Sky & Telescope - December 2024 - 65

chrome camera equipped with a color filter wheel loaded
with photometric filters. Most any cooled or uncooled
camera is suitable, as you won't be taking extremely long
exposures. So the one you may already be using to photograph
the planets should suffice.
You also need to know the pixel scale of your entire sysMethane
(CH4)
Baader Sloan Y
tem. For example, I use a 12-inch (305-mm) telescope operating
at a focal length of 1,500 with a camera equipped with a
Sony IMX290 sensor having 2.9-μm-square pixels. Using the
formula 206.265 × (pixel size) / (focal length) yields an image
scale of 0.4 arcsecond per pixel.
Key to getting good photometric results is to avoid overexposing
the planet. It's important to keep the level of the histogram
at a reasonable value, because as a pixel approaches its
saturation point, the signal becomes non-linear. I recommend
keeping the maximum histogram value below 80%.
It's helpful to calibrate both the target solar system body
and the comparison-star images with flat-field, bias, and
dark frames just like in stellar photometry. However, using a
low-noise CMOS sensor, I find I can forgo these calibration
frames by making sure my sensor is free of dust and stacking
video frames to produce a high signal-to-noise ratio result.
Don't perform any additional post-processing like sharpening
or wavelets, and save your stacked result in FITS format.
Finally, there is another important tip to use when you
take the videos of the comparison stars: They should be
recorded intentionally out of focus. Focused stars are vulnerable
to fast seeing variations, which result in a loss of light
and somewhat incorrect values. Unfocused stars behave
similar to planetary disks, which don't twinkle due to their
angular size as seen from Earth. The unfocused star will
require a longer exposure, which produces consistent intensity
readings. For the same reason, videos of at least 15 to 30
seconds are recommended.
Measuring Your Images
To measure the luminosity value of your target, open the calibrated
planet image in a program with photometric tools, such
as IRIS (https://is.gd/irisastro). Next, adjust the display of the
ADDITIONAL WAVELENGTHS The methane (CH4) and Y filters
are also useful additions to a planetary imager's arsenal of tools. The
methane filter passes a narrow swath of the spectrum centered at 890
nanometers and reveals areas deeper within Jupiter's atmosphere. The Y
filter passes light between about 950 and 1,150 nanometers.
image (the Threshold sliders seen on the bottom of page 66)
so that the background noise is just visible. This will make the
planet appear saturated, but don't worry, this is only for display
purposes, and the image itself isn't being modified.
Select Analysis > Aperture photometry from the pulldown
menu. A window opens where you set the areas to be measured.
Circle 1 should just encompass the entire object, for
example Jupiter. Circle 2 should be slightly larger. The third
circle sets the outer limit of sky background that will be
measured and subtracted from the target body's luminosity
measurement. The area between the first and second aperture
is ignored, as it generally contains light scatter around
your target that can contaminate the photometric measurements.
With these set, click OK, and then click on the object
to be measured. A small window will pop up containing the
measured information, the most important of which are the
intensity and magnitude.
However, the magnitude information is the relative, rather
than the actual, magnitude. For the latter value, perform
these same actions on your comparison star of known magnitude,
and use the formula provided from the AAVSO method:
M = (m − c) + C + T × D
where m is the relative (measured) magnitude of the planet, c
is the relative magnitude of the comparison star, and C is the
catalog magnitude of the comparison star, T is the transformation
of magnitude, and D the transformation of color,
producing M, the actual magnitude of the planet. T and D are
coefficients that are necessary to convert your system data to
the Johnson-Cousins or Sloan catalog system. You can find
these system values in a paper published by Anthony Mallama
in the journal Icarus found at https://is.gd/planetalbedo.
Then, once you know the apparent magnitude of the planet
Rc
Ic
in a given band, you can determine its geometric albedo. This
requires first calculating its normalized magnitude (the equivalent
of the absolute magnitude of stars but applied to solar system
objects). The normalized magnitude is how bright the planet
sk yand tele scope .o r g * DECEMBER 2024 65
https://is.gd/irisastro https://www.is.gd/planetalbedo http://www.skyandtelescope.org
Sky & Telescope - December 2024

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