The purpose of this post is to show the alignment procedure of an on-axis parabolic mirror. The methodology has been applied to align the collimator of two of our fibre linked spectrographs:
- LECHES which uses the full parabola and
- FLECHAS where the collimator works in an off-axis parabola. In this case we have used a full parabola because it is cheaper than a dedicated off-axis mirror.
An off-axis parabolic (OAP) mirror consists of a small section cut out from a larger, so-called “parent” parabolic mirror
List of used material:
- Fibre: Multi-mode – Ø50 µm, 0.22 NA, L 5 meters with SMA connectors
- Parabolic mirror: Edmund Optics Ref. NT32-065-533 Ø 4.25″ (108 mm). FL 17.5″ (444 mm)
- Optical target (circles on paper mask)
- Flat mirror: Edmund Optics Ref. NT69-251 76.2 diam. Surface Accuracy λ/ 4-6
- Flat mirror Ø 100 mm with a Ø 15mm central aperture
- Mirror Support: Newport EQ120-E Outer Bracket 90°
- Green laser 532 nm < 5 mW, preferably with regulated power supply
- Teleobjective f:500 mm, Ø 67 mm 1:8, e.g. Walimex or Beroflex with adapter to 1.25″ eyepiece
- Eyepiece f:20mm for teleobjective
- Optical table. E.g. Thorlabs PBG52511 – Breadboard – 1200 x 750 x 55 mm
The alignment was highly simplified by using a high quality (better than λ) mirror with a central aperture:
The steps we followed were:
- setting an optical axis with a laser beam
- alignment of the centre of the parabolic mirror with the axis of the laser beam,
- alignment of the hollow flat mirror in front of the parabolic mirror and perpendicular to the optical axis
- alignment of the fibre by auto collimation with the hollow mirror
- Preparation of the paper mask (optical target). Concentric circles were printed on a piece of paper where the outer circle matched exactly the diameter of the parabolic mirror (Ø 108 mm). Each concentric ring corresponded to specific F/#. In a parabola of 444 mm of focal length, a circle with Ø 44.4 mm represents an F10, another circle of Ø 55.5 mm for an F8 , Ø 74 mm for an F6 and so on.
- Cutting the paper mask. Centre the mirror with respect to the mask and with its face down to avoid touching the aluminium surface. With care and a very sharp knife cut half a diameter of the mirror. In the very same centre make a hole of Ø 2 mm.
- Mount the parabolic mirror on its support. Fix the parabolic mirror on its final support perpendicular to the optical table and hang the paper mask over it. The small hole Ø 2mm in the mask defines the centre of the mirror. We assume that the vertex of the parabola is on the centre of the mirror!
- Definition of the optical axis. The laser was mounted on a support containing X, Y displacements (green) and tip-tilt (yellow). The laser beam should be parallel to the optical table and its heigh must be the one defined by the centre of the parabola.
- Fix a ruler to the optical table and parallel to one side. Another ruler in “L” shape will be used to mark the height of the axis marked by the laser beam. This ruler will slide along the other ruler fixed to the table. Adjust the position in X, Y (red) of the laser so that the laser hits the edge of the “L” ruler along the fixed ruler. Adjust tilt (yellow) and height (green) until the laser beam hits the marked height in the “L” ruler along the fixed ruler. Once the axis is set, clamp the laser mount to the table.
- Alignment of the parabola with the laser. Remove the rulers and set the mirror assembly at the other end of the optical table. Set the paper mask on the mirror and adjust the position of the mount until the laser beam hits the hole in the centre of the paper mask. The laser beam is now reflected. A fine adjustment of the height of the laser beam is permitted until the laser beam hits with precision the very same centre of the mirror.
- While keeping the laser beam in the centre, tilt and tip the mirror assembly until the reflected laser beam returns in the same axis (auto-collimation). Once the auto-collimation is achieved clamp the mirror assembly to the table.
- Alignment of the hollow mirror. Set the flat mirror facing the parabolic mirror and at a distance beyond the focal length (444 mm) of the parabola, so that it will not interfere later with the fibre output and its support.
- Adjust the height of the mirror to pass the laser through the central hole. Place a small flat mirror in the centre of the hole. Tilt and tip the mirror to adjust the perpendicularity with the laser beam (auto-collimation with respect to the laser beam.
- Alignment of the optical fibre. Inject the laser beam on one end of the fibre. To increase the flux of the laser into the fibre we used a microscope objective: first, you place the fibre input end in front of the laser beam, place the microscope objective (< 10X) between the two with the entrance end of the objective pointing to the fibre. Adjust its high and lateral position until the output beam is centred with respect to the fibre. Approach the objective to the fibre to focus the beam into the fibre. For that look through the objective and look for the image of the fibre end. DO NOT look at the laser beam!! On the other fibre end we glued a small white screen on the SMA connector and made a small hole just in front of the fibre.
- The output fibre end was mounted on a mechanical support allowing tilt-tip and height adjustments. The fibre assembly is set in front of the parabolic mirror at a distance close to its focal length (444 mm). Use the laser beam projected on the paper mask to coarse adjust the fibre output by tip-tilt.
- Remove the paper mask from the parabola, the laser beam is now reflected from the parabolic mirror toward the flat mirror behind the fibre. And again back from the flat mirror toward the parabolic mirror and reflected back again toward the fibre. On the small white screen you should observe the image of the fibre. Adjust focus and lateral movements of the fibre output until the reflection concurs with the fibre position.
- To reach the sharpest focus position along the axis the best method is to visualize the image of the fibre output with an objective set at infinite and acting as a collimator. We have a 500mm focal distance telephoto objective where we put a reticle on the focal plane and an eye-piece t see the reticle.The focal length of the objective should be larger than the one of the parabola (444 mm) in order to increase the accuracy of the focusing. The calibrated objective was put just behind the fibre assembly and aligned in height with the axis. With a ruler along the optical axis (laser beam) we displaced the fibre until its image observed through the objective is in focus. When achieved we clamp the fibre assembly to the the optical table.
- Another method to achieve focus position can be obtained with a paper mask with two holes converting our on-axis parabolic mirror in two smaller off-axis parabolic mirrors. Each of these parabolic mirrors will produce individual images of the fibre.
- By displacing the fibre assembly along the axis, the focus position will be reached when both images coincide together with the same position of the fibre. Once this is achieved clamp the fibre assembly to the optical table. At this point the alignment is complete.
Below a gallery of pictures taken during the alignment of our collimator for our LECHES spectrograph
Carlos Guirao and Gerardo Avila
Alignment of an on-axis parabolic mirror by CAOS group is an article licensed under a Creative Commons Attribution-Non-Commercial-No Derivative Works 3.0 Germany License.
Based on a work at spectroscopy.wordpress.com