Signal and Noises. Calibration frames.


      Astrophotography of faint deep-sky objects like galaxies and nebulae is all about signals and noise. The more signal you collect, the better your photos will be. It's that simple. You should consider this to be your number one priority in astrophotography: collect more signal!!! In digital astrophotography, there are several ways to collect more signal: a.) Expose individual frames longer; b.) Combine many individual shorter exposures; c.) Use a larger aperture in a faster optical system; d.) Use a sensor that has a higher quantum efficiency.

     The signal from the deep-sky object recorded in our images comes from the photons of light that have traveled incredible distances across the universe to hit the sensor in the camera and get counted. This signal is recorded in a "light" frame - the actual exposure of the sky object.

      In digital astrophotography, there are other signals that also get recorded along with the object signal, such as the thermal signal. Thermal signal is generated by heat in the sensor even when it is not exposed to light. Thermal signal can be recorded by itself in a "dark frame", an exposure the same length and at the same temperature as the light frame, but with the lens or scope capped so no light reaches the camera.
      There are also other noises present in every image we take: BIAS and Flat field.

      Flat field frames are used to address unwanted problems in the optical path, including dirt/dust, vignetting and internal reflections. Any change in the optical path will require a new flat field frame. To obtain the "best" possible flat field frames would mean taking it at the same conditions as the light, dark and bias frames. To take flat field frames, first you need to get your scope into focus and then point it to an evenly illuminated surface. Flat field frames can be taken from the twilight sky.
      When using a CMOS/CCD chip, not all the pixels start out at a value of zero. The purpose of a bias frame is to apply it to a dark and light frame to bring all the pixels on the chip to an equal starting value. To make a bias frame, cover the aperture in the same fashion as the dark frame. Capture at the lowest possible time setting your camera will allow. Bias frames should be taken at the same temperature and settings as the light frames.

      With these frames (dark, bias and flat field) we can generate a master bias frame, a master flat filed frame and a master dark frame. The master frames are called calibration frames. These unwanted signals can be subtracted from our images by correct use of calibration frames and this is a way to calibration our light frames.