Alpy 600 - user guide
an extended object. When you’re close to the optimal
focus, the spectrum may disappear : This is because
the star image is now small, and may be outside the
slit. In this case, slowly move the telescope across the
slit: you should soon recover the star. If not, turn back
the focus, to extend again the object size, until you re-
cover its spectrum. Step by step, you will be able to
focus the telescope perfectly and put the star right in
the middle of the slit.
At this point, look through the flip-mirror’s eyepice,
and note the actual position of the star. This position
will have to be the same for the next observations. If
you put any star exactly at this position, you’ll have
its spectrum in the Alpy 600 camera each time.
Tracking the star
You’ve been able to put the star in the Alpy 600
slit... now, you have to keep it there during the whole
observation. This depends of course on the quality of
your mount, and if it is precisely aligned or not. You
need to adapt the exposure time to ensure that spec-
trum remains very thin, with no visual effects of track-
ing issues. The spectrum must be thin, and absorption
lines must be sharp.
If after some exposures, the spectrum disappears,
this is because the star moved out of the slit. You can
of course flip the mirror and look for it again, but in
practice, it is probable that the star always moves in
the same direction, due to a mount alignement error.
So to return the star to its original position, you can
just move the telescope in the right direction. You will
quickly find this direction and the level of required
corrections after some trials.
We may even suggest, in some conditions, to slightly
misalign the mount. This way, the star will slowly
move in the FOV, but always in the same direction.
Then, with experience, you will know in which direc-
tion you need to move the telescope when spectrum
level is decreasing.
Of course, if you have spent hours to align
your mount, this could be bad advice ! Only
misalign your mount if you consider there is no
such risk.
Pointing and tracking in wide slit mode with a flip
mirror requires some experience, but this is a real im-
provement compared to the slitless mode. You will
not be wasting your time if you do it !
Observing with the narrow (25µm) slit
This section is for people who like challenges ! It is
possible to observe in ”full slit mode” (using the 25µm
slit), but it requires still more accuracy and patience
compared with the wide slit. The process is exactly
the same as for the previous section, but the tolerance
for star positioning is more than ten times smaller
(300µm vs 25µm). However, this is really feasible -
we did it... even without a flip mirror (only a finder).
The main tip is to slightly unfocus the star. It be-
comes bigger in the focal plane (slit plane), and easier
to find. We recommend first working in slitless mode,
to be sure that telescope is close to focus. If the focus
plane is too far out, the star can be huge (even bigger
than FOV) and the light level very low.
Be careful : if you unfocus the telescope, only
a samll fraction of the light will enter the
spectroscope. This is applicable only for bright
stars.
Also, with some experience, and if your telescope is
precise enough, you can use your finder (or even bet-
ter, an electronic finder) to put the star very close to
the slit .
Then, depending on the spectrum level, you can de-
cide to refocus the telescope step by step. By doing
that, the star may go out of the slit, because it be-
comes smaller and smaller. Then, move the telescope
again until you recover it. You can make several iter-
ations, until you juge that the star size and spectrum
level are compatible with your observing conditions.
Again, proceeding this way is more of a a fun chal-
lenge than a recommended observing method. But it
can show you the significant difference between slit-
less and slit spectra: you’ll quickly see that in most
cases, there is a much higher potential in slit spec-
troscopy.
Then, keep in mind that by using the Alpy guiding
unit, you’ll get the high quality of a slit spectroscope,
in a very easy way, because you can permanently see
the star you are observing in your guiding image.
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