In this project, I aimed to explore how the brightness of the Moon changes with Moon phase. When we look at the Moon in optical wavelengths, we are viewing reflected sunlight. As the Moon phase changes the amount of visible reflected sunlight changes, resulting in a very bright Full Moon and dim New Moon.
In my last project, I viewed the Moon through these optical wavelengths. In this project, I opted to image the Moon in radio wavelengths. I wanted to explore how the brightness (flux density) changed over time in radio wavelengths as radio is not measuring visible reflected sunlight.
Radio waves have super long wavelengths --- some billions of times longer than visible light. Thus, radio waves are an important tool for astrophotographers as they can image the visible "invisible" parts of our universe as radio waves are unaffected by sunlight, clouds, and rain; their length allows them to pierce through the atmosphere and image objects with much less interference than we have with visible light.
Enough background, let us move ahead to the fun stuff!
Observing in the Radio
I worked with my group in class to take four radio images, one of the Moon now, one of the Moon in the future a few days ahead, one of Virgo A now (for flux calibration purposes), and one of Virgo later. My responsibility was to take the image for the later Moon observation. All other data was gathered from my group mates, Adrienne Brummett, Margaret Wright, and Glenn Lassiter.
I took my later Moon image using the 20-meter radio telescope at Green Bank Observatory (pictured below) remotely via Skynet. I used the 20m's L-band receiver, which detects radio waves with frequencies between 1300 MHz and 1800 MHz. I chose specifically to operate the telescope in low spectral resolution mode with the "HI" bandpass filter enabled to narrow the detector's range to better avoid radio frequencies that are human-made and would interfere with my data.
The 20m essentially has a "one-pixel camera" which we can aim wherever we choose to collect radio data. To collect data for the Moon I chose to follow a "map" path with a size of 6x6 beamwidths to give ample room for a background level to be established. Skynet automatically converts this number into terms of right ascension (RA) and declination (dec) so the telescope can actually stay within its mapped boundaries.
Now that we've established the size of our map we must next choose how to maneuver the one pixel camera across these constraints. A raster map sweeping through RA is our best bet as it puts the least amount of stress on the telescope. By choosing this option the telescope will sweep back and forth rather than up and down, negating the extra work that would be caused by gravity. I also had to designate how much of a gap there should be between each sweep and how often data is recorded at each of these sweeps: I chose 1/5 of a beamwidth for both.
The last step was to choose how long the telescope would expose at each data collection point, also known as integration. I selected an integration time of 0.3 seconds which is approximately 0.5° per second. It's better to keep this number under 1° per second or else the raster pattern (and thus the whole map) pretty much falls apart and is unusable.
I was submitting this observation on 1/13/23 so I used the delay feature on Skynet to delay my Moon observation until 1/17/23 so the moon phase would have changed.
Calibration and Cleaning
Here's the data I was immediately presented with via Skynet before any cleaning.
I was lucky to get fairly uninterrupted data, no radio frequency interference (RFI) was present in my data. Actually, that's an unfair statement. I was lucky to get no unexpected RFI in my data. Note on the rightmost graph the RFI peak present around 1420 MHz. This is an astronomical emission line caused by cold hydrogen. We expect to see this line in our data as cold hydrogen is the most abundant thing in our universe!
Below are my preliminary radio Moon maps from both channels of the 20m. These rainbow colors are certainly fun to look at, but what do they mean? Well, these colors technically mean nothing. They are completely arbitrary. Red corresponds to brighter areas while purple is for dimmer areas.
Let's fix these up and make em' pretty.
We'll just give them a quick facelift through some quick cleaning in Skynet first.
Both channels of data were successful and avoid of RFI so under Image Processing Basics I set the channel to sum between both. For Gain Calibration I chose to interpolate the data just to ensure everything is as accurate as possible. Since no RFI was present I did not have to cut any ranges of frequencies from our data
Afterglow time!
Here was my immediate view of my Moon map post-cleaning.
She looks so much better than before, don't you agree?
I completed the same cleaning process for the Virgo calibration data. Here's the image of Virgo after cleaning:
Analysis
We're doing more here than making a pretty rainbow picture. My main goal is to explore how the brightness of the Moon changes with phase. Thus, we must measure the brightness. I used the Photometry tool built into Afterglow to measure the flux of the Moon (later) observation. The same was done by my group mates for Moon Now and both Virgo observations.
We next converted all of our flux values into the form of Jansky (Jy), the "industry standard" physical form for radio wavelengths. We also determined the Moon flux in units of Virgo A. Virgo's brightness is constant which is why we are using it for calibration in the first place. Distance was obtained from Stellarium.
Just a quick note -- the Moon Phase of 61% is the same as the "Moon Now" observation. 20.60% is the "Moon Later" (my) observation. If you were interested, the Moon Now was taken at the phase of waning gibbous, Moon Later was taken at the waning crescent phase.
Well!
We've obtained our answer. The Moon's brightness does NOT change according to what lunar phase it is in. The Moon Flux Density (Jy) for the first observation was approx. 855 Jy and 950 Jy for the second observation. If the Moon's brightness was solely based on reflected sunlight this makes absolutely no sense as the Moon phase is decreasing the amount of reflected light decreases as well, however, this is not reflected (haha get it) in our data. If anything, the brightness actually slightly increased.
Flux and Janskys may be a little difficult to comprehend. To better familiarize our data, let's convert to units of temperature instead. My group got a temperature of approx. 233° Kelvin (K) for our first observation and 230° K for our second.
Anyways, if it isn't reflected sunlight that we're looking at then what's going on here?
It's nothing out of the ordinary. Our Moon doesn't only reflect sunlight, it actually absorbs some of it and re-emits it, acting as a thermal emitter. It can be said that it emits "blackbody radiation".
For Fun: Me vs. the Pros
How accurately did my group measure data compared to top astrophotographers in the field?
I'm surprised to say we did very well, maybe I should doubt myself less!
As a reminder: we calculated a value around 230° K for our temperature.
Both data from 1 GHz and 2 GHz are shown as we actually observed in a frequency somewhere between the two. Our data most accurately fits the findings of the Pros for 2 GHz, so perhaps our data was observed closest to this frequency.
Final Remark
This project was very fun and informative. I was shocked to find out just how accurate our group's observations were. Thanks 20-meter! I learned a lot and definitely look forward to getting to work more with radio observations.
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