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)

Thursday, August 16, 2018

Planning three tours of Arizona observatories; Photographing the Milky Way from a cruise ship? Underground liquid water lake on Mars; Star formation rate vs. redshift

Greetings from Palmia Observatory

Well this week has been again spent mostly inside, even though the smoky skies have pretty much abated, but we have been busy scanning the astronomy news and have been busy planning for travel and packing our bags.

We attended the Distinctive Voices lecture, completely sold out, series last night at the Beckman Center and heard Alex Stone talk about "The Science of Magic and the Art of Deception."  Pretty neat, and his performance of the "Three Card Monty" card game scam was totally mind blowing in that he baffled the whole auditorium.  We were all pretty sure where the Ace was, but his clever sleight of hand always resulted in us selecting the wrong card.

Afterwards, Math Whiz Dave reminded OCA, Learning to Tango, Brian, and this Resident Astronomer to get signed up for the upcoming Mars Society Convention 2018 held in Pasadena on August 23-26.  I went to the convention last year and they do a good job of covering all aspects of first of all getting to Mars and then many sessions on how to survive and thrive on Mars and create a colony which can be self sustaining and not relying completely on resupplies from Earth.  If you want to head to Mars, or just interested in what developments are being made, then check out the details at: http://www.marssociety.org/conventions/2018/

Ok, back to the travel plans, first up, after seeing the great photos, provided last week, by Retired Superintendent Jay, showing the sites around Moana Kea observatories, we decided to accelerate our previous plans for visiting three great observatories in Arizona.  Check out the map below which shows the general location of the three observatories, all within about 2 hours drive from Tucson.  Anybody else up for making the journey?

Proposed dark sky observing tour at Mt. Graham, Mt. Lemmon and Kitt Peak Observatories (Source: Palmia Observatory)
Proposed dark sky observing tour at Mt. Graham, Mt. Lemmon and Kitt Peak Observatories (Source: Palmia Observatory)

Each of the three observatories has a visitor website for tour details.  Mt. Graham International Observatory, a division of Steward Observatory, U of Arizona, has three main instruments, including the Large Binocular Telescope and a telescope operated by the Vatican.  We don't have an opportunity for any night viewing there, and they have limited access up the mountain, but they do provide an all day tour, with lunch.  For more details, check out: http://mgio.arizona.edu/

The Mt. Lemmon Observatory has a half dozen scopes in the 0.5 to 1.5 meter size and they do offer public viewing nights on some of the 24-36 inch scopes.  You can check out the details at: https://skycenter.arizona.edu/content/programs

Kitt Peak offers DSLR and CCD imaging opportunities, and option to spend the night in the astronomers' dormitory, if you choose.  We are currently just planning to do some visual observing on one of their 1/2 night tours, including dinner.  Other amateurs might want to take advantage of the option to do all night observing, which includes a stay in the astronomers' dormitory.  For details and cost for the Kitt Peak astroimaging sessions, check out their website at: https://www.noao.edu/kpvc/

So, while we continue to sort out the logistics of that three-day, three-observatory tour, we are busy packing our bags for an ocean cruise from Athens to Egypt and beyond to Mumbai.  It should be a lot of fun and exciting to visit some of these sites.  I've been to Egypt before, but this will be the first time for Resident Astronomer, Peggy to see the pyramids and other sites.  We have a couple of days in the Red Sea, with snorkeling and touring, and it is there that I hope to have very dark skies, unless the moon gets in the way, or the cruise ship lights get in the way, to do some Milky Way photography.

