Telescope Reviews: Re: Optimum Planetary Imaging Focal Length

Use Dawe's Limit to solve the resolving power of your telescope. For example, an 11" telescope can resolve 0.41 arc-seconds.

According to Sampling Theorem, you want to sample at twice the frequency of the signal being sampled. In other words, if 0.41 arc-seconds is the frequency, you want to use two pixels to sample that amount. Thus, the ideal resolution for our 11" telescope would be to sample at about 0.2 arc-seconds/pixel.

The NexImage has 5.6µ pixels.

If you use the formula

, you'll come up with 0.41"/pixel. (There's a handy calculator here.)

A 2x barlow makes our f/10 11" scope's FL 5600 for 0.21"/pixel, and a 2.5x Powermate yields a 7000mm FL, and 0.17"/pixel.

This is a long answer to get around to a general rule of thumb. Most webcams are properly sampled around f/20 to f/25. Too little, and you're undersampling the image--you're not recording all of the detail your optics can deliver. Too much, and while the image may be bigger, you're not actually capturing any more information--there's just not any more to be captured.

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