Transcript Procedure

Spectroscopy of Saturn
By Nick King
Purpose
Spectroscopic analysis of:
Saturn
 Saturn’s Rings
 Titan

This process will determine the composition
of these three objects
Saturn
6th planet from the Sun
Gas giant planet
9.4 times the diameter
of Earth
9.5 AU from the Sun
Saturn’s Rings
First observed by
Galileo
Extend as far as 81000
miles from Saturn’s
center
Formed by thousands
of “ringlets” of rock
and debris orbiting the
planet
Titan
Largest of Saturn’s
collection of Moons
Nearly half the size of
Earth
Holds an atmosphere
even thicker than
Earth’s
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
Gathering Data - Observing
Location: Rice Campus Observatory

Near Entrance 13 on the North side of campus
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
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
Gathering Data – Observing (cont.)
Object
Saturn (rings included)
Titan
Procyon
Moon
Dark Field
Flat Field
Neon Lamp
Exposure Times in sec (#)
75 (4x)
300 (removed), 1500
1 (2x)
3 (3x)
1 (3x), 3 (3x), 75 (3x),
1500 (3x)
50 (3x)
30
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)
Data Preparation
1.
Convert file format from .fits to .pix
Use “imhead” to find information lost in truncated
file name
2.
3.
4.
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
Data Preparation (cont.)
5.
6.
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
7.
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
Data Analysis - Extraction
1.
Run interactive “apall” extraction of reference
star Procyon
Background subtraction on
Set aperture size to fit width of star in image
2.
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
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
Data Analysis – Extraction (cont.)
5.
6.
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


7.
Background subtraction is off because it will be done
manually
Interactive controls are off
Subtract background spectrum from object
spectra for each image
Data Analysis - Calibration
“Identify” the features in one of the neon
spectra using the calibration sheet
2. “Reidentify” the remaining neon spectra
using the first as a reference image
3. Edit the header of object spectra to assign
calibration lamp information
1.
“hedit [image] REFSPEC1 [neon image] add+ ver-”
4.
Apply the dispersion solution to each spectra
“dispcor” will produce a spectrum of intensity vs
wavelength rather than intensity vs pixel
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
Data Analysis – Elimination (cont.)
1.
2.
3.
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
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”
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