FEROS on the MPG/ESO-2.20m

EUROPEAN SOUTHERN OBSERVATORY

La Silla - Science Operation Department

FEROS on the MPG/ESO-2.20m

Version: 1.1 2005-03-28 J.D.Pritchard

Highlights of the Commissioning of the FEROS ADC at the ESO/MPI 2.2m telescope

Commissioning I -- September 21 - October 17, 1998: Original installation at the ESO-1.52m
Commissioning II -- November 11 - 30, 1998: Original installation at the ESO-1.52m
Commissioning III -- October 22/23 - 25/26 2002: First commissioning of FEROS at the MPG/ESO-2.20m
Commissioning IV -- November 13/14 - 14/15 2002: Second commissioning of FEROS at the MPG/ESO-2.20m
Commissioning V -- March 08/09 - 10/11 2003: Mini commissioning for the Fibre Head Service Mission
Commissioning VI -- October 09/10 - 14/15 2003: First Commissioning of FEROS-II (VLT Compliant system)
Commissioning VII -- November 10/11 - 15/16 2003: Second Commissioning of FEROS-II (VLT Compliant system)
Commissioning VIII -- March 13/14 - 16/17 2005: Commissioning of the FEROS-II Atmospheric Dispersion Corrector

Summary

The ADC project is a success, with one significant pending action, the replacement of the current prisms with new ones (hopefully in June 2006) with a more appropriate broad spectrum cement to correct the up to 50% transmission losses blueward of 4000Å.
Recomendations for Visitor Mode use: For red objects with little or no flux below 3800Å, or any other observations where the wavelength range below approx 3800Å is less (or even not at all) important, the ADC should be used for Zenith Distances greater than ~25degrees. For observations where the wavelength range below approx 3800Å is important the ADC should be used for Zenith Distances greater than ~50degrees. Note, these recomendations will change once the current prisms are replaced with new ones.
Recomendations for Service Mode use: Due to the fact that Service Mode PIs can not know in advance at what ZD their observations will be made, the situation is a little trickier. For observations where the wavelength range below approx 3800Å is less (or even not at all) important, the ADC should be used. Set ADC=IN (or AUTO, currently there is NO difference between these two options) in all OBJSKY templates. For observations where the wavelength range below approx 3800Å is important, the ADC should not be used. Set ADC=OUT in all OBJSKY templates. When the new prisms are installed and have been successfully commissioned, FEROS Service Mode PIs will be given the opportunity to change the ADC settings in their remaining OBs since the ADC then should be used for all OBJSKY observations (unless an airmass constraint of 1.1 corresponding to a ZD of 25degress is specified, but this would be very restrictive and one could almost guarantee that such OBs would NEVER be executed!).
Note: Due to physical conflicts between the ADC and the SCSM, the ADC can NOT be used in Object-Calibration (aka simultaneous ThAr) mode.

Commissioning VIII -- March 13/14 - 16/17 2005

This commissioning is focussed on three main issues:

Verification that the ADC has no adverse effect on WFI (or FEROS) in terms of scattered light at least when the ADC in OUT
Verification of ZERO impact on the nominal functioning and performance of FEROS in terms of DRS initialisation stability and Measured Peak Efficiency
Verification of proper functioning of the ADC itself (of course)...

Scattered Light on WFI and/or FEROS

Scattered light resulting from the ADC is tested for using:

Pinhole exposures with WFI
Comparison of flatfield images in key bandpases (i.e. UBVRI of the calibration plan)
Measurement of the Zero-Point Variation Map

Pinhole expsoures

The graphic below shows pinhole expsoures made with WFI from three epochs:

Pre Baffling, with FEROS Adapter
Post Baffling with FEROS Adapter but before ADC.
Post Baffling with FEROS Adapter and after ADC.

The Pre Baffling, with FEROS Adapter is clearly the worst. Moreover it is also clear that the post ADC situation is, if anything better than the pre ADC situation.

