Alpy 600 - user guide
2.5.4
Third pass : Response curve
correction
The whole instrument, including telescope (if any),
spectroscope, and CCD camera, has its own response
to the light. It is called Instrumental Resonse Curve.
For instance, a CCD is much more sensitive to red
than to blue light. If you want to get the true spec-
tral profile of the source (the Sun in your case), you
must correct for this very important effect. The best
way to do this is to record the spectrum of a known
star (a reference star), and compare it to its “theoret-
ical” spectrum found in the literature.
In the current case, you’re observing the Sun... and
the Sun is a very well known G2V type star ! So, we’ll
make the correction from a G2V spectrum.
When you observe astronomical targets, it is neces-
sary to observe a reference star, generally a hot star
(A-type, with few absorption lines), close to your own
target to prevent atmospheric effects.
A reference star is required for both calibration and
response curve correction. Of course, the same star
can be used for both needs.
Each observing session must start by observ-
ing a known hot reference star, to allow the
correction of any instrumental effect.
ISIS includes a database of spectra recorded at dif-
ferent resolutions. Here is how to get a G2V star ref-
erence spectrum. In the “4. Profile” tab, click on the
database button. The database panel appears:
At the top left, you can select an G2V star from the
Pickles list:
This profile is different from yours. This is due to
the Instrumental Response curve and the wavelength
range covered.
Click on the "Display" button for reloading the sun
profile. Then click on the “Response” button. A re-
sponse panel appears : select the G2V profile as refer-
ence. Click on the “Response” button (red).
In the main window, you now have 3 profiles: your
Sun spectrum, the G2V theoretical profile, and the
division of both - which is indeed the raw response
curve.
Click on the OK button: only the response curve re-
mains. As you can see, this profile is very noisy, and
there are some “emission lines”. This is a side effect
of the division between the profiles, because the Alpy
600 and reference profiles do not have the same res-
olution.
To get the effective response curve, you need to re-
move these lines, and smooth the profile. This can be
done with the “continuum button”:
To remove the emission lines, double click on both
sides of each line. The line is replaced by a straight
line:
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