Posts Made By: Vladimir Sacek

Optically, advantages of the c9.25 come from its slower primary (~f/2.4). For one, it makes it easier to produce aspheric corrector within certain tolerances, the way it is manufactured. Then, its field curvature is milder, nearly 300mm, comapred to ~170mm in a "regular" 8" SCT. This makes for a better field performance.

There are speculations that its primary is aspherical, in which case secondary could be spherical (that would be a plus, since small aspheric convex mirrors are also tricky to manufacture) but it's not so. I knife-edged it at the radius of curvature, and the shadow is perfectly straight (characteristic of a spherical mirror). Secondary is more strongly aspherized than in a "regular" design, possibly even mild hyperbola, but it's more likely that they kept it a strong ellipse.

Also, due to lower secondary magnification, chromatism in the c.9.25 is about twice lower than in a "regular" sct. That shouldn't be a dramatic advantage by itself (due to relatively low chromatism in a "regular" sct), but combined with better, on average, corrector figure and milder field curvature, it is probably enough to give noticeably better impression.

You can do the little trick that Mike Hosea posted somewhere sometime ago. Place relatively bright star close to the edge of your 40mm ep and have it defocused. You'll see clearly enough how much of the defocused circle is cut off (missing) due to the baffle blockage for an approximate estimate of the severity of vignetting.

More precise method would be to measure up baffle diameters and positions in regard to the lens and focal plane, and apply appropriate formulas to the obtained values. But the quick look at the defocused star is much easier to do, and usually sufficient.

Ron, it takes more than one brief "test" to determine what is the effect of c.obstruction. Seeing is a factor that can play games with us. Also, we all have our biases: if you want to be really objective, such test should be a "blind" test (you don't know when the c.obstruction is on, when off).

A 33% obstruction causes nearly 11% light loss. It is equivalent to 5% increase in magnification. I am yet to experience "very noticeable" loss in brightness from a 5% magnification increase.

You also may want to check optical surfaces. If there is something on them that causes partial transmission loss, like a narrow segment towards the perimeter, it could result in a (visual) loss of a segment of the diffraction ring.

Chromacor is optimized for chromatic correction of an ~f/10 visual doublet achromat. This means a system in which red and blue foci are close one to another, and placed ~1/2000 f.l. outside the green-yellow focus. Neo-achromat is a four element system, so you'd need to play a little with color filters in order to find out approximate locations of the blue/red foci in regard to the green/yellow. If they are close to those in a comparable visual doublet achromat (~0.36mm separation), Chromacor should noticeably improve color correction, although not as much as in an ~f/10 system.
However, simpler and safer way is probably to ask the retailer.

Image scale refers to an image formed in the focal plane, not the one we see through the eyepiece. Image formed by the objective is real, and formulas that Jim gave are for it. Image seen through an eyepiece is magnified by both objective (Mo=fo/250) and eyepiece (Me=250/fe), resulting in the familiar total magnification formula M=MoxMe=fo/fe.
In other words, even if two instruments have identical total magnification of 80x (or any other), it is quite possible that one of them has larger image scale. As Jim points out, it would be one with longer f.l.

No, that would be too easy - Barlow magnifies the whole thing: diffraction disc and chromatic blurring around it to the same degree. It may add some chromatism of its own, but it is normally (with good Barlows) negligible.

Agreed.

Tan, it's almost like a fly-by; you take us out to the space
and don't even charge for the ticket Can you tell your feeling on how much (say, as percentage in a 100%) of the definition you achieve is due to (1)imaging instrument (2)imaging setup w/o camera, but including the output-to-medium match (3)medium (camera) itself and (4)processing.
Let's try to treat the seeing as a constant. I know it's sort of awkward and somewhat overlapping , but maybe you'll be able to make some sense out of it?

This mask turns the 11" into a multiple (triple) aperture telescope with asymmetrical aperture opening. The resulting diffraction pattern is likely to consist of some sort of triangular star-like pattern that fits in a circle nearly twice the Airy disc diameter of the 11" aperture. The spikes would be visible on brighter stars in between the points, and also in the direction of points. There would't be, however, continuing ring structure visible. With some turbulence, the pattern could appear more roundish, w/o ring structure.
It could perform, for general observing, similar to 6" to 7" unobstructed aperture resolution-wise, and for some purposes even better, due to less energy out of the circle containing the core pattern.