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Posts Made By: Vladimir Sacek

October 21, 2003 06:22 AM Forum: Equipment Talk

Eyepiece Extension??

Posted By Vladimir Sacek

If it's an 8" f/10, every inch of extending the focus will introduce ~1/16 wave of undercorrection, and increase the efective f# by ~5%. The final effect depends on what level and kind (overcorrected/undercorrected) of correction you have at the present focus position. If it is, for instance,
1/4 wave overcorrection, then a 2" focus extention would practically cut it in half. But if it is 1/4 wave undercorrection, you'd end up with 1/2.7 wave correction error.

For larger f/10 SCTs, the despace error diminishes proportionally (i.e. a 10" would have ~1/20 wave of undercorrection induced for every inch of focus extention).

October 21, 2003 06:45 AM Forum: Equipment Talk

Cassegrain Question for Rick and Vlad

Posted By Vladimir Sacek

Walt, you're right, they need to determine primary's figure and f.l. It should be as close as possible, because it determines needed secondary conic, but doesn't have to be - and, chances are, won't be - dead on. Final adjusting to secondary position with best correction will have to be done through a trial-and-error process. The closest they come to a real primary's parameters, the less deviation from the planned focal plane position.

If the primary is a sphere, the knight edge shadow will readily show it. But if it is parabolic, or hyperbolic, you'd need to make precise measurements to find out which one is, because an RC primary typically has quite small hyperbolic deviation.

October 28, 2003 09:47 AM Forum: Telescope Making

EQ Platform Finished

Posted By Vladimir Sacek

Very nice, indeed - a work of love. It sure deserves less of a tracking error. For near-perfect tracking, a point on the circle section needs to make full circle (360 degrees) in 24 hours. At present, it seems to be some 70 degrees behind or ahead of it.

November 1, 2003 10:39 AM Forum: Equipment Talk

masked reflector resolving power?

Posted By Vladimir Sacek

The only mask that comes close to the resolving power of the full aperture is hexagonal, around a 33% c.obstruction.
It is still inferior, due to roughly a magnitude brighter 1st ring. The fewer holes, the more inferior resolving limit
of a mask vs. full aperture. A two-hole mask (holes at the diameter) would have a sort of elongated central spot, with the length equal to that of the central spot of a single opening, and the width about 1/3 of it (depends on the holes' relative separation). The "empty" space of this shape (if you imagine it encircled) conatins a couple of very bright spikes. The resolution in one direction is similar to that of a single opening (slightly better, due to less energy outside the central spot), and somewhat better in another one, but not by much, due to those couple of bright spikes that get in the way.

Practice will inevitably differ from what the theory says, because there is a number of variables that affect the final result, which in a typical (read: uncontrolled) practical environment are partially or entirely ignored.

November 6, 2003 01:54 PM Forum: Telescope Making

To Vladimir

Posted By Vladimir Sacek

Donnie, I have c9.25 Ultima, Intes' MN5 and a home-made short-tube 8" Dob, at present in re-workings (want to make it so that the collimation is done only at the focuser; wish me luck). I like them all, but I do like most any telescope, even those that don't perform very well. There's something about them; if I'd try to put a finger on it, probably a feeling that a telescope can help me reach something, get to some very special, happy place.
And as we all know, they do, every once in a while...

November 12, 2003 09:26 AM Forum: Telescope Making

Homemade Hubble Space Telescope

Posted By Vladimir Sacek

Glenn, this boy's smile proves that such a little project can be worth its weight in gold. Or it goes under "some things money can't buy" (priceless) - more likley...

November 25, 2003 12:48 PM Forum: Equipment Talk

Help with Greory/Cass. Design Params.???

Posted By Vladimir Sacek

Ed, Scott is right - you can't have an acceptable two-mirror system with two spheres. A pair of spheres would give you horrendous spherical aberration. Even with only primary spherical (and aspherised secondary) coma is still exorbitant, worse than with an f/3 parabola.

Other than aspherising the primary, one thing you might try is to get one of those 6" short tube ota's with spherical primary and negative lens corrector in front of diagonal. I think those primaries are also ~f/3 and, if so, the corrector should work as well with an 8" sphere (as long as it is placed at the same distance in front of the primary's focal plane).

November 28, 2003 08:43 PM Forum: Telescope Making

a severe hyperbola

Posted By Vladimir Sacek

Phil, what the literature usually recomends is working the 70% zone, since this is the zone that takes the least degree of deformation during refiguring a sphere into parabola. Hyperbola is only an "overdone" parabola, which means that the center is deeper and the edge zone lower than what they are supposed to be (with the edge being more overdone than the center). To compensate for this, the 70% zone needs to be lowered too, in order to bring the curve closer to parabola. With hyperbola being shallower than parabola, it is the edge that needs to be worked the least, with most of glass being removed from around 70% zone towards mirror center.

To determine best course of action, it is necessary to know more specifically what the surface look like, and to have some practical experience with it. I'm sure Mark Harry would know what is the best way to do it.

January 3, 2004 12:18 PM Forum: Equipment Talk

D&G 6-inch f/15 Refractor

Posted By Vladimir Sacek

Unfortunately, a 6" f/15 achromat is not chromatism free. It doesn't compare to 60mm f/15; the larger aperture, the higher f# needed to keep chromatism at the same level. So, 60mm f/15 compares to 152mm f/38, and 152mm f/15 to 60mm f/6, or to 100mm f/10.

This amount of chromatism compares to somewhat worse than 1/4 wave s.a. level, which is not bad at all. Due to lower "sensitivity" of an achromat to other error sources, a 6" f/15 refractor is likely to outperform high-quality 6" Mak-Cass with ~35% c.obstruction.

January 8, 2004 02:01 PM Forum: Equipment Talk

Wavefront error of an average C8??

Posted By Vladimir Sacek

Francisco, there is no known correction error average for C8s. Exessive correction error usually is not the main problem with SCTs (it is normally possible to bring it within diffraction limited by a small change in primary-to-corrector/secondary separation). The "big uknown" is fabrication quality (mirrors/corrector surface roughness) and proper positioning/centering of the elements. It may be good - or very good - and may not. If it's not, it may be "fixable" to some extent, and may not, at least for an average amatuer. If you can return it, no problem, if can't, try to check it out before you buy. Otherwise, anything's possible - expect the unexpected too.