What should I observe? That is one of the most common questions the AAVSO receives. The answer however, is not a simple one and depends on several factors:
- Your experience level
- The equipment you have access to and it’s capabilities
- How often you are able to observe
- Your personal interests
These elements will shape your journey, and the information below is designed to help you get started.
What types of observing does AAVSO support?
The AAVSO supports a wide range of observational programs that enable amateur astronomers and students to contribute meaningful data to the scientific community. These programs include:
- Stellar Photometry: By measuring the brightness of variable stars over time—either visually or with digital detectors—your observations help monitor transient events, characterize stellar evolution, and support the operations of space missions.
- Stellar Spectroscopy: By splitting starlight into its characteristic wavelengths and measuring it with a digital detector, your data can reveal changes in temperature, chemical composition, radial velocity, and emission or absorption features.
- Exoplanet Photometry: By observing the brightness dips caused by transiting exoplanets using digital detectors, your measurements help refine orbital parameters and support follow-up efforts by space-based observatories.
- Sunspot Counting: By visually counting sunspots on the solar surface each day, your observations contribute to a continuous historical record that is used to calibrate modern space missions and study solar activity cycles.
If your interests lie outside these areas, let us know—we’ll be happy to direct you to a program or group that better matches your goals.
What is Your Experience Level?
Your experience level plays an important role in determining which observing projects will be a good fit. The AAVSO welcomes observers from all backgrounds, but we do assume a basic level of astronomical knowledge.
If you plan to observe the Sun, we expect that you have appropriate solar filters, a solar telescope, or other dedicated solar observing equipment, and that you know how to assemble and use it safely. Observing the Sun without proper protection can cause permanent eye damage or blindness, so safety must always be your top priority.
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, mirrorless, CCD, or CMOS camera; photoelectric photometer; spectroscope; or similar instrument—we assume you can locate objects in the sky, assemble your equipment, and operate it reliably.
If you’re still developing any of these skills, don’t worry—we’re here to help. We have manuals dedicated to each major observing type. Members of the AAVSO can also request a mentor to help them learn skills and get started observing.
What is Your Equipment and its Capabilities?
This is an important question because it defines what you are able to do and how challenging different types of observations may be. For example, a fourth magnitude star is easy to observe with your eye, but much more difficult with a camera attached to a telescope as it can quickly saturate the detector. Another important consideration is what equipment you might want. If you have a particular interest, tailor your setup to support it, assuming it’s within your budget.
The tables below can help you access which objects may be accessible to you under various conditions.
Typical Visual Photometry Limits
Visual stellar photometry is done by eye, with or without binoculars or a telescope. Because rods dominate low-light vision, the eye has a broad spectral response and is sensitive to background light. While less precise than digital methods, visual estimates are useful for long-term monitoring of bright variables. Table 1 below provides approximate limiting magnitudes for visual observers.
| Approximate limiting V magnitude for visual stellar photometry observations | |||
| Observing Method | City | Semi Dark | Very Dark |
| Unaided Eye | 3.2 | 4.8 | 6.7 |
| Binoculars | 6.0 | 8.0 | 10.6 |
| 6” / 152-mm Telescope | 10.5 | 12.0 | 12.5 |
| 8” / 203-mm Telescope | 11.2 | 12.7 | 13.6 |
| 10” 254-mm Telescope | 12.0 | 13.5 | 14.7 |
| 16” / 406-mm Telescope | 13.0 | 14.5 | 15.6 |
Table 1. Approximate limiting magnitude based on equipment and light pollution.
Approximate Limiting Magnitudes for Stellar Photometry
Modern CCD and CMOS sensors are extremely sensitive devices capable of detecting faint objects with high precision. Table 2 below provides approximate limiting magnitudes for a variety of telescope sizes. These values were derived from Michael Richmond’s SNR calculator for CCD (and CMOS) photometry which implements the 2D aperture photometry methodology described in Howell et al. (1989). When collecting stellar photometry, you should strive for a SNR of 100 or more.
