A Star Test Primer

by Thomas Back



Updated 6-29-03

The first and most important thing to remember about star testing a telescope is to never expect your telescope will star test perfect, as it will not. The test itself is just too sensitive to wavefront aberrations. W. T. Welford studied the sensitivity of the star test (see "Optical Shop Testing" pages 397 - 426), and found that under ideal testing conditions, the star test is sensitive to 1/20 wave for slowly varying aberrations (such as third order spherical), 1/60 wave for rapidly varying aberrations (like a sharp zone on the optical wavefront). These are wavefront measurements, not surface errors. Amateur optics that exceed 1/8 wave on the wavefront, or a better measure, a RMS wavefront of 1/40 wave are rare indeed, and are fully suited for high quality planetary work.

Most amateurs and authors of star test manuals make the mistake of using too little power during testing. If you use a low magnification, such as only 20x per inch of aperture, you are desensitizing the test. Much better still is to use 50x or slightly more magnification per inch of aperture (200+ power for a 4" telescope). The next mistake is the commonly heard recommendation of using a 2nd or 3rd magnitude star. This may be fine for large apertures of over 12", but in the most common telescope sizes, a bright star is a much better choice (Vega, Capella, Altar or Deneb are good choices). A bright star will show zonal aberrations much clearer than a dimmer star, and will allow other optical defects to be much easier to see. Use a well corrected eyepiece (Nagler, Radian, Plossl or Ortho) and don't use a diagonal or barlow in your tests (unless it is known without question, to be of very high quality). A prism diagonal will add overcorrected color and spherical aberration, therefore it is not recommended.

Give the scope a full two hours of cool-down time (or more if the scope has a long thermal equilibrium time or you live in a cold area), and make sure it is a night of good seeing, say 7 or better on a scale of 10. Testing under worse conditions can hide aberrations, and the poor seeing can mimic surface roughness on the optics. Atmospheric turbulence can also cause softening of the contrast in the ring pattern, outside of focus. This is due to focusing on the upper level turbulence. Just another reason to test on nights of good seeing. The higher the test star is in the sky during testing, the better. Using filters in the peak visual wavelengths such as a Wratten number 58 filter (dark green) or number 15 (dark yellow) will help in your testing, especially with refractors, because of their chromatic and spherochromatic aberration components, which can make the star test look worse than the optic really is. Focus on the star, and then focus out 5 or 6 rings on either side of focus (inside and outside). The most important area of the optic is the edge, ideally the outer most thick ring should appear identical on both sides of focus. That means the thickness, brightness, texture and hardness/softness should be the same. If they are just slightly different, the optic is a good one. A gross difference, however, is a bad sign.

Next, rack out 10 or more rings and look for any bright, thin rings (dim on the other side of focus) inside the diffraction pattern. These are zones, and can be common in mass produced and homemade optics. Then bring the focus down to 1 or 2 rings on either side of focus. Here you are compressing the aberrations down close to focus, and very few optics will pass the one ring test. Again, don't expect perfection. You are looking for perfect symmetry here, the breakout of the Airy disk to the first ring, with identical focus travel on each side of focus.

Now bring the scope to best focus. What you want to see is a well defined, round Airy disk, with no more than 2 or 3 diffraction rings around it. This is the real meat and potatoes of optical performance, that is, how dim are the first and second diffraction rings at best focus? If they are dim (a perfect optic would have 7% of the total energy in the first ring, 3% in the second), then you know the optics are good. You can also do the "snap" test, which is a subjective way to test optics, but still a good general test. If the star (or planet) snaps into focus without any ambiguity of focus, the optic is probably a good one. The same holds for high power viewing. A good optic can stand high magnification on a night of good seeing, and a poor one will get mushy at high powers.

Other aberrations such as astigmatism will show itself as oval images, just outside of focus, that shifts to a 90-degree angle oval on the other side of focus. The contrast of the ring pattern on both sides of focus is a good indicator of smooth optics. This is important for detecting low contrast details on the planets, because telescopes can have quasi-perfect symmetry in the star test, but if the surfaces are rough, the images will not show high contrast. Anything that deviates from a clean, round Airy disk at best focus, assuming the telescope is collimated, no tube currents or temperature boundary layer problems above the mirror, fully cooled down (optically at null), and is tested during excellent seeing, is a sign of an optical aberration.

Even if your telescope doesn't pass the star test with flying colors, please don't stop enjoying your telescope, and don't worry about the star test too much, unless it shows very obvious errors. Again, no optic can pass the star test perfectly. What really counts is how it performs in focus. If your telescope shows highly detailed lunar and planetary images, you have a very fine optic.

Happy star testing!

Sincerely,

Thomas M. Back
TMB Optical


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