Gran Telescopio CANARIAS

Observatorio del Roque de los Muchachos
La Palma

Index

• OSIRIS detector cleaning
• OSIRIS standard readout mode
• OSIRIS Multi Object Spectroscopy: final tests
• MOS mask manufacturing machine
• OSIRIS Multi Object Spectroscopy: initial tests
• OSIRIS Red Tunable Filter (RTF) wavelength calibration across the FOV
• OSIRIS Red Tunable Filter upgrade
• A new set of OSIRIS filters for common use
• Older news items

22^nd Jul 2019
Subject: OSIRIS detector cleaning

After periodic outbaking of OSIRIS detector on May 2019, some contamination appeared both at the cryostat window and detectors surface. Despite these contaminants were easily corrected via flatfielding during reduction process, a complete stand-down of the instrument on July 2019 was planned to clean those componente with the valuable support from IAC staff. Once this procedure was succesfuly completed, OSIRIS was included again on routinely operations from July 2019, with no major problems on this issue.

In order to help users when reducing imaging data taken with OSIRIS in the period June-July 2019, we have produced the following masterflats, enough to eliminate the contamination seen in the images taken on those dates.

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1^st Mar 2014
Subject: OSIRIS standard readout mode

In order to decrease the overheads during OSIRIS operation, from semester S14A onwards the standard OSIRIS readout mode will be 200 kHz in all the observing modes provided by the instrument. The initial purpose of allowing two different readout speeds was to provide a low readout noise mode for spectroscopic observations, different than the one used for imaging modes. However, the reasonable good performance of OSIRIS CCDs allows to get low readout noise levels either at 100 kHz or 200 kHz, hence the only real difference when using those modes is having different readout times. Several tests have been performed to ensure that this change will not affect at all to the instrument capabilities (see next Figures as an example).

Users must be aware of this change when preparing their proposals for S14A (although ETC estimations are nearly independent on using one readout speed or the other), as well as when filling the Phase 2 tool prior to the observations (overheads due to readout time will decrease noticeably).

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1^st Dec 2013
Subject: OSIRIS Multi Object Spectroscopy: final tests

After exhaustive tests carried out along 2013, the Multi Object Spectroscopy mode for OSIRIS is now fully operative. Both the Mask Designer tool for preparing the masks and the automated script for target acquisition at the GTC have been notably improved, enhancing their capabilities up to a more than acceptable level for their use in normal operation.

OSIRIS Multi Object Spectroscopy observations can be defined either using target coordinates based on pixels of an OSIRIS r' band image (pre-imaging mode), or using target coordinates based on equatorial J2000 coordinates (catalogue mode). In both cases, tests showed that differences between star positions and slit coordinates are less (worst case) than 0.4 pix (0.1"), and even lower in most of the targets. This result is excellent, in particular considering that these errors contain all sources of error, from design, through manufacturing and alignment (see examples in Figures below).

Despite of the good performance shown during MOS tests, it has been decided to define some practical limitations for Multi Object Spectroscopy observations during S14A (minimum slit width, fixed sky orientations, etc..) in order to ensure a complete success of this observing mode during their first operational stages. Detailed description on these limitations can be found here. It is expected that more flexible configurations would be accepted in a near future.

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25^th Feb 2013
Subject: MOS mask manufacturing machine

Multi-object spectroscopy (MOS) with OSIRIS is expected to become a key feature of GTC. The light-collecting power of the telescope, combined with the efficiency and multiplexing capability of observing tens of objects simultaneously will obviously be a huge advantage and open new observing possibilities for the GTC. MOS mode in OSIRIS works with focal-plane masks that contain many slits whose location corresponds to the location of the objects in the field. Each slit then produces a spectrum on the detector.

There are several components that are required before MOS-mode can be taken into operation, and GRANTECAN together with the OSIRIS team at the IAC is working towards making MOS mode available as rapidly as possible. One of the key components is the machine to cut the slits on thin, aluminum plates to high precision. In February this machine, produced by the company SMI-Herluce (Erandio, Vizcaya), was delivered to La Palma for final acceptance testing.

