Alpy 600 - user guide
5.1.3 Wavelength calibration
Take special care when performing the wavelength
calibration. Normally, you should have an RMS smaller
than 0.5 Å. Take the time to record a calibration quite
often, and use this image for the data reduction. Al-
ways double check that there is no major error in the
calibration. The simplest way is to look at the positons
of the balmer lines, visible in most types of stars.
Also, remember that the calibration law can change
across the FOV. The key is to always put the spectrum
at the same position in the image. If you work in slit
mode, the horizontal position is constrained by the
slit but also ensure that the spectrum is always at the
same position vertically.
5.1.4 Autoguiding (guiding module)
Autoguiding is very dependent on your equipment.
Autoguiding means that you send the guiding image
to the computer, which controls the telescope based
on star movement in the image. AudeLA software can
do that (like many other software).
Autoguiding is not only for comfort (when you acti-
vate the autoguiding, you don’t need to permanently
check the guiding image any more), but it also im-
proves the spectrum quality, because it always repro-
duces the same conditions. Consider autoguiding a
high priority for obtaining good results.
5.1.5 Cosmetic file
The CCD chip has some “hot pixels”. These pixels
are much more sensitive than others. These pixels
will give wrong measurements. It is better to remove
them from your image. This can be managed by ISIS
software, by creating a map of hot pixels. All pixels
considered as hot are replaced in the image by an av-
erage of the surrounding pixels. To generate the hot
pixel map, you use a dark frame and record all pix-
els above a certain value. In ISIS, go to the Master
images tab, and focus on the “Computing a cosmetic
file” area. Enter the threshold value - for instance 100
ADU (in the dark we use, most of the pixels are close
to 0 ADU). Click on the Go button, and look at the
console: you’ll see the number of hot pixels detected.
Around 100-500 hot pixels is considered normal.
The master dark is not really a dark frame,
but the thermal map of the camera after the
offset has been removed. This is why the aver-
age level of the image is close to zero.
You can use the hot pixel map in the “2. General” tab.
Simply enter the name of your map file in the field
“Cosmetic file”, and re-run the data reduction. ISIS
will now include this hot pixel list in its calculation.
5.1.6 Instrumental Response curve
We have already described a simple way to get the
Instrumental Response curve with ISIS. In fact, this
is a tricky operation. The response curve can evolve
with some parameters, like sky conditions, target po-
sition in the sky (air mass), reddening, and so on. To
be perfect, you need a reference star close to your
target, of the same spectral type, with similar mag-
nitude... and this never happens. Getting a bad re-
sponse curve will have an immediate effect: the gen-
eral shape of your spectrum will be distorted. It is a
too long story for this document, but keep in mind
that getting the proper Instrumental Response curve
requires some experience.
5.1.7 Write a log file
It can take various formats, but we strongly encour-
age you to write a detailed log file when observing.
Note all the conditions of your observations - setup,
weather, goal, target list, problems encountered dur-
ing the night. It will help you, in the future, to make
the best use of your data.
5.2 Improve productivity
5.2.1 Keep the same setup
The best way to improve your productivity is to al-
ways keep the same setup, and always process the
same way. Of course, observers often want to improve
their setup (this is a good behavior), but as soon as
you change parameters in your setup, you have to
adapt (to change...) your process. Sometime, it is prefer-
able to keep the same conditions, for both quality and
productivity.
5.2.2 Dark frames library
Dark frames are long exposure time images (at least
as long as the exposure time of your objects). And the
more darks you take, the better (you will improve the
SNR of the dark frame). This is a very time consum-
ing operation - something we should do at the end of
the night, when we know the longest exposure time.
But dark frames are an intrinsic charateristic of your
equipment. They only depend on the temperature and
exposure time. They don’t depend on the observing
conditions. So there is no need to do them during the
observing session itself: you can do them once, for in-
stance during a cloudy night (or even during the day
- just take care of light leaks).
We suggest that you make your own dark frame li-
brary, with different exposure times and camera tem-
peratures (you could even put the camera in the fridge
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