Image of the day

Captured by

M 81

My Account

New to Astromart?

Register an account...

Need Help?

Posts Made By: Vladimir Sacek

January 1, 2009 07:24 AM Forum: Refractors

FR equivalents for apo, semi-apo, achromatic?

Posted By Vladimir Sacek

The simplest way is to go with polychromatic Strehl. If we assume that an apo should have it 0.95 or better in the visual spectrum (400nm-700nm), an achromat pretending to this low chromatic error should be at this level.

For the standard Fraunhofer, polychromatic Strehl is closely approximated by ~1.3(F/D)^0.25 for (F/D)<0.25, and by (F/D)^0.07 for 0.25<(F/D)<1, where F is the focal ratio number, and D the aperture diameter in mm.

For 80mm f/15 that gives polychromatic Strehl of 0.86. Considering that most standard achromats - including 4" f/12 - are bellow 0.8 PS, it could be regarded as a semi-apo level.

For 0.95 PS, the 80mm would have to be f/40. But it would be real close at ~f/30 already.


March 16, 2009 04:04 PM Forum: Reflectors

Ronchi Test of an unknown mirror

Posted By Vladimir Sacek

I'm not a Ronchi expert, but I'll try to interpret the surface based on test's basic principles. Take it for what it's worth.

Widening of Ronchi lines on both sides of the central hole indicates that those
areas focus closer to the test grid than the
outer ~1/3 of the radius. At the same time, the lines above and bellow the central hole are slightly narrowing, indicating that light from those (much smaller) areas focuses farther away than most of the outer mirror area. If so, the surface around central opening is wavy, with fairly symmetrical cross-like pattern.

The error is largest, both in magnitude and the area, on either side of the central hole. With a typical 100 lines per inch Ronchi grating, some 20 intercepted lines would indicate that the illuminated grating area is about 5mm in diameter, which with an f/1.2 16" mirror (pressumably) at the r.o.c. indicates some 12mm focus-to-grating distance for the outer zone. If the line width next to the side of the opening is roughly 50% greater, this portion of the surface to a first approximation belongs to a surface of the sphere (or other conic) focusing about 3mm closer to the grating. Assuming it does belong to such a sphere, a quick calc gives that surface error at its max (at the side of the opening) could be as large as 0.016mm, or 30 waves in units of 550nm wavelength.

The actual error may and may not be close to this value, but it definitely has to be very large to show that clearly on such fast mirror, with that many intercepted lines. It would also vary with grating density, being only half as large with 200 lpi grating and so on.

The good part is that the deformation area is relatively small. If the rest of mirror surface is decent, it could still be useable. It's hard to tell though, with so many intercepted lines. Another shot with no more than 3 or 4 lines would probably give some more information.


March 25, 2009 07:48 PM Forum: Refractors


Posted By Vladimir Sacek

The "sphero" sounds like the violet is due to the shape of the optics, but I'm curious to know whether, at least on the drawing board where cost is no object, the use of a fancy FL glass in the corrector would diminish or control the violet error.

Not a single glass; corrector can be achromatized if made of two different glasses. But the wavelength specific error for a single glass corrector depends on
how accurately it's been made. Deviations in corrector depth will result in the shift to some other wavelength being optimized (i.e. w/minimum spherical aberration). Since in the denominator for needed curve profile there is (n-1), "n" being the glass refractive, a deeper corrector will shift optimum correction toward blue/violet, with the correction worsening in green and red, while a shallower one will shift best correction toward red.

This, of course, assumes that mirror contribution is exactly what it is supposed to be (i.e., that they are perfect spheres of specified radii). The final specific wavelength with the minimum spherical aberration in either case (accurate vs. deeper or shallower corrector) will depend on the amount and sign of mirror aberration contribution. It can be near the green e-line, but it could as well be in blue, violet or red. Your SCT could have, for instance, something like 1/20 wave PV in violet, 1/4 PV in green and 1/3 wave PV in red - or the other way around.


April 12, 2009 03:07 AM Forum: Equipment Talk

Recent Interferometic test results

Posted By Vladimir Sacek

Rms measured at a specific wavelength other than the optimized in telescopes with any form of chromatism is valid only for that wavelength. Depending on the overall correction for spherical aberration, the error at the wavelength that should be optimized - usually 550nm - can be both, smaller or larger.

For instance, the rms at He-ne line due to chromatism is 0.023 with spherical aberration of the system cancelled. If the system residual is non-zero, that will change. For, say, 1/5 wave p-v (0.059 rms) of overcorrection at 550nm, correction at 633nm goes down to 0.12 wave p-v (0.031 rms), going from under- to over-corrected.

Undercorrection in green would, on the other hand, worsen correction in red and improve it in blue/violet.

