Photometry

The tools provided by CCDSoft to measure the intensity of star light and quantify the measured value as a “magnitude” are limited only by the observational techniques employed in making the measurements. Using great care, a serious observer will be able to measure stellar magnitudes to accuracy of 0.01 magnitudes or better. There are several key factors for accurate photometric measurements with any instrumentation. These include:

1. The telescope’s aperture, focal length and transmission.  

2. The detector’s quantum efficiency, noise and pixel size.

3. Stellar spectral properties such as Red Giants, White Dwarf, or young blue stars.

4. Atmospheric conditions including background radiation, transparency and steadiness.

5. Exposure periods, precise tracking, the quality of the optics and precision of focus, as well as the use of optical filters.

6. The use of standard stars used for calibration and stellar databases for spectral magnitudes.

Many observers are just interested in “How faint a star did I record on my CCD image?” For this category of measure (the quick and easy approximation path), using CCDSoft along with TheSky Level IV with its Image Link provides a relatively good starting point. We are still required to pay attention to the factors listed above, but the rigors required for precise measurements are be reduced. Before discussing how to measure stellar magnitudes, a general list of instrument requirements is provided as a guide. Although exceptions do exist for different methods of photometry, these guidelines provide a sound foundation for those interested in more serious astronomical photometry.

1. Use all reflective optical systems. The Newtonian telescope is good, but a classical cassegrain is better. Other compound instruments and refractor optical systems fall short of the all reflective designs. Note: Any telescopic system can provide good results, but the all reflective system is generally the most efficient in terms of light gathering power; allows all wavelengths to be focused on the same plane; and has a very broad spectral response range compared to other systems.

2. Use longer focal ratios (or larger f-ratios). This translates into a longer focal length instrument that has a larger image scale and a narrow field of view. A classical cassegrain of f/12 to f/30 in modest 12” to 20” aperture telescope can do outstanding stellar photometry. The large scale, typically 1 arc second per pixel of a CCD camera, is advantageous because the of the following.

a) The star being measured is enlarged to cover several pixels so that an averaging effect takes place when the signal is summed and processed. Atmospheric conditions often limit a photometric measurement because the stellar source is “spread” out over several arc seconds in the telescope image plane. Ideally, we want 100 percent of the received stellar energy falling onto the CCD pixels in the same spot! Of course, this is a very rare event. 

b) The narrow field of view (large image scale) isolates the star being measured from nearby stars and other objects that might add unwanted energy to the observed star's intensity. At larger f-ratios (like f/15) sky glow and background effects are reduced and the sky appears darker. This provides for a better signal to noise factor in the measurement.

c) A telescope system designed at longer focal ratios generally provides better image characteristics because of the correction of optical aberrations (found at f/4 to f/8 ) and the use of smaller secondary optics which minimize diffraction effects. The image plane spot size does become larger as the f-ratio increases, but this is controlled by matching the CCD sensor size to the application.

d) What do you when your telescope does not have a long focal length? There are several approaches.

1. Use a good quality Barlow lens. Although the addition of auxiliary optics can compromise image formation and introduce scatter and spectral absorption, you can increase the f-ratio 2 to 3 times.

2. Use a smaller pixel size CCD camera. This is a compromise that costs significantly more than a Barlow lens! Often the use of smaller pixels represents much more data to be processed by the system. Furthermore, spectral response and sensitivity can be limited by using smaller size pixels. There are occasions where an excellent optical system using small pixels can provide good repeatable photometric results over limited kinds of observations.

e) Mechanical stability, telescope weight, distribution, flexure, and accurate tracking are all important factors when doing photometry. A classical cassegrain (like many compound telescope designs and refractors) does have a distinctive advantage of “loading” instruments at the back of the telescope. The Newtonian reflector is more awkward, especially with a Barlow lens, to balance unless other modifications are employed. For example, you can fold the optics down the tube length.

f) Larger pixel size CCD cameras like the SBIG ST-6 and ST-4x provide adequate sensitivity over a large spectral region, as well as being “well matched” to most amateur telescope systems. The ST-6 is an excellent choice for photometry with the following configurations.

1. Telescopes 6 inches and larger, focal ratios of f/10 and larger.

2. With or without optical filters.

3. Wide range of “seeing” conditions.

4. Short exposure periods minimize tracking errors.

5. High dynamic range provides for many calibration levels to be established within the recorded data.

6. Relatively smaller number of pixels allows for efficient data processing and management. The ST-4X is a preferred CCD because of its smaller area, although centering an object at longer focal lengths can be more tedious than the much larger area ST-6 CCD camera.

g) If filters are used in making photometric measurements, each filter must have a “calibration curve” indicating what wavelengths of light are being transmitted and at what efficiency level. To say that “a star measures magnitude 12 using a blue filter,” does not provide any meaningful results to the science of photometry.