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Title: Measuring the Flatness of Focal Plane for Very Large Mosaic CCD Camera

Abstract

Large mosaic multiCCD camera is the key instrument for modern digital sky survey. DECam is an extremely red sensitive 520 Megapixel camera designed for the incoming Dark Energy Survey (DES). It is consist of sixty two 4k x 2k and twelve 2k x 2k 250-micron thick fully-depleted CCDs, with a focal plane of 44 cm in diameter and a field of view of 2.2 square degree. It will be attached to the Blanco 4-meter telescope at CTIO. The DES will cover 5000 square-degrees of the southern galactic cap in 5 color bands (g, r, i, z, Y) in 5 years starting from 2011. To achieve the science goal of constraining the Dark Energy evolution, stringent requirements are laid down for the design of DECam. Among them, the flatness of the focal plane needs to be controlled within a 60-micron envelope in order to achieve the specified PSF variation limit. It is very challenging to measure the flatness of the focal plane to such precision when it is placed in a high vacuum dewar at 173 K. We developed two image based techniques to measure the flatness of the focal plane. By imaging a regular grid of dots on the focalmore » plane, the CCD offset along the optical axis is converted to the variation the grid spacings at different positions on the focal plane. After extracting the patterns and comparing the change in spacings, we can measure the flatness to high precision. In method 1, the regular dots are kept in high sub micron precision and cover the whole focal plane. In method 2, no high precision for the grid is required. Instead, we use a precise XY stage moves the pattern across the whole focal plane and comparing the variations of the spacing when it is imaged by different CCDs. Simulation and real measurements show that the two methods work very well for our purpose, and are in good agreement with the direct optical measurements.« less

Authors:
; ; ; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
984640
Report Number(s):
FERMILAB-CONF-10-191-A
TRN: US1005953
DOE Contract Number:
AC02-07CH11359
Resource Type:
Conference
Resource Relation:
Journal Name: Submitted to Proc.SPIE Int.Soc.Opt.Eng.; Conference: Presented at SPIE Astronomical Instrumentation, San Diego, California, 27 June -2 July 2010
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; ACCURACY; CAMERAS; COLOR; DESIGN; DEWARS; SIMULATION; SKY; TELESCOPES; Accelerators

Citation Formats

Hao, Jiangang, Estrada, Juan, Cease, Herman, Diehl, H.Thomas, Flaugher, Brenna L., Kubik, Donna, Kuk, Keivin, Kuropatkine, Nickolai, Lin, Huan, Montes, Jorge, Scarpine, Vic, and /Fermilab. Measuring the Flatness of Focal Plane for Very Large Mosaic CCD Camera. United States: N. p., 2010. Web.
Hao, Jiangang, Estrada, Juan, Cease, Herman, Diehl, H.Thomas, Flaugher, Brenna L., Kubik, Donna, Kuk, Keivin, Kuropatkine, Nickolai, Lin, Huan, Montes, Jorge, Scarpine, Vic, & /Fermilab. Measuring the Flatness of Focal Plane for Very Large Mosaic CCD Camera. United States.
Hao, Jiangang, Estrada, Juan, Cease, Herman, Diehl, H.Thomas, Flaugher, Brenna L., Kubik, Donna, Kuk, Keivin, Kuropatkine, Nickolai, Lin, Huan, Montes, Jorge, Scarpine, Vic, and /Fermilab. 2010. "Measuring the Flatness of Focal Plane for Very Large Mosaic CCD Camera". United States. doi:. https://www.osti.gov/servlets/purl/984640.
@article{osti_984640,
title = {Measuring the Flatness of Focal Plane for Very Large Mosaic CCD Camera},
author = {Hao, Jiangang and Estrada, Juan and Cease, Herman and Diehl, H.Thomas and Flaugher, Brenna L. and Kubik, Donna and Kuk, Keivin and Kuropatkine, Nickolai and Lin, Huan and Montes, Jorge and Scarpine, Vic and /Fermilab},
abstractNote = {Large mosaic multiCCD camera is the key instrument for modern digital sky survey. DECam is an extremely red sensitive 520 Megapixel camera designed for the incoming Dark Energy Survey (DES). It is consist of sixty two 4k x 2k and twelve 2k x 2k 250-micron thick fully-depleted CCDs, with a focal plane of 44 cm in diameter and a field of view of 2.2 square degree. It will be attached to the Blanco 4-meter telescope at CTIO. The DES will cover 5000 square-degrees of the southern galactic cap in 5 color bands (g, r, i, z, Y) in 5 years starting from 2011. To achieve the science goal of constraining the Dark Energy evolution, stringent requirements are laid down for the design of DECam. Among them, the flatness of the focal plane needs to be controlled within a 60-micron envelope in order to achieve the specified PSF variation limit. It is very challenging to measure the flatness of the focal plane to such precision when it is placed in a high vacuum dewar at 173 K. We developed two image based techniques to measure the flatness of the focal plane. By imaging a regular grid of dots on the focal plane, the CCD offset along the optical axis is converted to the variation the grid spacings at different positions on the focal plane. After extracting the patterns and comparing the change in spacings, we can measure the flatness to high precision. In method 1, the regular dots are kept in high sub micron precision and cover the whole focal plane. In method 2, no high precision for the grid is required. Instead, we use a precise XY stage moves the pattern across the whole focal plane and comparing the variations of the spacing when it is imaged by different CCDs. Simulation and real measurements show that the two methods work very well for our purpose, and are in good agreement with the direct optical measurements.},
doi = {},
journal = {Submitted to Proc.SPIE Int.Soc.Opt.Eng.},
number = ,
volume = ,
place = {United States},
year = 2010,
month = 6
}

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  • Meeting the science goals for the Large Synoptic Survey Telescope (LSST) translates into a demanding set of imaging performance requirements for the optical system over a wide (3.5{sup o}) field of view. In turn, meeting those imaging requirements necessitates maintaining precise control of the focal plane surface (10 {micro}m P-V) over the entire field of view (640 mm diameter) at the operating temperature (T {approx} -100 C) and over the operational elevation angle range. We briefly describe the hierarchical design approach for the LSST Camera focal plane and the baseline design for assembling the flat focal plane at room temperature.more » Preliminary results of gravity load and thermal distortion calculations are provided, and early metrological verification of candidate materials under cold thermal conditions are presented. A detailed, generalized method for stitching together sparse metrology data originating from differential, non-contact metrological data acquisition spanning multiple (non-continuous) sensor surfaces making up the focal plane, is described and demonstrated. Finally, we describe some in situ alignment verification alternatives, some of which may be integrated into the camera's focal plane.« less
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  • In this note I describe an inexpensive and simple laser-based method to measure the flatness of the LSST focal plane assembly (FPA) in situ, i.e. while the FPA is inside its cryostat, at -100 C and under vacuum. The method may also allow measurement of the distance of the FPA to lens L3, and may be sensitive enough to measure gravity- and pressure-induced deformations of L3 as well. The accuracy of the method shows promise to be better than 1 micron.
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