1. Spectroscopy of Saturn
By Nick King

2. Purpose Spectroscopic analysis of:
Saturn
Saturn’s Rings
Titan
This process will determine the composition of these three objects

3. Saturn 6th planet from the Sun Gas giant planet
9.4 times the diameter of Earth
9.5 AU from the Sun

4. Saturn’s Rings First observed by Galileo
Extend as far as miles from Saturn’s center
Formed by thousands of “ringlets” of rock and debris orbiting the planet

5. Titan Largest of Saturn’s collection of Moons
Nearly half the size of Earth
Holds an atmosphere even thicker than Earth’s

6. Gathering Data - Preparation
Starry Night
Before going out to observe coordinates are need to locate objects
Saturn (observing)- RA: 7h 28m Dec: 22° 0’
Titan is essentially at the same coordinates
Procyon (reference)- RA: 7h 40m Dec: 5° 13’
The moon can be found without coordinates
Starry Night also confirms that these objects will be visible during the observing time

7. Gathering Data - Observing
Location: Rice Campus Observatory
Near Entrance 13 on the North side of campus

8. Gathering Data - Observing
Equipment
Spectrograph CCD (charged coupled device)
This equipment collects photons of light and translates the signal into electrical current
Varying exposure time will be needed to obtain a sufficient signal to noise ratio because of the objects varying brightness
Meade 16-in telescope

9. Gathering Data - Observing (cont.)
March 23, 2005
Conditions
Clear Skies
Slight Haze
Near-full Moon
Fairly windy
Constant breeze
Occasional gusting
7:45 PM Measurements
22° C
29.85 mmHg
47% Humidity
10:00PM Measurements
18.5° C
29.91 mmHg
61% Humidity
Less wind

10. Gathering Data – Observing (cont.)
Object
Exposure Times in sec (#)
Saturn (rings included)
75 (4x)
Titan
300 (removed), 1500
Procyon
1 (2x)
Moon
3 (3x)
Dark Field
1 (3x), 3 (3x), 75 (3x), 1500 (3x)
Flat Field
50 (3x)
Neon Lamp 30

11. Gathering Data – Observing (cont.)
Problems Encountered
Auto-guidance – problems initiating procedure for moderate length exposure time
Dome obstruction – needed to rotate the dome over long exposure times to not obstruct view
File format – saved first file under the incorrect format for future use
Truncated File Names – in transferring files from the telescope to the Unix computers, all file names where truncated after 6 characters
Missing exposures – dark field exposures of 30 and 50 second length forgotten (neon and flat)

12. Data Preparation Convert file format from .fits to .pix
Use “imhead” to find information lost in truncated file name
Combine Dark field and Flat field exposures of the same exposure time
Scale a copy of the 75 sec Dark field into the missing 50 sec & 30 sec exposures
Object Image – Dark Image = Science
This procedure removes both the bias and dark current counts in the CCD
Same exposure time required

13. Data Preparation (cont.)
Normalize the Flat field image by dividing by the mean pixel value
Flat-field all object images by dividing each by the normalized Flat field image
This procedure counters the variation that occurs over the surface of the CCD
Combining the Moon images (“imcombine” sum) creates an image that contains all the absorption information of the Earth’s atmosphere and the light reflected from the Sun
Referred to as the Solar image for this point on

14. Data Analysis - Extraction
Run interactive “apall” extraction of reference star Procyon
Background subtraction on
Set aperture size to fit width of star in image
Shift each Saturn image so the area where the spectrum will be taken falls over the same pixel locations as Procyon (“imshift”)
Saturn, Rings of Saturn

15. Data Analysis – Extraction (cont.)
3. Shift each Saturn image so that the spectrum to be extracted will only cover the background
These spectra will be subtracted from the extracted object spectra to remove the background effect
This is done manually because the “apall” algorithm would take part of the rings or Saturn to be the background
4. Shift the solar image by the same amount as the object image and create a new file specific to each object file

16. Data Analysis – Extraction (cont.)
Shift the neon image by the same amount as the object image and create a new file specific to each object file
Use “apall” to extract a spectrum from each shifted image and its corresponding shifted solar and shifted neon images
Background subtraction is off because it will be done manually
Interactive controls are off
Subtract background spectrum from object spectra for each image

17. Data Analysis - Calibration
“Identify” the features in one of the neon spectra using the calibration sheet
“Reidentify” the remaining neon spectra using the first as a reference image
Edit the header of object spectra to assign calibration lamp information
“hedit [image] REFSPEC1 [neon image] add+ ver-”
Apply the dispersion solution to each spectra
“dispcor” will produce a spectrum of intensity vs wavelength rather than intensity vs pixel

18. Data Analysis - Elimination
Spectral features are clearly visible in the object spectra, but some are due to atmospheric and solar effects
These can be removed using the solar spectra extracted earlier
The object spectrum and the solar spectrum must first be scaled

19. Data Analysis – Elimination (cont.)
Fit a continuum to each spectrum by using “splot” and selecting sections that don’t have features
Divide each spectrum by it respective continuum fit to obtain a spectrum scaled around the value of 1
Using the amplitude of one of the common spectral features in the object spectrum and the solar spectrum find the proper power to raise the solar spectrum to

20. Data Analysis – Elimination (cont.)
Multiplication can be performed using the “imarith” IRAF operation
Logarithmic and exponential functions can be performed using the “imfunction” IRAF operation
Divide the continuum scaled object spectrum by the continuum scaled and power adjusted solar spectrum
Atmospheric and solar features should be almost if not completely removed
This process may need to be repeated if the spectra are not laterally aligned
Lateral adjustments can be made using “imshift”

21. Data Analysis - Measurements
Once the atmospheric and solar effects are removed, absorption patterns from only the object will remain
Use known absorption wavelengths to identify these features
Using the functions in “splot”, find the equivalent widths of the remaining features to measure the relative strengths