Gratings

The 1.5-meter Cassegrain spectrograph has a fixed $55^\circ$ angle between the optical axes of the collimator and camera. For sufficiently large grating tilts e.g., when observing at high dispersion, the beam coming from the collimator can overfill the grating resulting in the loss of a small amount of light. For the 1.5-m Cassegrain spectrograph, the collimated beam begins to overfill the grating for tilts greater than $20.\!\!^\circ6$ (grating tilt readout $> 29.\!\!^\circ85$).

The amount of light which might be lost due to overfilling the grating is almost always negligible, typically a few percent. In practice, this small loss would be more than offset by the ability to observe using a wider slit while preserving spectral resolution that results from the anamorphic demagnification at large grating tilts.

Table 2 lists the twelve $127\,{\rm mm} \times 152\,{\rm mm}$ Bausch & Lomb plane reflection gratings available with the spectrograph. Gratings tilts for the 1.5-m Cassegrain spectrograph are listed in Table 3.

 

Table: 1.5-Meter Cassegrain Spectrograph Gratings, 160mm Camera, Loral 1K ( $1200 \times 800$) CCD

 

			$1^{\rm st}$ Order
No.	Lines/mm	Blaze1	Resolution	Coverage
		(Å)	(Å)	(Å)
13	150	5000	17.2	6900
112	158	8000	16.4	6550
09	300	4000	8.6	3450
32	300	6750	8.6	3450
22 $^{\thempfootnote}$	300	10000	8.6	3450
58	400	8000	6.5	2590
16	527	5500	4.8	1965
26	600	4000	4.3	1725
35	600	6750	4.3	1725
56	600	11000	4.3	1725
47	831	8000	3.1	1245
363	1200	7500	2.2	860

¹ The value given for the blaze wavelength is the first order Littrow blaze. Effective blaze wavelength when used in the 1.5-m Cassegrain spectrograph is 0.89 of the Littrow value.

² Gratings 11 & 22 are silver coated and thus does not reflect very much light below $\sim 4000\,{\rm\AA}$.

³ Grating cannot be tilted far enough to be used in $2^{\rm nd}$ order.

 

 

Table 3: 1.5-Meter Cassegrain Spectrograph Gratings Tiles

Center Wavelength		Grating Tilt Readout
Order	(Å)	36	47	58	26	09	35	32	56
0	--	$9.\!\!^\circ25$	$9.\!\!^\circ25$	$9.\!\!^\circ25$	$9.\!\!^\circ25$	$9.\!\!^\circ25$	$9.\!\!^\circ25$	$9.\!\!^\circ25$	$9.\!\!^\circ25$
1	3800				$15.\!\!^\circ07$	$12.\!\!^\circ16$
	4000				$15.\!\!^\circ38$	$12.\!\!^\circ31$
	4200				$15.\!\!^\circ69$	$12.\!\!^\circ46$
	4400				$16.\!\!^\circ00$	$12.\!\!^\circ61$
	4600				$16.\!\!^\circ31$	$12.\!\!^\circ66$
	5800	$27.\!\!^\circ34$					$18.\!\!^\circ46$	$13.\!\!^\circ67$
	6000	$27.\!\!^\circ96$					$18.\!\!^\circ77$	$13.\!\!^\circ83$
	6200	$28.\!\!^\circ58$					$19.\!\!^\circ08$	$13.\!\!^\circ98$
	6400	$29.\!\!^\circ20$					$19.\!\!^\circ39$	$14.\!\!^\circ14$
	6600	$29.\!\!^\circ82$					$19.\!\!^\circ70$	$14.\!\!^\circ29$
	6800	$30.\!\!^\circ44$					$20.\!\!^\circ01$	$14.\!\!^\circ44$
	8400								$21.\!\!^\circ83$
	8800								$22.\!\!^\circ46$
	9200								$23.\!\!^\circ10$
	9600								$23.\!\!^\circ73$
	10000								$24.\!\!^\circ36$
	10400								$25.\!\!^\circ00$
2	3800		$25.\!\!^\circ58$	$16.\!\!^\circ99$
	4000		$26.\!\!^\circ49$	$17.\!\!^\circ40$
	4200		$27.\!\!^\circ40$	$17.\!\!^\circ82$
	4400		$28.\!\!^\circ31$	$18.\!\!^\circ23$
	4600		$29.\!\!^\circ22$	$18.\!\!^\circ64$