For imaging, it makes more sense to think of light as particles, photons or quanta of energy. CCD chips are essentially photon counters. Each photon of visible and infrared light (about half of them anyway) knocks loose an electron from the silicon substrate. These electrons accumulate as the charge read off each pixel on the chip and are converted in to digital values stored in the computer. My Cookbook camera registers one digit for every 30 or so photons, known technically as the CCD’s gain. Impressive, but modern, high-end sensors count almost every photon, and that’s incredible. With detectors that good, the only way to see deeper is to collect more photons with longer exposures and bigger telescopes.

So how many photons do we get from distant stars and galaxies? Consider a bright and familiar example, the star Vega high overhead tonight at twilight, just 27 light years away. Professionals certainly have: Vega is the archetypical A-spectral class, zero-magnitude star for flux calibrations, and its spectral characteristics are well known. From Vega, we receive about 1 million photons of visible light per second for every square centimeter. For an 8” Schmidt-Cassegrain telescope, that’s 280 square cm of aperture, or 280 million photons, each taking about 3 billionths of a second to enter, reflect off the primary and secondary mirrors and enter the eyepiece. So on average then, at any one instant, there is one photon in the telescope.

Focusing Vega onto a 2 x 2 group of pixels in my Cookbook camera means that a count of 2333 units should appear on the screen for the shortest exposure possible, 1/1000 s (1 millisec). Vega will be nicely registered, but any longer exposure will saturate the electronics (a 10-bit A-D converter) and would cause the ugly streaks of charge bleeding so characteristic of CCD over-exposures. A 15th magnitude star, for example, the faintest stars in the Hubble Guide Star catalog, delivers a million times fewer photons and would require 17 minutes of exposure to appear as bright. Although it will be easily detectable in exposures just a fraction that long, say 5 minutes, it’s important to make every photon count for such dim targets.

Since we can account for almost every photon from far away, seeing deeper requires bigger telescopes and longer exposures, and we strive to do both. However, imaging distant objects is easy compared to spectroscopy, which is in fact how we actually learn almost everything about them: composition, speed and distance. Rather than being concentrated on a few pixels, light is spread into its spectrum and dispersed across hundreds or thousands is pixels for analysis. This is why aperture always wins, and why amateurs can’t compete with pros for astrophysical studies. Since the light is spread out a hundred fold or more, spectrographic studies are limited to objects five magnitudes or more brighter than that which can imaged directly. This puts amateurs at a considerable disadvantage although they have produced amazing results. But that’s no reason not to try, and all the more reason to build bigger telescopes. So many photons, so little time…

Above flux calculations are approximate, and CCD performance on dim objects is limited by chip and sky factors. Moondark is written by Doug Miller, published on the web, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 22 September 2002. All text and images copyright © 2002 Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.