1. CASE spectrograph Spectrograph Optical Specifications
Spectrograph Optical Design
Spectrograph Mechanical and Electrical Design
Calibration System: Lamps, Sky Lines, Stars, CCD Flat Field Illuminator By
Justo Sanchez
Instituto de Astrofísica de Andalucía (CSIC)
Granada

2. Spectrograph on Telescope, Specifications
Basically, the CASE spectrograph will be based on the DESI spectrographs, but using only two arm, BLUE and RED. The telescopes, focal plate & positioner specification are:
parameter
DESI
CASE
Telescope/FOV
MAYAL (Ø4m)/3⁰
Schmidt (Ø0.8m)/8⁰
Focal Plate (Ømm ) // Scale
Primary focus (808mm) // μ/”
Schmidt focus (335mm) // 11.6μ/“
Fiber sampling μ,“ /// F#input-F#output
107μ → 1.52” /// F3.9 – F3.74 FRD
90.2μ→ 8” (??) /// F3 - F2.8 FRD
Corrector lens and ADC
booth
neither
#N positioner x spectrograph
500 x 10
350 * 1 (??) (inclu. 40 ?? fiducials fixed fibers)
Actuator pitch /patrol area
10.4mm / 12mm
15.4 mm / mm (AVS NEW)???
Fiber View Camera (FVC) // central IFU
Needed // not provided
Needed // yes 350 fibers ??, hexagonal Pack.
Positioner precision //
FVC fiber centroid precision
5μ //
<3μ
6.5 μ //
<4(??)
Schmidt
Mayal
FVC on Schmidt

3. Spectrograph Optical Specifications
Apart of the two cameras, there is a double parallel pseudo- slit, one for positioner fibers and other for central IFU fibers.
parameter
DESI
CASE
Resolution (λ/∆λ)
360 nm < λ< 555 nm: > 1500
555 nm < λ< 656 nm: > 3000
656 nm < λ< 980 nm: > 4000
360 nm < λ< 600 nm: 1500 (????)
600 nm < λ< 950 nm: 3000 (????)
Output Fiber diameter, focal #
Pseudo-Slit dimension
107μ→ 1.52” , F 3.74
120.9 mm
90.2μ → 8” , F FASTER COLIMATOR,
101 mm BUT SMALL PSEUDO SLIT
F# mirror collimator , F# camera
Demagnification factor, slit on detector
F , F1.7
2.2 , mm
F 2.8????? , F 1.55 ????TBC
1.7 , 59.1mm NEEDED demag. To this slit=101mm
CCD, Pixel detector, side
3 * 4k x 4k , 15μ, 60mm
2 * 4k x 4k , 15μ, 60mm
Minimum resolution elements
3 pixels > Nyquist
3 pixels
Fiber spacing (slit plane)
230μ to prevent cross talk
193μ
Number of fibers (spatial)
500 singles fibers(positioners)
Singles fibers (positioners) ???
OR 350 in the central IFU ??? (for a 101mm slit long)
Sky + standard star on target selection
40 sky + 10 standard star
20sky (????)+ 5 standard star (????) (CASE FOV is bigger)
217 fibers IFU =2.55mm→3,66’
BLUE
Pseudo-slit
RED
331 fibers IFU =3.15mm→ 4,5’

4. Spectrograph Optical Design (DESI)
The design considerations to reduce cost of DESI f/1.7 !!! cameras:
To minimize: the number of lenses, aspheric surfaces, the volume of glass; use low cost glass with frequent melts.
To maximize the throughput the number of lenses was minimized and only glasses with high transmission in the band pass were used.
The last lens was fused silica since it is the vacuum seal to the cryostat.
The DESI spectrograph use Volume Phase Holographic (VPH). They have the highest throughput over the relatively broad band ∆β <16⁰. The fringes of the gratings are tilted to remove the Littrow ghost. The VPH grating is made by sandwiching a gelatin between two fused silica optical flat plates. ∆β = dispersión angle, 1/σ is the line density, and Ø is the tilt angle.
Radius Spot RMS of the three cameras vs. wavelength
Resolution vs. Wavelength

5. Spectrograph Optical Design (CASE)
The more important difference with the DESI spectrograph is coming from the focal number of the fiber at the output, that is dependent of the focal number at the input and the Focal Ratio Degradation (FRD) in the figure:
For DESI: f input = 3.9 and 40 meters → f out=3.74
For CASE: f input = 3 and 10 meters → f out=2.8???
FAST COLIMATOR AND CAMERA DESIGNS are NEEDED
Grims and VPH compatible with CASE, in function of Wavelength range, resolution, etc FINAL RESOLUTIONS AND RANGES CHOICED ARE NEEDED
VPH efficiency and wavelength range

