As one of the AAVSO’s four primary observational programs, stellar photometry involves precisely measuring the brightness of stars. By collecting many of these measurements over time, we can watch as the stars’ brightnesses change in response to a huge variety of astrophysical processes. Stellar pulsations, eruptions, accretion disks, supernovae—photometric observers study all of these, and much more!

Throughout the long history of the AAVSO, stellar photometry has been our primary focus. Our photometric database, the AAVSO International Database (AID), houses multiple centuries of data in an unparalleled record of stellar photometry across time. The AID continues to grow at an accelerating rate, with over 5,000,000 new observations contributed every year by people around the globe.

Most of the people who contribute these observations are not professional astronomers. Instead, they’re students and teachers; parents and children; retirees, busy professionals, and everyone in between. If there’s one word which describes stellar photometry, it’s “accessible”. Stellar photometry is an ideal project for stargazers in their backyards, and indeed, we rely on the contributions of these stargazers to advance astronomical research.

You, too, can make a real contribution to astronomy. If you’re interested in doing so, read on.

Photometry: Visual and digital

Broadly speaking, there are two ways to record stellar photometry.

The first method, visual observing, involves visually looking at each star of interest, and comparing its brightness to stars of constant brightness called comparison stars. Because this method requires nothing more than the human eye, it is the easiest and least expensive way to contribute to scientific astronomy.

The second method, digital photometry, involves using a camera to photograph the target star and its comparison stars. When done carefully, this has several advantages, including allowing you to measure a star’s brightness much more precisely than would be possible with the human eye. However, if you’re not already familiar with astrophotography, the learning curve can be steep.

Notes on terminology

Individual measurements of brightness are known as photometry (as in “let’s take a look at the photometry”). The act of recording this data is also known as photometry (as in “AAVSO observers do photometry”).

Strictly speaking, it does not matter whether you observe visually or digitally; if you are measuring the brightness of stars, you are doing stellar photometry. However, to avoid confusion, we tend to reserve the word “photometry” for digital photometry, while photometric data points recorded with the human eye are known as “visual observations” or “visual magnitude estimates”.

Guidelines

If you plan to observe visually – using the unaided eye, binoculars, or a telescope – we expect that you know how to identify constellations, use star charts, and locate objects in the sky by star hopping or similar techniques.

If you plan to observe digitally – using a DSLR, CCD, or CMOS camera; photoelectric photometer; or similar instrument – we assume that you can locate objects in the sky, assemble your equipment, and operate it reliably.

Equipment

Stellar photometry is the most accessible of the AAVSO’s programs, and it is possible to collect useful data with little to no specialized equipment.

Visual observers are encouraged to make naked-eye observations of the brightest stars in the sky, many of which are interesting variables in need of additional data. You are also encouraged to use binoculars and telescopes of any size. The ideal aperture for visual observing depends on the target; some observers report that their magnitude estimates are most accurate when working close to their magnitude limit.

Digital photometric observers may use any combination of digital camera and optics, ranging from a lone DSLR camera to an observatory telescope with a cooled CMOS camera. Smart telescopes can be used, as well. The most important criteria when choosing a camera is that it allows you to obtain raw, unaltered images (this rules out some consumer-grade digital cameras, including most cell phone cameras).

To maximize data quality, we recommend using a monochrome sensor in combination with standard photometric filters. Additionally, like in other forms of astrophotography, your signal-to-noise ratio and magnitude limit will be best if you use a telescope which tracks the sky; larger apertures help, too. However, it is important that the field of view is wide enough to image both your target and its comparison stars simultaneously.

If you want to start doing photometry but don’t yet have access to photometric equipment, consider becoming an AAVSO member. AAVSO members have access to a global telescope network, AAVSOnet, which is optimized to provide high-quality photometric data in a variety of filters.

Data quality

Much more important than your specific equipment configuration are your data quality practices. The subject of data quality is lengthy and complex, but simply taking care and consideration as you observe is a great start.

No matter what mode of observing you plan to do, you should strive to:

Observe your targets near culmination, when the airmass is lowest.
Choose comparison stars which are similar in color and magnitude to your target, and nearby on the sky.

