When using SimSpec is the basic calculation and the adjustment to the
existing telescope and the CCD camera is implemented, this raises the
question of the detailed design. Now, as important aspects come into play,
such as mechanical stability, adjustability, light leakage (stray light
from the outside), weight (how much tolerates the focuser ?)....
I must warn here: The beginners tend to underestimate certain "trivial"
facts. These include the weight problem, torsion effects by the moving
telescope, external light comes through leaks in the housing, stability
of the mount, wind influences, sufficient means for adjusting the system
... failings in the planning phase no longer consider solvable problems
in the practical handling. Such problems destroy the joy of spectroscopy!
Therefore we will often build two spectrographs, the second one will take
into account (hopefully), what is important.
My advice is to build spectrograph from aluminum. Wood is always working
in its structures and we are often measure in wet nights when temperature
falls during the night and dew formation occurs. Aluminum makes little
thermal changing, it is specific lightweight and easy machinable, and
relatively inexpensive. Be sure to look for sufficient rigidity. During
an hour turns the polar axis of the mount by 15 ° and thus the orientation
of the spectrograph on the Earth's gravity! This caused changes in the
geometry of the spectrograph can greatly disturb.
It is also important that all optically important distances and angles
are adjustable as possible. So do not simply believe that the lines of
a grating in its holder are exactly perpendicular to the optical axis
of the collimator, just because you have designed it that way. It is better
to be able to adjust this alignment mechanically with set screws. Similarly,
collimator and camera along its optical axis must be movable to fractions
of a millimeter - for proper focusing. With adjustment devices you are
independent of small manufacturing errors.
Here is an example of experienced mechanical instabilities.
My first slit-free spectrograph was attached by 2" bush with 1-bolt clamp
on the eyepiece of a 5" Schmidt-Newtonian (Intes Micro MN 58). As a mount
used was a HEQ-5 (see photo above left). The combination of instable mount
(wind effect), not very stable attachment of the bush the instabilities
of the spectrograph and housing were also raised during the spectra recording
mechanical jumps. The graph shows 50 such consecutive recorded images
of the H alpha emission line of zet Tau (as extracted raw spectra). The
spectra stripes jumps on the CCD along the dispersion axis to within a
range of about 200 pixels and forth. Similar jumps also occurred in the
y-axis, but here they are no longer visible because they are removed by
the extraction of the spectra already. The rotation of the spectrograph
/ telescope within the 30 sec exposure time followed by a "trigger" by
the electromechanical shutter of the Audine camera was enough vibration
that in both axes of the CCD the spetra stripes was moved on. Jumping
was then compensated by processing software (second graph below left).
This mechanical instability has meant that the exposure times was limited
to 30 sec. With this exposure I have had about 1 / 3 of the recordings
due to jumps to sort out as useless. Longer exposure times lead to jumps
within the exposure time and movings in two axes according to dual or
multiple stripes of spectra. The "normal" host series consisted of 50
to 100 individual images, therefore, à 30 sec. What was the consequence?
At each frame is the normal noise of the signal the readout noise added.
he result was the sum spectrum, obtained from all 30 to 70 usable shots,
dominated by the readout noise. I had caught myself unnecessary noise,
which did not occur in one or a few long exposures.
The 50 scattered spectra in the above graph were correlated by the MIDAS
script "opa" (opa see the page Links / software), so that in
the dispersion direction are all concruent. The next step, they were added
to a sum spectrum. This is a beautiful example of what can be corrected
later with modern software inadequacies of the apparatus.
At my current aluminum spectrograph (LHIRES III) are
avoided bushes and replaced by more stable screws. The 100 kg-mount is
much stiffer. In continuous shooting series are now no dislocations of
the spectrum on the CCD chip recognize more. The spectra stripes coincide
exactly. And exposure times are from 5 to 60 minutes no longer a problem.
The readout noise is now negligible.
In this example, I want to show that, in practice, not
only the theoretical optical criteria play a the major role as they are
calculated by SimSpec. The very simple things make the biggest problems.
All practitioners within spectroscopists can sing that song, and everyone
can contribute their own verses about this!