The CCD camera control software, CCDOPS, has the capability of determining approximate magnitudes for stars and diffuse nebulosities. The process used involves a number of simplifications and ultimately requires that a reliable magnitude be known for at least one star in the image. The magnitudes determined are effectively differential magnitudes with respect to the chosen "standard." Within a given image this process can produce magnitudes relaible to within a few tenths of a magnitude (say, +/- 0.2). Actual errors can be smaller than this. This process cannot be used to compare magnitudes for stars in two different fields, unless each field has its own "standard." These magnitudes are approximations and should in no way be considered as standard magnitude values.
Since a magnitude value is a measure of the energy detected coming from an object, a magnitude can be determined by summing the pixel values for pixels which record the object. This sum is a measure of the total number of photons that struck the detector during the exposure. This sum can be converted into a magnitude value by applying appropriate calibration factors.
In general, one considers a box or circular aperture centered on the object of interest and simply adds up the pixel values from the pixels within this analysis aperture. This number is then corrected for the sky or background determined to contribute to each pixel in the analysis aperture. This background corrected number is then adjusted for the exposure time and presumed sensitivity of the CCD-telescope combination and a magnitude value is determined.
This procedure is available from the X-hairs sub-menu in the Analysis Display mode. This procedure involves several steps.
Move the cursor to an area that is representative of the background surrounding the object of interest. For asn isolated star, merely move the cursor to a region of sky that contains no stars. For a star embedded in nebulosity, or for a galaxy, you will need to identify a background area with roughly the same brightness as the nebulosity surrounding the object of interest. Once the cursor is positioned, press the "B" key (for background, case does not matter in this case). You should see the magnitude value for this region change to 99. This is indeed a faint magnitude and effectively corresponds to "no light" received. It is also possible to double click the left mouse button to set the background level at a particular location.
Position the cursor over the object of interest. The system will not automatically centroid an object, so you need to carefully center the object. The display will show a magnitude value that has been corrected for background.
The size of the analysis box considered for the measurement can be changed by using the "T" key (for toggle box size). For stellar objects you want the box to be large enough to include all the light from the star. However, you do not want the box to include light from other objects. As you increase the size of the box, you will see the displayed magnitude value decrease. Of course, a smaller magnitude value indicates a brighter object. As the box size is increased more light from the star is included and the object should display a brighter magnitude (smaller value). Eventually the magnitude values will reach a plateau as all the light from the star is considered in the analysis. You should attempt to use a box size that has just reached the plateau region.
A plot of magnitude versus analysis aperture size is termed a growth curve. This type of analysis is termed a growth curve analysis. You should perform a growth curve analysis and determine what size box (analysis aperture) should be used for the target objects in your image. For this approximate analysis you should select a fixed box size for the magnitude determinations for each image.
Adjust the Sensitivity Calibration