Additionally, digital photometric observers should:

Be vigilant about avoiding saturation and staying within the linear regime of your sensor.
Use standard photometric filters.
Transform your data into a standard color system.
Calibrate your images using flat frames.
Measure and report uncertainty.

This is not an exhaustive list. To learn more about photometric data quality, see the AAVSO Guide to CCD/CMOS Photometry, and Matthew Templeton’s article “Good Data, Good Science”. We also welcome questions about data quality on the AAVSO forums, and encourage you to discuss data quality with others through our peer learning structures.

Charts

Please use AAVSO charts and comparison star magnitudes. Visual observers can use the comparison star magnitudes as written on the chart itself, while photometric observers should use the more precise values provided in the associated photometry table.

You can generate charts and their associated photometry tables using Variable Star Plotter (VSP).

Your first observation

We provide illustrated tutorials designed to help you learn visual observing; these are known as the “10-star Training Tutorials”. These are an excellent place to start, even for digital observers, because the concepts covered in the 10-star Training Tutorials are applicable to photometry of all kinds.

Once you have an AAVSO observer code, you can submit your data from within the AAVSO app portal. Simply log in to the website, fill out the form, and click “Continue” to review your data in context. Once you’ve reviewed your data and confirmed that it is free of transcription errors, click “Submit”.

Pro tip: Submitting observations individually can eventually become tiring, and it increases the risk of typos. Fortunately, you can also upload observations in bulk. To do so, select “File” mode within the upload form, and submit a file in AAFF format (LINK).

Choosing targets

Every star is variable on a long enough time scale, and every class of star is of interest to astrophysicists somewhere. This represents a golden opportunity for observers to define their own observing programs. Would you like to watch unpredictable eruptive variables, like cataclysmic variables and novae? Track binary stars as they flex with tidal forces? Witness young stellar objects emerging from their dusty cocoons?

For your first targets, we recommend simply choosing stars which excite you. If you’d like some ideas, we have compiled tables of exciting, rapidly-varying stars on our page “Your First Observing Target”.

As you gain experience as an observer, you may wish to add more stars to your list, or take on longer projects with a specific science focus. For guidance on this process, please see Creating a Target List.

Relevant Special Interest Groups

Most of the AAVSO’s Special Interest Groups (SIGs) engage with stellar photometry in some way. Joining a SIG is a great way to learn about targets of interest, participate in research projects, and socialize with other observers who share the same interest.

Visual observers might be especially interested in the Long Period Variables and Young Stellar Objects SIGs, which focus on bright and interesting targets classically well suited to visual observation.

For photometric observers, we particularly recommend the AAVSOnet SIG, because they can help you develop a successful proposal and begin receiving data from our global remote telescope network. We also encourage newer photometric observers to investigate the Eclipsing Binaries SIG, which offers the opportunity to contribute substantially to science through eclipse timing, even if you haven’t yet figured out filters or transformation.

Other SIGs relevant to stellar photometry include:

To browse the full list of SIGs, see Special Interest Groups.

Learn more

The AAVSO offers a wide range of educational materials to support observers interested in stellar photometry.

Software

We have developed several online tools to support photometric observers:

Variable Star Plotter (VSP) is our online chart creation tool, useful for both visual and digital observers.
VPhot (members only) is our powerful cloud photometry software. It takes much of the difficulty out of learning digital photometric data reduction.
The AAVSO Application Portal offers unified access to our webapps, including the data upload form, the JD calculator, and much more.

Courses

Our online CHOICE courses offer intensive, individualized training in various subjects. Photometric courses include:

“Developing a Visual Observing Program”
“CCD Photometry”
“Photometry using VPhot”
“Observational Best Practices”

Each course accepts only a limited number of students, so we recommend registering early.

Webinars

We frequently discuss stellar photometry as part of our interactive webinar series. Each webinar is recorded and uploaded to our YouTube channel, resulting in a free library of information-rich lectures.

Visual observers might be particularly interested in these recorded lectures:

While photometric observers may find the following videos to be especially valuable:

Stellar Photometry