Frequently, with the typical amateur telescope, the most appropriate method to match the Airy disk size with the detector pixel size is the technique known as eyepiece projection. An eyepiece is placed between the chain of telescope optics and the detector which steepens the effective input angle to the detector, increasing the effective focal length of the optical system. Mathematically, f _ deye X /obj
J eye where deye is the eyepiece-image plane distance, and /eye is the eyepiece focal length.
Using the example optical system from the earlier section on Telescopes, in conjunction with a 28 mm focal length eyepiece, a 2000 mm effective focal length can be attained by setting deye to 43.1 mm:
./effective X ./eye /obj effective eye deye _-v---(17.4)
2000 mm x 28 mm
Eyepiece projection systems can be bought commercially (see the list of suppliers at the end of the chapter) or custom-built using standard machine shop tooling.
In practice, deye is fixed to give the desired feffective, and the whole projection apparatus is moved to adjust the focus. Finally, keep in mind that not all eyepieces will physically fit in a commercial system. The wide angle, large eye relief eyepieces (TeleVue Nagler, Meade Ultra Wide Angle, etc.) have significantly larger housings than the more traditional Kellners, Orthoscopics, and Plossls.
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