Posts Made By: Vladimir Sacek

Tom, you are probably talking about eyepiece astigmatism. It is nearly in inverse proportion to the f#, so it would be some 11% greater in an f/4.5, compared to f/5. It is much less of a factor than coma, which would make quality field in an f/4.5 nearly 30% smaller linearly, and nearly 20% smaller angularly (counting in eyepiece astigmatism, quality field size gets another 2-3% smaller).

An f/4.5 would be also about 30% more sensitive to miscollimation. The only disadvantage of an f/5 is ~7" extra length/focus hight.

Ricardo, Schmidt-corrector in combination with spherical mirror is quite sensitive to decenter. Even a few mm off can result in more than 1/2 wave worth of center-field coma, and that can be noticed with bright star-like objects even at low magnifications. You'll probably have to correct the decenter through trial-and-error process. It is possible that these Schmidt-Newtonians have diagonal offsetted within the corrector. In such case, the diagonal would be not in the middle of the aperture opening, but somewhat farther away from the focuser side. The amount of offset is nearly approximated by A/4F, where "A" is the minor axis, and "F" the f#. You could try that first. But, before you start fiddling with it, it is probably better to triple check the collimation.

When coming from close object, rays from every point diverge. As the object gets closer, angle of divergence becomes significantly larger for marginal than for paraxial rays. It results in marginal focus, after refraction, being moved farther away from the objective. The effect is overcorrection.

Suiter gives a formula for the distance at which overcorrection grows to 1/4 wave (it is strictly for Newtonians, but he says it can be used as a general guide for fixed systems). It gives the distance in units of telescope's f.l. as d=14D/F^3, for D in mm. For an 80mm f/7.5 it gives d=2.65 f.l. or as little as 5 feet. If so, 20 feet should be fine. But you'd need very small point source:
not more than 0.05mm in diameter (definitely less than 0.1mm, which would equal the Airy disc angular diameter).

Another thing is that the focus would move some 2.5 inches farther back from the infinity focus location, and the converging cone would become somewhat wider inside the tube. If the baffles are tight, that could cause slight stopping down of the aperture, due to widening of the converging cone at the baffle location. Such stopping down would somewhat reduce inherent spherical aberration, although not likely significantly.

Try testing with green filter. Going out of focus brings you closer to red/blue/violet foci (while green/yellow disperses), and it usually more or less ruins pattern clarity with achromats.

Neither one. They both state standard for "tolerable" chromatism in achromats, not "color free". The more demanding criterion comes from Conrady, who made an arbitrary (but, of course, educated) decision on how much of blue/red defocus can be tolerated. The less demanding criterion seems to be based on the red/blue blur size at the best focus: three times the Airy disc, which, accidentally or not, nearly coincides with the blur size at 1/4 wave of spherical aberration. Calculation shows that not only blur size, but also energy distribution within it (nearly 80% in the central 1/3 of the blur), nearly coincides with that
of 1/4 wave of s.a. (taking into account eye's color sensitivity). So the f~3D^2 is an approximate equivalent for 1/4 wave of spherical aberration. The f~5D^2 is probably comparable to a 1/7 to 1/8 wave of spherical aberration. It comes to near apo levels.

Of course, this applies to nearly perfectly executed achromats. If actual glass indici deviate from the design requirement, it will result in more chromatism. Also, depending on the kind of color correction, chromatism can be more or less apparent to the eye, without it being significantly different in its effect to the image quality.

You may want to check out Lumicon's version. They claim larger clear aperture and less coma than other commercial reducers. Goes for $100.

Jon, looks like astigmatism. If you haven't already, you could check out if the objective and focuser are aligned with the tube. Achromats this fast can have that much of off-axis astigmatism at ~1 degree off-axis.

Rich, you could place the Cheshire into focuser, cap the objective, and flash some light from the side onto Cheshire's 45-degree reflective surface. Curved objective lens surfaces will reflect small amount of this light and form images of the 45-degree surface as circular doughnut shaped images of diferent sizes, intensities and colors, that will float in front of the objective. You may not see more than two or three, and they'll likely be small or hard to see due to relatively small/fast lens. If these reflections are nearly centered and concentric, the alignment is also near perfect. If not, either the lens or the focuser (or both) are not aligned. Small amount of misalignment probably won't do much harm, but if it looks fairly asymmetrical, it is probably good idea to have it fixed.

Here's one procedure that should work. To check out focuser's alignment with the tube, you could suspend a crosshair on the front tube end (w/o objective cell) and see if its center nearly concides with that of Chesire's crosshair. If it does, the misalignment comes from the objective, which should be cemented, so it can't be caused (or shouldn't be) by inter-lens misalignment. What you need to make sure is only that the objective is well centered in the cell. If asymmetry persists, chances are that the cell is not square with the tube. You can confirm that by rotating the cell: direction in which reflections diverge should follow the rotation. Now detrmine which pair of adjustment screws this direction of divergence most closely points to. By loosening or tightening that pair, through trial and error, you should be able to have reflections nearly centered and concentric.

Hi Terry,

Sounds like win-win situation. Glad you like the mighty little scope - may it give you many more satisfying moments...

Either the scope has a very bad case of spherical aberration, or (more likely) the problem is in focusing. You probably need to refocus when switching between the two eyepieces. Also, the focuser may not have sufficient in-focus (towards the tube) travel. If that is the problem, you can have it fixed by moving the primary slightly closer to the front tube opening.
Just a couple of possibilities...