The first data you should take are the ARRAY_TESTS data. When the
first of these frames arrives, two of the aforementioned display
windows will be opened to display them.
When the array tests have completed, oracdr will analyse them and
print out a message telling you whether the array performance is as
expected. The actuall text should end with something like this:
Double correlated readnoise = 40.1 electrons is nominal
Median Dark current = 0.89 is nominal
Modal Dark current = 0.44 is nominal
If it declares any of the values to by HIGH, then you should consult
your support scientist, or the CGS4 instrument scientist, for advice.
LOW values are beneficial, not a problem!
Flat and Arc
After completing the array tests, you will probably want to take a
flat field and an arc spectrum. These serve as a good example to
explain what gets displayed.
First of all, a GAIA window opens up showing your flat field
frame. If you move the mouse cursor over the image, you can read off
the data value at the current mouse location in the box labeled
Value towards the top right of the numeric display area.
The window should look somewhat like this:
Next a KAPPA window opens. The top left quadrant shows a
histogram of the data values in the flat field frame, the bottom left
quadrant shows you a histogram of data values in the normalised flat
fied, and the bottom right shows you an image of the normalised flat
field. It should look somewhat like this:
You should use the GAIA window and the Histrogram display to
check that your flat field frame has sufficient counts - should be
around 3000 in the brighter areas, and not more than around 4500,
where the array responce starts to go non-linear.
If the count rate is not suitable, then go back to the OT, and edit
the flat field component - adjust the exposure time, or the black body
lamp apperture if necessary.
Next, your Arc spectrum should arrive. Again, the frame will be
displayed in the GAIA window and a histogram of data values
will be plotted in the top left of the KAPPA window. Again, you
should check for a suitable number of counts in the data and adjust
the exposure time if necessary. You should also check for a suitable
number of lines to wavelength calibrate the spectrum. You could try
one of the other arc lamps if you are low on lines. Do these by
editing your program in the OT.
The lower right panel in the KAPPA window shows you the arc
spectrum with an estimated wavelength scale on it (this is derived
from the grating parameters (angle, order etc) reported by the
instrument). Use this to identify a few lines and check that your
wavelength region has been selected with suitable accuracy.
What actually gets displayed
OK, time for a word on what's actually getting displayed.
First up, the GAIA window. We do simply display "raw" data
here, but this isn't necessarily easy to interpret - there may be
multiple integrations in your observation ( see the note on exposures, integrations,
observations and co-adds), and/or the data might be oversampled by
array stepping. The pipeline replaces the display of raw data in the
gaia window with the data in a form more easy to interpret as soon as
it has been reduced into such. In fact, this goes through several
iterations. Usually, you end up with a _wce frame in the gaia window,
which is the most reduced form that single frames go to.
Next up, the KAPPA window. the top left quadrant of this
allways shows you a histogram of data values. The actual image that
this comes from is the same one as what gets displayed in the
GAIA window. Note that especially with the echelle, the
histrogram can be changed significantly by the flat-fielding step, as
echelle flat fields can have significant CVF gradients across them,
and thus when normalised, still contain a range of values. Thus, if
you want to check whether you're saturating, make sure you check a
pre-flat-fielding frame when using the echelle.
The top right of the KAPPA window, shows you the "bgl"
file. This is a frame that shows you how background limited your
observations were. The colour scale for this image goes from 0 to 2,
where 1 is when the Poisson noise from the number of detected
photoelectrons equals the readnoise of the array. For maximum
sensitivity to faint objects, your exposures should be long enough to
make almost all the pixels background limited - ie this frame should
be well filled with values greater that 1. Of course, when observing
bright targets, this is not possible as to do so whoudl saturate the
array on the target.
When you're observing a Flat, the lower half of this window shows
you the histogram of pixel values from the normalised flat field frame
on the left, and an image of the normalised flat field frame on the
right. You shouldn't be able to see large amounts of structure in this
image other than the bad pixel mask which will be apparent.
Other than when taking flats, the lower right panel of the
KAPPA window shows an extraction of rows 139-141 of the lastest
image, with an estimated wavelength scale applied. Thus, this is
useful for checking wavelength regions with arc observation. During
normal observations of a target on the sky, this will show either a
simple extracted spectrum of your target, with no sky subtraction, or
a sky spectrum, depending on whether you're observing in the main or
offset position at that time. This assumes that you are using the
conventional peakup row for your targets (row 140).
The lower right of this KAPPA shows you the y-profile of
your sky subtracted group image, once you have observed sufficient
data to form the group image. Generally, this is useful to see how
well you're detecting your object and to check that you're getting
equal flux in all beams if you're nodding (or chopping) along the
slit. If you see here that your beams are unequal, you may need to
stop and do a 2-row peakup.