Comparison of key bandpass flatfields

Details of the results of these tests to be provided by F.Selman.

Zero-Point variation map

Details of the results of these tests to be provided by F.Selman.

Nominal Functioning and Performance of FEROS

DRS Initialisation

Wavelength

2005-03-02 (Pre-Intervention)	2005-03-15 (Post-Intervention)
Wavelength Solution RMS: OBJECT fibre

Wavelength Solution RMS: SKY fibre

Order Profiles

Flat field ratios: OBJECT fibre

Flat field ratios: SKY fibre

Flat field Signal to Noise

Instrument Focus

solution: OBJECT fibre

Throughput/Efficiency

Observing conditions have been "quite" favorable, with photometric skies and good (but not execellent) seeing (typically 1.0 - 1.5 arcsec). In such conditions, based on experience, we would expect to achieve upward of 15% peak efficiencies. Indeed, during the two nights of scientific commissioning we have been able to achieve a measured peak efficiency of approximately 21%:

Telescope Focus for FEROS with and without ADC IN.

Using the FEROS_img_cal_focus template which measures Image Quality in the technical TCCD to estimate optimum focus we estimated the nominal focus values for ADC OUT and ADC IN to be 1440eu & 1880eu respectively, thus giving a focus offset between the two of 440eu.

ADC Performance

Overall the ADC is performing WELL.

Prism Transmission

Prism transmission has been measured by acquing SolarSpectra with and without the ADC IN during the middle of the day when the sky brightness is to a good approximation constant over a period of a few minutes, during which the expsoures are acquired.
The two ratios shown below are from an ADC=OUT/ADC=IN/ADC=OUT sequence taken approx 3hrs before sunset. They are in the ratio 2nd over 1st (left) and 2nd over 3rd (right). The two plots provide the LOWER (2nd/1st) and UPPER (2nd/3rd) limits to the absolute transmission curves due to the fact that the sky even 3hrs before sunset is slowly getting darker. Close examination of the 2nd/1st image shows that the total transmission of the TWO prims is above 90% for wavelengths above ~3900Å thus achieving the Design requirement of 90% or better transmission over the range 4000-9250Å. Below 4000Å there is a strong fading of the transmission down to ~50% at 3500Å due to the cement used to bind the two pieces of glass together to make each prism. A new set of prisms to be binded using a more appropriate cement have already been ordered from the manufacturer and we expect to install the during the Mirror Maintenance technical time scheduled for June, 2005 when WFI and the FEROS adapter must be dismounted from the telescope anyway. The overall modulation of the transmission curve is due to the antireflection coating.

ADC.In2Oot.ratios/2005-03-17/FEROS.2005-03-17T194625.667o2005-03-17T194625.667.fib1.png

ADC.In2Oot.ratios/2005-03-17/FEROS.2005-03-17T194828.858o2005-03-17T194625.667.fib1.png

ADC Performance

The following set of plots shows the normalised ratios of pairs of exposures of the star DM -3311708 taken with ADC OUT the ADC IN. The plots have been normalised in the sense that they have been scaled so that the ratio at the reference wavelength (i.e. the at which the guiding is made, 550nm defined by the Johnson V filter in front of the TCCD) is 0.95, the lower limit on the transmission efficiency indicated by the solarspectra above. The sequence plainly shows that the ADC performs well and provides an increased flux at 9000Å relative to the ADC OUT for ZD greater than about 25degrees. Due to the low transmission of the cement shortward of 4000Å, the improvement in the blue does not exceed the ADC OUT situatio until a ZD of approximately 40degrees ZD. However the indication below is that once the new prisms with a correct cement are installed, the improvement in the blue will be very good and quite small ZD, possibly for ZD even as small as 10-15degrees.

Last revision: J.D.Pritchard 2003-03-28
Comments
[ESO |LaSilla |SciOp ]


Last revision: J.D.Pritchard 2003-03-28 Comments	[ESO \|LaSilla \|SciOp ]