DSLR Photometry Tutorial

Photometry by itself answers a single fundamental question: “How bright is it?” and if you do photometry over time you can answer another fundamental question: “How does brightness change with time?” These answers to these simple questions can be obtained from just visual inspection of a light curve [1], or through more advanced techniques, but the main reason why we do photometry [2] is to gain additional insight to the behavior of various astronomical objects. In this tutorial we focus on stars, but the same approach we discuss here can be applied to planets, asteroids, comets, or even galaxies.

The AAVSO has already posted several documents [3] discussing the importance of variable star work therefore we won't elaborate further here. Instead we wish to discuss a relatively new technique for observing variable stars that appears to be a middle-grounds in terms of ease and precision: DSLR photometry. (Even though our tutorials specifically say “DSLR” cameras, keep reading on. Your point-and-shoot camera may indeed be able to work with these techniques!) Unlike single-channel and CCD photometry which require expensive sensors and specialized astronomical filter sets (more on these later), DSLR photometry can produce high quality data in a fairly short period of time at modest cost. The DSLR Documentation and Reduction Team [4] has created a series of tutorials to guide you through setting up your camera, imaging target and comparison stars, processing your data and submitting the results to the American Association of Variable Star Observers (AAVSO). Other tutorials on this website will walk you through analyzing your data [5] to produce scientific results and publishing your results in scientific journals [6].

As you read through this tutorial you may find it to be out of sequence: we don't actually discuss taking images until the end of the tutorial. This is highly intentional as any experienced photometrist will tell you that frustration begins with bad data. Therefore we want you to work with known good data first so you can get a feel for the procedure as a whole before you take your first frame.

The tutorial is arranged into six major sections. We first introduce you to the broad concepts of how DSLR photometry works and what you will need [7] if you decide to do it on your own. Then we give you an overview of the data reduction packages [8] used to extract photometric magnitudes from images. Then we walk you through starting [9] and finishing [10] analysis and how and finally how to submit data [11] to the AAVSO database for long-time archiving. After this is all complete we discuss the procedure for taking images [12] and things you should check during and after the imaging process.

If you have questions as you go through this tutorial, please post them to the photometry forum [13] as many of our team members regularly visit there.

What you will need

The camera itself
While these tutorials focus on DSLR cameras, any digital camera will do as long as it meets the following criteria:

Produces images in a RAW data format.
Focuses semi-manually.
Can manually select a shutter speed/exposure time of several seconds.
Has a wide enough field of view to get a variable star and comparison star in the image. A typical 50-90 mm lens will do just fine.

For reference, several Cannon point-and-shoot cameras meet this criterion. Although you can do this type of photometry using JPEG images, we don't recommend it. JPEG images offer a very small dynamic range (256 levels) whereas most DSLR cameras offer several times that (4096 or more levels is typical). Such a low dynamic range makes it difficult to observe the small change in brightness that is typical in variable stars.

Something to hold your camera
A simple photo tripod will do. Even a pocket tripod will work, as long as it can support the weight of your camera. Our tutorials focus on bright stars for which exposure times are typically less than 10 seconds and a little star trailing actually helps. For our tutorials, you do not need a telescope or expensive motorized mount, but as your skill grows and you start to do photometry on faint stars, a better mounting scheme will probably be required.

Two other items that will make imaging much easier are a cable release and right angle viewer. The viewer makes locating your stars in the viewfinder a much more comfortable exercise.

A photometric analysis software package
After you snap a few images with your camera, you need to extract the information from the images. In our tutorials we discuss free (IRIS), entry level (AIP4WIN), and high-end (MAXIMDL) reduction packages. We’ll show you how to use each of these suites to reduce the data. A few of our first-time participants have found Richard Berry's AIP4WIN book plus software suite to be helpful for answering fundamental questions about image processing, theory, and practice.

A Computer Capable of running the software
Almost all of the analysis suites run in Windows, but some can be made to run on Macs and Linux machines. You will need to check that your computer meets the requirement of whichever suite you decide to use. Although not explicitly required, we also ask that you submit our data to the AAVSO electronically over the Internet (instead of by mail) to help reduce transcription errors.

