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ALFOSC grisms #18 - #20

As of January 2015 we have a new set of VPH grisms available for ALFOSC. The grisms were designed and assembled by Alessio Zanutta and Prof. Andrea Bianco, from the Osservatorio Astronomico Di Brera (INAF, Merate, Italy). The VPH gratings were made in Merate as part of the PhD project of Alessio, aiming to develop new techniques for fabricating VPH gratings.

Funding for the new grisms has been made possible by Prof. Johan Fynbo and Prof. Hans Kjeldsen through an IDA grant (Instrument Center for Danish Astrophysics).

The new grisms #18 (blue), #19 (green), #20 (red) aim to replace grisms #16, #7, #8, respectively, and are designed to give best throughput while covering the optical wave range 353-984nm at a 1-arcsec slit resolution of R=800 - 1000. The achieved gain in peak throughput is more than a factor 1.3 for the new blue and green grisms.

The new red grism outperforms the old grism #8 redwards from Halpha, and by a factor 1.4 at 750nm. This new grism will be valuable in combination with the new red-sensitive fringe-suppressed CCD that is anticipated to arrive at NOT in 2015.

The new grisms have inbuilt order blocking filters, and consequently the spectra are free of polluting second-order contribution.

The plot below shows for the new grisms the on-sky measured total system efficiency of ALFOSC, the telescope and at airmass=1, while using the 10-arcsec slit.

Design parameters

Preliminary design study by A. Zanutta, March 2014
Lab report by A. Zanutta, Sept 2014
Final report by A. Zanutta, Mar 2015


These grisms displace the object spectrum a few tens of pixels in the spatial direction, with respect to the position of the object on the acquisition image.


Thanks to everyone involved!
In particular to: Alessio Zanutta, Andrea Bianco, and Emilio Molinari
and to: Johan Fynbo and Hans Kjeldsen

John Telting, Jan 2015

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