If we know whether this other-than-optimized color tested is over- or under-corrected, and what it is supposed to be in a perfectly corrected system, we can obtain the error in the optimized wavelength by simple arithmetics. In the above case, the differential between 0.023 wave rms under-corrected and 0.031 rms over-corrected is 0.054 wave rms of over-correction added to the system in units of 633nm wavelength, which for 550nm translates to 0.062 wave rms, or little over 0.2 wave p-v (a bit more due to rounding off).

What makes this diffcult is that wavefront errors other than spherical are likely to be present. Tested C9.25 has 1/4 wave p-v which, if it would be spherical aberration alone, would result in the rms smaller by a factor of 3.35, or 0.0745. Since it is significantly smaller than that, it indicates the presence of some type of local error, or errors, which inflate the p-v value significantly more than the rms. It is mixed with spherical aberration, making determination of the error in the optimized wavelength more uncertain.

There is nothing wrong with Suiters calculation. It is diffraction based, and still valid since it is still diffraction that determines intensity distribution. His figure for 1/4 wave p-v is for 1/4 wave p-v of spherical aberration, which corresponds to 0.0745 wave rms. The aberration in the C9.25 of a mixed form, with significantly lower rms; it is altogether different animal.

Btw. Strehl ratio is used to express quality of the wavefront (and, by that, quality of optical surfaces). It does not account for the effect of obstructions, except in the part in which they change the rms by taking out portion of the aberrated wavefront. Consequently, the Strehl and the final relative peak intensity of an obstructed system are two different things. If the Strehl (from wavefront quality) is S, and c.obctruction in inits of the aperture "c", the final peak intensity is I=S(1-c^2)^2.


April 23, 2009 04:02 AM Forum: Telescope Making

baffle for a nasmyth cassegrain

Posted By Vladimir Sacek

There is no reason for Nasmyth not to require baffling. The flat acts as an opening letting light reach the focuser and focal plane just the same. Both, baffle in front of it and around secondary are still needed.


June 14, 2009 05:15 AM Forum: Reflectors

Mirror math

Posted By Vladimir Sacek

Is it possible to calculate the accuracy of a spherical mirror in waves given the following parameters:
R=314.00" +- 1%
Smallest blur circle size =.06"
light source diameter=.03"

What can be expected in terms of spherical accuracy of either slope error or waves.

This makes it 59" f/2.66 sphere. The smallest blur diameter for point source is:

B=D/128F^2 for D in mm

which gives 1.65mm. The blur is twice as large, 3.3mm, at best (diffraction) focus. The PV wavefront error of lower order spherical is:


or 0.0389mm. In units of 0.00055mm (550nm, 0.55 micron) wavelength it is a bit over 70 waves.

The surface PV error is twice the best focus error (half the paraxial focus error), or 0.0778mm.


August 4, 2009 09:43 AM Forum: Reflectors

Aperture Mask Question??

Posted By Vladimir Sacek

Most of the improvement comes from the reduction in seeing error. It changes with (D1/D2)^(5/6), which means that 3" aperture has 2.7 times smaller average error than 10". That is like difference between 1/2 and 1/5.4 wave PV of spherical aberration. Very obvious. In effect, by reducing the aperture you switched from poor to very-good seeing.


September 4, 2009 07:33 AM Forum: Equipment Talk

Oblique viewing phenomenon?

Posted By Vladimir Sacek

It could be also influenced by the shift of the image formed on the retina, which inevitably follows tilting the eye w/respect to the eyepiece axis. Looking at the distribution of retinal photoreceptors

(Fig. 155), two different scenarios seem possible. If we intuitively tilt the eye so that the image formed by eye lens falls onto the center of vision, the halo might be showing due to high cone density and the ensuing maximum level of (bright) light absorption. Tilting the eye shifts the image onto retinal area of significantly lower cone density, while the rods are still too few and/or sufficiently deactivated by bright object to produce appreciable visual effect.

Alternately, if we don't intuitively bring the image to the center of vision, we might be seeing due to the presence of low-ligh-sensitive rods away from it (they are certainly active to some extent), and lose it by bringing the image near the center of vision. Obviously, in this scenario the halo would disappear only when tilting eye in one particular direction.


September 20, 2009 06:32 PM Forum: Equipment Talk

Need your help: Meade LX200 ACF 12"/Celestron CGEM

Posted By Vladimir Sacek

2)According to what you could suggest or make me understand, instead of mentioned before, we could buy 2 Meade LX 200 ACF 12"...or at the opposit, buy 2 CGEM 11" Edge HD???

I wouldn't be buying two EdgeHD's just yet. Most of what we have right now is advertising talk. It isn't that far back when Meade came up with a new system (RCX, so called "advanced RC"), which in their sales talk was the best thing that ever happened...until it hit the market. It doesn't mean something similar will happen with the HD, but it is certainly possible. Addition of a sub-aperture corrector to a commercial SCT is something new, and is yet to show up how it will work out in practical use.