Celestron CGX Equatorial 800 HD Telescopes Whitepaper EdgeHD Optics - Page 16

Appendix A: Technical Profiles of EdgeHD Telescopes When evaluating astronomical telescopes, astroimagers must bear in mind the many factors that influence image quality. The major factors at play are: • The image formed by the telescope • The sampling by pixels of the image sensor • The diffraction pattern of the telescope • The "seeing" quality during exposure • The guiding accuracy during exposure To aid astroimagers, this Appendix presents a spot matrix plot for each of the telescopes in the EdgeHD series. To determine the size of the images that you observe in your exposures, these must be compounded with the other factors that affect your images. In the spot matrix plots we have provided, each large gray box is 64µm on a side, and consists of a ten small boxes 6.4µm representing a pixel in a "typical" modern CCD camera. The black circle represents the diameter of the Airy disk to the first dark ring. It is immediately clear that for each of the EdgeHDs, two 6.4µm pixels roughly match the diameter of the Airy disk. This means that under ideal conditions, a CCD camera with pixels of this size will capture most of the detail present in the telescopic image. Referring to Figure A1, the left column shows the Airy disk for a telescope with a central obstruction of 34%. Because the light in the Airy disk is concentrated into a smaller area in the center, capturing all of the image detail in a planetary or lunar image requires using a 2x or 3x Barlow lens to further enlarge the Airy disk. Unfortunately, ideal conditions are fleeting. During a typical CCD exposure, atmospheric turbulence enlarges the image of all stars, and furthermore, it causes the images to wander. On the steadiest nights, the "seeing" effect may be as small as 1 arcsecond. In Figure A1, the "superb seeing" column shows blurs with a FWHM (full-width half-maximum) of 1 arcsecond. The next column shows excellent seeing (1.5"), and the right column shows 2" seeing blurs, typical of many nights at most observing sites. It is important to note that as the focal length of the telescope increases, the diameter of the seeing blur increases in proportion. With a small telescope, seeing plays a smaller role. With the large apertures and long focal lengths of the EdgeHD series, nights of good seeing become particularly valuable. Airy Disk and Seeing Blurs 14-inch 11-inch 9.25-inch 8-inch Airy Superb Excellent Average Disk Seeing Seeing Seeing FIGURE A1. Shown at the same scale as the matrix spot diagrams are the Airy disk and the point-spread-function of seeing disks for average (2.0"), excellent (1.5"), and superb (1.0") seeing. 16 I The Celestron EdgeHD