The machine works with precision cutting tools that drill and cut the slitlets to the desired size and shape, and in the correct location. An important aspect is that the machine can produce a mask with minimum human intervention thanks to its robust interface and controls. Moreover, the machine autonomously carries out the calibration of the cutting tools so that the end result is always reliable.

The following pictures show the machine in its (temporary) location at the observatory, and an example mask from the first tests just after installation.

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31^st Jan 2013
Subject: OSIRIS Multi Object Spectroscopy: initial tests

During 2012, several advances have been made in the multi-object spectroscopy (MOS) observing mode. A nearly completed Mask Designer Tool (the main tool for design and preparation of the manufacturing process of the slit masks) was made available, and the automated script for MOS target acquisition at the GTC is fully operative (see next Figure).

These tools have been used in producing several MOS masks, using the IAC facilities and in a very valuable collaboration with the OSIRIS instrument team, for testing this operational mode with satisfactory results (see example below). These show that the slit positioning and drilling are reasonable good within specs for the scientific exploitation of this mode.

With the recent acceptance of the final mask drilling machine in January 2013, and its shipping to GTC on middle February 2013, we are entering in the final stages of the commissioning of this highly demanded observing mode. It is expected that the commissioning will be finished during the next few months, and, if all goes well, can be offered to the general community for scientific observations in semester 2014A.

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12^th Jul 2012
Subject: OSIRIS Red Tunable Filter (RTF) wavelength calibration across the FOV

When using the OSIRIS RTF, the observed wavelength varies across the FOV, becoming shorter towards the extremities of the field. This can be seen as a purely geometrical effect that depends only on the incident angle that is determined by the ratio between the telescope and the instrument collimator focal distances. The wavelength dependence can be approximated by the following formula:

This formula has been used for positioning and calibrating in wavelength the targets across the effective OSIRIS FOV for the RTF observations (4 arcmin). However, some OSIRIS users detected small but non-negligible deviations from this formula when analyzing their scientific data, and some work has been done to solve this apparent discrepancy.

For calibrating the wavelength dependence across the FOV for the OSIRIS RTF, images of different emission lines at different wavelengths, covering the full OSIRIS wavelength range, were obtained during commissioning work. For each emission line, the RTF was tuned at different wavelengths. The results were checked against OSIRIS RTF scientific data of cluster galaxies covering the whole OSIRIS FOV, courtesy of M. Sánchez-Portal, with spectroscopy available from the literature. From these data, thanks to the extensive work done by Dr. Cepa and the OSIRIS instrument team, the following wavelength dependence across the OSIRIS FOV for the RTF is derived:

where λ₀ is the central wavelength tuned, and r is the distance in arcminutes to the optical centre of the TF. Equation 2 implies that the wavelength dependence is in fact weaker than expected.

Within the inner ~ 2 arcmin, this expression is very accurate for any wavelength (see Figure above). Even at the edge of the 8 arcmin diameter OSIRIS RTF FOV, the maximum error is of the order of the tuning accuracy (about 1 to 2Å) for most wavelengths, and always within ± 6Å in the worse cases. Experience to date shows that this accuracy is sufficient for most applications. Besides, if images are dithered an additional wavelength shift depending on the distance to the optical center is produced. For example, at the edge of the 8 arcmin diameter TF FOV, the shift is of order 7Å for a dithering of 10 arcsec.

For those specific projects requiring more accuracy, that use no dithering, an additional chromatic term a₃(λ),

where λ is in Å, allows obtaining accuracies of the order of the tuning error (±1Å) over the whole OSIRIS TF FOV. Note that this expression is iterative, as it depends on the wavelength obtained at a certain distance r from the RTF center (although the value for λ₀ can be used as an initial estimate).

This deviation from the pure geometrical relation described by Equation (1) was also reported by Mendez-Abreu et al. (2011, PASP, 123, 1107). In this study the difference in wavelength dependence over the field is attributed to variations of the focal distance of the camera, in the same manner as it was previously done in Veilleux et al. (2010, AJ, 139, 145), who were the first to report such a deviation from the geometrical expression for the TF of the Magellan-Baade 6.5m telescope, that depended non linearly on etalon gap and wavelength.

