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GRIM II Spectroscopy

Subject: GRIM II Spectroscopy

From: Alan Watson

Submitted: Tue, 24 Oct 95 17:43:25 -0600

On 19/20 October 1995 we used GRIM II to attempt spectroscopy of the
2.3 micron CO absorption band in stellar clusters.  We used the f/10
optics and slit.  On the whole, things worked well, although there are
some peculiarities to be overcome.  Here are some comments that may
help other observers.

Alan Watson & Jon Holtzman

Throughput

   We obtained a peak signal of 500 DN/s/pixel (28 DN/s/A) and a total
   signal of 2500 DN/s/pixel (140 DN/s/A) from a K = 6.5 star under
   conditions of clear skies and 1.2ish arcsec seeing.

Columns 128 and 256

   Columns 128 and 256 seem to be offset down by one row.

Read Noise

   Our typical backgrounds in 60s were about 100 DN (about 500 e), and
   so for faint objects our noise is utterly dominated by the read
   noise of about 110 e.  We can think of three ways to improve this
   situation: integrate longer; reduce the read noise; or implement
   multiple reads.  However, since we would have to integrate for 1500s
   for the background noise to equal the read noise, reducing the
   effective read noise seems very desirable.  Implementing a quadruple
   read scheme or halving the read noise, neither of which are at all
   unreasonable goals, would increase the efficiency of spectroscopy by
   a factor of four.

Bias Fluctuations

   In our object-sky subtractions we saw banding parallel to the rows
   in almost every exposure.  The pattern of the banding was similar in
   each quadrant, suggesting fluctuations in the bias.  About 20% of
   our exposures had banding with a peak-to-peak magnitude of about 100
   DN in the low rows of each quadrant.  In the remainder, the banding
   had a peak-to-peak magnitude of 15-30 DN and more variable
   structure.  Since our primary targets (K about 12.5) had only about
   100 DN peak signal in 60s, this banding was a very serious concern.

   We were able to significantly reduce the level of the banding by
   averaging portions of rows well away from the spectrum to determine
   a bias level for each row.  Since the spectral dispersion is not
   perfectly parallel to the columns of the detector, this will only
   work well if the residuals from sky lines in object-sky subtractions
   are small.  For stellar objects, a background aperture close to the
   object aperture may work well.

   Do people find this banding a problem in low background (i.e.,
   narrow band) imaging?

Flats

   The quartz lamps were too dim for flats.  We used the incandescent
   lights which gave a few thousand DN in 10s.

Wavelength Calibration

   Wavelength calibration of CO band spectroscopy is difficult because
   of the paucity of emission lines in the 2.3 micron region in each of
   the three standard sources of emission lines: planetary nebulae, HII
   regions, and the night sky.

   We attempted to obtain an Ar spectrum using the conventional set up:
   lamps shining onto the enclosure from behind the secondary.  We
   detected four lines in a 300s lamps on/off pair, with peaks of about
   100 DN in the brightest two.  At the suggestion of Dan Long, we
   obtained much better spectra by removing the instrument from the
   telescope and placing a lamp on a step ladder directly in front of
   the dewar window; we obtained good signal on about a dozen lines
   spread across the K window in exposures shorter than 1 second.  In
   retrospect, we think it might be possible to do this without
   removing the instrument from the telescope by pointing the telescope
   close to the horizon and placing the lamp between the instrument and
   the tertiary.

   Bruce and Karen are investigating Xe and Kr lamps, as these may turn
   out to have brighter lines and be suitable for use with the
   conventional set up.

   We can supply line lists for OH, Ar, Xe, and Kr.
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