Astronomy

University of Michigan

Dr. Nathalie Thureau

phone: 734 615 1583

fax: 734 763 6317

email: nthureau@umich.edu

February 16, 2005

To: Optical manufacturers.

Ref: Request for quote, MIRC spectrograph Optical Fabrication.

This is a request for a quotation on the fabrication of custom lenses for an infrared spectrograph, which we are currently building for the 6 telescope CHARA interferometer on Mount Wilson, California. This infrared spectrograph, called the Michigan Infra-Red Combiner (MIRC), is a fiber based instrument optimized to work in the H spectral band. Funds for completion are available immediately.

The combiner includes two air spaced achromat, each composed of a biconvex CaF2 and a Fused Silica negative meniscus. Included with this letter are drawings of each element and specifications for their fabrication. The lenses are 25mm in diameter. Because the optical quality of the lenses will be critical to the performance of the instrument, we will require a report of he final testing and measurements of the lenses, including interferograms or other data characterizing the finished elements.

We designed these lenses using Zemax and did a test-plate fitting based on the ISP test-plate list available on ISP's website. We would like to make sure that the test-plates for the radii we have chosen are available. Additionally, we would like to have the measured glass parameters in order to update our zemax lens design.

We are particularly interested in the following issues: (1) the fabrication method you will use to generate, grind and polish the elements; (2) the surface figure accuracy and finish quality which you can confidently deliver; and (3) the testing methods and equipment available in-house to document the characteristics on the finished elements. Please note that we expect all the elements to be coated in the same batch.

Please feel free to comment on any aspects of our specifications which appear particularly difficult or costly to achieve, or which would drive up the cost without contributing significantly to the optical performance. We particularly would like to know if any of our specifications will require the use of manufacturing techniques which will significantly increase the cost. We will expect the fabricated element to adhere to agreed-upon specifications on a best-effort basis. If you intend to subcontract any aspects of the production, please state so explicitly.

Element	Material	Diameter mm	R1 mm	R2 mm	Quantity
1	CaF2	25	57.687	46.244	3
2	Fused Silica	25	37.667	91.204	3
1-2.4μm AR coating that withstands cryogenic temperatures, with spectral performances @ 1-2.4μm Transmission: 98% average; Reflection: 1.0% average per surface

Please comment on the fabrication time needed and provide a fixed price quote on all the costs related to the production and delivery. We expect the quote to include separate prices for:

the glass blanks,

the manufacturing of the lenses,

the coating,

shipping and handling.

We would appreciate receiving the quote within a week time due to a tight schedule on our project. Please do not hesitate to contact me for clarification of the above request or if anything mentioned requires discussion. I look forward to hearing from you.

Best regards,

Nathalie Thureau.

LENS 1

Central thickness	5mm Radius 1	57.687mm Radius 2	46.244mm Diameter	25mm

Material:	IR grade CaF2 Surface Figure:	λ/20 at 633nm – Document and report final	figure. Radius Tolerance	± 0.06% Surface Quality:	60-40 Surface finish:	micro ripple 1nm RMS or better. Document	and report measured values. Diameter Tolerance:	+0/- 0.1mm Thickness Tolerance:	± 0.075mm. Report to ±0.010mm Concentricity:	± 0.013mm maximum runout. Report	measured value. Bevel:	0.25mm at 45° Clear Aperture	Exceeds central 90% of dimension Coating:	AR 1-2.4μm coating

LENS 2

Central thickness	3mm Radius 1	37.667mm Radius 2	91.204mm Diameter	25mm

Material:	IR grade Fused Silica Surface Figure:	λ/20 at 633nm – Document and report final	figure. Radius Tolerance	± 0.06% Surface Quality:	60-40 Surface finish:	micro ripple 1nm RMS or better. Document	and report measured values. Diameter Tolerance:	+0/- 0.1mm Thickness Tolerance:	± 0.075mm. Report to ±0.010mm Concentricity:	± 0.013mm maximum runout. Report	measured value. Bevel:	0.25mm at 45° Clear Aperture	Exceeds central 90% of dimension Coating:	AR 1-2.4μm coating

Regarding glass properties:

Striae grade A. per Mil-G-174A. Report bubble quality group.

Index homogeneity (n) = ±2x10-5 or better.

Refractive index measurements (to the 6th decimal place) within 0.8-2.4μm wavelength range. Record temperature with index measurements.

Duplicate blanks from the same melt and anneal.

Fine anneal. Maximum birefringence 10nm/cm.

Internal transmission data within 0.8-2.4μm wavelength range.

Nathalie Thureau, University of Michigan, Astronomy Dpt, 913 Dennison building, 500 Church St, Ann Arbor, MI 48109.

2005-01-26T14:29:00