Because system performance depends on numerous variables—such as optics, detector characteristics, and sky conditions—we recommend using the calculator to simulate your specific configuration and assess its limitations more accurately.
| Telescope Aperture | Limiting Magnitude | |||
| Imperial | Metric | SNR ~ 100 | SNR ~ 10 | SNR ~ 3 |
| 2-inch | 50-mm | 9.9 | 13.5 | 15.1 |
| 4-inch | 102-mm | 11.4 | 14.7 | 16.4 |
| 6-inch | 152-mm | 12.3 | 15.3 | 17.1 |
| 8-inch | 203-mm | 12.8 | 15.7 | 17.5 |
| 12-inch | 305-mm | 13.6 | 16.2 | 18.0 |
| 16-inch | 406-mm | 14.0 | 16.5 | 18.3 |
| 24-inch | 610-mm | 14.6 | 17.0 | 18.9 |
Table 2. Approximate limiting magnitudes for filtered CCD or CMOS sensors on various f/10 telescopes under heavily light polluted sky conditions using single 10 second exposures. See the “Rules of Thumb” section below to scale to other configurations.
Exoplanet Photometry Limiting Magnitudes
Exoplanet transits are subtle events, typically blocking less than 2% of a host star’s light. To maximize the throughput and improve signal to noise ratio, most exoplanet observers collect data without photometric filters in place. Table 3 provides approximate limiting magnitudes for a range of telescope apertures, based upon simulations as described in the Stellar Photometry section above. When conducting exoplanet photometry, aim for a SNR between 50 – 100 to reliably detect the shallow transit signals.
| Telescope Diameter | Limiting Magnitude | |||
| Imperial | Metric | SNR ~ 100 | SNR ~ 10 | SNR ~ 3 |
| 2-inch | 50-mm | 11.6 | 14.9 | 16.2 |
| 4-inch | 102-mm | 12.9 | 15.8 | 17.2 |
| 6-inch | 152-mm | 13.5 | 16.3 | 17.7 |
| 8-inch | 203-mm | 13.9 | 16.7 | 18.0 |
| 12-inch | 305-mm | 14.5 | 17.1 | 18.4 |
| 16-inch | 406-mm | 14.8 | 17.4 | 18.8 |
| 24-inch | 610-mm | 15.3 | 17.9 | 19.2 |
Table 3. Approximate limiting magnitudes for unfiltered CCD or CMOS sensors on various f/10 telescopes under urban sky conditions using 10 second exposures for photometry. See the “Rules of Thumb” section below to scale to other configurations.
Stellar Spectroscopy Limiting Magnitudes
The tables below present approximate limiting magnitudes for spectrographs used with telescopes of various aperture sizes. These estimates are based on simulations performed using Christian Buil’s SimTrans spreadsheet for transmission (slitless) spectrographs and Ken Harrison’s SimSpec spreadsheet for reflective grating spectrographs. All simulations assume an f/10 optical configuration, urban (Bortle 8) sky conditions, and coadded spectra from 10 individual exposures each 100 seconds in duration.
Because stellar spectroscopy is highly sensitive to the specific optical configuration of your setup, we strongly recommend simulating your own system’s performance using these tools to better understand its capabilities and limitations.
We recommend that observers aim for a SNR of 100 for all spectroscopic observations to ensure high-quality data suitable for a wide range of analyses. However, basic spectral typing of transient events (such as novae) can often be performed with spectra down to SNR ~10. As a general guideline, spectra with SNR below 3 are typically not scientifically useful.
An additional constraint for spectroscopic observations is the spectral resolution, R, which describes how finely a spectrograph can separate different waves of light. Resolution can be broadly constrained to the following:
- Low resolution (R∼200R): Provides a broad overview of the spectrum. Useful for detecting strong features like emission lines and general spectral typing, but fine details are not resolved. This is typical of what can be obtained with Star Analyzer 100/200.
- Medium resolution (R∼1000): Resolves more spectral features, allowing for better line identification and some velocity measurements. This is typical of what can be achieved with the Alpy 600, LISA, and LowSpec.
