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Gumball, Gumball Manual.
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Gumball: The User's
Manual |
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The User's Manual for Gumball
At the beginning of 1998, an upgraded version of the Gumball
calibration unit was commissioned at CFHT. Not only
optomechanics and electronics components were modified or changed
but a new Pegasus interface was also implemented. For an observer,
the major changes include the possibility to define different exposure
times for each lamp for the same calibration frame, pre-defined setups
for diverse intrumental configurations to optimize the utilisation
of the Gumball, and the availability of two Fabry-Perot interferometers
providing regularly spaced calibration lines over large spectral
ranges. The manual below describes in more detail the new
Gumball and ... how to use it!
Table of Contents:
- Foreword
- Chapter 1: The Upgraded Gumball
- Chapter 2: Using Gumball through Pegasus
- Other Information
This manual presents the upgraded calibration system for the Cassegrain
environment at CFHT. During 1997, major changes have been made on the
calibration unit (alias "Gumball") so it can be used for all the
instruments mounted at this telescope focus. For the observers, a
new interface has been developed which allows more flexibility for
acquisition of calibration frames. In the following, Chapter 1
briefly presents the new system, a short overview of the
new interface, and the Gumball database. The interface and how
to use the new Gumball are more extensively described in Chapter 2.
Purpose
The Cassegrain calibration system ("Gumball") was designed and fabricated by
the Observatoire de Marseille as a visible spectral calibration unit for
the MOS/SIS spectrograph. The system was operated from the MOS/SIS
controller allowing remote utilisation of the different lamps from a Pegasus
session. However, with the recent development of other instruments
at CFHT like the Adaptive Optics Bonnette
(AOB) and
OASIS, or for
planned instrument like CFHT-IR, it
became evident that the actual Gumball unit was not well suited to
provide calibrations for all these systems. In particular, the
transmission characteristics of the optics did not allow calibration in the
near-infrared domain and not enough light (or not enough
spectral lines) was produced for the OASIS
calibrations. Thus, the main purpose of the
upgrade was to modify Gumball so that it can function as a universal
system for all the instruments mounted at the Cassegrain focus without
any dependance of the MOS/SIS optics and controller. To achieve this,
significant modifications in the optics, electronics and
software have been carried out, as described below.
Optomechanics
According to the specific needs of each instrument already in use or
planned, the specific objectives during the upgrade were:
- Provide spectral and continuum coverage between 360nm and
2.2 microns.
- Provide 5-8 lines in the usable spectral range of each instrumental
configuration.
- Provide sufficent flux in all spectral regions to minimize the exposure
times.
- Provide an universal pupil of the correct size and location without
any specific instrument support.
- Provide an independant control of the calibration unit.
To achieve these goals, a new optical layout and controller were
designed. The transfer optics has been replaced with a wide bandwidth
glass to increase the blue and near-IR transmission. All the lamp control
sequences and status are now controlled via a ProLog based computer,
directly connected to the Gumball. Moreover, two fixed Fabry-Perot
etalons were installed to provide channelled spectra for OASIS (but
available also for MOS/OSIS). Both etalons (one optimized for the
blue, the other one for the red) are illuminated be a continuum
sourve via two optical fibers.
The new arrangement for the lamps inside Gumball is illustrated
here. An important constraint of this design
is the removal of the Helium lamp to make room for the FP etalons.
Two pencil Mercury (Argon) lamps have been added to provide spectral lines
in the blue. To provided a more powerful and uniform illumintation,
two similar lamps are provided. For each lamp, a backside
reflector can be installed to increase the light flux, if necessary.
Interface: An Overview
The ProLog computer implemented in the new Gumball controller allows a
much better control of the lamps and provide a feedback on the real
status of the different spectral sources. To fully exploit these
advantages, a new Pegasus interface was designed. The user's interface
was created on the following guidelines:
- Provide the possibility to define individual exposure times
for the different lamps for a single calibration frame.
- Provide an utilisation mode identical to the old Pegasus interface
as an option (i.e. lamps controlled by the CCD shutter).
- Provide a real-time status of the lamps (on-off)
- Provide pre-defined setups established by the CFHT
staff to obtain an optimal calibration frame for different
instrumental configurations.
Except for the pre-defined setups, all these options are
now implemented.
The Database
During the upgrade, it was also clear that calibration spectra should
be made available on-line for reference during data acquisition and
reduction. As presented earlier, this database
includes the most common frames taken with different instrumental
configurations. This database will evolve when more and more data
are being gathered...
