Adjusting the SDSS Fiber Throughput Tester

Sloan Digital Sky Survey Telescope Technical Note 19970416-05

Russell Owen

Contents

• Unpacking and Set Up
• Adjusting the Detector Stop
• Adjusting the Photodiode
• Replacing the Bulb
• Document History

Unpacking and Set Up

Tools required: 1/16", 1/8", 5/64", 9/64" hex wrench, 1/4" flat-bladed screwdriver, two 8-32 x 3/4" screws

• Attach parts to the optical rail.
• Remove the shipping link from the linear stage and store it in a safe place. Be sure to replace all screws after the link is removed; only the link is removed. Use a 1/8" hex wrench to remove the front plate and a 1/4" flat blade screwdriver to remove the link screws.
• Adjust the detector stop, as described below.
• Connect electrical and electronic cables:
  • Plug Keyspan USB-Serial adapter into USB port of computer.
  • Attach Anorad IStep stepper motor driver to port 1 of the Keyspan USB-Serial adapter (using RJ-11 phone cord and labelled adapters).
  • Attach Keithley picoammeter to port 2 of the Keyspan USB-Serial adapter (using RJ-11 phone cord and labelled adapters).
  • Connect stepper motor and encoder to Anorad IStep stepper motor driver (Phoenix screw-terminal connectors plug into top of Anorad).
  • Connect source to Keithley picoammeter using BNC cable.
  • Connect stabilized lamp to lamp power supply.
• Attach the source optical fiber between stabilized lamp and source module.
• Turn on the stabilized lamp.
• Adjust the microscope eyepiece (1/16" hex wrench) so that the ferrules are in focus.
• Adjust the source fiber so the input beam is centered on the ferrules and is in focus. Screw two 8-32 x 3/4" screws into the holes and push sideways on them to turn the eccentric rings after loosening the set screws with a 5/64" hex wrench. Check your work after tightening the set screws. Remove the 8-32 screws when done.
• Confirm input beam quality using an SLR or video camera (with no lens). Examine the cross section of the beam for uniformity. If the uniformity is significantly worse than 5% (ignoring the blips at the edge of the beam), try shrinking the band that squeezes the source fiber coil together.
• Adjust the photodiode, as described below.
• Turn on everything and test the fiber tester.

Adjusting the Detector Stop

No tools required.

• Tighten down the linear stage with its rail carrier at the approximate position 75. Tighten it down very securely, as it must stay fixed for a long time.
• Loosen the detector stop.
• Gently push the detector up against the linear stage.
• Measure the position of the detector on the rail (using the vernier scale).
• The desired position of the detector is this measured value plus 1.0 mm. Record this position as the "desired position of the detector".
• Move the detector to its desired position (1.0 mm to the right, using the vernier scale) and tighten down the detector.
• Gently push the detector stop up against the detector and tighten down the detector stop very securely. It must stay accurately fixed time, despite being repeatedly pushed up against.
• Record the stop's position as the "desired position of the stop".
• Check your work by removing and replacing the detector. Make sure it returns to the desired position, as recorded above.

Adjusting the Photodiode

Tools required: 9/64" hex wrench.

Preliminaries

• Mate the detector to the source.
• Make sure the detector is connected to the Keithley picoammeter and the stabilized lamp is on.
• Put Keithley picoammeter into local mode:
  • If the picoammeter shows 0...0 VA:ZC then press the ZCHK button
  • If the picoammeter shows "remote" then press the "local" button once; at this point the display should start updating.
  • If you are getting a reading of at least 20 uA then press AUTO once to disable auto-ranging (I find it more convenient to have a fixed range so I can tell at a glance what the current is doing).
• Remove the photodiode cover (at the back end of the detector) using the 9/64" hex wrench (two screws).
• This exposes an x-y-z positioning stage with three adjustment knobs (vertical, horizontal and focus). Note: when turning these knobs always make the final turn in the clockwise direction, to eliminate backlash.
• Record the position of all three adjustment knobs as "original position" (in case you wish to revert). To record knob position sketch the position of the scratch on each knob, being sure to include a zero reference in the sketch. Then turn each knob clockwise gently until it stops, recording the number of turns as you go. Then return the knob to its current position (remember to always end by turning clockwise).

Locate Edges

The basic idea is to locate the top and bottom edges of the photodiode, center the photodiode between them and apply some final adjustments.

Why the top and bottom edges? The range of motion of the translation stage is not quite enough to reach both the left and right edges of the sensitive area of the photodiode. However, the range of motion probably is sufficient to see the top and bottom edges. It is true that the photodiode sensitive area is circular, but the photodiode is tilted such that the top is closer to the input end of the detector module (the tilt is necessary to prevent light reflected from the photodiode from bouncing off the v-groove block and returning to the photodiode, giving an incorrect reading; the orientation of the tilt reduces dust accumulation). This tilt reduces the range of motion needed to see the top and bottom of the sensitive area of the photodiode.

