GeminiFocus July 2017 | Page 24

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