The Pegasus Interface
By selecting the Lamps button in a
Pegasus session for
a specific instrument mounted at the Cassegrain focus, the following
form appears:
Two basic modes of operation are offered:
- Timing Off: This mode corresponds to the way Gumball was
used prior to the upgrade. By selecting one or several lamps,
their exposure time is determined by the integration time
given for the CCD shutter in the Expose form of the pegasus session.
- Timing On: By selecting this mode, the individual exposure
time for each spectral lamp can be entered. Many lamps can be
used for a single calibration frame.
For MOS/OSIS, the integration time for the CCD, that is, the longtest
time for a given lamp, has to be entered manually in the
Expose form. Before the beginning of the exposure, the selected
lamps are turned on and progressively turned off during the exposure
according to their respective time.
There are 10 spectral sources available in the upgraded Gumball:
- Halogen Spectral: Bright halogen lamp for spectroscopic
or for narrow-band imaging flat-fields.
- Halogen Imaging: Same as halogen spectral except for an intensity
smaller by about a factor of 4. To be used when the halogen spectral
saturates the detector.
- Neon Wide: Classic Neon lamp.
- Neon 660/2.2: Neon lamp with a narrow band filter centered at 6600
Å. To be used for MOS-OSIS Fabry-Perot modes.
- Argon 1 and 2: Two identical Argon lamps located on both sides
of the Gumball. The lamps can be used simultaneously when
necessary.
- Mercury 1 and 2: Two identical Mercury + Argon lamps located on
both sides of the Gumball. the lamps can be used simulataneously
when necessary. The Mercury lamps have many bright lines in the blue
domain (see the database for a list of
spectral lines). Please, note that these lamps are a combination
of Mercury and Argon lines!
- Blue FP (360-500nm): One of the fixed gap Fabry-Perot etalon
for channelled spectra. The blue FP can only be useful for the
spectral domain specified. Also, it is highly recommended to
take some normal calibration frames with the blue FP to be able
to identify the spectral lines and correct for any shifts due
to external elements affecting the gap (i.e. temperature variations).
- Red FP (500-869nm): One of the fixed gap Fabry-Perot etalon
for channelled spectra. The red FP can only be useful for the
spectral domain specified. Also, it is highly recommended to
take some normal calibration frames with the red FP to be able
to identify the spectral lines and correct for any shifts due
to external elements affecting the gap (i.e. temperature variations).
The Pre-defined Setups option is still not available but
should be implemented soon. It will provide a quick way to get the
setup (i.e. lamps + exposure times) for a specific configuration.
At the beginning and during the exposure, the real status of the lamps is
gathered by the ProLog computer. If a problem occurs (e.g. lamp could
not be turned on), a red message will be displayed in the instrument
status window located on the right terminal.
The Fabry-Perot Etalons
The Gumball offers the possibility to get channelled spectra from two fixed gap Fabry-Perot
etalons. These etalons are illuminated by a common Xenon lamp via two 800 micron optical
fibers. The light is collimated, passed through the etalon and is re-imaged
onto fiber bundles which illuminate the internal sphere in Gumball. When using a
dispersor, the resulting spectrum consist in a series of regularly spaced spectral
lines (see this example).
Although the FP etalons were primarly installed to be used with the small spectral
ranges offered by OASIS, they can also be helpful with MOS/OSIS if necessary. However, since
the etalons have not been fully characterized yet, for example we do not have data
on the influence of temperature variations, normal
calibration lamp exposures must also be taken during an observing run.. These
lamps will allow to detect any variations in the spectral line positions produced
by the etalons. Also, they will offer an absolute reference for the wavelength
calibration. It is also very important to use the FP etalons in the spectral range
they were designed for!
Since data reduction using the channelled spectra will be a
bit more complicated, for the time being we recommend that you use the FP etalons
with MOS/OSIS in case of absolute necessity...
The Predefined Setups
Here is some supplementary information that can be useful about
Gumball:
- In the new Gumball, the flux delivered by the different
spectral sources is much larger than with the previous system. For
MOS/OSIS, neutral density filters have to be put in front of the
spectral lamps to diminish the intensity of the lamps. The backside
reflectors are also taken out. For OASIS, these filters are taken out
and the reflectors are mounted. We try to always keep the same
filter/reflector configuration for a given instrument in order
to have reproducible exposure times.
- The absence of the Helium lamp appears as a severe problem for MOS/FP
observations if the [OIII] line must be observed. It is, however, not
that big of a problem: a Helium lamp can be mounted in one of the
Gumball cell without too much of an effort for the CFHT staff. This
should be done on the day of the setup so you MUST specify this
on your observation preparation form!!