An unobstructed view
Although you don't need a fully unobstructed view of the sky, you do need to be able to see the variable star and at least one comparison star. Having a good "dark sky" spot out in the country is not necessary. Successful photometry can be done from a backyard with typical suburban light pollution. Darker skies will make the analysis easier, especially when targeting dimmer variables. Light pollution in a city sky will make faint star photometry nearly impossible, but you can still do meaningful bright star photometry with a DSLR camera.

Overview of Reduction Packages

Digital images of a star field contain all the information needed to determine an accurate magnitude of the target star. Obtaining this magnitude is accomplished in a two-phase process. The tutorial refers to phase one as “Starting Analysis” and phase two as “Finishing Analysis”.

Starting Analysis [9]: In this step of the tutorial we will use one of three software packages to obtain raw instrument magnitudes (a measure of the brightness of the stars that is camera-dependent) from our images.

Finishing Analysis [10]: Step two employs an Excel Spreadsheet to calibrate the instrument magnitudes and produce an accurate V-magnitude.

There is currently one options available in Finishing Analysis. The Basic Procedure [14] provides color calibration only and does not include corrections for variations in atmospheric attenuation whereas the Intermediate Procedure (to be completed soon) adds in a correction for atmospheric extinction. We have also planned for an Expert Procedure which will present you with the theory behind the reduction and let you develop your own routines for reducing data.

Image Processing Software

In order to extract magnitude from your photos, you will need a photometric reduction package. Below we have highlighted three of the most used photometric analysis packages. One is entirely free, whereas the others are commercial. Once you have selected your analysis package you can start analyzing your data [9].

IRIS

Iris is a general purpose astronomical image processing software suite by Christian Buil with both English and French translations that runs on Windows, and under WINE in Linux. It is full featured, updated frequently, and has a modest number of "how to" articles written for it. The program is free to download and use.

Where to get it:
The download and installation instructions can be found on Christian's AstroSurf webpage [15].

AIP4WIN

AIP4WIN comes as software bundled with the book The Handbook of Astronomical Image Processing written by Richard Berry and James Burnell. It is a very thorough book that discusses image analysis, astrometry, photometry and spectroscopy. The bundle sells for $100.00

Where to get it:
The book and software bundle may be purchased from Willmann-Bell Inc. [16] (the publisher) or through other book sellers.

MaximDL

MaximDL is a full camera control suite with complete observatory integration. There are several versions available, depending on your needs. This software is commercial, and the lowest priced version that includes photometric analysis is $300.00

Where to get it:
The software may be purchased in either boxed or downloadable form from Cyanogen Imaging Products [17].

Starting Analysis

Before you start imaging and analyzing your own data, we suggest you use one of our tutorials and sample data sets to learn the steps for obtaining a raw instrument magnitude. We have created two sets of data that include a minimal number of images for this purpose. The Beginner data file was taken under ideal observing conditions and may be reduced by any Final Reduction Method. The Intermediate level data was taken at higher air mass and requires the Intermediate Level Final Reduction method to get good results. Both data sets may use either the AIP4WIN, IRIS, or MaximDL tutorial listed below to extract instrument magnitudes.

If this is your first time going through these tutorials, we suggest using the Beginner Data File [18]. Once you become comfortable with this one, you might give the Intermediate Level Data [19] file a try. Either way, please download a copy of the sample data and extract it (to a convenient directory) before you proceed through the tutorials. Also have the standard stars page [20] open in the background so you can refer to it when needed.

Final Reduction Level	Description	Size (MB)
Beginner [18]	12 RAW Cannon (CR2) files taken as 5-second exposures at F/4.0, ISO 800 on 26 Jan 2010.	74.8
Intermediate [19]	Nine RAW Cannon (CR2) files taken as 5-second exposures at F/4.0 ISO 1600. These data were taken at air masses of ~2 and require the intermediate-level final reduction spreadsheet.	63.1

The tutorials below contain step-by-step procedures for processing digital images to yield instrument magnitudes, which are then used in the next step, Final Analysis. All of the tutorials assume you know how to load your camera’s RAW format images into your computer.

Reduction Software

AIP4WIN [21]

IRIS [22]

MaximDL [23]

After you have finished the tutorial in one of the reduction packages proceed to the next step, Finish your Analysis [10]. Here you will create calibrated visual or V-magnitudes for the variable star(s) in your image.