This section provides an outline of the steps used in Starlink Figaro to reduce UHRF data taken for the source 1 Sco, observed by Ted Snow on 18th June 1997.
Observing informationCCD detector = Tek 1024
Object frame = frame 28
On Source Time = 1200 s
V magnitude = 4.6
Lambda = 5896 Angstroms
Bias frames = frames 32-37
ThAr arc = frame 30
Quartz flats = frames 50 - 56
Data Reduction Summary
- Start up Figaro:
unix: setenv D /vaxdata/ccd_2/970618
unix: cd /data/sse/1/jmc/970618
- Examine file headers:
- Reorientate the ThAr arc frame:
The standard orientation for Figaro files is for the dispersion direction to be aligned horizontally with wavelength increasing from left to right and from bottom to top.
- Reorientate the object frame in the same way.
- Form the median bias frame and re-orientate it using
figaro: medsky, rotate, irevx
output median bias frame = medbiasrx
- Check the bias statistics:
This gave a mean = 167 and sigma = 1.32
- Subtract bias from object and arc:
figaro: isub soc28rx medbiasrx sco28rxb
figaro: isub arc30rx medbiasrx arc30rxb
- Examine object frame:
Note that the order is tilted but the spectral lines are parallel to the edge without the curvature typical of UCLES spectra. This is because the cross-dispersers for UHRF are located AFTER the echelle.
- If necessary straighten the spectrum:
- Clean the object frame to remove cosmic rays.
figaro: bclean scro28rxb sco28rxbc
- Flat field the data. Form the median flat field and re-orientate
it as for the bias frames, output flat field frame = medflatrx
Subtract the median bias frame from the median flat field frame:
figaro: isub medflatrx medbiasrx
Smooth ff frame:
output = medflatsrxbs
Divide the flat field by smoothed flat field:
figaro: idiv medflatsrxb medflatsrxbs flatf
Divide the object frame by the final flat field frame, output = sco28rxbcf
- Sky Subtraction:
The measured signal = source + noise + bias + sky. To fit the sky level, use a
region on the frame which is outside of the slicer.
Take spatial cut for one pixel:
Determine mean sky level:
Subtract sky level pixel by pixel from object frame:
Output = scorxbcfs.
- Estimate the signal/noise:
To convert counts to photons - multiply by the gain factor. For the TEK1024
with a SLOW readout speed the gain factor = 1.36.
Output = scorxbcfsm
Estimate count rates:
For single y pixel - max counts = 60 so S/N ~ 8 per pixel
Do extraction over a suitable range of rows (used 70 to 210):
plot spectrum - counts = 5000 so S/N ~ 70
Plot the extracted spectrum:
For this source - the absorption was saturated in line centre - so counts in the line centre were zero as expected.
- Wavelength Calibration:
Make sure that initial arcs have at least two well identified lines. If
necessary take several stepped frames and append/overlap. Then do fit
to intermediate lines. Once you have sufficient lines, set the wavelengths
of identified lines in the ThAr spectrum:
Hit Q to quit the arc routine and enter filename for calibrated arc spectrum.
Copy wavelength calibration from arc frame to object frame:
- Linearise spectrum:
It is not really necessary to correct UHRF spectra to a linear dispersion
since the wavelength range covered is so small. If you want to do this:
set minimum and maximum lambdas for the ends of the spectrum and the desired number of channels.
Return to main cookbook page.
Stuart Ryder, sdr -@- aao.gov.au