Astro 3310Fall 2015
LAB #6:
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Please copy this document to the REPORT sub-directory from the expanded LAB6_Data_Package_FA15.tar.gz. Then, edit it to write your answers in all the "______" areas. When finished, create a tar.gz archive of the REPORT directory and all of its contents,then scp thefile to datafarm.astro.cornell.edu and place it in:
/data/Courses/A3310/FA15/”your netid”/LAB6/
Remember that you will only get credit for the files that you put in the REPORT sub-directory and copy to datafarm. Please make sure that youkeep a Matlab workbook with all of the commands you used to answer the questions in the lab. Feel free to comment and organize your workbook so that it will be easy for us to follow your algorithms when we execute the doe. If you generate any functions for the lab, ensure that they are also in the REPORT sub-directory and properly called from the workbook file. For you convenience, there is a template for the workbook file already in the REPORT sub-directory.
YOUR NAME: ______YourNetID: ______
The data package includes four datacubes of Titan, taken with the SINFONI instrument on the European Very Large Telescope (VLT):
MOV_Titan2_20140715_NGS_OBS_OBJ.fits
MOV_Titan2_20140715_NGS_OBS_OBJ_1.fits
MOV_Titan2_20140715_NGS_OBS_OBJ_2.fits
MOV_Titan2_20140715_NGS_OBS_OBJ_3.fits
And a telluric standard (star with a smooth spectrum to calibrate the Earth’s atmosphere):
Telluric_Standard_OBS_STD.fits
Each fits file is a self-contained observation, with small offsets inthe pointing of the telescope. There is some variation in the
performance of the adaptive optics system from one observation to the next, so the image "Point Spread Function" (PSF) varies somewhat fromone datacube to the next. For the problems below, you may use one datacube, or all of them. You may also combine the four into a single datacube for improved signal to noise and bad pixel rejection.
Each datacube consists of a 2172 wavelengths of a 64 x 64 pixel image of Titan. The wavelength calibration is described in the FITS headerkeywords crpix3, crval3, and cdelt3.
Familiarize yourself with the datacubes. You will need to be able to extract a spectrum of a given pixel, and extract images at a given wavelength.
Part 1:
Make an average (full disk) spectrum of Titan from one of the datacubes. Note that there are some bad pixels and invalid data (NaN values) that you need to handle in order to extract a valid spectrum. Your spectrum should look mostly like the spectrum in
Note that some of the features you see in the spectrum are features of the Earth’s absorption, not Titan.
Use the ESO Sky transmission calculator:
to calculate an atmospheric absorption spectrum at a similar spectral (wavelength) resolution as the datacube and compare it to the Titan spectrum.
Answer: Insert a figure of your raw spectrum annotated to identify features in the Earth’s atmosphere and Titan’s atmosphere.
Part 2:
The telluric absorption can be corrected by measuring a star with a smooth spectrum, and dividing the observed spectrum by the observed telluric absorption. Hot (O, B and A-type) stars have nearly featureless spectra that are close to a blackbody.
Construct a telluric spectrum using the datacubeTelluric_Standard_OBS_STD.fits, and compare it to the model Earth’s absorption spectrum. Note that there are some features that are present in the stellar spectrum that are not in the Earth’s absorption spectrum (e.g. the 2.16 μm “Brackett-gamma” feature due to atomic Hydrogen absorption). For the most careful analysis, these features should be excised from the calibration spectrum either by interpolation or using a theoretical model of the spectrum, but for the purposes of this lab, these features may be ignored, but they will produce small artifacts in the final spectrum.
To calibrate out the Earth’s absorption in the Titan spectrum using this stellar spectrum, the Titan spectrum needs to be divided by the stellar spectrum. Note that this also divides the spectrum by the spectrum of the star, which is brighter at shorter wavelengths. This can be corrected by multiplying the spectrum by a Planck function with the appropriate temperature for the star, approximately 10700K for a B9V star. There is a function to calculate the planck function in the SUBROUTINES directory.
Answer: Insert a figure of your calibrated spectrum
What is the difference between the bright and dark regions of the Titan spectrum? Hint: imagine what you would see if you were looking at a reflected spectrum of the Earth illuminated by the hot spectral calibrator star observed above.
Part 3:
There are archival observations of Titan going back nearly 20 years using adaptive optics on large ground based telescopes, including the VLT and also the Keck 10m telescope in Hawaii. Nearly all The Keck data becomes publically available after an 18 month proprietary period. There are many more imaging observations than spectroscopic observations, and those images are taken in filters that probe different regions of Titan’s atmosphere.
Use the Keck archive search tool to find observations of Titan with the NIRC2 (adaptive optics imaging) instrument. Sice Titan is a moving object, note that you need to search on the TARGNAME, not the Object Name, which is resolved into astronomical co-ordinates.
These observations are taken in a variety of filters, which are described at
Choose 2 filters that have a substantial number of observations in the archival data, and are within the spectral coverage of the titan datacube, preferably one filter that covers brighter parts of the spectrum and one that covers darker.
Download at least 5 different epochs of observation in these filters, and make a figure that compares the images in the different filters at different times. Note that most observations from the archive will be delivered uncalibrated, and without much quality control. You may find that some of the observations are of low quality, and have to pick and choose. Occasionally the observer may not have correctly updated the TARGNAME field, mis-pointed the telescope, or there could be clouds in the way so some of the observations will be faulty and you need to exercise a little care. Typically the calibration data is also available from the archive to dark/sky subtract and flatfield the images. This processing, and averaging multiple images will yield superior quality images, but is not essential for the purposes of this lab.
Answer: Insert a figure of your archival data
Discuss what you have found, and what changes with wavelength and time.
Part 4:
In the datacube in part 1, there is a bright feature, a cloud. Extract and compare the spectra of the bright and dark features in the datacube.
Answer: Insert a figure comparing the bright and dark spectral features
Part 5:
Due to absorption in Titan’s atmosphere, the observed radius of Titan changes as a function of wavelength (see the discussion in ). Using the
datacubes above, determine the radius of Titan as a function of wavelength.
Answer: Insert a figure showing the radius of Titan as a function of wavelength.
Explain the algorithm you developed to measure the radius of Titan.
Background Reading:
Radiative transfer in atmospheres:
TitanOccultation -
Adaptive Optics Imaging of Clouds in Titan: