first tests with vph grisms for afosc

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ISSN 1594-1906

Padova and Asiago Observatories

Technical Report n. 20

April 2002

Document available at: http://www.pd.astro.it/

Vicolo dellOsservatorio, 5 35122 Padova Tel. +390498293411 Fax +390418759840

FIRST TESTS WITH VPH GRISMS FOR AFOSC

Giro E., Pernechele C., Desidera S., Chiomento V., Frigo A., Traverso L.Astronomical Observatory of Padova Italy

Molinari E., Conconi P., Crimi G., Bianco A.Astronomical Observatory of Merate Italy
http://www.pd.astro.it/http://www.pd.astro.it/


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Technical Report N. 20 First tests with VPH grisms for AFOSC

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1 Introduction

Since 1997 when AFOSC (ref 4) was installed at 182 cm telescope, a limit in the highest resolution possible, about3600 in echelle mode (see fig 1), was seen. In fact the previous installed instrument, the B&C(ref 2), with the 1200grooves/mm was capable to obtain dispersion of 42 /mm. After the installation of the polarimeter (ref 5), whichcannot be used with echelle grisms, this fact arises more evidently. The principal limitations in designing new grisms

are the pupil dimension of the spectrograph (28 mm), the maximum number of grooves/mm for classical grisms and themaximum dimensions of the grisms which AFOSC could house. For this reasons only a new technology would be ableto overcome the limit in the resolution of the spectrograph. The Volume Phase Holographic (VPH) gratings (ref 1)promise to reach this target.

Figure 1:The actual set of grisms of AFOSC. A comparison in resolution and range is proposed.

2 Volume Phase Holographics (VPH) gratings principles and fabrication

Diffraction in VPH gratings is obtained by fringes created by modulations in the refractive index inside a volume.The light diffracted follows the same equation for a classical grating i. e. for a transmissive grating in air:

m= sin() - sin()

where and are the incidence angle the diffraction angle, the wavelength, is the grating frequency and mis theorder of diffraction.The depth of the volume, the contrast of the fringe structure together with the angular and spectral relationship of theincident light to the Bragg condition however modulate the efficiency spectral curve of the grating. In particular for aplane, parallel grating with fringes normal to the grating surface the Bragg condition will be:

m= 2sin()

with the previous notations. In this case we can write efficiencies in the parallel and perpendicular polarization planeswith respect to the plane of incidence in the following way:
http://merlino.pd.astro.it/asiago/2332.htmlhttp://merlino.pd.astro.it/asiago/2332.htmlhttp://merlino.pd.astro.it/asiago/2332.html


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s= sin2((n d)/(cos())

p= scos(+ )

with d the thickness of the grating and n the modulation in refractive index. These last two parameters are strictlyrelated to the optical set-up of the holographic system. Theoretically efficiencies better than 90% are possible.The typical substrate used in the VPH grating fabrication is the Dichromated gelatin (DCG). A thin film of this

sensitized gelatin is deposited onto a glass substrate and exposed into an holographic system. Wet processing transformsthe exposed fringes into refractive index modulations. A glass sandwich is then laminated over the gelatin to protect itfrom humidity. DGC in fact is hygroscopic. Transmittance of DCG is very high in the range 300 2800 nm (better than95%).

3 Optical Layout of VPH for AFOSC

To house VPH inside AFOSC in place of standard grisms, central wavelength must cross undeviated inside the opticalelement, moreover Bragg condition must be satisfied. Two prisms before and after the grating may fulfill thisrequirements as shown in figure 2. Moreover in the AFOSC case prisms and grating dimensions must not exceed themaximum thickness of 5.5 cm.

figure 2: optical layout of a VPH grating (VPH grism) for AFOSC

The angles of the prisms and their refractive index permit to tune different grating for different wavelengths. In table 1we report the six configurations designed for AFOSC. The last configuration is not planned to be build together with theother configurations.

Conf # central () min () max () mm-1 px-1 Spectral feature VPH (mm-1) Angle Glass

1 4680 4430 4930 20 0.5 467.0 HeII @ R=5000 2310 31.38 PBH71

2 5100 4849 5351 20 0.5 500.7 OIII @ R=5000

520.0 Mg @ R=5000

2310 34.60 PBH71

3 5890 5521 6258 30 0.7 589.0 Na @ R=5000 1720 30.67 PBH71

4 6600 6238 6961 29 0.7 660.0 H@ R=5000 1720 34.60 PBH71

5 8700 8195 9205 41 1.0 860.0 Ca @ R=5000 1280 34.60 PBH71

6 6600 6381 6818 18 0.4 660.0 H@ R=8200 2600 30.96 ZnSe

Table 1: the six VPH grisms configurations designed for AFOSC

In Figure 3 the optical components of a VPH grism are shown. The prisms and the VPH are not cemented butmechanically fixed together with a matching index oil. The better way to align the optical components is to use a lightsource behind them. When the system is aligned, the spectrum generated by the VPH is not curved and it overlapsperfectly the light source in the direction perpendicular to the dispersion (no shift in that direction).


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Figure 3: The three optical elements which compose a VPH grism.

A mechanical mounting for the first prototype has been designed and built in the Ekar workshop. This device permits tofix together the optical elements composing the VPH grism and to align it with high precision with respect of the slitdirection in the spectrograph. In Figure 4 the first VPH grism inside its mechanical mounting not yet anodized isshown.

Figure 4: The first VPH inside its mechanical mounting not yet anodized.

4 Efficiency measurementsThe efficiency measurements are currently carried out at Merate Observatory using MODI, an appropriate devicedesigned to measure transmission curves of optical elements. For a complete discussion on the capabilities of this


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instrument see ref 3. A first example of these measurements for the 2310 grooves grating is shown in Figure 5. As onecan see the maximum of efficiency is about 75% a bit below the possibilities of VPH technology but anyway a goodvalues for this kind of grating. In particular such values are very difficult to achieve with ordinary technology.

Figure 5: The efficiency curves for the 2310 grooves grating as function of different incidence angles

Figure 6 report the efficiency measurement obtained for the first prototype of VPH corresponding to configuration 4 of

Table 1. The peak in the curve is about 75% at 680 nm.

Figure 6: Efficiency curve for the first VPH prototype.


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The complete set of measurements will be added as appendix in the AFOSC manual (ref 4).

5 First astronomical observations

During the period March/April 2002 the first VPH grism corresponding to configuration number 4 (VPH4) of the table1 has been tested at telescope both for spectroscopic observations and spectropolarimetric measurements.

In Figure 7 the VPH grism mounted on the grism wheel of AFOSC is shown.

Figure 7: The VPH grism mounted on the grism wheel of AFOSCTests carried out with calibration lamps shown the presence of lateral chromatism due the high refractive index ofprisms as shown in Figure 8. This effect anyway is easily fixed during data reduction.

Figure 8: Calibration spectrum of VPH4 obtained with Ne lamp. Lateral chromatism is evident.


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Four targets have been observed. We will show the results for comparison with grism 9 (echelle, resolution about 3600)or grism 8 (resolution about 1800).

5.1 HD 98281

HD 98281 is a G8V star with a visual magnitude about 8. It is has been observed with VPH4 and grism 9. In figure 9

and 10 the two spectra are shown. Exposure time for the VPH4 spectrum was of 60 seconds with a slit 1.26 arcsec wide,while using grism 9 we exposed for 120 seconds and with a slit 2.5 arcsec wide. The seeing was about 3 arcsec. Thedispersion was 0.66 /pxl for VPH4 and 0.88 /pxl for grism 9.

Figure 9: HD 98281 observed with VPH4, technical data of exposure in the text.

Figure 10: HD 98281 observed with grism 9, technical data of exposure in the text.


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5.2 AG Draconis

AG Draconis is a symbiotic star of K0 spectral type. In the red part of the spectrum its main features are a weak Hinemission, He I 667.8 nm in emission and the band at 682.5 nm due to scattering Raman. The star was observed both inspectroscopy (VPH4 and grism 8) and spectropolarimetry (VPH4). In Figure 11 and 12 we report spectroscopy withVPH4 and grism 8. The exposure time was the same, 240 seconds, as the slit (1.26 arcsec). The dispersion measured on

the spectra was about 0.66 /pxl for the VPH4 and 1.9 /pxl for grism 8. Using efficiency measurements obtainedfrom data sheet of grism 8 from comparison of the two spectra an efficiency a bit less than 80% for VPH4 has beenestimated which is in good agreement MODI measurements.

