David's Astronomy Pages CCD Image Notes - Session 91 (2005-08-08)

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HD 209458 (Osiris) - Magnitude Measurements during Extra-Solar Planet Transit (2005-08-08)

Abstract

A more complete write up will appear here in due course

• Introduction

• Method

• Measurements

• Analysis

• Discussion

• Conclusions

• References

Introduction

Method

Fig 1.   CCD Image showing starfield around HD 209458 Comparison star HD 209346 (C) and Check Star GSC 1688:1766 (K) that were used during study are highlighted (20s exposure, clear filter, #90014)

Fig 2.   Example V Filter Image acquired during observing session (15s exposure, V filter, #91090)

Measurements

Fig 3.   Graph showing measured raw instrumental magnitudes for HD 209458 and reference stars

Fig 4.   Graph showing measured signal to noise ratio (SN) for HD 209458 and reference stars

Analysis

Fig 5.   Graph showing changing sky conditions through observing period The measures of 'cloud' are semi-quantitative and were derived from measurements of  HD 209458's SN and the number of astrometric stars matched during plate solving,  They are checked against visual observations.

Fig 6.   Quality control graph showing raw magnitude of Comparison Star (C). The data noticeably deviates from the ideal line which would apply  if the atmosphere was consistent through the observing session.

Fig 7.   Graph showing uncorrected differential magnitude for HD 209458  (uncorrected for B-V colour difference between HD 209458 and Comparison Star) Whilst at first sight there appears to be the expected dimming of HD209458  during the transit period, the deviation of the check star from  in the same general direction and by a similar amount,  suggests that there additional calibration is required.

Fig 8.   Graph showing raw magnitude of Comparison Star (C) against airmass A plot of raw instrumental magnitude against airmass should show and an  extinction trend, (ie decreasing brightness with increasing airmass) Due to the poor quality and variable sky conditions, the extinction trend is only seen when  looking at data acquired at points of clearest visibility.

Fig 9.   Extinction graphs for two reference stars (C & K) based on data  from two 10 minute periods with best sky quality HD 209346 (B-V=0.25) is bluer than GSC 1688:1766 (B-V=0.58) and correspondingly shows higher levels of  extinction

Fig 10.   Graph showing calculated Zero-Point Adjustment Under ideal sky conditions data would plot as a near horizontal line.  The calculated data clearly deviates from the ideal, particularly for period after 01:00h UT

Fig 11.   Graph showing differential magnitudes for HD 209458  (Corrected for B-V colour difference between HD 209458 and Comparison Star) Graphs shows all data points, together with points representing the average  differential magnitude in each 5 min period. High/Low lines are plotted at +/- 1 S.D.  Differential magnitudes for Check Star show a horizontal trend, suggesting data is well calibrated. Data for HD 209458 possibly shows an overall dimming of ~ 0.01 mag during  the time of the transit, but this is unfortunately within the uncertainty range (of +/- 0.01 to 0.02 mag) which results directly from the poor sky conditions. (The wider scatter seen in the check star data is due to it being a less bright star - with lower SN).

Fig 12.   Graph showing Estimated V Magnitude for HD 209458  Data points are coloured by relative data quality.  (Transform Values were not available and therefore aTransform Correction of -0.137 mag  was assumed, this being the correction required to correct pre-transit calibrated V magnitudes to produce a value of +7.65

Discussion

Fig 13. Graph showing differential magnitudes for HD 209458  (Corrected for B-V colour difference between HD 209458 and Comparison Star) Graphs shows points representing the average differential magnitude  in each 5 min period. High/Low lines are plotted at +/- 1 S.D.  Data may possibly shows an overall dimming of ~ 0.01 mag during  the time of the transit, but this is unfortunately within the uncertainty range (typically +/- 0.01 to +/- 0.02 mag) which results directly from the poor sky conditions.

Conclusions

Data has now be corrected for the colour differences between HD209458 (B-V=0.58) and the comparison star (B-V=0.25).  The reference star (K Star) has a more or less consistent magnitude now (though with considerable scatter) due to it lower S/N. HD 209458 has less scatter due to its higher S/N. 

Now do I see a 0.02 mag dimming during the predicted transit period ? 

Well, possibly (with an eye of faith), but the graph isn’t convincing. I suspect that the sky quality just wasn’t consistent enough to get the required level of magnitude detection. Also I didn’t have an adequate lead-in period ahead of ingress to see ( ‘prove’ ) a step change. Unfortunately deteriorating sky conditions ruined any hope of spotting egress. 

On this occasion I can not claim that I have detected the extra solar planet HD 209458b.. However I have probably learnt enough and given myself enoug confidence to believe that it would be worthwhile to make future attempts  (next suitable observing dates are in October 2005).

References

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Sky Condition Graph

Sky Condition Graphs for Observing Session 91

Notice  1) Sky darkening between 21:45 and 22:30h UT 2) Fairly consistent Seeing (FWHM ~ 4 arc secs) 3) Deterioration in sky quality from 01:00 / 01:15h UT as shown by smaller Mag Zero Point values

• Airmass of image. Purple Line
  1 is zenith, with higher number indicating positions progressively towards the horizon
  

• Sky Brightness.  Red circles/line.  
  Sky Magnitude in mag per sq. arc sec. Higher numbers (towards top of chart) indicate darkest skies.
  

• Sky Brightness (V).  Yellow circles/line.  
  Sky Magnitude in mag per sq. arc sec. (V Filter), corrected to equivalent C Filter sky brightness. 
  

• Mag Zero Point. Green circles/line.   
  For each Clear Filter image this is the average difference between the catalog magnitude of stars and their instrumental raw magnitude normalized to a 1 sec exposure.  Only sharp star-like objects are used in this calculation.  Larger numbers (towards top of chart) indicate more transparent skies .   Lower numbers indicate either a less transparent sky or the formation of condensation/dew on the front lens of the telescope.
  

• ZP. Small green circles
  These are magnitude zero points for individual stars in Clear Filter images. The range is due to a number of factors including catalog magnitude error, star colour, impact of star brightness on S/N
  

• FWHM (clear). Blue diamonds
  There are the Full-Width Half Maximum measurements of individual stars in Clear Filter images.  Only sharpest star-like objects above a S/N threshold are shown. Lower number (towards top of chart) indicate best seeing (most stable atmosphere) and best focusing.  Note that seeing & focusing may drift over an observing session.  
  

• FWHM (RGB). Orange diamonds
  There are the Full-Width Half Maximum measurements of individual stars in Colour Filtered (RGB) images.  Only sharpest star-like objects above a S/N threshold are shown. Lower number (towards top of chart)
  

• FWHM (RGB). Small white circles
  These are objects that fail to meet sharpness & S/N thresholds.  They might include galaxies or low S/N stars 
  
  

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Master Flat

Master Flat (Dusk Sky Flats near Zenith, with TeeShirt)

11 x 20s (average combine), clear filter, -15 deg C  2005-08-14 (#92001-11)

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