DFPS Spectrograph

Undergraduate project: Mason Jelken

Our lab partnered with Open Source Instruments (OSI) to test their Direct Fiber Positioning System (DFPS) prototype at McDonald Observatory. We repurposed an ORIEL MS125J spectrograph and Teledyne Kuro sCMOS detector to collect spectra for up to four objects at a time, one spectra for each fiber of the DFPS prototype. The readout of the detector is then sent through a custom data pipeline to extract the unique spectra from each object. This data can be used to determine the four objects’ relative elemental abundances via observations of emission and absorption features that give insight on the formation and behavior of these celestial bodies.

DFPS Prototype at McDonald Observatory 82" Telescope

On October 11-15, 2024, we observed at the McDonald Observatory 82" Otto Struve telescope with the DFPS prototype and spectrograph. The DFPS prototype controls the positions of four optical fibers using piezoelectric ceramic cylinders attached to carbon fiber masts that hold the fibers. Four guide sensors in a box around the fibers monitor reference stars in the sky. Two monitoring cameras point inward at the fibers to monitor their positions. The fibers exit the DFPS prototype and enter the spectrograph.

20241011_091717 — Inside DFPS prototype. The fibers are held in carbon fiber tubes that are installed in ceramic piezoelectric actuators. Applying voltage to the piezoelectric actuators moves the fibers precise distances.

muCyg_5s — DFPS first light. Spectrum of Mu Cygni in fiber 3.

Spectrograph Hardware

Our part in the prototype testing was assembling the spectrograph, done by undergraduate Mason Jelken. There are three main hardware components for this system. The spectrograph to spread the light into its spectra, the detector to collect the light information from the spectrograph, and a fiber-optic USB relay device. The spectrograph is an ORIEL MS125J spectrograph, a compact system that has an adjustment screw to change the bounds of the spectra. The original optics were made to observe only one source at a time, so a cylindrical lens is added to reduce effects of the inherent astigmatism in the images. The detector is a KURO 2048B sCMOS detector from Teledyne Princeton Instruments. The fiber-optic USB relay device allows for control and data collection at a greater distance. These components are joined via two aluminum plates designed and fabricated in the lab that also mounts this system to the OSI DFPS.

osidfps_hardware — Spectrometer with the spectrograph (a), sCMOS detector(b), and optical relay (c) ready for testing in the lab.

SOLIDWORKS assembly of the completed spectrograph attached to the OSI Direct Fiber Positioning System (DFPS) instrument.

Spectrograph Software

The KURO 2048B detector utilized for data collection was also used on ETSI, so the ETSI control code is repurposed to control the spectrograph's detector. Additional capabilities such as viewing single exposures is added with basic data reduction methods; i.e., accounting for dark current. To accurately obtain information about the spectra observed, a relation between the position of an emission line on the detector, the wavelength of the line, and the setting on the spectrograph’s micrometer have to be found. This is done by observing the readings of a 650nm and a 780nm laser at a range of settings for the micrometer.

Experimental setup for obtaining the scale of the system through the use of two lasers of a known wavelength. Light from the lasers are sent through an integrating sphere to reduce variability in the experiments.

The pixel-wavelength relation is found by linear interpolation in the placement of the two peaks. The wavelength-micrometer relation is found through fine adjustments of the micrometer. This relation is extrapolated and tested with a 450nm laser. A program for obtaining the desired setting on the micrometer adjustment screw allows the user to input the wavelength that should be in the center of the detector, and the program outputs the setting that best fits the desired wavelength.

osidfps_calibration — Result of the calibration, showing the relation between the micrometer setting and the wavelength response.

Image Correction

The ORIEL MS125J has an astigmatism in the optics as a tradeoff for its compact design, which is not an issue when viewing a single source. However, we need to view four sources. A cylindrical lens partially corrected this, and post-processing software corrections further improve image quality. Due to the unique properties of this system, a new correction algorithm is constructed for the best results. The correction results in an increase of spectral resolution by approximately 25%. A HgAr lamp was used for testing the correction software.

osidfps_correction — Detector reading of HgAr lamp centered at 740nm. The top frame is the original image image before running through the software. The bottom frame shows the corrected image.

Publications

Design and Fabrication of the Back-End for a Prototype Multi-Object Spectrograph, Mason D. Jelken, Luke M. Schmidt, 2024 AggieSTAAR and Texas A&M LAUNCH poster session