By using calibration data from emission lines at different wavelengths, Mendez-Abreu (private communication) obtained that the expression presented in their study is also wavelength dependent (as expected), and the results showed that there are no differences between using the new expression given by Equation (3) and the ones suggested by them (but taking into account that for each wavelength a new calibration constant has to be calculated, and hence the one presented in their paper cannot be considered valid for other wavelengths).

This is shown in the next Figure, that represents the difference in wavelength between using the new expression given by Equation (3) and the wavelength-dependent expression of Mendez-Abreu et al (but again, this is not the one presented in their work, that only can be used for the wavelength range covered in their study). Only for r > 3 arcmin and λ > 900 nm there are notable differences, but at these distances from the RTF center, the wavelength variation due to dithering is even larger than these.

Note that the differences in using the geometrical expression given by Equation (1) and the new expression given by Equation (3) are negligible for r < 1 arcmin at all wavelengths, and also for r < 2 arcmin and λ < 800 nm. This means that for point source studies (that usually place the targets at distances to the RTF center in the order of 1 arcmin) the use of either the new expression (Equation 3) or the geometrical one (Equation 1) will produce indistinguishable results. Hence, for these kinds of projects there will no significant changes in the wavelength calibration of the scientific data obtained up to date. This is what is shown in the next Figure, that represents the differences (in Å) in using the original geometrical formula (Equation 1) and the new derived one (Equation 3).

Summarizing, for those projects that aim to take advantage of the complete FOV available with the OSIRIS RTF, the new expression given by Equation (3) is recommended as it provides the most accurate wavelength calibration across the whole FOV:

As this expression is iterative, when defining the positioning of the target within the OSIRIS FOV in order to get the wavelength observed at a certain distance of the RTF center, Equation (2) is recommended:

This one is easy to handle with, and its accuracy is good enough for this purpose. Even more, as it was also stated above, depending on the wavelength range and for distances no larger than 1-2 arcmin from the RFT center, the old geometrical expression used up to date is still perfectly valid for positioning the targets in OSIRIS.

The effect in the wavelength dependence along OSIRIS FOV for the RTF that has been described here is expected to be also observed in the OSIRIS BTF. The corresponding expressions for the OSIRIS BTF will be derived as soon as it is commissioned.

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11^th Jul 2012
Subject: OSIRIS Red Tunable Filter upgrade

In order to increase the flexibility in the use of the OSIRIS Red Tunable Filter (RTF), following comprehensive tests a new table has been produced that defines for each wavelength setting the corresponding order sorter filter (OS) that is to be used in combination with the RTF.

With this new table it is now possible to extend the use of the RTF by defining narrower FWHMs at the reddest wavelengths (λ > 800.0 nm) that were up to now limited to a FWHM of 1.2 nm (mainly for λ > 850.0 nm) so as to avoid contamination by other interference orders within the effective FOV of the RTF. As shown in the next Figure, now the lower limit achievable can be as small as 0.8 nm for 840.0 nm < λ < 880.0 nm, and 0.9 nm for λ > 910.0 nm.

According to this new configuration, the currently available RTF widths can be summarized as follows:

These values are approximate ones. For a precise estimate it is recommended to use the TF Setup Tool that is provided as part of the OSIRIS Exposure Time Calculator.

The new table that defines the TF wavelength ranges where a certain OS has to be use is the following one:

This new upgrade does not affect any of the observations taken to date with the OSIRIS RTF, as FWHMs as small as 0.8-0.9 nm were not allowed for scientific observations. Even when there are some changes with respect to the ranges of use of the RTF defined at the beginning of the scientific observations with OSIRIS, there are no differences in using the present configuration instead, as both configurations are valid for the higher values of tuned FWHMs. For this reason, the long term programs that are currently being executed with OSIRIS RTF can continue using the previous OS configuration without problem or, alternatively, move to the new one if requested by the user.

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15^th Jun 2012
Subject: A new set of OSIRIS filters for common use

It is a pleasure to announce that the number of filters available for general use with the OSIRIS instrument has been drastically extended thanks to a generous gesture by Dr Pablo Pérez González from the Universidad Complutense de Madrid to make available his private optical filters. Dr Pérez González designed and purchased (using funding from the Spanish Government through projects CSD2006-00070 and AYA2009-07723E) a set of medium-band filters for the SHARDS science program that is currently being executed on the GTC.

