ESO Faint Object Spectrograph and Camera
EFOSC Arc Lamp exposures tests

The below describes the tests done on the He Ar lamp exposures on 15, 16/10/2001, with help from Andres, Ernesto, Gorky and Juan. Thanks.

The problem that we have been encountering is that the Ar lamp seems to be "getting to bright" so that even with the shortest exposure the Ar lines on the right are saturated.

The above is a typical He Ar spectrum. Marked are the lines I use to invesitgate the lamp intensity. These are He5875, He6679, Ar6965 and Ar7948. Please note that the line right of Ar6965 is a blend of He and Ar (ie wrong in the manual). For the following tests, I measure the peak intensity of the lines. During each test, a sequence of arc lamp exposures of the same exposure times are taken, and the intensity of each line is plotted vs the number in the sequence. Typically it takes 2 minutues between each exposure.


The above is the line intensities for the setup we have been using, tested in the morning of 15/10/2001. I cannot remember exactly the exposure times, but are either 0.8s or 0.4s for He, and 0.4 or 0.2s for Ar. It is obvious that 20% of the time the line intensity has a dramatic change, dropping to about 25% of the normal value. Note also that the Argon lines are very bright in comparison to the He lines, and Ar7948 is nearly saturated. Notice that the He lines are very stable.


We then installed a new Ar lamp and repeated the experiment. The above graph shows that the erratic behaviour is not due to a bad Argon lamp-- it happens with the new lamp too. An examination of the way the OB is executed shows that the following happens:

1 - The He lamp is turned on
2 - About 1-2 seconds later the shutter is opened and shut for the He.
3 - He lamp turned off
4 - Ar lamp turns on and _at_the_same_time_ the shutter opens and shut.
5 - Ar lamp turned off, and the CCD is read out.

Since there is no delay between Ar lamp on and the exposure, and the Ar exposure is very short (0.2s), it may be part of the problem -- the lamp is not ready when the shutter is opened.

I then took Ar exposures using the engineering mode. The Ar lamp is turned on continuously throughout the sequence (about 30 minutes), and I use the OS control panel to take 0.2s exposures. The result is shown above. It is obvious that the exposures are very stable. This shows that either leaving the Ar lamp on for a long time makes it stable, or that there is a problem with the shutter speed when controlled by the OB. (The He lamp is turned off so the intensities are zero).

Note that now the Ar lines are much fainter. The explanation may be the following: in the first few seconds after the Ar lamp is on, it hasn't reached steady state and the lines have more energy in the red (probably because it is cold). Now, if we shorten the exposure it makes the problem worse-- the spectrum is even redder and the red Ar lines become very strong. This is very counter intuition-- one would think that shortening the exposure time reduces the flux. It takes a few seconds for the gas to ionise, and then the spectrum becomes more blue. This I think maybe the explanation for the "Argon lines seem to be getting too bright" problem. Especially when using a wide slit, the shutter time is reduced, but then the exposure time is so short that we are in a very instable fraction of the second when the lamp is switched on.

I then changed the template to give it a 10 seconds delay between lamp-on and shutter open, for both He and Ar. The results are shown above. Sadly the behaviour seems to be erratic, and now the He lines are behaving strangly too. Very strange... but...
see below.

In the following afternoon (16/10), Juan checked the Ar lamp and screwed in the small mirror (which was not 100% screwed in), and he also checked the connections, which looked fine. I then repeated the last experiment (lamp on for 10 seconds before shutter open), and the results are shown above. Note the He and Ar exposures are 0.4 and 0.2 seconds respectively like yesterday. Now the arcs are stable. Note also, that the intensities are now low-- we are nowhere near saturation.

To increase the stability, we decided to put a Neutral density filter of factor 1 (10 time decrease in intensity) (notice the typo in the title of the plot), and increase the exposure by a factor of 10 (eg 4s He and 2s Ar), keeping the 10s delay. The above shows the result: the stability is very acceptable.

Send comments to : ls-spectro

Last modified: Fri Jan 18 04:01:32 SAT 2002

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