- High resolution (R∼10,000): Can separate very closely spaced spectral lines, allowing detailed analysis such as measuring radial velocities or studying line profiles. This is typica of Shelyak eShell, LHIRES 3, and similar devices.
Tables 4 – 6 below give limiting magnitudes for each resolution category.
| Telescope Diameter | Approximate Limiting Magnitude | |||
| Imperial | Metric | SNR ~ 100 | SNR ~ 10 | SNR ~ 3 |
| 2-inch | 50-mm | 5.3 | 8.1 | 9.4 |
| 4-inch | 102-mm | 6.9 | 9.7 | 11.0 |
| 6-inch | 152-mm | 7.7 | 10.5 | 11.9 |
| 8-inch | 203-mm | 8.4 | 11.1 | 12.5 |
| 12-inch | 305-mm | 9.2 | 12.0 | 13.4 |
| 16-inch | 406-mm | 9.9 | 12.6 | 14.0 |
| 24-inch | 610-mm | 10.8 | 13.5 | 14.9 |
Table 4. Approximate limiting magnitudes for a R~200 transmission grating spectrograph on various telescopes under conditions as described in the text above.
| Telescope Diameter | Approximate Limiting Magnitude | |||
| Imperial | Metric | SNR ~ 100 | SNR ~ 10 | SNR ~ 3 |
| 2-inch | 50-mm | 7.5 | 11.2 | 12.6 |
| 4-inch | 102-mm | 8.8 | 12.6 | 14.1 |
| 6-inch | 152-mm | 9.5 | 13.3 | 14.8 |
| 8-inch | 203-mm | 9.8 | 13.6 | 15.1 |
| 12-inch | 305-mm | 10.4 | 14.2 | 15.7 |
| 16-inch | 406-mm | 10.6 | 14.4 | 15.9 |
| 24-inch | 610-mm | 11.1 | 14.9 | 16.4 |
Table 5. Approximate limiting magnitudes for an R~1,000 reflective grating spectrograph on various telescopes under conditions as described in the text above.
| Telescope Diameter | Approximate Limiting Magnitude | |||
| Inches | Millimeter | SNR ~ 100 | SNR ~ 10 | SNR ~ 3 |
| 2” | 50 | 4.9 | 8.7 | 10.2 |
| 4” | 102 | 6.3 | 10.0 | 11.5 |
| 6” | 152 | 6.9 | 10.7 | 12.2 |
| 8” | 203 | 7.2 | 11.0 | 12.5 |
| 12” | 305 | 7.8 | 11.6 | 13.1 |
| 16” | 406 | 8.1 | 11.8 | 13.3 |
| 24” | 610 | 8.5 | 12.3 | 13.8 |
Useful Rules of Thumb
Table 6. Approximate limiting magnitudes for an R~10,000 reflective grating spectrograph on various telescopes under conditions as described in the text above.
The following guidelines may help you estimate how changes in your observing strategy affect limiting magnitude and signal-to-noise ratio (SNR):
- Exposure Time: Increasing the exposure time by a factor of 2.5 improves the limiting magnitude by approximately 1 magnitude, up to the point where sky background or instrumental noise becomes the limiting factor. For example, a 25-second exposure will typically detect stars about 1 magnitude fainter than a 10-second exposure.
- Image Stacking: Stacking N images increases the SNR by a factor of √N. For instance, stacking 100 images improves the SNR by a factor of 10 compared to a single image.
- Sky Darkness: For telescopes 8 inches (203 mm) or smaller using filtered detectors, there is little to no gain in limiting magnitude when moving from Urban (Bortle 8, limiting Mag V ≈ 18.1) to Rural ( Bortle 3, limiting Mag V ≈ 21.8) skies. However, unfiltered detectors and filtered detectors on larger telescopes do benefit from darker skies, with improvements proportional to the reduction in sky background brightness.
What are Your Time Constraints?
Not all observations require the same level of time commitment, so it is important to ask yourself: how much time do I have and when is it available? When picking targets, try and find those that fit into your schedule and life.
