Observing with Street Lights

Observing with Street Lights
Dark sky sites not always necessary to see the Milky Way (This image was taken ouside of a B&B in Julian, CA)

Wednesday, November 29, 2017

Quantum gravity lecture; Lessons learned trying out the piggyback camera; Exoplanet transit light curve spectra analysis; Thermal and non-thermal radio emission sources; Happy Birthday Kendra (and me)

Greetings from Palmia Observatory

This week's CSULB physics colloquium was a presentation on supergravity by Professor Zvi Bern, UCLA.  Now I had tried to read some of his papers on the archive and found the topic so far beyond my current understanding that I was thinking of not attending the colloquium, but Math Whiz Dave convinced me that Bern would tailor his presentation for his audience of physics students.  So, I gave in and attended the colloquium and it turned out to be much more informative than I had imagined.

There is no way I could summarize the lecture though, it was just too weird.  Although I had heard most of the terms used before, and the lecturer did a good presentation, it was still too much for me.  I will try just one summary of the lecture which is "...quantum gravity can be described by the square of the gauge representation..."  Ok, so there, aren't you feeling better now just hearing that as a summary?  Anyway during lunch, I asked Professor Bern what kind of study path is needed for students as they try to get an understanding of the ongoing search for a theory of quantum gravity.  He said that one only needs a little bit of general relativity, but that the key subject is quantum field theory.  Ok, I have just a little bit of understanding of general relativity and after attending Math Whiz, Dave's, QFT study group, I can sort of understand the need for more detailed understanding of QFT.  Wow, more work is going to be required in QFT for this physicist wannabe or anyone else hopping to get a grasp on quantum gravity.

Well, after the physics colloquium I was ready for some nighttime observing so that evening I rolled the telescope tripod assembly outside for a first look with the piggyback mounted camera.  There are a lot of city lights, but my main worry was whether or not the piggyback adapter, nuts, washers and screws would all hold together or vibrate apart, not to mention the other tripod and wheely bar adapters.

Wheeled out the tripod with piggyback camera attached (Source: Palmia Observatory)
Wheeled out the tripod with piggyback camera attached (Source: Palmia Observatory)

I tried doing just a rough polar alignment, but before I could slew to the star Altair, which I was using as a proxy location of the Milky Way, the mount battery shut down because it had not been charged in over a month.  So, grabbed my spare battery and slewed to Altair.  The 20 second image below shows how bright the house lights illuminate the white telescope tube and the house roofs.  At least this image and another 60 second image don't show much star trails since the telescope mount, even with imperfect polar alignment, did a pretty good job of tracking Earth rotation.  But no Milky Way is visible in the vicinity of Altair, the target location in this frame, since the sky is assumed to be too bright.

This 60 second exposure shows some stars but no Milky Way (Source: Palmia Observatory)
This 60 second exposure shows some stars but no Milky Way (Source: Palmia Observatory)

Ok, it seems we have to look for darker skies somewhere.  We have tried Julian and Borrego Springs and even the OCA location in Anza, but maybe we should look at this available property in Pioneertown, CA, near Joshua Tree.  I'm not sure if the local zoning ordinances allow for a telescope dome or not, but the bigger problem would be getting water and sewer hookups.  Regardless, we still plan to coordinate a couple of observing nights in the Joshua Tree area because of the super dark skies.

Ok, maybe this is a possible dark sky observatory site in Pioneertown (Source: OC Register)
Ok, maybe this is a possible dark sky observatory site in Pioneertown, Ca. (Source: OC Register)

In the meantime, the first test of the piggyback adapter on this small 80mm telescope offered its own learning lessons.

First learning lesson:  The initial mounting of the camera resulted in unbalance in the right ascension axis.  The mount right ascension axis is aligned parallel with the telescope tube. The camera should have been mounted opposite to the direction to the counterweight so that any rotation about the axis would not result in additional torque.  But in this first attempt, the torque was too great and the mount slipped out of alignment.

Second learning lesson.  The TeleVue camera piggy back mount locking screws were not adjusted tight enough to keep the camera position fixed as the scope moved.  The camera started to sag as the scope rotated.  Additionally, the lockdown screws interfered with the full opening of the camera Liveview screen.  As far as the locking screws letting the camera position shift, I found that I could just tighten them down a bit more and the camera seemed to stay in place.

Well these issues were fixed when I brought the assembly back inside.  The clam shell ring on the scope tube was rotated to bring the camera in opposition to the counter weight and the TeleVue adapter camera mounting plate was rotated 180 degrees so that the locking screws did not interfere with the opening of the Liveview screen.  See the final fix in the photo below.

Aligned piggyback camera with counter weight and fixed Liveview screen constraint (Source: Palmia Observatory)
Aligned piggyback camera with counter weight and fixed Liveview screen constraint (Source: Palmia Observatory)

So, with these fixes in place, we could try once more to see if we could find the Milky Way in city light skies with long term exposure and a tracking mount, but the weather was just not cooperating.  So, maybe what I will plan to do in the next week or so is to actually measure the sky brightness and compare it with the surface brightness of the Milky Way.  This will be a neat kind of science type  measurement and should be fun!  The weather forecast is for cloudy skies the next week or so, so, the only thing to do was wait and return to some astrophysics study.  Are you ready for some astrophysics?

