Posts Made By: Vladimir Sacek

That is probably because the camera lens in an afocal setup
works with converging cones coming from the eyepiece. The camera lens - and autofocus mechanism - are designed for midely to moderately diverging beams. Converging beams will result in shorter camera lens to image distance, which may put it out of range for the auto-focus mechanism. Also, resulting change in image position for a given camera lens adjustment is different for the two different beam shapes.
It doesn't make it better that lens optimized for diverging beams will have increased level of image aberrations when working with converging beams.

It may be enough just to re-adjust the focuser; it may be off the objective's optical axis.

Looks like hyperboloid (outer ~50%) that contains a sphere
(inner ~50%).

On a closer look, the mirror is probably ellipsoid up to ~60% radius, transforming into hyperbola up to ~90% radius, after which goes into TDE. I don't quite get the inside focus pattern; looks like the screen hasn't been centered, as it is with the out-of-focus shot.

If I got it right, defocus between marginal and paraxial rays with the source at the r.o.c. should be 10-11mm for this mirror. With the screen ~5mm out of the marginal focus (which is longer than the paraxial for ellipse/parabola/hyperbola), it is about three times closer to the screen than the paraxial focus. This would make top of the central line appear about three times thicker than its middle. It is about there at the 90% radius, but the line is not thickening at a "proper" rate: deformation is supposed to go in proportion to the square of off-center distance. So if it is 3+ times thicker (corresponds to
150%+ deformation) at the 90%, it should be about half that much at the 60% (about 80% deformation), and it is somewhat less. This indicates that defocus diminishes faster, probably as a resolt of midely hyperbolical ~60%-90% zone.

On the other hand, bellow ~60% zone, change in the line width is barely noticeable, indicating that defocus within this zone diminishes somewhat slower than it should for a parabola. Since some mild curves are noticeable on both central and side lines, it is probably an ellipsoid.

Assuming it is so, profile-wise, the mirror seems deeper than needed up to about 60% zone, then turns down with a stronger slope than a parabola would have. Seems logical to try to flatten ~30%-80% zone, in order to bring it closer to a parabolic surface. I'm sure some other people here know better abot the way to accomplish that (including TDE and not nearly as important central depresion).

1/32" is way too big. Suiter suggests that angular size of the pinhole is half that of the Airy disc. It also needs to be placed far enough for not to induce significant spherical aberration (since telescopes are optimized for objects at infinity). Parabolic mirror generates 1/4 wave of spherical aberration when object is placed at 28(D/F)^2 feet away, for D(aperture) in inches. If you don't want to have more than, say, 1/20 wave in your setup, you need to place the star at about five times greater distance.

Whatever distance (S) you choose, pinhole diameter should be about S/1500D (make sure that both distance and diameter are expressed in same units).

The above "distance" formula doesn't apply to refractors and Cassegrain-like systems. 1/4 wave distance varies with the system, but will be significantly greater. With these systems, distance to artificial star shouldn't be less than a couple hundred feet.

To produce a pinhole small enough, you can use a high-power microscope objective or eyepiece. For instance, if you place a 40x (4mm f.l.) microscope objective 50mm in front of 0.8mm (1/32") pinhole (so that objective's back faces the pinhole), it will form an image of the pinhole that is 50/4=12.5 times smaller, or 0.064mm in diameter.

Or, placing, say, 5mm eyepiece 50mm in front of the pinhole (facing it with its field lens) will produce a pinhole image in front of the eye lens that is about 50/5=10 times smaller, or 0.08mm in diameter.

Such artificial star should also be brighter than one produced by a real opening of that size.

If you use a reflective sphere, the "pinhole" distance/diameter requirements remain identical.
A circular light source of the opening diameter "L" directed at a reflective sphere of radius "r" placed at a distance "S"(in a setup in which the light source directed towards the sphere is by the observer's side) will produce artificial star of diameter ~r^2/2S.

You may need to do a little experiment with the eyepiece out of the assembly. Place it in front of a projection screen, separated from the eye lens as much as it is the
eye lens-to-film separation within the assembly. Place some bright object in front of the field lens, at a distance where its image created by the eyepiece focuses at the screen. The object-to-field lens distance in this setup is identical to the focal plane-to-field lens distance when the eyepiece is placed in its photo-assembly. The magnificatin factor M is given by S/S', with S=eye lens to the image (film)separation, and S'=object-to-field lens separation. The effective F# is MF, F being the F# of the telescope.

John, with diagonal-to-focus distance of 228.5, your min diagonal minor axis with an f/5 scope is 228.5/5=45.7mm.
This means that a 47mm diagonal edge would overlap the axial cone converging from the primary by as little as 0.65mm. That is not a desirable situation. Keeping the diagonal edge at least a couple of mm away from the light forming diffraction limited field guards you from having likely imperfections around the edge erea impair your high-quality field. With diffraction limited field diameter for an f/5 of ~2.6mm (from F^3/47), you'd have that minimum achieved with 52.6mm or, say, 2 inch minor axis. That would give an 8.4mm (0.39 degrees) fully illuminated field diameter, and 62% illumination 17.5mm off-axis (the very edge of your widest eyepiece).

Most people agree that pushing the diagonal size to the very minimum is likely to be counterproductive. In this case, difference between the two sizes in terms of influence on image quality is similar to that between 1/6.6 wave and 1/7.2 wave of spherical aberration. Bringing possible diagonal edge imperfections into
diffraction-limited field can sure have more damaging consequences.

Tom, I'd worry about the upper cage diameter in regard to thermal issue more than in regard to potentil vignetting. If you'd have a circular 13.1 stop at the front opening, it would cause 10% wignetting 24mm off-axis. Unlike vignetting by the focuser bottom, which kicks in only when it gets larger than that at the diagonal, vignetting at the front opening adds to that at the diagonal; so, if vignetting at the diagonal is 40% 24mm off-axis, the total vignetting would be 40%+10%=50% - not a big difference (BTW, such stop would help you get rid of most of primary's off-axis astigmatism, which would probably be more of a plus for the edge definition ).

More important is to have any thermally unstable surface farther away from the incoming bundles forming your high-quality field. That would be a reason not to go bellow 14" ID. Somewhat more wouldn't hurt either.

A 2.55" sounds a bit too tight, although it wouldn't have the very edge area "participating" in forming scope's diffraction limited field (which is here less than 2mm in diameter). But, like you said, that's the closest you could get, and that should work well too (even if you go for 14.5" ID, to accomodate the filter slide).

There is a graphic description of Paracorr's performance with different F# at TeleVue's site:

http://www.televue.com/images/accessories/Paracorr%20Graphs.jpg

According to it, improvement in off-axis sharpness should be noticeable at f/5.

Ray, what is the SCT, the reducing factor you'd like to get,
and the primary purpose (photo or visual)? SCT reducer has to have quite a bit of astigmatism of appropriate sign in order to keep the field nearly flat. That's because a positive lens w/o such astigmatism "content" tends to worsen the field curvature, which is already quite strong with SCTs. In other words, you need to know specifics of a lens you intend to use (radii, thicknesses, indici) to find out what effect on field curvature it will have. Given location, it's easy to figure out what f.l. will result in desirable f# reduction (counting in necessary defocus to bring it to the eyepiece field stop); it's just that you don't know what kind of aberrations in general an "anonimous" lens would induce.