The introduction of charged-coupled devices (CCDs) have significantly advanced the field of astronomy. Today, virtually
all astronomical images are taken using a CCD. A CCD is a two-dimensional array of pixels on
a silicon waffer. Each pixel is a photon detector. When photons are incident upon the silicon surface, valence electrons are excited and collected using
a potential well. CCDs have a much higher quantum efficiency when compared to photographic plates, which allows for shorter exposure times and
the imaging of fainter objects. However, CCDs are not perfect. They have inherent properties and limitations that any aspiring astronomer should be aware
The first limitation is the saturation of pixels. During each exposure, each pixel collects a certain number of photons, which are converted into
photoelectrons. There is a ceiling to the number of photoelectrons that can be contained in the potential well of the CCD. If the CCD is exposed for
too long or if the intensity of light is too great, then no image can be resolved. In practice, one needs to know the level at which each pixel saturates
and pick exposure times that will not lead to saturated pixels.
The second limitation is known as dark current and is due to electrons created by thermal agitations inherent in the silcon material. The amount of dark
current present in a CCD image is proportional to the exposure time. In practice, we can minimize the dark current contribution by cooling the CCD to
limit the number of electrons created by thermal agitations. However, since astronomers usually deal with rather long exposure times, we
must correct for dark current by subtracting off a "dark frame." The dark frame is created by taking multiple dark exposures with the same exposure time as
the science images and combining them.
The third limitation is called the bias, which is the non-zero value of each pixel even when the exposure time is zero. The bias is caused by the
detector electronics. In practice, the bias or zero correction is done in two steps. First, an average bias or zero level for the entire CCD is computed
and subtracted off from each pixel. Then a "zero frame" is created from combining several zero second exposures. The zero frame contains the relative
variation in zero level between each pixel. The science images are bias-corrected once the zero frame is subtracted off.
The CCD electronics convert photoelectrons to data numbers. The conversion factor between the two quantities is known as the
gain of the CCD.
Since CCDs are not perfect, each pixel has a slightly different gain. The process of correcting for this is known as flat-fielding. A flat-field is an
image with a median of 1. It contains the relative gain of each pixel. Flat-fields are created by imaging uniformly illuminated surfaces. This can either
be done using a white screen on the inside of the telescope dome (dome flats) or the twilight sky (sky flats). CCDs have different sensitivities
to different wavelengths so a flat is created for each filter.
The last correction to CCD images involves removing cosmic rays and bad pixels. Raw CCD images frequently contain very high-valued single pixels. These high
values are either caused by cosmic ray hits or the pixel may have gone bad. In
either case, the these pixels should be masked and a file should be created
containing the location of bad pixels so that they are not considered in any subsequent analysis.
Images of the polar ring galaxies UGC 7576, NGC 2685, and NGC 3718 and calibration frames
were taken using the WIYN 3.5 meter telescope at Kitt
Peak National Observatory in Tucson, Arizona with the Mini-Mosaic Imager. A
summary of the observations are given in the Table 1 below.
Mini-Mo consists of two SITe 4096 by 2048 pixel CCDs separated by a small gap. The telescope has a total field of
view of 9'.6 by 9'.6 and a best possible resolution of 0.141'' per pixel. The telescope is fitted with a set of Harris UBVRI filters.
The transmission spectrum for these filters is shown below in Figure 2.
| Object || Date || Bandpass || Exp. Time (sec.) ||# of Exp.
| UGC 7576 || 21 Feb 01 || B || 900 || 2
| R || 600 || 2
| NGC 2685 || 21 Feb 01 || U || 900 || 2
| B || 600 || 2
| NGC 3718 || || U || ||
| V || ||
| R || ||
Table 1. This table is an observing log for the objects we imaged using the WIYN 3.5 m telescope at KPNO in Arizona.
Figure 2. This figure shows the transmission spectrum for the set of Harris UBVRI filters on
the WIYN 3.5m telescope at KPNO in Arizona.