CHAPTER 2. ALPY 600 - SPECTROSCOPY IN SLIT MODE
2.4.3 Flat field spectrum
A flat field spectrum is obtained by illuminating
the spectroscope with a continuous spectrum of white
light. Ideally, it should be a lamp that has the same
energy level in each wavelength. It doesn’t exist in
real life... As a compromise, we use tungsten or halo-
gen lamps (but not power saving, calibration lamps,
or sunlight !). The exposure time must be choosen to
give 80% ot the max level of the camera.
This image is mainly used to determine low fre-
quency non uniformities, caused by dust on the CCD
front glass for example or overall instrumentation vi-
gneting. Make sure to take a series (at least 5 expo-
sures) of flats. The signal has to be as high as possible
without any saturation.
The Alpy calibration module contains a halo-
gen lamp which gives a perfect flat field spec-
trum.
2.4.4 calibration spectrum
As explained in section 2.5.3, you will process your
Sun spectrum using the calibration information con-
tained within itself. But you can also use a calibration
lamp - if you are working in slit mode. For instance,
below is a neon spectrum
The Alpy calibration module contains a cal-
ibration lamp made of neon, argon and hy-
drogen which makes it very easy to calibrate in
slit mode.
2.4.5 Typical observation
To get all the data you need for a high quality ob-
servation, here is a suggested list of all the images
that you should have before switching off the equip-
ment. This is applicable for your Sun observation, but
is also valid for night observations:
– 7 offset images (or more)
– 7 dark images (or more)
– 7 flat field spectrum images (or more)
– 1 calibration spectrum image
– 5 reference star spectrum images (or more)
– and of course all the images of your object !
For each image, you must check that there is no satu-
ration.
A scientific measurement on an astronomi-
cal object spectrum requires not only a sin-
gle spectrum of this object, but a whole set of
images. If you make sure you have all these im-
ages during an observing session, data reduction
will be easy and fast.
2.5 Data reduction
You’ve now acquired all images for a full data re-
duction of the Sun observation. In this section, you
will see how to proceed with ISIS software. You will
proceed with these images as if they were from any
star.
2.5.1 Prepare master files
This step is to convert the series of reference im-
ages into single images, called master files. It is done
very quickly in ISIS. Go to the “Masters” tab, and se-
lect each series of images (offset, dark, flat), to create
a master. Click on “Go” for each series:
21
To Next Page
Loading...
Need help?
General