PFU

The LBT provides regular spectroscopy through a permanent focal station in the bent Gregorian foci. Three spectral resolutions can be chosen by selecting different fibers with different image slicers; R=50,000 (through a 2.3″ aperture on the sky sampled by 12 pixels on the CCD), R=130,000 (1.5″ aperture with a 4.2-pix sampling), or R=250,000 (0.74″ aperture, 2.0-pix sampling).

Table 1: Fibers, slices, resolution and readout time

fiber	arcsec	R	number of slices	slicer width(“)	R sample (pix)	Binning	Readout Time (sec)
100µm	0.74	250000	7	0.11	2	1	~77
200µm	1.5	130000	5	0.30	4.2	2	~50
300µm	2.3	50000	3	0.77	12.1	5	~20

Full wavelength coverage is possible with three exposures for all resolution modes. A single exposure always employs two cross dispersers (CD) simultaneously, one in each of the two spectrograph arms. The two cross dispersers in the “blue” and the “red” arm of PEPSI that are adjacent in wavelength, i.e. CD-III and CD-IV, cannot be used simultaneously because of the dichroic beam splitter. All other combinations are possible.

The entire spectral range of the PEPSI is from 381.9 to 906.4 nm but it can not be covered by a single exposure. PEPSI has two arms, blue and red, that can cover the full spectral range in 3 exposures. The blue arm covers the spectral range from 381.9-542.5 nm with the cross dispersers CD1, CD2, CD3. The red arm covers the range from 536.1-906.4 with the cross dispersers CD4, CD5, CD6. The red and blue arms can be observed simultaneously with the exception of CD3 and CD4.

The spectral ranges of each cross-disperser are given in the Table below (measured with the 100 µm fiber with DX, I.Ilyin, 09/2022). From 15-16 orders fall on the 10K x 10K detector. The SX and DX are slightly offset by a few Angstrom (4 in the blue and 16 in the red)

Table 2:  Cross-Dispersers

CD #	spectral range (nm)	width (nm)	order range	# orders
1	381.9-426.4	44.5	159-145	15
2	421.9-478.7	56.8	144-129	16
3	475.2-542.5	67.3	128-114	15
4	536.1-631.6	95.5	113-98	16
5	623.2-742.8	119.6	97-83	15
6	735.1-906.4	171.3	82-68	15

Because PEPSI has two beam splitters that are selected automatically depending upon the CD selection, no wavelength gaps exist. The user may also specify that the sky fibers provide fringes from a Fabry-Perot Etalon (FPE) instead of sky background light. This is what we call the high-precision RV mode. The simultaneous FPE spectrum is automatically chopped into sub-exposures of total duration equaling that of the science exposure and need not be specified by the user. This ensures proper exposure time for the fringes and comparable photo centers between science and calibration spectrum.

The possible wavelength coverage is from 381.9-906.4. The very edges of the spectrum are not really useful because the respective orders only partially fit on the CCD and appear asymmetrically truncated. The last measured wavelength coverage (measured with tracing part of them is possible but strongly depends on the S/N.

The basic modes described here are Integral-light spectroscopy in the wavelength range 383–907nm with spectral resolving powers R = λ/Δλ, of either 50,000 (2.3″ aperture on the sky), R =130 000 (1.5″), or R = 250 000 (0.74″).