Stellar photometry is highly flexible in terms of time commitment, making it ideal for observers with varying schedules. Many variable stars, like Miras, semi-regulars, or long-period eclipsing binaries, change slowly and only require brief observations once every few days or even once a month. Other variable stars, like cataclysmic variables, benefit from more frequent monitoring or time-series observations over several hours. The cadence depends on the type of star you’re observing, but in most cases, you can tailor your contributions to the time you have available.
Exoplanet photometry is time-intensive and requires advanced planning. Observing a single transit can take anywhere from 10 minutes to 12 hours of uninterrupted imaging. Transits are infrequent for any given star, happening every few days to every several weeks, so you must plan your observations in advance. As a result, exoplanet photometry is best suited for observers who can commit to longer sessions and plan around predicted transit times.
Stellar spectroscopy varies in time commitment depending on the target and desired resolution. Low-resolution spectra of bright stars can be acquired in short sessions of 15-30 minutes, making it possible to gather useful data almost any time you can observe. High-resolution spectroscopy or monitoring evolving events like novae typically require longer sessions of 2-3 hours in duration. The flexibility provided by spectroscopy makes it a favorite among many advanced observers.
Sunspot counting is well suited for those with daytime availability and limited time. A typical sunspot count only takes a few minutes and can be done visually through a projection setup or with filtered instruments in your backyard. Ideally, observations are made daily at the same time, but even observations every two or three days are useful to the scientific record if continued for many years. Also, because sunspot counting is a daytime activity, it leaves your night time available for other observational activities.
What Excites You?
While equipment and lifestyle are important constraints, the most important factor in picking an object to observe is what you find truly interesting. If you pick a topic because it’s doable”, but you have no real desire to do it, you will find yourself losing interest fairly quickly. If you truly enjoy what you are doing, you might find the lack of sleep tolerable, or you might find yourself scheduling around your observing time and not the reverse. This activity is a great way to learn about and contribute to astronomy and science in general, but remember that first and foremost it should be fun.
Putting It All Together:
By now, you’ve thought carefully about your own observing world detailing the experience you bring, the equipment you have, how much time you can devote, and what kinds of objects spark your curiosity. The next step is simply combining all of that into a clear, realistic plan for getting started.
Think about what your answers are telling you. If your setup is modest and your time is limited, you’ll get the most satisfaction from bright stars that change slowly, letting you build skills without pressure. If you have a telescope and camera, you can aim for fainter stars or those that vary more quickly. And if you can observe often or for long stretches, you might enjoy projects that need more continuous monitoring or quick responses to sudden events. The goal isn’t to pick the “perfect” target right away, but rather to find something that fits naturally into your life so you can learn and stay engaged.
Once you have a general sense of what fits you, it’s time to look for actual stars. If this is your first ever observation we recommend you start from a list of appropriate targets. Otherwise we recommend you start by Creating a Target List.
Lastly, start small. Choose just one or two stars and get comfortable with the observing routine and data submission process. You’ll quickly discover what works for you and what doesn’t, and you can always adjust as your skills and confidence grow.
Additional Advice
Figuring out what you want to observe should be a fun process, but it can also come with a bit of anxiety. Here is some advice to help keep everything in perspective.
- Avoid Buyer’s Remorse: There is a lot of specialized equipment out there for variable star observing of varying levels and at varying price points. Before buying everything brand new, start with what you have available. Many people have binoculars or a DSLR camera from other hobbies. Maybe you have an old telescope lying around. If nothing else, just naked eye observing is an option. If you do want to buy a better setup, check on second hand sites or at your local astronomy club. This should give you a better idea of what works for you without spending lots of money from the outset.
- Have Fun: The AAVSO is a great way to contribute to astronomy and science in general, but it can have a steep learning curve. There are quite a few things to consider, but just figuring out what to observe is a great first step. Allow yourself to enjoy each step of the process and don’t get too bogged down in all of the details, at least not right away. Instead, use this as a learning experience and allow yourself to not be perfect from the start.