6. Spectrograph general design
Booth DESI and CASE, adopt the same solution for the pseudo-slit, just in front of a mirror collimator. But in CASE there is two pseudo-slit parallel and close together, One receives light front the positioners, and the other from the central IFU. A laminar switching mechanism permit to work only one at same time.
Another mechanism provide light to retro - illumination of the fiber (by the spectrograph side) and the fiducials at same time.
Two pseudo-slits together
Mirror collimator for F5 input and 100mm pseudo-slit long. Dimensions (100x310x60mm)
DESI 3D model spectrograph :1.5 m wide × 1.4 m deep × 0.5 m height

7. Calibration System: Lamps, Sky Lines, Stars, CCD Flat Field Illuminator
Multiple calibration sources will be used in CASE, including sky lines, stars, continuum and line (arc) lamps, and a CCD flat field illuminator. The spectrograph is mounted on stationary optical bench in a temperature controlled environment, their calibration should remain quite stable.
used
obtained
Calibration lamps
illuminate the pupil of the telescope, over a flat field screen mounted inside the dome
establish the wavelength and to measure
the point-spread-function (PSF)
Continuum lamps
e.g. quartz iodine halogen
illuminate the pupil of the telescope, over a flat field screen mounted inside the dome.
to measure the xy trace location of the spectra on the CCDs and to correct for relative fiber-to-fiber throughput variations.
Stars
Standard stars will be selected as main-sequence F-stars based upon color and magnitude,
By comparing the observed standard star spectra with their modeled spectra and photometry. To provide spectro-photometric calibration to the spectra
CCD Flat Field Illuminator
illuminates the CCD in the spatial direction with
a smooth spectrum such that each pixel receives the same wavelength
By comparing each pixel to the median of other pixels with the
same wavelength. The wavelength-dependent pixel level response is measured. NEED to dismount the pseudo-slit plate. Once per year???

8. Comparisons. CASE DESI SDSS MUSE
Telescope (m); f/#
Schmidt 0.8m; f/3
Mayal 4m; f/3.9
SDSS 2.5m; f/5
4th unit VLT 8m; f/15 (582μ/”)
Scale
FOV
11.6μ/” 8⁰
70.14μ/” 3⁰
60μ/”
The FOV is sliced in 24 IFU &spectrog
1’x1’ WFM or 7.5”x7.5”NFM
Fiber sampling (μ,“)
sampling“
90.2μ??
7.7”
107μ
1.52”
180μ 3”
Each pixel in IFU element =
0.2”x0.2”WFM 0.025”x0.025”NFM
Positioner precision
6.5μ RMS <=9μ??
5μ RMS
By hand
Slicer
Range /arms
nm VPH? /2arms
nm VPH /3 arms
nm/2arm
nm /24 channels sliced
Resolution (λ/∆λ)
1500 at λ< 600nm ???
3000 at λ >600nm ???
1500; 360 nm < λ< 555 nm
3000; 555 nm < λ< 656 nm
4000; 656 nm < λ< 980 nm
1500 λ <600nm
3000 λ >600nm
1750 at 465nm
3750 at 930nm
Slit size
Fiber output, f/#
Colimator f/#
Camera f/#
Resol. element
101mm
90.2μ, f/2.8 ?
f/2.8 ? to reach given
f/1.55 ? fibers –resol.
48μ
120.9mm
107μ, f/3.74
f/3.74
f/1.7
125mm
180μ, f/4
f/4
f/1.5
67.6μ
24 channels * 48 15” x 0,2” mini slits
slice
(spatial resolution)
15μ=( )”WFM;( )”NFM
CCD
2 * 4k x 4k , 15μ
15 * 4k x 4k , 15μ
2k x 2k, 24μ
24 * 4k x 4k, 15μ
Fibers, Elements
350 positioner or 350 IFU
10 * 500
320
90000 spectra at same time
Pixel / elem. resol. 3 1
If CASE can use a 6k x 6k, 15 CCD, for all the range, with only a Grating, the expected resolution would decrease to (more calculi are needed).

9. Conclusions and work to do, for previous optical design
Some Telescope parameters are not known in details:
no opto-mechanic drawings
We need to decide specifications to the spectrograph system:
Sky sampling → fiber diameter → number of fibers on ???
Range and resolution/resolutions, single/multiple grating???
Estimation of the FRD for this application, bend, path across telescope, distance???
Collimator and camera focal f/# and its focal distances, to appropriate demagnification ???
etc…
Thank you