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Testing the Optics of a telescopeTesting is usually carried out on a bright star at night. Before this is done, several conditions should be met: Thermal equilibrium The t emperature of the interior of the tube assembly must be at outside temperature. This is to ensure that there are no circulating air currents within the telescope. For the U.K. this means leaving the system outside for a minimum of two hours. Atmospheric refraction. The star that is selected for star-testing should be at least second magnitude and must be located at least 60 degrees from the horizon. Centred image. During the course of the test, the star should
be kept at the centre of the eyepiece field. What you will see in the eyepiece?The star test is performed by examining the host star at high
power (at least 200 times). When a star is focused, you will see
a bright central disk (Airy Disk) surrounded by a couple of faint
circular diffraction rings.
Airy Disc Visibility of the rings is dependent on the collimation of the optics, warm air currents in the tube and the steadiness of the atmosphere (seeing). Under turbulent conditions, the out of focused image will tend to "boil", and thus it is difficult to discern any fine detail of the image. For large-aperture telescopes (for example 8-inch and above), the diffraction pattern would be easier to see by covering up some of the aperture. To begin the star test, the star is to be racked in focus (call this IF). In this out of focus star image, you will see an expanding series of rings. As you defocus up to seeing four to six rings, stop to examine the image. Besides seeing a relatively broad outermost ring, the light is spread more or less uniformly among the rings. Next, rack through focus to the same place on the other side of focus (call this OF). Both IF and OF unfocused images are identical in a perfect optical system. An optical system like the refractor will have an unfocused
image that is filled in. An optical system like the reflector with
a secondary mirror will have an unfocused image that resembles a doughnut. The following sections highlight the optical quality of the system and the various optical errors that can be present. Spherical AberrationThe out of focus image is compared both IF and OF. The most common optical error, spherical aberration is present when an optical element (mirror or lens) has not been figured to the desired shape. Light rays from the perimeter of the element would then focus either closer in or further out than rays from the centre. Under-corrected mirror or lensWhen the main mirror or lens is under-corrected, you will see a weak central disc with rings increasing in brightness from the centre outward for IF. It is extremely prominent at the outer edges of the diffraction pattern. For OF, the light is concentrated at the centre of the image, and the central disc is nearly as small and bright as at the focused position. Brightness of the outer rings diminish from centre outward, with the periphery being faint and ill-defined.
Over-corrected mirror or lensFor an over-corrected mirror or lens, the opposite happens for the IF and OF to the undercorrected case. The effect of spherical aberration causes fuzzy images. Stars
and planets never snap into focus. A system that is free from spherical
aberration would have identical IF and OF extrafocal images, irregardless
of the presence of an obstructive secondary. Pinched Optics
When the out of focus image (either IF or OF) looks more
like a triangular or polygonal shape instead of being circular,
the optics could be suffering from mechanical stresses - pinched
optics. This is evident particularly in Newtonian systems whereby
the mounting mirror cell holds the mirror too tightly. However,
this can also be present in refractors when the lens cell grips
the objective excessively. AstigmatismAstigmatism is present when the image is racked quickly between IF and OF. You will see that the out of focus image will appear like an ellipse that flips over at right angles as you rack from one side of focus to the other. At the point of focus, it looks cross-like. Astigmatism occurs when an optical surface is polished on an uneven backing with uneven strokes. This problem might be common in Schmidt-Cassegrains that have frontal corrector plates that are poorly matched to the primary mirrors or are disorientated. Note however that astigmatism may be visible with an uncollimated telescope or from pinched optics. In Newtonians, this may be indicative that the secondary diagonal is not flat (either slightly concave or convex). To further evaluate that this rotate the main mirror by 45 degrees. If the pattern does not rotate by the same amount, the diagonal is either bad or poorly mounted.
Astigmatism displaying 90 degrees asymmetry at IF and OF positions. There is also a possibility that astigmatism originating from
the eye may effect the test. However, mild astigmatism in your
eyes will not show when you use high power, since the telescope’s
exit pupil will be so small. But to be sure that your eye is not
a contributing factor to the nature of the extrafocal image, rotate
your head around the optical axis as you look. If the elongation
does follow your head around, then your eye is astigmatic. Secondly,
the eyepiece could be tested for astigmatism by rotating it. If
the image follows the rotation, then there is a problem with the
eyepiece. Zonal errorIn this part of the test, you should look for one or more rings that looks weaker at either IF or OF than compared to the other side of focus. Zonal errors result when an optical surface is figured harshly during machine polish methods. This results in little valleys or small hills running concentric in rings on the surface. Turned-down edge
For a reflector: For a refractor:
Turn-down edge occurs when the edge of an element is rounded off
by undued polishing
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