DECal

DECal is a spectrophotometric calibration system for the DES DECam project at the Blanco 4-meter at CTIO. We successfully deployed a prototype of this system at the Swope and DuPont telescopes at Las Campanas Observatory. This provided data to help reduce the systematic error budget of the Carnegie Supernova Project.

Design

The system uses a 2nm wide tunable source to measure the instrumental response function of the telescope from 300nm up to 1100nm. A monochromator based tunable light source provides illumination on a dome flat. The dome flat is monitored by calibrated photodiodes to measure the throughput as a function of wavelength.

DeCal_layout_2 — Layout for the DES Calibration Project

Light Source

DECal operates continuously over a very wide wavelength range (300 nm < λ < 1100 nm). We use two light sources to cover the entire wavelength range. A 150W compact xenon arc lamp light source (Optical Building Blocs Corporation, PowerArc series) covers the region from 300 to 800nm. A 250W Quartz Tungsten Halogen lamp (Horiba, LSH-T250) is used to cover the infrared from 800nm to 1100nm.

Monochromator

We use a fully automated f/4.1 Czerny-Turner monochromator (Horiba, iHR-320) having a high reciprocal dispersion of 2.3nm/mm when used with a 1200g/mm grating. We adjust the bandwidth of the light by varying the input slit width. The output slit width is fixed to 900 microns (the width of the linear fiber bundle). The monochromator has two input ports that host the two lamps; a flip mirror allows us to change from one lamp to the other without interruption.

JP_2 — J-P Rheault and Steven Villanueva working on the DECal electronics

Fiber Bundle

The fiber bundle consists of 87 fibers with 300 micron cores. At the input end, the fibers are arranged in 3 parallel lines of 29 fibers. The bundle brings the light from the calibration room (located under the telescope main floor) up to the telescope top ring. At a length of 65 meters, the fiber bundle splits into 4 branches, each 10 meters long and containing 21 fibers. These 21 fibers are placed in a compact circular arrangement and coupled to the projection system. Each of the 4 branches sample the monochromator slit evenly to ensure that the four projection units each have the same intensity and spectral content.

DeCal_fiber_slit_ends — Fiber bundle slit end

Projection System

A projection unit containing the fiber projection optics, the photodiodes and a LED cone for the daily flat field system was built specifically for DECal. 4 of these units are installed on the top ring of the telescope and allow us to illuminate the dome flat field screen in a uniform fashion. The fiber projection unit uses an Engineered diffuser from RPC Photonics that projects the light uniformly into a 20 degree cone. The photodiodes monitor the light from the screen. LED cones are included on the units to project flat-field light for daily flat-field calibration.

Photodiodes Modules

The 4 reference photodiodes are located in the photodiodes modules. We use 10mm diameter silicon photodiodes (Hamamatsu, S2281) to measure the light from 300nm to 1100nm. The photodiodes have a NIST-traceable calibration performed by Gentec-EO. We anticipate signals on the order of 1 nW on the photodiode. These signals are amplified by a two-stage, low noise, trans-impedance amplifier with a variable gain from 108 to 1011. The amplifier has a cut-off frequency around 20 Hz to reduce noise levels. The amplified signal is read by a 16-bit analog-to-digital converter (ADC, National Instruments, ENET-9215) acquiring signal at 100 kHz. The signal is averaged to 100Hz in the ADC and sent over Ethernet to the control computer where data are saved and analyzed.

Screen illumination uniformity

A ZEMAX non-sequential model of the illumination pattern from 4 fiber bundles using the 20° engineered diffusers was made to evaluate the illumination uniformity. In the model, the fibers are placed on the top ring of the telescope, 2.5 meters from the telescope axis; the distance from the flat field screen from the telescope ring is 7 meters. The figure below shows the results of the model. It confirms that the screen illumination should be uniform to 95% in the central 4 meters of the screen.

Flat Field Screen

The flat field screen is made of a lightweight aluminum honeycomb panel coated with a highly reflective and almost perfectly Lambertian coating called Duraflect from Labsphere. It is a coating that was developed for use in integrating spheres. The spectral reflectivity of the Duraflect coating is excellent with reflectivity over 95% from 350 to 1200nm and over 85% from 300 to 2200nm. The screen was shipped to Chile in Summer 2011, and was installed in November 2011.

DeCal_Screen — J-P Rheault working on the screen for DECal

decal_screen_ctio — Screen installed at CTIO

Software and automation

DECalS (Dark Energy camera CALibration Software) is a LabVIEW software that has a user-friendly interface that controls the different components of the system with minimal intervention required by the telescope operator or observer.

Prototype

Before being installed on the 4-meter Blanco telescope at CTIO, DECal was installed as a prototype on the Henrietta Swope 1-m and the Irénée du Pont 2.5m telescopes at Las Campanas Observatory as part of the Carnegie Supernova Project.

Publications

Calibrating DECam Data: the DECal and aTmCam Calibrations Systems
Marshall, J.L., Li, T.,2015
(PDF) (Poster)

Spectrophotometric Calibration System for DECam
J.-P. Rheault, D. L. DePoy, J. L. Marshall, T. Prochaska, R. Allen, J. Wise, E. Martin, P. Williams; 2012SPIE.8446E..6MR
(PDF) (Online) (Poster)

DECal: A Spectrophotometric Calibration System For DECam
J. L. Marshall, Jean-Philippe Rheault, D. L. DePoy, Travis Prochaska, Richard Allen, Tyler W. Behm, Emily C. Martin, Brannon Veal, Steven Villanueva, Jr., Patrick Williams, and Jason Wise; arXiv:1302.5720
(PDF) (Online)

DECal: A Spectrophotometric Calibration System For DECam
Jean-Philippe Rheault, D. L. DePoy, J. L. Marshall, T. Prochaska, R. Allen, J. Wise, E. Martin 2012 AAS
(Abstract) (Poster)

The DECalS Software for the Dark Energy Survey Spectrophotometric Calibration System
Jason Wise, J. P. Rheault, D. L. DePoy 2012 AAS
(Abstract) (Poster)

Spectrophotometric Calibration System for DECam
Rheault, Jean-Philippe; DePoy, D.; Marshall, J.; Carona, D.; Cook, K.; Behm, T.; Allen, R. 2011 AAS 21743309
(PDF) (Poster)

Spectrophotometric calibration system for DECam
Rheault, J.-P.; Depoy, D. L.; Behm, T. W.; Kylberg, E. W.; Cabral, Kris; Allen, Rick; Marshall, J. L. Proc. SPIE, Vol. 7735, 201
(PDF)