On the nights of 1996 November 1-2, standards Feige 110, G191B2B, and Hilter 102 were observed with 300R, 600R, and 1200R line gratings. The extracted spectra were then flux-calibrated to give the above-atmosphere AB magnitudes yielding 1 photon/sec/Ê. We have combined these data with previous throughput measures in the following table. All AB mags are scaled to a median fibre, and we have not included any 'slit' losses due to seeing or fibre mis-centering. The R1200R throughput is lower than expected; this is being investigated.
Grating Dispersion Spectral Range Peak AB (A/pixel) (A) Magnitude R300B 5.5 5000-9000 17.3 R316R 5.5 5000-9500 17.1 R600R 3.0 6000-8500 16.75 R1200R 1.5 6000-7250 16.1
For comparison, ISIS (with 158 line grating and TEK chip) has a peak AB mag of 17.8 - 17.9 (similar for blue and red arms), and LDSS (with high grism giving 2.4 Ê/pixel, and TEK chip) has a peak AB mag of 18.1.
We also observed two radial velocity standards with each of the R1200B and H2400B gratings (centered at 4500 and 4100Ê respectively, and using the same fibre for all measurements) to investigate the precision of measuring radial velocities with WYFFOS. For each grating, we computed the observed velocity difference, (V1 - V2)obs, between the two standards via cross-correlation, and compared with the true velocity difference, (V1 - V2)true, determined from the known (heliocentric) velocities. For the R1200R grating, we find that (V1 - V2)obs and (V1 - V2)true differ by 24 +/- 25 km/second, and for the H2400B grating the difference is 11+/-12 km/secondd. In each case then, the velocity precision is good to ~ 0.2 of a pixel. One could expect to do better by observing in the red and by choosing velocity templates carefully.
Transmission mode
On the night of 1996 October 31, we commissioned the WYFFOS transmission mode, where instead of the usual reflection gratings, a grism is used in straight-through mode with a second collimator to give a low dispersion of ~5.5Ê per pixel and a total coverage of ~5500Ê. First, Landolt field SA113 was observed for 600 second total, and as expected, we found a linear relationship between the extracted counts for each star/fibre and the star magnitude (this is a combined test of the input astrometry, accuracy of fibre positioning, and the performance of the data reduction).
We expect transmission mode to have higher throughput than reflection mode, but observations of flux standard BD+75û325 in fact reveal transmission mode to be slightly less efficient than reflection mode with similar dispersion (ie the R316R grating). The reasons for this are not known, but may be due to a combination of poor seeing and fibre mis-centering; further tests will be carried out. Thus, while transmission mode is now characterised and available for observers, at this time we do not see any advantages of transmission mode over reflection mode using the R300B or R316R gratings, save for the lower background levels with transmission mode; observers should also consider using LDSS with medium grisms for smaller fields. Users should note that the change between reflection and transmission modes is a day-time job, and thus we do not encourage applications that require both modes in one observing run.
Software
All of the WYFFOS commissioning data were reduced using the WYFFOS package written by Jim Lewis at RGO. This is an IRAF reduction package specifically designed for WYFFOS, and contained within the RGO package (note that other IRAF packages, such as DOFIBRES, will not work with WYFFOS data because of the staggered slit end), and is publicly available via anonymous ftp. Please contact Jim Lewis (jrl@ast.cam.ac.uk) or Terry Bridges (tjb@ast.cam.ac.uk) for more information. There is also a new IRAF script to measure fibre throughputs from spectrophotometric standards, to be included in the next release of the WYFFOS package.
Echelle grating
In March 1997 we plan to commission a so-called 'echelle' grating for use in reflection mode. This is actually a 632 lines/mm grating blazed near 57 degrees. Five orders are accessible covering the wavelength range 3500 - 9500 angstroms, giving a range in dispersion of 23.73Ê/mm in order 3 to 10.17Ê/mm in order 7. Orderûsorting filters have been purchased for this grating, to allow observations of each of these five orders.
The parameters for each order are shown below.
Order Wavelength Dispersion Free Spectral CCD coverage (A/ blaze) A/mm A/pix Range (A) (A) 3 8830 23.73 0.57 2940 580 4 6623 17.80 0.43 1650 435 5 5298 14.24 0.34 1060 350 6 4415 11.87 0.28 740 290 7 3784 10.17 0.24 540 245
We welcome proposals for the echelle grating (with the proviso that the success of the commissioning will not be known at the time of PATT submissions), but ask that proposers contact Dave King (king@ast.cam.ac.uk) or Terry Bridges before applying.
CCD upgrade
We are discussing the possibility of upgrading the present TEK 1k CCD with a thin Loral 2k device. This would give better QE, particularly in the blue, and the smaller pixels would give better sampling with the present 2.7 arcsecond fibres. However, it's not clear when RGO will receive the Loral devices, and in what numbers, so this upgrade is probably at least a year away.
Second module of 1.5 arcsecond fibres
We are planning to commission a second module of 1.5 arcsecond diameter fibres in the latter half of Semester 97B. These fibres will be much better suited for the observation of stellar sources, since the sky background will be reduced by a factor of four, with a gain in limiting magnitude (for sky-limited observations) of 0.75 magnitude. The small fibres have been ordered, but the timescales at present are very uncertain and we will give an update in the next PATT newsletter. We anticipate soliciting science proposals for the small fibres for Semester 98A; please do not submit such proposals for Semester 97B. The AUTOFIB-2 gripper has been made to function more reliably and more accurately, and allocations of commissioning time before science runs are being used to improve and refine the fibre positioning in readiness for the small fibre module.
Further Information
We are presently writing a WYFFOS/AUTOFIB-2 Users Guide which will be available shortly in postscript form. In the meantime, potential users are urged to contact Terry Bridges (tjb@ast.cam.ac.uk) or Don Pollacco (dlp@ing.iac.es) for more information about WYFFOS/AUTOFIB-2. You can also find the information you need to write proposals on our WWW page, http://www.ast.cam.ac.uk/RGO/wyffos/wyffos.html