1.0 Overview
The advantage of long-slit spectroscopy have been known for decades and long-slit spectroscopy in the near-infrared is becoming a new and exciting area of investigation. The Mount Wilson and Mount Laguna Observatories make use of the University of Illinois (UIUC) and San Diego State University (SDSU) Near InfraRed IMaging camera (NIRIM) developed by Dr. Margaret Meixner of UIUC and Dr. Robert Leach of SDSU. This camera is designed around a 256x256 NICMOS array and includes the capacity for a large number of filters. At present this camera does not have spectrographic capability, but it does already have 4 broad-band photometric filters (I, J, H, K') that are ideal to be used for ordersorting in a spectrometer, and by replacing one of the existing narrow band filters with a grism the power of a spectrometer can be added.
In order to provide SDSU and UIUC astronomers with near-infrared spectrographic capabilities, a grism has been designed that can be inserted into NIRIM with very few modifications. This document describes the design of the grism and provides status information.
2.0 Designing the Grism
The grism was designed using an in-house program, Xspect that runs under unix. Xspect is an application of Snell's Law across each of the interfaces in the grism accounting for index of refraction changes, groove angles, determines the optimal prism angle for an undeflected specified, central wavelength and provides transmission efficiencies and a blaze function. The code was validated by developing a simple grism spectrometer from "off the shelf components" based on the specifications provided from Xspect.
The initial grism to be used in NIRIM has been designed more as a proof of concept rather than being optimized for specific scientific measurements. The grism will provide coverage for the 4 major band passes, I, J, H, K', using the existing photometry filters for order sorting.
The performance parameters for the grism within NIRIM have been set to provide a Seeing Limited Resolution: R > 500 over the 4 broadband regions. This was matched by the following design parameters for the grating and prism components of the grism. There is still some confusion over which material will be selected for the prism. Fused Silica (KUVI) provides good transmission characteristics over the wavelength range from 0.7 to 2.5 microns and seems to be the easiest material to work, with while ZnSe, BaF2, CaF2 all can be more challenging and can be harder to obtain making them less desirable options. Should the chosen manufacturer prefer any of these other materials the specifications below will be altered to accommodate their choice.
Prism:
KUVI fused silica (index
of refraction ~1.46 @ 656nm use 1.44 @ NIR)
size = 25mm disk by 22mm thick
prism angle = 35.6 degrees
thick end of prism = 20mm (so no sharp edge at the other end)
flatness = lambda/5 for lambda = 635nm
scratch and dig = 40-20
beveled edges (~0.5mm wide)
Grating:
NIR resin (Epotec 301-2) index of refraction 1.56
Grooves run perpendicular to the line defining the maximum prism
slope
Grooves per mm: 40 grooves per mm
1st order blaze: 22.5 microns
Blaze angle: 26o 45’
The builder of the grism has been selected as Diffraction Products, Inc. owned by Dr. Edward Leibhardt. I have used reflection gratings manufactured by DP, Inc. in the past and they made a wonderful visual range grism as part of the test project to qualify them for this project.
I have been accumulating more information about how to handle the issues surrounding using the grating resin and glass combinations at cryogenic temperatures. Discussions with Epotec confirms their belief that there will be no problems with their resins and the test information they sent to me supports this opinion. DP, Inc. has provided information about how to chemically 'heal' the naturally occurring fine cracks in the prims so that they will handle the thermal changes. Now it it time to see if all of this pans out in making the NIR grating..
3.0 Project Status
Optical development and construction phase has ended and grism is in its final testing. New holder being machined at the Mount Wilson machine shop. Anticipate ready for testing in NIRIM this summer. Observing planned for this fall or winter.
May 21, 2001: Mail out of the final project report.
May 17-20, 2001: Developed final project report for submission
to the AAS.
May 15, 2001: Received collimation tester. Used to collimate
the optical bench to test the NIR Grism in red light.
May 10, 2001: Testing grism found to be challenging due to limited
ability to collimate beam. Ordered shear plate from Edmund Scientific,
Inc. to knock out collimation issues.
May 8, 2001: Received grisms from DP, Inc. 1 was damaged in
packing the other appears good.
