Photometry

Photometry is a beautiful thing: equal parts science and art, equal parts frustration and reward.  It is the core measurement technology of astronomy and provides much of the observational evidence behind the theories.

There are three main methods of doing photometry of a very bright star like epsilon Aurigae:

• Single Channel Photoelectric Photometry (PEP): PEP was the first mainstream optical photometric technology and in widespread use in astronomy by the 1940's. It is not very sensitive, but it can work in very large fields of view. It is also very precise and simple to operate. Some modern PEP systems also have the unique capability of working in the near-infrared, a capability that may prove key to studies of epsilon Aurigae. Infrared measures dust (among other things) and epsilon Aurigae may be a dusty system. PEP was basically made for a project like this!
• Photometry w/Charge Coupled Devices (CCD): CCDs are currently the standard technology for photometry these days. The CCDs in an astronomical CCD camera are close cousins to the CCDs in digital cameras. They are also very sensitive and precise. However, the sensitivity (combined with small fields of view) makes CCD work on bright stars very challenging.
• Digital Single Lens Reflex (DSLR) Photometry: These are your high end digital cameras bought at a consumer electronics store. They are basically a CCD with a lens in front of it and a small computer operating it. They are not as sensitive and precision can be a challenge, but they have large fields of view.  If you are interested, the DSLR Documentation and Reduction team has created a set of tutorials to help you get started doing photometry with your DSLR.

Feel free to ask for advice on our photometry discussion forum.

More details below...

Photoelectric Photometry (PEP)

If you own an Optec SSP3 photometer then you probably already know how to operate it. Here are the comparison and check stars we'd like you to use. Report them as "10star_digital" in the chart field of your observation report.

Comparison Star:

Check Star:

Submit your data through the AAVSO's Blue/Gold system, a web-based system that will automatically take many PEP upload files and convert them into the proper format. For more information, consult the AAVSO's PEP Committee web pages and/or contact us.

The AAVSO helped pioneer the development of the SSP-4 near-infrared photometer. Working with J and H band filters, IR PEP observers are the first amateur astronomers to acquire scientific grade infrared data. Only a handful of instruments currently exist across the world. Visit the AAVSO's infrared PEP web pages if you are interested in joining this elite, pioneering group.

CCD Photometry

Whether you can observe epsilon Aurigae with a CCD system depends mostly on one thing: your field of view. Since the star is so bright it is difficult to find comparison stars that are of similar brightness.

First, find the maximum exposure time of your system before your CCD becomes nonlinear on epsilon Aurigae.  If you do not know, then set your exposures so that your ADU counts are 50% or less of the well depth. This is especially true if you have a camera with anti-blooming gate (ABG). (Click here for info on ABG and saturation).

Once you know this, take 10-100 frames and stack them. Next, look at the field and identify as many comparison stars as you can. Since they are far away from epsilon Aurigae, you may need to offset your field of view so that epsilon Aurigae isn't in the center. If you can find an offset that allows you to have at least one (preferably two) comparison star in the same field as epsilon Aurigae, then you're in business! If not, then you can still participate in one of the many other phases of Citizen Sky, such as analysis.

This type of photometry will likely push the limits of your system. So we recommend paying extra careful attention to the best practices of good CCD photometry: dark frames, flat fields, etc. Be sure you do them well if you want your data to be valuable to researchers.

Use our Variable Star Plotter (VSP) to plot charts and comparison stars. Set the field of view to the value that matches your system, then plot the field. Remember you can offset the center of the field by typing in the coordinates of the center of the field.

Hint: Right now the 62 comparison star is the closest one we have to epsilon Aurigae. To get it in the field, set the center of the chart to RA=05:04:30, Dec.=+43:24:24 and set a field of view of 1 degree. This will show you the smallest chart you can use for epsilon Aurigae at this time. (Click here for a quick link to the chart made with VSP's default settings.)

Click here to read the AAVSO's CCD Manual for Observing Visual Stars for more detailed information.

Here is our latest photometry for the 62 star. Report this as "10star_digital" in the chart field of your observation report.

DSLR Photometry

Filtered photometry with digital cameras is relatively new for amateur astronomers. In order for photometric observations of epsilon Aurigae to be useful to researchers they need to be made with a standard filter. This allows anyone doing research to combine observations made by different people over different nights and with different equipment. A common saying at the AAVSO is "V is V is V", meaning any observation made with a V photometric filter is photometrically the same as any other observation made with a V filter, regardless of the instrument used.

Sadly, they don't make standard V filters for DSLR cameras (if you find one, let us know!). However, DSLR cameras do have filters of another type: tricolor (red, blue and green). The tricolor filters are similar to photometric filters, but not exactly the same. The DSLR camera software saves images from each filter in the .RAW file that is loaded into the photometry software.

So when using photometric software, you need to determine which tricolor filter you want to use. It is vital that you report this information to the AAVSO. If you use the red filter, you want to choose the TR filter code in your observation report. If you use the blue filter, choose TB. For the green filter, choose TG.

There is another, better solution for those who want to make their observations even more useful for researchers: mathematically convert your tricolor filtered data to the same bandpass as filtered data. This is called transformation and can be a lot of effort. But the result is more useful data. This will be discussed at a future workshop. We will use that material to create a tutorial for this web site. Des Loughney has kindly given us permission to post his description (.DOC Word file) of how he determined transformation coefficients for a Canon camera using AIP4Win software.

DSLR photometry can use any comparison star that VSP plots. However, we recommend that you start out using zeta or eta Aurigae as they are well placed in most DSLR fields of view.

If you have a DSLR camera and would like to learn about how to get started from scratch, post a message in our photometry forum! We will also post video here from a DSLR session at the upcoming workshop in Chicago. Until then, here are four references of photometry projects with DSLRs:

• John Hoot, "Photometry With DSLR Cameras". SAS 2007 Proceedings.
• Don Collins, "Intrinsic Variability of beta Lyrae Observed with a Digital SLR Camera", SAS 2009 Proceedings.
• Des Loughney, DSLR photometry of two Eclipsing Binaries.
• Christian Buil's pages have lots of information, including exoplanet transit work with a DSLR. The pages are in French. 

More Information

The following are a few sources of more information on photometry:

• Epsilon Aurigae: A Mysterious Star System: includes a detailed discussion of different forms of photometry.
• AAVSO CCD Observing Manual: A manual for those brand new to CCD photometry.
• Citizen Sky photometry discussion forum