry where a guy discusses that he has made a Schmidt plate on his own. The discussion does not go that much into details, but basically he states that it is not more complicated than making a standard telescope mirror (what many amateurs already have done):

http://www.astro-foren.de/showthread.php?t=9519

BTW I find it interesting that the secondary mirror is not only attached via the Schmidt plate. With all SCTs that I know, the secondary is just mounted in the midst of the Schmidt plate, with no additonal spider or the like.

kind regards,
Thomas

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