Figure 11: AG DRA observed with VPH4, technical data of exposure in the text, ordinate scale is logaritmic.

Figure 12:AG DRA observed with grism 8, technical data of exposure in the text, ordinate scale is logaritmic.


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Spectropolarimetry of AG Draconis with VPH4 is reported in figure 13. In this case a spectropolarimetric slit 2.5 arcsecwide has been used.

Figure 13: AG DRA spectropolarimetry, the polarization on the Raman band at 682.5 nm is well evident.

Flat fields obtained in spectropolarimetry permit to have an estimation of the different efficiency due to polarization. Adifference about 10 % between P-plane and S-plane polarization direction is present. In figure 14 a spectropolarimetricFlat field is reported.

Figure 14: Spectropolarimetric flat field of VPH4 is shown. A different efficiency among polarimeter channels due tothe grism is evident.


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5.3 V838 Mon

V838 Mon is a very peculiar star which has shown many spectral variations after its outburts in Jan 2002. In particularin the H region it presents a weak Pcyg profile. In Figure 15 and 16 a comparison between VPH4 and grism 9 is

shown. A well visible double Pcyg profile is present in the VPH4 spectrum, not detectable in the grism 9 spectrum.For the VPH4 a slit 1.26 wide has been used and the exposure was of 120 sec. The same exposure time was used for thegrism 9 spectrum and a short slit 2.5 arcsec wide.

Figure 15: V838 observed with VPH4 , technical data of exposure in the text.


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In Figure 18 we report a comparison between the rotation curve obtained from the previous VPH4 spectrum and an oldspectrum obtained with B&C in 1995 with a 1200 grooves/mm. The two spectra are largely comparable showing thatthe target to reach the B&C resolution with AFOSC has been fulfilled. Moreover as one can observe the new rotationcurve presents a better S/N ratio than that one obtained with B&C spectrograph.

References

Ref 1: Claudi R.U., Cremonse G.,Handbook of the Padova and Asiago Observatories1992Ref 2: Barden S. C., Arns J. A., Colburn W.S., Volume-phase holographic gratings and their potential for astronomicalapplications, Proc. SPIE Vol. 3355, p. 866-876, 1998,http://www.noao.edu/ets/vpgratings/papers/spiepaper.pdfRef 3: P. Conconi, G.Crimi, L. Mantegazza, E. Molinari and F.M. Zerbi, mOd a new tool to characterise opticalelements, Presented at "Telescopes, instruments and data processing for astronomy in the year 2000" S.Agata (Naples),12 to 15 May 1999, http://golem.merate.mi.astro.it/publications/ps/conconi1.ps.gzRef 4: Desidera S., Fantinel D, Giro E.,AFOSC User manual, 2001, Osservatorio Astronomico di Padova,http://merlino.pd.astro.it/asiago/man01.ps.gzRef 5: Giro E., Pernechele C.,AFOSC Polarimeter, Technical report n18, Osservatorio Astronomico di Padova,http://merlino.pd.astro.it/asiago/5200/techrep18.pdf
http://www.noao.edu/ets/vpgratings/papers/spiepaper.pdfhttp://www.noao.edu/ets/vpgratings/papers/spiepaper.pdfhttp://www.noao.edu/ets/vpgratings/papers/spiepaper.pdfhttp://golem.merate.mi.astro.it/publications/ps/conconi1.ps.gzhttp://golem.merate.mi.astro.it/publications/ps/conconi1.ps.gzhttp://merlino.pd.astro.it/asiago/man01.ps.gzhttp://merlino.pd.astro.it/asiago/man01.ps.gzhttp://merlino.pd.astro.it/asiago/5200/techrep18.pdfhttp://merlino.pd.astro.it/asiago/5200/techrep18.pdfhttp://merlino.pd.astro.it/asiago/5200/techrep18.pdfhttp://merlino.pd.astro.it/asiago/man01.ps.gzhttp://golem.merate.mi.astro.it/publications/ps/conconi1.ps.gzhttp://www.noao.edu/ets/vpgratings/papers/spiepaper.pdf

first tests with vph grisms for afosc

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