UIST has two 9-slot grism wheels which contain the polarimetry prisms and the grisms for spectroscopy. 1, 2, 4, 5 and 7-pixel slits are available for use with each grisms (the exceptions being the IJ and JH grisms, which have their own 2 and 4-pixel slits – see previous page). The table below lists the currently available grisms.
Note that with the wider slits, the spectral resolution is reduced, though with the narrower slits, it is usually improved, roughly by the ratio of the slit widths. Unfortunately this is not the case with the IJ and JH grisms, where only a ~30% improvement in spectral resolution is seen when the 2-pixel slit is used instead of the 4-pixel slit.
The spectral resolution with the IFU is roughly equivalent to a 2-pixel slit, so is double that given below.
Click on the name of a grism below… to see the relative transmission across the passband of that grism. Each plot shows a spectrum of a bright standard star. The spectrum has been “normalized” via division of an appropriate black-body function, thus giving the transmission (though note that the absolute scale on the y-axis is arbitrary). Absorption due to the atmosphere plus the telescope and instrument optics (especially the grism and spectral blocking filter) all contribute to the overall shape of each plot. Photospheric absorption lines associated with the standard have not been removed. Note that UIST’s throughput drops quite considerably towards the I-band, and that the long-wavelength end of the JH grism is suppressed by the blocking filter (see below).
Current Grism set: 27 May 2005 – present
|Short J||1.024-1.177||1500||2||Long J||1.162-1.315||2000||2|
|Short H||1.423-1.625||1900||2||Long H||1.603-1.803||2000||2|
|Short K||2.007-2.260||1800||2||Long K||2.204-2.513||1900||2|
|Short L||2.905-3.638||700||1||Long L||3.620-4.232||1200||1|
** PLEASE NOTE: The throughput of the JH grism is 1.5 to 2.0-times worse than the IJ and HK grisms in the J and H-bands respectively. Therefore, wherever possible, the IJ and HK grisms should be used in preference.
Also, the blocking filter in use with the JH transmits between 0.85 and 1.80 microns; this impacts JH data in two ways: (1) emission above 1.80 microns is blocked completely, and (2) lines between 0.85 and 0.90 microns may be seen in your data in second order between 1.70 and 1.80 microns.
A comparison of IJ, JH and HK spectra obtained through a 4-pix slit of HIP 87895 (G2V, V=6.3), after division by an A0V star (HIP 85382, 5.9 mag) for telluric correction, is given here (wavelength scales are approximate).
Note also that in RAW data frames the wavelength increases to the LEFT ; the pipeline software will, however, display the wavelength increasing to the right (further details are given in the pages on Data format).
Low versus Moderate-Resolution Grisms
Should I use a moderate-resolution or a lower-resolution grism? The answer depends on your needs. The relative transmissions are similar. However, with most of the higher-resolution grisms background-limited performance is essentially impossible, so (read)noise on the array can be a dominating factor. The higher-resolution grisms work well if one is trying to detect line emission superimposed on continuum emission, and obviously they offer higher spectral resolution.
To compare the performance of the HK and short-K grisms, a young star with a line-emission jet was observed with both grisms, using the 4-pixel slit and 60 sec exposures in both cases. In both datasets, continuum from the star and weak line emission from the jet were detected (Fig.1). However, in extracted spectra from the jet (Fig.2 – left) a low-frequency “ripple” is evident in the short-K data which isn’t apparent in the HK data: this is produced by a “chevron” readnoise pattern across the array, which though variable can dominate the noise at very low flux levels.
The short-K spectrum extracted from the star fairs much better (Fig.2 – right): in this case the main source of noise is the continuum from the star. At the higher spectral resolution of the short-K grism, the (spectrally-unresolved) line-emission towards the star is more prominent in the short-K data than it is in the HK spectrum.