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Starting Observing

Starting Observing

Array Tests

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.

Contact: Tom Kerr. Updated: Wed Oct 6 11:54:13 HST 2004

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