The gratings shown below can all be used with Hydra. The dispersions shown are those obtained when the gratings are used with the 400mm Bench Schmidt camera and the 2x4K SiTe CCD.

200 and 100 µ wide slit plates are available to put in front of the output of the fibers.

With the 400mm camera and no slit plate the images will be approximately 7 pixels wide. With 200µ and 100µ slits they will be about 4.5 and 2.5 pixels wide respectively.   Thus, with (say) KPGL3 which has a dispersion of .70Å/pixel at 5500Å, the resolution would be .70X7=4.9Å (R=1100) with a bare fiber, .70x4=2.8Å (R=1900) with the 200µ slit plate and .70x2.5=1.75Å (R=3000) with the 100µ slit plate.  The efficiency of the system will be reduced by about 25% when the 200µ slit plates are used and by roughly 60% with the 100µ slits.

In selecting a grating, you should take into account the tabulated grating efficiencies or those shown in the charts at the bottom of this page. These charts were made with the R/C spectrograph. For Hydra the wavelength scale should be shifted by about 9% towards the red.

The 229mm Air Schmidt camera with the 1Kx3K Loral CCD is now available a backup only.  With the Air Schmidt the dispersions are 1.75X the values shown and the images are approximately 4 pixels wide. The slit plates do not improve the resolution with the Air Schmidt because of the poor images and thus cannot be used with this camera.
 

GENERAL: All the gratings used in the R/C spectrograph will fit on Hydra. However, gratings 250 and 400 have been left off the list because their dispersion is too low to be useful. Grating 420 is not included because it is redundant with KPGLF and less efficient.

Dispersions will be half the values shown when the gratings are used in second order.
 

First Order at blaze wavelength with 400mm camera: For Air Schmidt (retired) multiply dispersion by 1.75X, coverage by 1.31X
Grating #	Lines/mm	Blaze (See note #4 below)	Dispersion (Å/mm)	Coverage (4096 pixels)	Comments
510	300	10000Å	1.24Å/pixel	See note #1 below
181	316	7500Å	1.18Å/pixel	See note #1 below	LOW THROUGHPUT!!!
KPGL2	316	4400Å	1.19Å/pixel	See note #1 below
KPGL3	527	5500Å	.70Å/pixel	~2800Å
KPGL1	632	4200Å	.59Å/pixel	~2400Å	See note #2 below
KPGLF	632	8200Å	.57Å/pixel	~2300Å
450	632	11000Å	.56Å/pixel	~2200Å
KPGLD	790	8500Å	.45Å/pixel	~1800Å
KPGLG	860	11000Å	.42Å/pixel	~1700Å
380	1200	8000Å	.27Å/pixel	~1100Å	See note #3 below
ECHELLE	316	56120Å	(Wavelength/105,000) /pixel	~.038 x Wavelength	See note #5 below

Sometimes it is necessary to use an order-separating filter. Available filters are shown here.  Don't use a filter unless you must, since they cause a slight loss even at their best wavelengths and can cause suprious reflections.

Looking for spectra taken with the comparison lamps?

NOTE #1 Care must be taken when using gratings 510, 181 and KPGL2. They have too much wavelength coverage for the CCD. Second order contamination limits the coverage to less than the full width of the chip and a blocking filter is normally required.

For example, if KPGL2 is used with a central wavelength of 6000Å, coverage from 4000-8000Å can be obtained but a filter blocking wavelengths below 4000Å will be needed. Wavelengths beyond 8000Å will be seen on the CCD but will be strongly contaminated by second order blue light.

Grating 181 as currently used in the Hydra spectrograph has very low throughput, and should not be used until further notice. (KAO 1/25/06)

NOTE #2 KPGL1 should always be used with a blocking filter, EVEN in first order. Tests done by P. Massey and K. Olsen in March 2005 demonstrated that a few percent of the light in the red appears in the blue at half the wavelength (not to be confused with second order diffraction, which goes the other way). The problem was likely introduced by the machine that ruled the grating. None of the other gratings have been found to have this problem.

NOTE #3 Grating 380 is not very efficient in second order and produces very bad ghost reflections at some wavelengths.

NOTE #4 Blaze wavelength is shown in "Littrow", i.e. normal incidence. This is how they are used in the spectrograph and is approximately the peak wavelength.

NOTE #5 See the echelle mode documentation for more information about how to use the echelle grating.

RELATIVE EFFICIENCY OF R/C SPECTROGRAPH GRATINGS (Wavelengths for Hydra = scale x 1.09)

Blue - relative to grating 250

Red - relative to grating 400