SITCOMTN-152

Collimated Beam Projector Installation and ComCam Testing#

Abstract

Report on the initial installation and testing of the laser and Collimated Beam Projector during the ComCam campaign.

Introduction#

The Collimated Beam Projector (CBP) is a Schmidt Reflector run in reverse. We use an Ekspla tunable laser connected to an integrating sphere to illuminate the CBP focal plane. We design custom masks that we place at the focal plane and then CBP sends out collimated beams of light that can be reimaged by ComCam. The beamsize is 240 mm, so we illuminate only part of the Simonyi Telescope mirror. However, by doing a copointing dance, we can illuminate any part of the mirror and any point on the focal plane. A NIST calibrated photodiode in the integrating sphere allows us to know exactly how much light we are sending into the telescope, allowing us to perform very precise photometry of our spots.

The main goal of the CBP is to measure the throughput of collimated light with high precision, but more generally with the CBP we can:

  • Measure the wavelength-dependent throughput of collimated light.

  • Look for reflectance and transmission variations across the mirrors and filters.

  • Study ghosting.

  • Measure cross-talk nonlinearity.

  • Look at pointing offsets created on parts of the mirrors when AOS applies bending mode corrections.

CBP Design#

The CBP, a Schmidt reflector, has a focal length of 625 mm and an aperture of 240 mm. It has slots for five different masks. We use an Ekspla NT242 tunable laser fiber fed into a 6” integrating sphere to illumminate the focal plane. A NIST-calibrated photodiode in the integrating sphere is read out by an electrometer and can then be used to determine the amount of flux sent to the Simonyi Telescope (see SITCOMTN-106).

_images/CBP_preshipment.png _images/mask_holders.png

Figure 1: The CBP in the lab in Tuscon before shipment (left) and the CBP mask holder (right).

CBP Repair#

The CBP arrived on the summit badly damaged. The dew shield that held the Schmidt corrector fell off during transit, many screws came loose, the mask changer stage drive arm broke, the azimuth drive shaft was damaged, the azimuth and elevation disks were damaged, and there were metal shavings and sawdust everywhere.

_images/damage_body.png _images/damage_screws.png _images/damage_mask_changer.png

Figure 2: The CBP arrived on summit damaged. From left to right, the dew shield fell off, screws came out, and the mask changer stage arm broke.

This damage is consistent with one hard drop during shipping and with the CBP crate having spent many hours on a truck on gravel roads with poor suspension. Fortunately, although the planar mirror was superficially scratched, there was no major damage to the optics, and thus we were able to repair the CBP. We transported the CBP back down to La Serena, where we disassembled the CBP. The machine shops in La Serena and Tucson made replacements for the broken parts, and added many new holes for screws both to replace the stripped holes and to make the CBP more secure in case of future earthquakes. We finished the repair in La Serena before transporting it to the summit.

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figure:: images/CBP_dome_front.png :width: 400px

Figure 3: The repaired CBP. Notice that there are many more screws visible around the dew shield.

CBP and Laser Installation on Dome#

Once the CBP was repaired, it was driven to the summit and then after functional tests and throughput measurements with the laser, it was installed on the dome.

_images/CBP_dome_side.png _images/CBP_dome_top.png
_images/CBP_point_telescope.png _images/laser_dome.png

Figure 4: The CBP and laser installed on the dome. Top row shows the CBP in the dome from different angles. Bottom row shows the CBP and telescope pointing at each other (left) and the laser installed on the dome (right).

Mask Design and Installation#

The magnification factor of the CBP mask to the ComCam image is a ratio of the focal lengths, so approximately a factor of 16.

For ComCam, we left one slot empty, and we filled the remaining four slots with four different masks. One mask is a 1mm pinhole from Thorlabs, which we will use with a calibration system to calibrate the CBP to an external source, but did not use during the ComCam campaign. The other three masks are custom designed 1” photolithographic masks that we used for tests during ComCam. They are:

  1. A single 100 um pinhole for every amplifier on ComCam, offset so that when properly aligned the crosstalk does not intersect with any other pinhole.

  2. A single 150 um pinhole for every CCD on ComCam.

  3. A test mask with two lines of pinholes with sizes 10 um, 20 um, 50 um, 100 um, 150 um, 200 um, 250 um, 300 um, 500 um, and 1 mm.

_images/one_per_amp_mask.png _images/two_line_mask.png

Figure 5: The one-pinhole-per-amp mask illuminated with a blue LED (left) and the design for the mask with two lines of pinholes of varying sizes (right).

CBP Testing with ComCam#

With heroic effort from the electronics, electrical, IT, and construction teams, we were able to install the CBP and laser, hook them up, and then take data with the CBP during the last four nights of the ComCam campaign.

Here is a list of the major tests and milestones we were able to achieve:

  1. We were able to copoint the CBP and the Simonyi Telescope.

  2. We demonstrated that the copointing locations were repeatable from night to night even as the dome, TMA, and CBP had moved during the day.

  3. We swept through focus for all of our masks.

  4. We made small movements of the CBP and TMA to see how repeatable and accurate the movements were.

  5. We performed a copointing ‘dance’ where we moved the CBP and telescope at the same time and kept the CBP spots on ComCam.

  6. We swept through wavelengths in the g and r bands, to find our in-band, out-of-band, and band-edge throughput. We observed a red leak in the g band.

  7. We took images to investigate nonlinearity in the crosstalk.

  8. We took images while applying different AOS bending modes and saw how the spots moved.

  9. We looked at diffraction off the channel stops from red light.

_images/one_per_amp_image.png _images/two_lines_image.png

Figure 6: The one-pinhole-per-amp imaged on ComCam (left) and the two lines of pinholes of varying sizes imaged on ComCam (right).

_images/one_per_CCD_image.png _images/filter_throughput.png

Figure 7: The one-pinhole-per-CCD mask with a scaling so that the ghosts are clearly visible (left) and (right) the filter transmission curves for g and r band, calculated by doing photometry on images like the left. Photometry of spots with the filter in were divided by photometry of the spots with the filter removed.

Summary and Next Steps#

Overall, the testing campaign with ComCam was very successful. We were able to demonstrate that the CBP works and to take the data that we need to design the masks we want for LSSTCam. We also took data to find out exactly what the relevant

We are currently in the process of analyzing the data. We are also working on software to make the process of data collection more automated during LSSTCam.