• First center the bottom edge of the photodiode sensitive area in the eyepiece:
  • Keep your eye on the current meter while you raise the photodiode most of the way.
  • Find the point whether the current is halfway between max and min. This gives you the rough position.
  • Look through the eyepiece and change the focus until you see the edge. The edge looks like the edge of a large black disk; when you get it right it will it should appear as a thin bright curved line centered in the light spot.
  • Center the edge in both x and y and bring it into focus.
  • Warning: there are several visible edge-like features. The one that you want is next to the black sensitive area of the photodiode. Confirm you have the right one by translating the photodiode slightly up/down and observing that the current goes between max and min. The transition should be quite abrupt if the edge is in focus!
  • If you cannot turn the vertical knob far enough to find this edge then read If You Can Only Find One Edge.
• Record the position of all three adjustment knobs as "position of bottom edge".
• Lower the photodiode and turn all three knobs to place the center of the top edge of the sensing element in focus and centered on the eyepiece (roughly 4 1/2 turns CW vertically and 1 1/3 turns CCW in focus). If you cannot turn the vertical knob far enough to find this edge then read If You Can Only Find One Edge.
• Record the position of all three adjustment knobs as "position of top edge".
• Compute the center position as the average of the position of each edge.
• Center the photodiode in all three axes. This centers the photodiode, but we are not done yet!

Final Adjustment

• Defocus the photodiode by 2 turns (1 mm) counter-clockwise. Defocussing makes the signal somewhat immune to small surface variations in the photodiode.
• Move the photodiode no more than 1/2 turn up and down (but not sideways) to maximize the reading. This prevents a local sensitivity minimum on the photodiode from causing artificially high throughputs.
• Record the adjustment knob positions as "final position".
• Replace the photodiode cover.

If You Can Only Find One Edge

If the vertical adjustment is too small to measure both the bottom and top edges, then you'll have to work from just one edge, as follows:

• Measure whichever edge you can (as per the instructions above).
• Center the photodiode as follows:
  • Assume the vertical distance from either edge to the center is 2 1/4 turns: CCW from the top edge or CW from the bottom edge.
  • Assume the horizontal distance is the same for the top and bottom edge. Leave that knob alone.
  • Assume the focus distance from either edge to the center is 2/3 turns: CW from the top edge or CCW from the bottom edge.
• Proceed with to final adjustment.

Replacing the Bulb

How to replace the bulb in the Newport 780 stabilized white light source.

Equipment Required

• Replacement bulb: Newport recommends Osram HLX 64610 (50W 12VDC quartz halogen) which is only rated at 50 hours of life. I have found one long-life replacement: Divine Lighting 64610-BRL-500 which is rated for 500 hours. A long-life bulb is strongly recommended because changing the bulb is a headache.
• DC voltmeter with safe clip leads (insulated push-to-grab type, not not alligator clips).
• 1/16" hex wrench.
• 3/32" hex wrench.
• 3/16" hex wrench.
• Phillips #1 screwdriver.
• Small flat-blade screwdriver.

Procedure

• Make sure lamp is off and housing is cool.
• Remove source fiber from lamp (do not detach the other end—at the source module—because that end requires delicate adjustment).
• Lay lamp housing on its side (base towards you, lens to left or right).
• Detach base from lamp housing (remove one 3/16" hex bolt).
• Remove bulb assembly from housing by removing three 1/16" button-head hex screws from bottom of lamp housing.
• Remove bulb (loosen two 3/32" hex screws holding bulb's wires).
• Insert new bulb, being sure not to touch or contaminate it (use a cloth).
• Make sure bulb wires are fully inserted, so bulb is down as far as it can go.
• Tighten the two bulb wire screws.
• Insert bulb assembly, making sure line of bulb filament is perpendicular to optical axis. A/C cord should come out near the mounting post.

  Adjust the lamp as follows:

• Make sure lamp intensity trim control (on front of lamp housing) is at 1/2, which is its recommended position.
• Remove top cover of lamp control (remove four #1 Phillips sheet metal screws).
• Locate the four test points on the circuit card (along right edge near the front and top).
• Put a DC voltmeter across test points 1 and 3 (ground to 3 if you want a positive voltage).
• Remove fiber optic adapter from lamp (loosen at least two of the three 1/16" hex set screws) and aim the lamp's lens at a flat white surface at least 1 foot away.
• Turn on lamp.
• Adjust lamp lens to focus the spot on the white surface.
• Adjust bulb position by turning the thumbscrews at bottom to approximately maximum brightness and get the filament visible in the center of the circle of light.
• Turn off the lamp and let the bulb cool a bit.
• Replace lamp base plate and set lamp upright.
• Replace fiber optic adapter on lamp lens and attach source fiber.
• Put source on detector and use picoammeter to tweak up measured intensity by turning the thumbscrews on the base of the lamp housing (you can also try adjusting the focus). You should be able to get at least 30 uA.
• Find trimpot R38: it is accessed by a through hole in the bottom panel of the lamp controller, near the A/C cord.
• Adjust trimpot R38 until the voltmeter reads 0.415 VDC. (This is a shortcut; the original instructions said to adjust it for 0.425 VDC with the intensity trim control set to maximum. You can do that, but be sure to turn the intensity trim control back down to 1/2 afterwards.)
• Turn off lamp.
• Replace top of lamp housing.
• Replace lamp on base.
• Let bulb burn in for 1/2 hour before using (as usual).

Document History

• 5: Improved the section Adjusting the Photodiode. 2010-03-01 R. Owen.
• 4: Added section Replacing the Bulb. 2009-06-11 R. Owen.
• 3: Updated for SDSS-III system. Improved instructions for centering photodiode (in particular if you can only find one edge). Removed spaces from html anchor names. 2008-01-12 R. Owen.
• 2: Made compliant with HTML 4.01 Transitional. 2008-06-02 R. Owen
• 1: First release, for SDSS (software written in LabVIEW). 1997-04-16 R. Owen