This set consists of no less than 25 filters spanning the wavelength range from 500 to 940nm with bandwidths from 14 to 34nm. Interested parties who would like to use of any of these filters should contact Dr Pérez and GTC to request their use, and write the appropriate credits in any paper that may result from the use of these filters. Further details about the filters and their general use will in the future be posted on the GRANTECAN OSIRIS web pages.

GRANTECAN thanks Dr Pérez González for his generous and colaborative gesture that will benefit many scientists in the GTC user community.

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1^st June 2011
Subject: OSIRIS cryostat repair status

The cryostat of OSIRIS has had a difficult youth, with serious problems affecting the day-to-day operation of the instrument and requiring continuous baby-sitting to ensure that temperature and pressure remained within operational bounds. This was an untenable situation and hence about a year ago a project was initiated to plan an intervention to repair the cryostat. With the successful completion of this work we can now put this bad episode behind us and look forward to a more mature exploitation of the OSIRIS instrument.

Early users of OSIRIS may remember the problems with high dark current and occasional loss of vacuum. Following several months of testing and modeling to identify the problem it was concluded that high thermal conductivity between the CCD and the outside world was likely the main culprit. A possible vacuum leak was also suspected. The planning and testing was complicated by the fact that throughout the process leading up to the intervention the engineers had to carry out their work while the cryostat was operational, limiting the scope and duration of any test that could be done.

In May the final intervention was carried out, dismantling the guts of the cryostat, including the CCD itself, the cold link to the liquid Nitrogen container, and, a very critical part, separating the lens that acts as a cryostat window from the CCD assembly. The mechanical tolerances being very small for the excellent image quality of the OSIRIS instrument meant that much effort had to be expended on modeling and on building a special tool to ensure that the re-assembly would reproduce the original image quality.

The thermal insulation between the CCD package and the exterior was much improved by replacing materials at the contact points with the external parts of the cryostat. In the process, the CCD surface was gently cleaned, removing some of the particles that had accumulated during the early life of the cryostat.

The work went according to plan and the objectives were fully achieved. The CCD temperature is now stable and the cryostat holds its vacuum over long periods of time. The CCD is currently being characterized again and is back in service.

We are grateful to our colleague engineers from the Isaac Newton Group of Telescopes for their assistance in the project. This work was made possible thanks also to the outstanding work carried out by Kevin Dee from Engineering & Project Solutions Ltd.

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9th Mar 2011
Subject: Final Call for ESO-GTC proposals

A final call has been issued by ESO for time under the ESO-GTC agreement. Details on how to apply may be found at the following URL: http://www.eso.org/sci/observing/phase1/lbn.html

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15th Dic 2010
Subject: New VPHs for OSIRIS

Five new VPHs for OSIRIS have been taken into operation. These are R2500U,V,R and I and R2000B. These VPHs provide important and much-demanded medium-resolution capability for the instrument. Initially the VPHs suffered from a quite strong ghost image of the spectrograph slit, but this has been corrected by adapting the baffling of the optics. This has been a delicate and precise task in order to avoid any vignetting that could be detrimental to the throughput. The ghosting has been nearly fully removed; what remains is a very small fraction of the total flux of the object that normally will have a negligible impact on the quality of the spectra. The efficiency of the grisms is shown to be good. Technical details will be posted and updated on the OSIRIS instrument web page as they are obtained. As an example, below these lines you see a image and the extracted spectrum taken with the R2500 grism of a planetary nebula.

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17th Mar 2010
Subject: OSIRIS CCD Dark Current status

Recently a new cryostat for the OSIRIS detectors was taken into use with the aim to address a number of problems, the most critical of which for regular science observations was the very high dark current resulting from an excessive temperature of the CCD that was not correctly reported by the CCD thermometry system. A redesign of the thermal coupling between the liquid Nitrogen container and the CCD has resulted in a notable improvement of the dark current, which is now at acceptable levels of about 6 ADU per hour for a 2 x 2 binned pixel. Concerns remain present about the vacuum hold time and the temperature stability of the CCD, but in spite of that we can conclude that the detector is fully operational for for astronomical purposes.

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Last modified: 20 September 2019