Posts Made By: Vladimir Sacek

Darren Drake said:

Which configuration between the 2 gives better off axis images given that the f ratios are the same after the newtonian's f ratio has been multiplied by 1.15X?

Typical MN corrector induces coma opposite to that of spherical primary (which has coma identical to that of a parabola as long as the stop is at mirror's surface), cancelling ~30% of it. Another coma-cancellation factor comes from the position of corrector: if placed at mirror's r.o.c. the primary would have zero coma, and the only coma of the system would be that introduced by the corrector. Corrector placed at about 1.4 times the primary f.l. would result in a near-zero system coma. At the typical corrector position, somewhat inside the focal plane, primary's coma is reduced to somewhat more than a half (~60%), so that total coma of a typical MN is approximately 30% of that in a comparable parabola. In other words, its linear coma-free field is ~3.5 times greater than in a comparable parabola. That puts its diffraction-limited field radius at ~F^3/26 in mm, "F" being the F#. That makes it comparable to that of a parabola of ~1.5F in regard to the linear coma-free field, and to ~1.8F parabola in regard to the angular field size.

Paracorr graphs would indicate that it still produces roughly 50% greater linear field at a comparable F#. But it is hard to say without knowing what kind of aberration is behind the corrected geometrical blur size.

Vlad

Tilt of the primary mirror in an SCT
changes its relative position in regard to both
corrector (aperture stop) and secondary mirror.
Relative tilting of the corrector has no appreciable
effects, but tilting of the fast primary does result
in axial coma, which is partially compenseted by that on the secondary, which has identical amount of opposite tilt relative to the primary, and is also now decenterd relative to it. If I got it right, for the typical configuration with an f/2 primary and spherical secondary at about 3/4 primary's f.l. from it, coma resulting from the primary tilt is more than double that created at the secondary. Final axial coma (full blur length) comes to ~460tD in Airy disc diameters (t=tilt angle in radians, and D=aperture diameter in inches).

In this case, t=0.0004 and D=12, giving axial coma
of ~2.2 Airy disc diameter, which is bellow 0.80 Strehl (the RMS is 2.2/20.9=0.105, and the Strehl
0.64). Since the coma is propotional to the tilt angle and aperture, a "diffraction-limited" tilt
level for the primary would be given by 0.0034/D
in radians, for D in inches. Of course, the less the better.

Vlad

There is no chance that the front
opening causes any significant vignetting. Even if you leave it to be equal to the mirror
diameter, in this case 18.75", it would result in as little as 6%
vignetting 23mm off-axis for an f/4.5. The effect is entirely invisible. Moreover, since a stop
placed at the focal length distance in front of paraboloidal mirror cancels
its astigmatism (nearly 3 waves 23mm
off-axis), edge definition
may get slightly better - but it ultimately will depend on the eyepiece used.

Vlad

The numbers imply an excellent f/4 mirror, at the level of 1/8.5 wave
PV of spherical aberration. The squareness most likely results from
pinching by the mirror holder - interferograms show no cause for it whatsoever.

Vlad

Don't know where Newt's numbers come from, but
the data indicates ~8.5" diagonal-to-focus separation. This would result in ~0.25" field of
full illumination diameter, and ~76% illumination
12mm off-axis (all you need to worry about with 1.25" ep barrel).

As for the contrast effect, 26% obstruction results
in 0.87 contrast degradation factor, and 21% in 0.91
contrast degradation factor. This is comparable to
1/5 wave p-v and 1/6.1 wave p-v of spherical aberration, respectively. Not much of a difference
practically, but it could be as a part of an effort to minimize all error sources inherent to the system.

Vlad

In general, obstructed telescopes need higher
wavefront quality than 0.8 Strehl to meet
1/4 wave p-v of spherical aberration level.
Needed rms error of spherical aberration is not hard to estimate: it is

rms=sq.rt.{[0.0055+0.058log(1-c^2)^2]/(1-c^2)^4}

where "c" is the relative size of c.obstruction.
For the p-v error it only needs to be multiplied
by 3.354. So 20% obstructed telescope (c=0.2)
needs rms=0.064 and pv=0.214 or 1/4.7 wave of
spherical aberration to achieve 0.8 Strehl level.

A 30% obstructed telescope needs 0.015 rms or
1/20 wave level. A 33% obstruction would require
better than perfect wavefront - unfortunately,
not an option.

