graduate student presentation: subtracting the sky light international mercury watch meeting...
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Graduate student presentation:Subtracting the sky light
International Mercury Watch Meeting
Student: Cesare GravaSupervisor: Cesare Barbieri(Università di Padova - Dipartimento di Astronomia)
Collaborators: François Leblanc (CNRS, Università di Trieste) Nick Schneider (University of Colorado)Valeria Mangano (IFSI, Università di Padova)
IMW 06/05/2008 Cesare Grava
Paris, 5th and 6th of May 2008
IMW 06/05/2008 Cesare Grava
My PhD programme and subject of this presentation:
My PhD programme is the study of the exospheres of the bodies of the Solar System (specifically Mercury, Io, Moon).
For Mercury, during my PhD I will try to: correctly reduce the TNG – SARG Mercury spectra obtained so far (2002, 2003, 2005, 2006); to calibrate them (using the Hapke method) in order to get information on spatial and temporal distributions of the column density (Na atoms/cm2) and on the intensity (Rayleighs) and eventually to perform new observations of the body.
Purpose of this presentation is to show the result of the method that I’ve developed for the extraction of the Hermean Na emission outside the disk, namely in the sunward and antisunward (tail) direction. This procedure has not been implemented so far in the reduction of the TNG data. Therefore it will provide new information on our spectra, supplementing what has been already presented by Valeria Mangano (this conference) and by Cesare Barbieri (conference of 2006). The procedure is exemplified using a spectrum acquired in 2005, but it can be applied to all TNG-SARG spectra.
IMW 06/05/2008 Cesare Grava
Sky subtraction: main interest to measure the Na outside the disk (sunward, tail, poles)one example from TNG-SARG
Spectrum taken the 29th of june 2005
Slit dimensions: 26.7 x 0.4 arcsecResolution: 115000Dispersion: 0.022 A/pixNa filter
Heliocentric Distance: 0.422 A.U.Radial Velocity: 8.112 km/sApparent Diameter: 6.8 arcsecPhase Angle: 81.5°Slit Position: 0.5” southward SEPTAA = 124°
IMW 06/05/2008 Cesare Grava
First of all, I’ve tried a linear interpolation (sky = + *j, where j is the column – spatial direction). from one side to the other of the original spectrum, that is one column on the left and another one on the right.The constants and are calculated one time per each row (from 5880.62 to 5902.98 A).
Flaw: it clearly subtracts also the sodium emission,as seen from the residual emission in the night sky
Extraction of the exospheric tail Need of an accurate sky subtraction method: first test
Raw data Sky model Subtracted spectrum
Sunward
5888.23 A
5897.15 A
D1
D2
IMW 06/05/2008 Cesare Grava
Then I’ve tried with a couple of and for each row, likethe preceding method, except for the fact that in this case I’ve used more columns: 6 on the left and 6 on the right.
Flaw: it still subtracts too much sodium.
Extraction of the exospheric tail Need of an accurate sky subtraction method: second test
Raw data Sky model Subtracted spectrum
Sky = + *j where j is the column.
Sunward
5888.23 A
5897.15 A
D1
D2
IMW 06/05/2008 Cesare Grava
In third test I’ve took mean values of and . I’ve chosen 6 columns on the left and 6 on the right and the rows avoiding the regions with the Na emission lines. I’ve calculated the means of the object alongthose columns and from these means I’ve calculated and from a fit.
Flaw: it doesn’t subtract too much sodium anymore, but the subtracted spectrum presents very negative values
Raw data Sky model Subtracted spectrum
= -2.62 +/- 0.02 = 880.48 +/- 1.16
Extraction of the exospheric tail Need of an accurate sky subtraction method: third test
Sunward
5888.23 A
5897.15 A
D1
D2
IMW 06/05/2008 Cesare Grava
The fourth method is a mix of the preceding twos: (the slope) is calculated from a fit of the mean values along wavelength direction; q, on the contrary,is calculated row per row.
Sky (j,q) = (q) + *j j is the column, q is the row.
Flaw: as second method, it clearly subtracts too much sodium.
Raw data Sky model Subtracted spectrum
Extraction of the exospheric tail Need of an accurate sky subtraction method: fourth test
Sunward
5888.23 A
5897.15 A
D1
D2
IMW 06/05/2008 Cesare Grava
In the last method, I’ve used a polynomial of order 2. For each of the useful rows (those far from the Na Lines more than 1 Angstrom),I’ve calculated the three coefficients of the polynomial and then I’ve done a linear fit along wavelength direction.
Sky (j,q) = a0(q)+ a1(q)*j + a2(q)*j2
Acceptable result: it doesn’t remove Na emission
Raw data Sky model Subtracted spectrum
Extraction of the exospheric tail Need of an accurate sky subtraction method: final test
Sunward
5888.23 A
5897.15 A
D1
D2
IMW 06/05/2008 Cesare Grava
Examples - Column # 65: sunward direction
IMW 06/05/2008 Cesare Grava
Examples - Column # 35: planet’s disk
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Examples - Column # 15: Na tail. This corresponds to approximately 8.3” from the SEP anti-sunward
IMW 06/05/2008 Cesare Grava
Counts in ADUs (to show the tail)
sunwardtail