Telescope Performance Factors
Central Obstruction and Wave Error

The ideal telescope has no central obstruction and no wave error. The only telescopes can that can come close to this ideal are high quality APO refractors such as those produced by Astro-Physics, Takahashi and TeleVue. The problem is that the cost of producing a high quality APO increases exponentially with the aperture diameter. If you want a telescope with over about 6" aperture, you have to start to compromise various design factors. The newtonian designs represent the least level of design compromise. It is possible to produce large mirrors with 1/10 wave error for a reasonable cost. These Newtonian can also be designed with central obstructions in the 20 to 25% range. The commercial SCTs represent the greatest compromises in design factors. The central obstructions of the typical 8" SCT range from 34% to 43%. Because the typical SCT is mass produced with a low target price, the wave error target is 1/4 wave at the primary mirror. There is also the wave error created by the corrector and secondary to be considered. Often, more often than anyone would care to admit, the overall wave error on many commercial SCTs is 1/3 to 1/2 wave.

What is the big deal with central obstruction? When a point source of light is filtered through a optical system like a telescope, the image of that point source is not a point but a disk of measurable size called the Airy disk. Surrounding the Airy are diffraction rings. A central obstruction has the effect of transferring light from the Airy disk to the diffraction rings. This light transfer causes the image to loose contrast. The bigger the central obstruction , the more contrast that is lost. To get a feel for what contrast variations are all about, try adjusting the contrast control on you monitor. Even better, locate a set of astro images on the web and adjust your monitor contrast while looking at these images.

Harold Suiter, in his excellent book, Star Testing Astronomical Telescopes , has the following to say about the central obstruction....

" The most obvious and potentially the most damaging kind of transmission change is caused by the centrally place diagonal or secondary mirror.... the negative consequences of central obstruction can be readily and precisely calculated....they worsen considerably beyond a fractional obstruction of 20 to 25% of the aperture (diameter)...the bottom does not drop at the middle frequencies until the obstruction is beyond 25%...but they are saving their full fury for the obstructions beyond 30%." (p153..156)

For an interesting discussion about how a central obstruction effects the size, see David Whysong's  excellent paper complete with graphs.

Some Typical Central Obstruction Sizes as a percent of diameter.
Meade Starfinder, 8" f/6 Newtonian - 18%
Celestron, 8" f/10SCT - 34%
Celestron, 9.25" f/10SCT - 37%
Meade, 8" f/10 SCT - 37%
Celestron, 5" f/10SCT - 38%
Meade, 8" f/6 SCT - 43%

The March 1995 issue of Sky and Telescope magazine had an article by Mark D. Russell, Ph.D., about the effects of central obstruction and wave error on telescopes. Mark used a computer to generate sample images of the effects of various wave errors and central obstructions on an 8" telescope. The resulting picture is on page 93 of that issue. I have scanned that image and am presenting it here as a service to the amateur astronomers of the internet.

For a closer look, select one of the links below.
72 dpi jpeg, 46 kb
150 dpi jpeg, 117 kb
200 dpi jpeg, 174 kb
400 dpi jpeg, 454 kb




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