Vlad

David Pope said:

Can anyone tell me in layman’s term what is “Diffraction Limited Optics”?

"Diffraction limited" is somewhat vague expression, that no one seem to be able to trace direct origin from. Conventionally, it
is accepted as reffering to wavefront quality of 0.8 Strehl (1/13.4 wave RMS, or 1/4 wave p-v of spherical aberration), or better. In effect, "diffraction limited" without further specification covers anything from good to excellent optics.
Difference within can be noticeable, but not significant to the
extent that would amount to a gross inferiority/superiority.
The fact is that actual quality level is bellow "diffraction-limited" for all but smallest apertures (<4"), regardless of
the optical quality. This is due to the seeing and other error sources. Better quality optics helps to keep actual quality higher: it does matter if that actual quality is, say, 1/3 wave vs. 1/2.5 wave p-v level.

One has to keep in mind that "diffraction limited" optics with
obstructed telescopes doesn't necessarily mean "diffraction-limited" performance. Due to the effect of c.obstruction, "diffraction-limited" level for obstructed apertures requires better optics, as approximated with the Strehl given by 0.8/(1-c^2)^2, "c" being the relative size of
c.obstruction (in units of aperture). So a 25% obstructed telescope (c=0.25) would need optics of 0.91 Strehl (1/6 wave p-v) to perform at a true 0.8 Strehl level. Obstructions greater than 32% make it impossible to reach true "diffraction-limited" minimum, as defined above.

Vlad

>I was reading in the Celestron forum that because the center baffle is 27mm on this scope, that purchasing 2" EP's would be a waste since this scope will never be a wide-field. I'm no expert, but I think the author was saying that the center baffle would have the same effect as a 27mm field stop.<

Not quite. Size of illuminated field is determined by the rear baffle opening, not the front. Since the C6 is of similar proportions as the 8", only 25% smaller, its 50% illuminated field will be also about 25% smaller, or ~34mm in diameter. That corresponds to angular field diameter of ~1.3 degrees.

So some 2" eyepieces would work, but it wouldn't make it wide-field. For that, Antares 6" f/6.5 refractor would do much better.

Vlad

Doug Peterson said:

A recent ad for a Takahashi "figured to an almost perfect strehl ratio of .992" and a similar spec for the TEC140 reminds me that nominal design specs extracted from a Zemax CAD model (which these are) and actual results in the real world (which would require an interferometric test report, NOT a zonal focal length smoothing type test used by some mirror makers) are not the same thing.

The real meaning gets lost in "translation". Most marketers won't mention that this is for the optimized (usually) e-line only. Even the manuals often won't specify the polychromatic Strehl for the visual spectrum, which is at 0.95-0.96 Strehl level for the top apos. Combined with wavefronts of up to ~1/50 RMS tops, the top apos have actual Strehl at the ~0.95 level. Guess it doesn't look good enough put aside 0.98ish and 0.99ish Strehl ratios claimed by most mirror makers. In fact, somewhat inflated apo figures have a compensatory effect, and only partly so. Why should the apo makers and marketers play it "straight", and make these instruments look of inferior optical quality, if it is not so? Actual mirror instruments suffer from the added effect of central obstruction, and the combined error from other sources is significantly greater. Neglecting the seeing factor, a mirror claiming 0.99 Strehl, even if true, will have hard time to reach 0.9 actual Strehl in the field, or even 0.8 for larger apertures. On the other hand, an apo claiming 0.99 Strehl while having 0.95 actual one, will be pretty close to the latter figure in its actual performance.

Vlad

It is easy to find out what is the level of error induced to this type of system. For 11" f/2/10 spherical-mirror SCT, OSLO gives about 1/20 wave P-V of over-correction added for each inch of focus extention beyond the optimum focus.

The problem is that manufacturers don't give
the information about where this best focus is, nor what is the actual level of correction. My guess is that best focus location varies somewhat from one unit to another, depending on the mirror match-up. And if your system is, say, under-corrected at the regular focus location, extending the focus would actually have corrective action (the opposite if it is over-corrected at the regular focus location).

So, it takes a bit of testing. If your unit is over-corrected at the regular focus, you don't want to worsen it by adding significant extentions (spherical aberration adds up arithmetically: if thescope is, say, 0.2 waves over-corrected and you add 0.1 waves by extending the focus, it nets 0.3 wave P-V error).

Vlad