Graduate student presentation: Subtracting the sky light International Mercury Watch Meeting Student: Cesare Grava Supervisor: 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 06/05/2008Cesare Grava Paris, 5 th and 6 th of May 2008

IMW 06/05/2008Cesare 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/cm 2 ) 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 06/05/2008Cesare Grava Sky subtraction: main interest to measure the Na outside the disk (sunward, tail, poles) one example from TNG-SARG Spectrum taken the 29 th of june 2005 Slit dimensions: 26.7 x 0.4 arcsec Resolution: Dispersion: A/pix Na filter Heliocentric Distance: A.U. Radial Velocity: km/s Apparent Diameter: 6.8 arcsec Phase Angle: 81.5° Slit Position: 0.5” southward SEP TAA = 124°

IMW 06/05/2008Cesare 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 to 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 dataSky modelSubtracted spectrum Sunward A A D1 D2

IMW 06/05/2008Cesare Grava Then I’ve tried with a couple of  and  for each row, like the 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 dataSky modelSubtracted spectrum Sky =  +  *j where j is the column. Sunward A A D1 D2

IMW 06/05/2008Cesare 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 along those 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 dataSky modelSubtracted spectrum  = /  = / Extraction of the exospheric tail  Need of an accurate sky subtraction method: third test Sunward A A D1 D2

IMW 06/05/2008Cesare 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 dataSky modelSubtracted spectrum Extraction of the exospheric tail  Need of an accurate sky subtraction method: fourth test Sunward A A D1 D2

IMW 06/05/2008Cesare 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) = a 0 (q)+ a 1 (q)*j + a 2 (q)*j 2 Acceptable result: it doesn’t remove Na emission Raw dataSky modelSubtracted spectrum Extraction of the exospheric tail  Need of an accurate sky subtraction method: final test Sunward A A D1 D2

IMW 06/05/2008Cesare Grava Examples - Column # 65: sunward direction

IMW 06/05/2008Cesare Grava Examples - Column # 35: planet’s disk

IMW 06/05/2008Cesare Grava Examples - Column # 15: Na tail. This corresponds to approximately 8.3” from the SEP anti-sunward

IMW 06/05/2008Cesare Grava Counts in ADUs (to show the tail) sunward tail