I. Introduction

CCD photometry has the big advantage of being able to acquire simultaneous data on multiple stars. The sensitivity of the CCD also allows short exposures of brighter stars and the ability to work with very faint stars. One disadvantage is the low dynamic range. For photometry comparison and program stars must be within a couple of magnitudes of each other. Photometric imaging is very similar to regular astro imaging except images are taken through special photometric filters.

Learning CCD photometry involves several stages:

1. Learning the equipment - telescope, mount, camera and software. While most any telescope can be used, reflectors are favored. For mounts, a fork mount will be superior to a German Equatorial Mount (GEM). This is because the best photometry is near the meridian and that is where the GEMs are weakest as they must do a median flip to continue past the meridian. A fork mount has no such problem. A polar/equatorial mounted telescope is preferred to an Alt./Az mount although it is certainly possible to do photometry with an Alt./Az. mounted telescope. A permanent setup is preferred , if the telescope is removed each night. Setting up and aligning can take a fair amount of time each night. Some protection against wind and stray lights is useful. Horizon-to-horizon visibility is not needed. At the most plus and minus 60 degrees from the zenith would be fine. In fact plus and minus 30 degrees will work fine most of the time as that is the best region for photometry.

2. Learning to take good images of star fields involves taking dark and flat fields and using them to calibrate the image.

3. Once you have mastered the imaging you are ready for practice photometry. Pick stars that are high in the sky when they cross the meridian. The closer to the zenith, the better. The further from the meridian the poorer quality the images will be and thus poorer photometry. Try to plan your observing so the star is to the East of the meridian and will cross during the observing session.

4. Take multiple sets of images of a star field with a program and comparison star. Be sure to save the images as FITS. If you save the images as a JPG or GIF or other than a FITS format, you cannot do photometry on the image.

5. Practice getting data from a single image until you can repeat the magnitude values. They should repeat exactly for the same image.

6. Practice getting data from several images of the same star field taken near the same time and with the same filter. The magnitude data for the same stars should be the same between images. Most likely the data will not be the same. Work to find out why and how to minimize the differences.

7. Once you have repeatable data and are confident in knowing what your are doing, start some serious photometry.

The following is a suggested procedure using a Meade DSI Pro or DSI Pro II monochrome camera and AutoStar software.

Created 1 February 2007

Modified 10 February 2007

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