I have been trying for sometime now to calculate how much light from ground based light sources can travel upward and then be back scattered to arrive in our telescope tubes, but still can't quite work out all the details.  My back of the envelope calculations indicate that about 1% of the upward bound light will be scattered in the first 10 km of the atmosphere and only a tiny fraction of this light will come back down on the right path to enter our telescope tubes, and yet that small amount might still be as large as the dim deep sky objects we are after.  I calculated that about  4% of photons reach high enough to be in the right position to have line of sight into my see into my 10mm camera lens and for any of the photons at those locations, only about 1 in a hundred billion will be scattered randomly so that the scattered photon arrives at the camera lens.  So, if these assumptions, including my assumption of about 10 kW of outside ship lighting, then only about something like less than 1 photon per pixel per second will arrive due to scattered shipboard lighting.  I hope to prepare a little summary report of all of my assumptions and calculations later on showing how all of this scattered light can affect our telescope viewing.  We will let you know how that estimate turns out and if the ship is stable enough, when away from port, in even darker skies, to make doing any astrophotography at all.

Measuring night sky brightness on Journey through the middle east and beyond (Source: Cruise ship itinerary)
Measuring night sky brightness on Journey through the middle east and beyond (Source: Cruise ship itinerary)

There has also been some exciting news about the discovery of liquid water on Mars.  This is pretty neat news and furthers the thinking that colonizing Mars is becoming more feasible.  But, how does one determine underground water on Mars from an orbiting satellite?  The diagram below shows the concept of radar reflections from the buried water layer standing out from normal waterless return signals.

How MARSIS radar data shows subsurface water layer  (Source: Anja Diez, "Science, Vol 361, 3 August 2018)
How MARSIS radar data shows subsurface water layer  (Source: Anja Diez, "Science, Vol 361, 3 August 2018)

Ok, so it at least seems possible given the MARSIS radar system orbiting Mars, but what does the actual detected signal look like?  The Science article goes on to describe what the received signal actually looked like.  Ok, if you look at the data plot below you can see that the signal reflected from the buried water (blue data curve) jumps much higher than the background surface signal (red data curve) for the track distance between 45 to 65 km.  Hmmm, I'm glad there are experts that can interpret this data and determine that that little blip indicates below surface liquid water!

Radar data showing liquid water layer (Source: R. Orosel, et al, "Science, Vol 361, 3 August 2018)
Radar data showing liquid water layer (Source: R. Orosel, et al, "Science, Vol 361, 3 August 2018)

Finally, another very interesting article showed up in Astrobites on measuring the star formation rate (SFR) in galaxies and how the rate changes during the evolution and history of the galaxy.  Now, the first thing I do when I see some strange chart in a paper is try to understand what the horizontal and vertical axes represent.  The horizontal axis in this case is pretty interesting because it has redshift, from 0 to 8, along the bottom and then the corresponding lookback time, in billions of years, across the top.  That is pretty neat!  So if you are seeing an object with redshift of say, 4, that is the same as saying that object is seen as it was 12 billion years ago.  Also we know that as redshift gets larger and larger, the lookback time approaches the age of the universe, around 13.7 billion years.  A redshift of about 1100 corresponds to the formation of the CMB, which is about 380,000 years after the big bang.

Now the vertical axis is the star formation rate in units of the logarithm of the stellar mass per year per cubic megaparsec.  So a parsec is a unit of distance equal to about 3.26 lightyears, so a megaparsec is just a million times bigger.  The Milky Way galaxy is about 30,000 parsecs in diameter, so it would easily fit in a box 30,000 parsecs on a side, so a cubic megaparsec is a volume large enough to contain something like the equivalent of 30,000 Milky Ways.

So what was the peak star formation rate and when did it occur?  Well it seems the measured curve seems to peak at -0.8, which corresponds to about 0.16 stellar mass stars per year at redshift 2, which occurred about 10 billion years ago.   By conducting these large surveys and collecting the numbers of stars at various redshifts, we get a sense of the evolution of galaxies and of how young galaxies form lots of stars and old galaxies, having tied up most of the gas, form relatively fewer stars.

Neat chart showing star formation rate, redshift and lookback time (Source: Avery Schiff, astrobites)
Neat chart showing star formation rate, redshift and lookback time (Source: Avery Schiff, astrobites)

Until next time,
Resident Astronomer George

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