Spectrograph assembly guide

Optical bench mounted on active support system: PFA51507 - 700 mm (27.5“) Active Isolation Frame 900 x 1200mm (36” x 48“). Requires compressed air regulated to 50-100 psi.

External table/shelf for camera pump, computer relay/thermister/watchdog (on DIN rail?)

Power needed for:

1. computer (and monitor for testing)
2. power supply for QHY camera (power to camera is routed through watchdog with relay reset)
3. power for cooling pump
4. power for alignment laser
5. power for LED source
6. power for FPU iodine stage (FPU testing only)
7. power for FPU calibration stage (FPU testing only)
8. power for calibration box (FPU testing only)
9. power for high voltage source (FPU testing only)
10. power for iodine cell temperature controller (FPU testing only)

Ethernet connection needed for spectrograph computer.

USB:

1. USB3 to hub : alignment camera, autocollimator camera, Esatto focuser, Zaber stage (FPU testing only), Atik camera (FPU testing only)
2. USB2 to hub: QHY power relay, QHY thermister, keyboard, mouse
3. USB3 : QHY camera

1. 4×3 foot optical table on vibration isolation legs
2. upper table and legs
   1. input fiber unit
   2. input LED source
   3. 100mm lens assembly Thorlabs ACA254-100-A
   4. (removable) beam splitter
   5. laser input
   6. (removable) beam splitter
   7. slit viewing camera with 150mm reimaging optics
   8. right angle unit and 100mm reimaging optics (same as above)
   9. slit
3. collimating telescope mount, rail with two diagonal mirrors
4. FM1 and mount
5. OAP1 and mount
6. grating and mount and grating mirror and mount
7. FM2 and mount
8. OAP2 and mount
9. FM3 and mount
10. prism and mount
11. camera and mount
12. ESATTO focusser
13. QHY camera
14. liquid cooling system
15. computer and monitor

• Prepare optical table with acceptable support and sufficient access on all sides (while attempting to minimize total footprint
  1. position legs in desired final spectrograph location (legs can be moved short distance by hand)
  2. put table on legs (table requires lifting fixture, but can be adjusted a bit once on the legs by hand)
  3. connect compressed air (50-100 psi)
  4. adjust valve on leg D to get ~9mm clearance from bottom of table to leg structure (takes a few minutes for air to raise table)
  5. adjust valves on legs C and B to get level table
• Mark optical table with locations of mounts based on lab measurements, previous installation, or design
• Install collimating telescope behind table.
  1. install laser for collimator mirror alignment at right back corner of table
  2. adjust laser to ensure correct height and (175mm) light parallel to table. Use pentaprism to get long baseline measurement of beam height at ~2 table lengths with reflection, Laser to pentaprism on top of tube with 1-inch circular mirror at bottom, then light comes back up and back towards laser: rotate pentaprism slightly to measure height with measuring stick.
  3. Align auto-collimator to provide a reference parallel to the table.
     1. adjust far diagonal tip-tilt to return laser back to source
     2. rotate far diagonal to direct towards diagonal under collimator for all positions of far diagonal
     3. adjust diagonal under collimator to get beam in collimator.

• With aligned collimator install flat (grating flat, FM2, and FM3 on table in desired location. Also install prism mount for convenience before adding upper table. For each of the three flats:
  • slide collimator diagonal to point at flat, tighten screws
  • Focus autocollimator to get retroreflection reticle
  • adjust tip-tilt of flat to get alignment

• set up upper table on main optical bench in correct location. Install:
  1. fiber input,
  2. collimating assembly with 1-inch 100mm lens in tube. Clear area is ~22mm, giving f/4.5 beam (or slower if input beam is slower)
  3. laser input via removable beam splitter,
  4. camera output via removable beam splitter with 150 mm focusing lens
  5. output focusing lens, also 100mm
• Alignment of laser: laser must be in location of chief ray to use it to align spectrograph optics!
  • use laser and white light source, adjust laser and two input diagonal mirrors so that laser is in center of white light beam, but also in the center of a diffuser attached to beam splitter: use first diagonal to put laser in hole of diffuser, second mirror to put laser in center of white beam (at large a distance as possible!)
  • laser should be in center of white beam on table; if not, something is not right!
  • put flat mirror on table at beam, look for reflected laser at laser source, adjust diagonal down-pointing mirror until laser reflection returns exactly to laser.

• Install slit to ensure focal plane is at desired height above optical table
  • measure distance from bottom of slit holder to slit (10mm)
  • using imaging camera, adjust slit to lens distance to get sharp image
  • position slit-lens assembly to put slit at 235mm above the table
• Install FM1
  • at desired height (175mm): this sets the plane of the spectrograph.
  • correct tilt to ensure laser stays at same height at range of distances
  • Rotate FM1 to get desired deflection angle per table marking
  • lateral position so that full white beam barely fits on left edge of FM1
• Check grating mirror alignment
• Install OAP1. Adjust initial pitch and yaw using autocollimator off mirror on back of mount. Adjust lateral so laser falls at appropriate location on the OAP (62mm from edge).
• Align OAP1 using 10micron fiber input to minimize aberration
  • inspect images through focus
  • move images with lateral and inspect images through focus
  • adjust lateral to get best images
  • same with height
• Rotate spectrograph mirror to get 15mm separation between input and output laser. Adjust further to ensure that reflected beam misses FM1.
• Install OAP2. Adjust initial pitch and yaw using autocollimator off mirror on back of mount. Adjust lateral so laser falls at appropriate location on the OAP.
  • Rotate FM3 so that return beam goes back into camera.
  • Align OAP2 using 10 micron fiber input to minimize aberration
• If desired, install camera at non-final location to do imaging through full system. Imaging of 10um fiber is good for assessing single-pass image quality.
• Install prism. Rotate FM3 to ensure correct beam location.
• Install camera. Adjust to ensure correct beam location: no light falling outside any optic
• Image cross-disperser fiber. Ensure expected width. Adjust detector to make vertical spectrum. Adjust FM3 further to ensure good wavelength coverage on chip (laser is at 632nm, should be near red end, LED peak is at 450nm, should be near blue end)
• Install grating.
• Adjust grating tilt and prism to ensure good spectral coverage: grating so that peak of blaze function near center of detector, and prism to ensure good wavelength coverage.

Photos of the fully assembled spectrograph at APO on May 7, 2025.