MTH 448/563 Data-Oriented Computing

Fall 2019

Day 16, Monday, October 21

Your movie questions

Whiteboard from last time: https://docs.google.com/spreadsheets/d/12lZirnXrP8uHQuJkk8xLNcrNnraHSahi8A3JLljfQsk/edit?usp=sharing except change first Z to U

SQL

Test on Monday, Oct 28. Please prepare thoroughly for this.

Review of queries on movie database.

Sloan Digital Sky Survey (SDSS)

An extensive moderate (0.5 arcsecond) resolution survey of the sky: an example of big data with a SQL API.

SDSS_telescope_new.jpg

http://skyserver.sdss.org/dr15/en/tools/started/startedhome.aspx

Querying the SDSS

Sloan Digital Sky Survey, SkyServer DR15: SQL Search

SQL Search Tutorial

What do all those variables mean?

Astronomical coordinate system

My own observations of Halley's comet using binoculars

Quiz "Halley": In which year did I make those observations?

Right ascension and declination

equatorial coordinate system

ra = right ascension, a sky coordinate comparable to geographic longitude (traditionally measured in hours, minutes, seconds, where 24h = 360 degrees)

dec = declination, a sky coordinate comparable to geographic latitude

Exercise: your patch of sky

Stake out a small patch of sky for yourself: choose an (ra,dec)-rectangle about the size of your thumbnail at arm's length. somewhere in the DR14 footprint, (What is the solid angle of a small (ra,dec)-rectangle?) Best if you avoid the plane of our galaxy (Milky Way) and do not include any bright foreground star.

Your answer will be in the format: a < ra < b, c < dec < d. You could look at a sky map like this one to get your bearings.

Exercise: Please enter the coordinates of your own patch of sky (all in degrees) on this spreadsheet (change the Z to a G).

Brightest objects

Exercise: Get the coordinates and some properties of the brightest objects in your own patch of sky.

The Schema Browser is useful.

Measures of brightness: Apparent magnitudes is the negative logarithm of the brightness as seen from here: larger number therfore means dimmer. Magnitude 9.5 is about as faint as you can see with binoculars. Measures of flux and magnitude in SDSS: explanation here. Here is a chart of the filters used by SDSS.

Can you make a plot of your brightest objects?

Galaxy count

Exercise: Guess how many galaxies the SDSS found in your own patch of sky, and enter it in the spreadsheet. Then find out the actual number and enter that.

Algorithms for recognizing things in data

Examples:

Recognizing stars in photographs

Group exercise: identify the stars in a photo with no metadata. Assuming you have a list of all the bright stars in the sky with their coordinates, how would you identify the stars in any photo of a portion of the sky, given no info except the photo itself?

big_dipper_asterism.jpg keith-breazeal-perseid-meteor-big-dipper.jpg

Preparatory exercise:

Here is a photo of Halley's comet taken on March 8.

Figure out the correspondences between objects in the photo and objects in a star chart. Then, using the photo and the chart we saw earlier, determine the location in celestial coordinates (Epoch 1950.0) of (the nucleus of) Halley's comet on March 8. Give the ra and dec.

Let's use Inkscape to try to line up the photo and the chart.

NOTE for Mac users: You will need to install XQuartz in order for Inkscape to run.

Save my_halley_observations_detail.jpg and wikipedia_commons_2_2a_Lspn_comet_halley_march_08.jpg, and import them into Inkscape. Then shift, rescale, and rotate as necessary to bring them into alignment.

Back to the goal of automatically recognizing the stars in a photo.

Can we recognize a single star? I took this photo on the NY State Thruway at twilight one evening.

20150505_203344_singleton_star.jpg

How about a pair of stars?

2stars.jpg

How about a triangle of stars?

How to make a canonical description of a triangle that is invariant under translations, rotations, and dilations? A moduli space.