This section leads you step-by-step through the star test. Before we get to that, there are three things you should do in preparation.
Even minor aberrations appear obvious during the star test. And a major problem, such as a seriously miscollimated telescope, will overwhelm all others. You should collimate your telescope before the star test. Setting up the telescope an hour before the test allows the optics time to cool to the ambient air temperature. The optics cool by releasing heat to the surrounding air. If you do the star test too soon after setup, all you'll see are the debilitating effects of turbulent air in front of the mirror. Use a high magnification eyepiece (1-mm exit pupil) and a moderately bright star--about 3rd magnitude--for the test. I recommend using 20X to 25X per inch of aperture. If the seeing is unstable even after the scope has been outside an hour, try fitting a #23A Wratten color filter to the eyepiece. The red light image should be steadier.
The Star Test: What Should I See? |
If the slightly defocused image of a moderately bright star at 25X per inch of aperture looks similar to this illustration, the telescope's collimation is good. You should see an identical pattern of concentric rings of light around a central dot of light. My illustration probably exaggerates the separation between the rings but it is only meant to approximate the appearance. Also an obstructed optical system, such as a reflector or catadioptric telescope, will produce a diffraction pattern with very faint inner rings. The faint inner portion of the diffraction pattern is the caused by shadow of the secondary mirror. |
The Star Test: Collimation Problems |
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The Star Test: Astigmatism |
The other possibility is that your telescope's optics are astigmatic. If your scope is a refractor, this is justification to return the instrument for repair, exchange or refund. If you own a reflector or a catadioptric, the astigmatism may be correctable. Newtonian owners should first check the primary mirror by making small adjustments to its collimation screws. Adjustments to the secondary mirror are best left to the light of day until you become more experienced. Owners of SCTs can make adjustments by tweaking the secondary mirror collimation screws. |
The Star Test: Spherical Aberration and Turned Down Edge |
Spherical aberration is present to some degree in all optics. The amount of error in the optical figure is measured in wavelengths of light. A telescope producing an image with 1/4 wave spherical aberration at the eyepiece is still diffraction limited. Spherical aberration produces a sharp pattern (illustrated at left) inside focus and a softer, ill-defined pattern outside focus. The greater the disparity, the worse the aberration. Unacceptable spherical aberration will produce a diffraction pattern inside focus with a very bright outer ring and a pattern outside focus that is very soft. Acceptable spherical aberration will produce patterns that are still very similar in appearance. The pattern outside focus will be slightly soft. The pattern inside focus will have inner rings that are similar in brightness to a soft outer diffraction ring. A turned down edge is exactly that. The outer edge of the optic slopes downward. The result is scattered light, and loss of contrast on fine lunar and planetary detail. Turned down edge produces a diffraction pattern similar to that produced by a telescope suffering from spherical aberration. A turned down edge will produce a more defined diffraction pattern outside focus. Also, the inside pattern will present rings at irregular intervals encased within an overall diffuse glow. This aberration is common to telescopes that use mirrors, so Newtonian and catadioptric owners will likely encounter turned down edge. What should you do if your telescope suffers from spherical aberration or turned down edge? First, don't panic. The star test makes even acceptable imperfections look obvious. Your telescope may have an aberration that does not significantly affect performance. The best way to determine your telescope's overall quality is to point it at the Moon or Jupiter. These objects show a wealth of subtle detail through a good amateur telescope. If you are pleased and impressed by the views you get of the Moon and planets, you're optics are probably fine. If you're not sure what you should be seeing, check out my Planetary Observing page. |
The Star Test: Poor Seeing |
Bad seeing can have local causes. Are you setup on concrete or asphalt? These materials absorb lots of solar energy during the day and release it at night, causing the air near ground level to be unsteady. Are you observing over rooftops and buildings? These are also heat sources that can muck up the seeing. Try to observe from a location with lots of grass, trees or fresh water nearby. An elevated deck places the telescope above the ground seeing. |
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Revised: February 11, 2002 [WDF]