Remember from our previous November 27 post, that the gentlemen we met in Julian, the Julianites, Dave and Dennis, had asked about an article by Caleb Scharf and we discussed some of his various YouTube lectures.  I liked his style and wanted to look in astrobiology a little bit more so I also asked my friends at Amazon to get me his textbook, "Extrasolar Planets and Astrobiology", which just arrived.

Palmia Observatory Resident Astronomer recommeds Caleb Scharf, "Extrsolar Planets and Astrobiology"
Excellent Astrobiology Textbook

It turns out that this is an excellent textbook and is at just the right technical level for a physicist wannabe like me.  In fact, with only reading a couple of hours into the book, I found the answer to a problem that I had been wrestling with since a previous Coursera course on spectra of extrosolar planet transits.  I always got the wrong answer to the homework when we were supposed to predict one possible planetary orbit orientation based on the observed spectra light curve.  Well, here in this book, on Page 173, was an explanation that had eluded me in the previous course.

Finally this Resident Astronomer understands transit spectra (Source: Caleb Scharf, "Extra solar planets..."
Finally this Resident Astronomer understands transit spectra (Source: Caleb Scharf, "Extra solar planets..."

This diagram from the textbook shows three different transit paths and the resulting spectrum light curve that would be observed from Earth.  Take a look at Panel (a) in the diagram, in which the rotation of the star is shown as well as the exoplanet transit path across the star.  The thing I always got wrong in the previous course was how as the planet crossed the limb of the star on the left hand side, that the spectrum would be redshifted.  It seemed obvious to me that since the star was rotating in the direction as shown, that the spectra would be blue shifted.  Aha, what I missed was that the observed spectrum was the average of the whole stellar light curve, because the star's limb is approaching us.  When you consider that the measured light curve is the sum of the blue shifted light from the left limb and the red shifted light from the right limb, that the sum would be seen as red shifted because the transiting planet blocked out some of the blue shifted stellar light.  Finally, I started to see what is going on and how just by looking at the integrated light curve from the star and its spectra you can begin to build a model of what path the exoplanet takes in its transit.  This is a very clear example of why I have had to rely on multiple books to explain and finally get the concepts that I suppose full time students are able to pick up when it is explained in class since I don't have that kind of study environment.

The 2nd astrophysical topic comes from the recently received Great Courses DVD on "Radio Astronomy - Observing the Invisible Universe" as discussed in the recent November 22 blog post.  After I made that post, I got a note from Telescope still packed up in the garage, Frank, who told me that he just received his copy of the DVD too!  Neat, now we can compare notes and ask questions as we proceed through the course.  Did anyone else get their DVD while it is on a steeply discounted sales price?

One of the first topics that caught my attention was the discussion about how the continuous radio emissions from stellar objects fall into two broad categories, depending on the source, which include thermal emission sources of radio signals and non-thermal sources of radio signals.  Now I had heard this before, but really didn't understand what this meant, so, after a little bit of internet searching, I found the answer.  Check out the illustrative spectra for some typical stellar objects below.

Two distinct types of spectra are shown in this chart.  The thermal radio emissions, as seen from the sun and the Orion Nebula, have upward sloping spectra, which are indicative of thermal emission sources defined by the Planck blackbody radiation equations.  These sources have flux that is proportional to temperature (actually temperature to the 4th power).  The higher the temperature, the radiation peak moves higher in frequency, so that for the sun, for instance at 5800 K, the peak emission is in the visible wavelengths.  Higher temperature gaseous sources would tend to emit in x-rays and beyond.  Radio wave emissions tend to peak at low temperatures, such as 10-20 K, similar to the peak cosmic microwave background radiation which peaks at about 3 K.

The other spectra type, which slopes down with increasing frequency, is identified with non-thermal sources, such as found in observations of Cas A, Crab Nebula, M31, and 3C273, for example.  These non-thermal sources are typically associated with synchrotron radiation, which is proportional to the magnetic field strength of the source, and not its temperature.

Stellar object spectra caused by synchrotron and thermal emission (Source: Swinburne University of Technology)
Stellar object spectra caused by synchrotron and thermal emission (Source: Swinburne University of Technology)

The screenshot below shows how synchrotron emission from many different particles adds up to the downward sloping spectra line shape.  This range of emission sources is why radio astronomy can be so useful in mapping the processes that are found throughout the universe.  These two sources of what is called continuous emission sources are both caused by the acceleration of charged particles, which results in electromagnetic radiation.  Note that this continuous emission is different from the discrete emission due to quantum mechanical transitions between atomic energy levels.  One type of quantum spin transition of the hydrogen atom is responsible for the 21 cm emission line, which also is a major source for investigation by radio astronomy.

How non-thermal synchrotron radiation gets its spectra (Source: Swinburne University of Technology)
How non-thermal synchrotron radiation gets its spectra (Source: Swinburne University of Technology)

Finally, after all of that study and beginning to feel much, much older and ready to turn another year older, we had a chance to relax and celebrate the upcoming birthday.  Luckily and enjoyably we were able to celebrate with great niece Kendra and enjoy a combined birthday party.  Happy Birthday Kendra (and me)!

Great niece Kendra and Resident Astronomer celebrating 87 years at combined birthday party
Kendra and Resident Astronomer look forward to their cake for 87 years at combined birthday party

Until next time, when we hope to be able to report on measuring the city lights night sky brightness measurement (weather permitting of course),

Resident Astronomer George


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