Star Testing Telescope Optics
Star testing a telescope can be a great way to determine if any errors are present in the optical system and what those errors are. The star test is very critical and can show even slight aberrations. It does require a sharp eye and some experience to really get the most out of it. This section will describe how to star test a telescope and give examples of the most common aberrations. For more details on the errors you might see, check out the Optical Aberrations page.
Note: First a warning. The images shown below are idealized examples. Unless you paid as much as a small car and waited on a list for the better part of a decade to get your telescope, do not expect flawless optics. Most mass produced telescopes--even good ones--can show minor flaws if you look closely enough. The real test of a telescope is what you see when you look through the eyepiece under normal conditions. Almost without exception, all telescopes from reputable manufacturers will perform well enough that you could never notice any errors. If you plan on using your telescope to look at high-power, out-of-focus star images, maybe a star test is a practical examination of the scope's optics. But in reality, as long as the telescope shows good images under normal use, a star test doesn't mean too much. However, for advanced users who want to compare different instruments or determine what the limiting factors of their optics may be, the star test is a great way to learn a lot about a telescope.
Performing a Star Test
A star test is simple to perform, but for critical results the conditions must be right. Atmospheric turbulence can ruin a star test, so optimal seeing conditions are a must. Likewise, allow the telescope to thermally stabilize. A telescope that is still cooling down will produce a heat spike from the warm air rising off the optics, distorting the star image.
Use a high-power eyepiece, producing around 200-300x. This will allow the fine details of the star image to be seen more clearly. Do not use more magnification than the optics or seeing conditions allow. Anything over 300x is probably excessive.
Often it is recommended to conduct a star test without a diagonal in place on a refractor or Schmidt-Cassegrain type telescope. We would recommend leaving the diagonal in place for two reasons. One, having a diagonal in place simply makes it easier to look through the telescope. Second, you will normally have the diagonal in place when viewing, so it is a part of the optical train. If you see an aberration with the diagonal in place, you can later remove it and test again without it to determine if the aberration is in the diagonal or the telescope.
Finally, choose a fairly bright (1st magnitude) star for the test. Select a star that is high in the sky, preferably at least 60° above the horizon. This will minimize atmospheric effects.
How a Star Test Works
To test the optics, defocus the star enough to see the details of the star image. You will examine the details within the defocused star disk to determine what, if any, aberrations are present. Refractors will present a slightly different star image than reflectors, since reflectors will have acentral obstruction. Other than this aspect, the test is the same for all telescopes. On the assumption that there are more reflectors than refractors out there, the diagrams on this page will show the view through an obstructed telescope. Aside from this, the patterns to look for in the test will be the same no matter the telescope.
The patterns of the star image are examined on both sides of focus. Deviations from a theoretically perfect test, and variations from one side of focus to the other will reveal what aberrations are present and to what degree.
What to Look For
Above: The appearance of a perfect star image through an obstructed telescope such as a Schmidt-Cassegrain telescope (SCT)
Note the features of a perfect star test. Foremost, the images are identical on both sides of focus. This is the easiest thing to check for with a star test. Also, note the concentric rings within each star image. These are clearly defined. The perfect star test is pretty simple. Adding aberrations to the system will produce deviations from this ideal, so keep in mind what the optimal star test looks like.
Shown and described below are the appearances of stars aberrated by less-than-ideal optical systems. While each diagram shows an instrument suffering from only one aberration, this is often how a real star test will look. It is unlikely that a telescope will suffer from any aberration enough to be very distinct in a test, let alone suffer from several aberrations at once.
Above: Star test of a telescope suffering from (overcorrected) spherical aberration. See text below for more details.
Spherical aberration is the easiest aberration to test for. It appears as a difference between the images inside and outside of focus. It is also the most likely aberration to exist. Most telescopes will suffer from a very slight amount of spherical aberration which will have no effect on the in-focus image quality.
There are two types of spherical aberration: overcorrected and undercorrected. Overcorrected spherical aberration will appear as well-defined rings in the star image outside of focus, but poorly-defined rings inside of focus. Undercorrected spherical appears the opposite, with sharp rings inside focus and mushy rings outside focus.
The greater the difference between inside and outside of focus images tells the magnitude of the aberration. Observers talk of "waves" of spherical aberration. The size of the aberration depends on how much the shape of the optics deviate from the ideal. It takes very little to ruin a star image, which is why making a telescope isn't easy. A difference between the ideal and actual optical surface of just one wavelength of visible light is a tremendous deviation. An error of less than one-quarter wavelength (1/4 wave) is considered acceptable and is typical of mass-produced telescopes. Most observers, even experienced ones, will not be able to detect 1/4 wave of spherical aberration during normal use, although this amount of spherical can be detected fairly easily with a star test. The diagram above shows about a half wave of error, a considerable amount. A full wave of error would produce a pretty horrible image, even in focus. The best telescopes will have 1/8th wave of error or less, imperceptible in normal use and hard to detect even in the critical star test.
Above: Star test of an astigmatic optic. See text below for more details.
Astigmatism is relatively easy to detect in a star test, but in a different manner than the normal test. Astigmatism can be difficult to see with the star very far out of focus. By taking the star only slightly out of focus in each direction, astigmatism can be seen, if it exists. This is a fairly rare aberration and when it exists it is often the result of a poor-quality diagonal. Removing the diagonal from the optical path is the first recommended course of action if astigmatism is detected in the test.
Astigmatism appears as a star elongated in one direction on one side of focus and elongated at a 90° angle on the other side of focus. Again this is easiest to see at high magnification just barely on either side of focus. Few telescopes will show this aberration.
Above: Chromatic aberration in an achromatic refractor
In an achromatic telescope objective, all wavelengths of light are not brought to the same point of focus. This results in a blue halo surrounding the star image. This is seen especially on bright objects, including planets. The larger the aperture and faster the focal ratio of the telescope, the worse the blue halos will appear. A 3" f/11 refractor shows much less aberration than a 6" f/8 objective of the same design. Apochromaticrefractors use special extra-low dispersion (ED) glass and different optical designs to minimize this aberration. A true apochromatic refractor should show little to no blue halo effect.