Figure 7 .
Wavemapper display for the GMOS IFU-2 mode , the B600 grating , and G-band filter , for a central wavelength of 475 nm . The two spectral banks overlap near the center of the display ( red shaded area ).
certain wavelength will fall onto the Gemini Multi-Object Spectrograph ( GMOS ) detectors , depending on the chosen grating and central wavelength ( CWL ). It works for all GMOS modes including long-slit , Integral Field Unit-R ( IFU-R ), IFU-2 , and MOS . Principal Investigators can now accurately plan their observational setup , avoiding important spectral features being lost due to detector gaps and boundaries .
The tool ’ s creator , Gemini astronomer Mischa Schirmer , used dedicated arc line observations to build the various mathematical models for each mode and grating . In the case of the MOS mode , he designed a special slit mask containing 135 slits on a tilted grid , covering the entire slit placement area . Mischa then observed arc lamp spectra with each grating , tightly stepping the CWL through the 380 – 950 nanometer range . More than 17,000 arc spectra were automatically calibrated for the MOS mode using a third-order polynomial . The coefficients of the polynomials are in turn functions of the slit position and CWL with their own polynomial dependencies , resulting in a 60 parameter model for each grating .
The models predict the wavelength positions with an accuracy of a few pixels — much smaller than the diameter of the GMOS detector gaps . It is also smaller than the long-term stability of the grating mechanism when establishing a certain CWL setting . The interactive tool allows the user to adjust the CWL and visualizes the wavelength grid of the spectra on the GMOS detector arrays . Individual wavelengths , atomic line series ( optionally redshifted ), and 2nd order overlap can be displayed as well .
The IFU-2 mode in particular benefits from the new tool , making the selection of a suitable grating / filter / CWL combination much easier . A substantial challenge inherent to the IFU-2 mode is that two spectral banks are mapped simultaneously on the detector array ( Figure 7 ). The spectra are cut asymmetrically by the detector gaps , such that a certain spectral feature might be lost in one of the two spectra . This can be avoided by fine-tuning the CWL , but only within a certain limit before one of the spectra gets pushed off the detector array . Previously , finding the optimal balance has been a tedious , if not impossible , task ; also because the two spectra have different dispersion factors . With the GMOS WaveMapper , this task has been much simplified , making IFU-2 a significantly more powerful , attractive , and less scary mode .
The GMOS WaveMapper is a plugin for the European Southern Observatory ’ s ( ESO ) Skycat tool . It is distributed together with the Gemini mask making software , GMMPS ( see news item starting on page 20 of this issue ). GMMPS uses the WaveMapper models for its internal calculations , but the mask design process is entirely independent of it otherwise .
22 GeminiFocus July 2017
The models predict the wavelength po-
sitions with an accuracy of a few pixels
— much smaller than the diameter of
the GMOS detector gaps. It is also small-
er than the long-term stability of the
grating mechanism when establishing a
certain CWL setting. The interactive tool
allows the user to adjust the CWL and
visualizes the wavelength grid of the
spectra on the GMOS detector arrays. In-
dividual wavelengths, atomic line series
(optionally redshifted), and 2nd order
overlap can be displayed as well.
Figure 7.
Wavemapper display for
the GMOS IFU-2 mode,
the B600 grating, and
G-band filter, for a central
wavelength of 475 nm. The
two spectral banks overlap
near the center of the
display (red shaded area).
certain wavelength will fall onto the Gemini
Multi-Object Spectrograph (GMOS) detec-
tors, depending on the chosen grating and
central wavelength (CWL). It works for all
GMOS modes including long-slit, Integral
Field Unit-R (IFU-R), IFU-2, and MOS. Principal
Investigators can now accurately plan their
observational setup, avoiding important
spectral features being lost due to detector
gaps and boundaries.
The tool’s creator, Gemini astronomer Mis-
cha Schirmer, used dedicated arc line obser-
vations to build the various mathematical
models for each mode and grating. In the
case of the MOS mode, he designed a spe-
cial slit mask containing 135 slits on a tilted
grid, covering the entire slit placement area.
Mischa then observed arc lamp spectra
with each grating, tightly stepping the CWL
through the 380–950 nanometer range.
More than 17,000 arc spectra were automat-
ically calibrated for the MOS mode using a
third-order polynomial. The coefficients of
the polynomials are in turn functions of the
slit position and CWL with their own polyno-
mial dependencies, resulting in a 60 param-
eter model for each grating.
22
GeminiFocus
The IFU-2 mode in particular benefits
from the new tool, making the selection
of a suitable grating/filter/CWL combi-
nation much easier. A substantial chal-
lenge inherent to the IFU-2 mode is that two
spectral banks are mapped simultaneously
on the detector array (Figure 7). The spectra
are cut asymmetrically by the detector gaps,
such that a certain spectral feature might be
lost in one of the two spectra. This can be
avoided by fine-tuning the CWL, but only
within a certain limit before one of the spec-
tra gets pushed off the detector array. Previ-
ously, finding the optimal balance has been a
tedious, if not impossible, task; also because
the two spectra have different dispersion fac-
tors. With the GMOS WaveMapper, this task
has been much simplified, making IFU-2 a
significantly more powerful, attractive, and
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