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Testing the homogeneity of the Hereaus-Infrasil 301 blank for the GIANO Cross-Dispersing Prism

P. Sandri (1) Gestione SILO srl, Scandicci Firenze.

( 1)

, E. Oliva

(2)

(2) INAF, Telescopio Nazionale Galileo INAF, Santa Cruz de La Palma, Spain. In this report we briefly summarise the results of two interferometric measurements performed on the Heraeus-Infrasil 301 blank for the GIANO Cross Dispersing Prism. We have investigated the 160mm-long side. In fig. 1 we report the interferogram acquired in a double pass configuration (as described in the following fig. 2). The blank has been polished on two opposite sides and then sandwiched between two fused silica 0.1 PV on a 100mm clear aperture plates. The matching between the infrasil 301 blank and the two fused silica plates has been done by means of a mixture of glycerine (88%) and distilled water (12%) which is expected to have a refractive index of 1.4547, very close to the refractive index of fused silica [Ref. 1].

Fig. 1: Interferogram of the Heraeus-Infrasil 301 blank. The interferogram has been obtained in a double pass configuration as better described in the following fig. 2.

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8 8 % g lycerin 1 2 % d istill ed water TF Zyg o

RF Zy go

Fizeau d ig it al in terfero meter 1 00 mm beam

H ereau s Infrasil 3 01 blank

p l at e 1 ­ fu sed silica 16 0 mm

p l at e 2 ­ fu sed sil ica

Fig. 2: Schematic arrangement adopted for testing of the homogeneity the Hereaus-Infrasil 301 blank.

Fig. 3: First measurement: Measured wavefront in double pass configuration.

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Fig. 4: First measurement: Measured Zernike Polynomials.

Fig. 5: Second measurement: Measured wavefront in double pass configuration.

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Fig. 6: Second measurement: Measured Zernike Polynomials.

From the above measurements we can estimate, with a good approximation, a homogeneity of [Ref. 2]:

n =

k 0.32 = 6.3E - 7 = d 2t 320

being k/d the deformation, and 2t the travel inside the blank.

References [Ref. 1] etd.rau.ac.za/theses/available/etd-12102003-110543/restricted/ CHAPTER6PhDrevised8.pdf table 6.4 page 25. [Ref. 2] M.V. Mantravadi in D. Malacara Optical Shop Testing Second Edition. Chap. 1.2.8, John Wiley Inc.

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