April 16-27, 2001: Spent considerable time working on optical
test system alignment, new spatial filters, better alignment fiducials,
etc. Test system operating well, simple shearing interferometer built,
not great for beam collimation as plate is a window not a wedge.
April 6, 2001: Sent 2 prisms to DP, Inc to have grating applied.
March 30, 2001: Testing that new prisms meet specifications
before having grating applied.
March 29, 2001: Received prisms from Almaz Optics, contacted
DP, Inc. to apply grating.
Mid February - mid March: Ported Xspect to Linux and optimized
and tested and optimized and tested.
February 2, 2001: GROUNDHOG DAY!
January 30, 2001: Ordered prisms to be made by Almaz Optics.
January 29, 2001: Requested grant extension from AAS SMRG till
May 31, 2001 due to lead time for delivery of optical parts.
Janurary 28, 2001: Almaz Optics agrees to manufacture prisms
for very good price, 8 weeks delivery.
January 11, 2001: Original optician no-bids project.
December 20-30, 2000: TDY at MLO to increase NIRIM operation
experience on telescope.
December 18, 2000: Request quotation from optician for prisms
grinding.
December 11, 2000: 1" dia x 1" high KUVI blanks from AlmazOptics
arrive. (They are perfect!)
December 4, 2000: Order 1" dia x 1" high KUVI blanks from AlmazOptics.
November 30, 2000: Completed resimulations using Xspect for
NIR grism.
November 25, 2000: Visual grism loaded into test spectrometer.
Results are very positive.
November 22, 2000: Received lab space, unpacking optical bench
etc. Lots of cleaning and unpacking.
November 16-21, 2000: TDY at MWO & MLO for optical design/
testing work.
November 17, 2000: DP, Inc. delivers visual grism. (Beautiful
work!)
November 1, 2000: Job change, all the flurry of moving to NMT
introduces some new challenges.
October 27, 2000: Wedge prisms shipped to DP, Inc. to have grating
replicated on surface.
October 26, 2000: Components from Edmund Industrial Optics arrived
and are being checked.
October 17, 2000: Ordered components from Edmund Industrial
Optics for test system.
October 13, 2000: DP, Inc. provides quotation for test grism.
October 10, 2000: DP, Inc. recommemds several changes to the
quote to ensure quality product. RFP updated and submitted.
October 6, 2000: Requested official quote from DP, Inc. to coat
stock wedge prism.
September 24-25, 2000: TDY at MLO for NIRIM operation/ instructional
meeting.
September 19, 2000: Set up manufacturing test for DP, Inc. to
coat a stock wedge prism.
September 7, 2000: Designed a test system for evaluation of
grisms.
August 22, 2000: NASA SMRG funding received and the default
project completion date is set for February 16, 2001.
August 21, 2000: Quote for KUVI fused silica blanks received
from Almaz Optics.
August 20, 2000: Engineering drawings prepared for optical
components of grism. These will form basis for prism quote.
August 16, 2000: Almaz Optics, Inc of New Jersey, has agreed
to quote on KUVI fused silica blanks, may quote on finished prisms.
August 15, 2000: AAS announces that the grism spectrometer project
has been awarded a NASA SMRG.
August 3, 2000: Investigating whether a stock wedge prism of
BK7 would be suitable for use in this initial investigation, the transmission
above 1.6 microns would be marginally acceptable, but the cost 1/10th
of having one made makes it look significantly better.
August 2, 2000: DP, Inc. responds with a quote. For a very good
price a grating can be replicated onto a prism, but it would be expensive
to have the prism made.
July 25, 2000: Send out a request for quotation to DP, Inc.
to have them manufacture the grism.
July 24, 2000: Sent out a querry letter to DP, Inc. to see if
they could manufacture a grism. DPI, Inc. confirmed the same day.
Ancient History: Many conversations and emails with another
manufacture that went nowhere.
1Acknowledgement: This research was supported by a grant from NASA administered by the American Astronomical Society.
Copyright (c) 2000, Scott W. Teare
Created:July 27, 2000
Last Updated: May 21, 2001