OSIRIS+ stands for the complete upgrade of OSIRIS instrument carried out along 2022, with the installation of OSIRIS at Cassegrain focal station first, and with the use of a new blue sensitive monolithic detector later. In this manner, from January 1st 2023 all the science observations with OSIRIS are obtained with this new OSIRIS+ configuration, with the need of adapting all the related information (data reduction, data format, etc..) accordingly.

Observing modes are the same as the ones for OSIRIS. The following table summarizes the available modes and features for imaging and spectroscopic observations with OSIRIS+. Also, the current optical elements available in OSIRIS+ can be found here.

Imaging Spectroscopy
Broad Band Imaging LongSlit Spectroscopy
Medium Band Imaging: SHARDS Filters Multi-Object Spectroscopy

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icon Index


icon Instrument Features

The following table summarizes some basic parameters of the instrument.

Spectral range 3650-10000 A
F.O.V. 7.8' x 8.5' (imaging; 7.8' x 7.8' unvignetted)
Plate Scale 0.254" (imaging and spectroscopy). Standard OSIRIS+ observing modes use 2 x 2 binning, hence plate scale can be reduced if needed.
Detector 4096 x 4096 Deep-depleted E2V CCD231-842 (astro-2 coating)
Pixel Size 15 µm/pix
Detector Quantum Efficiency (QE) 70% (400 nm), 90% (600 nm), 80% (800 nm), 50% (900 nm)
Image quality EER80 <0.3" (Imaging mode). Distortion <2% in all the detector

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icon Instrument Detector

The new detector for OSIRIS+ consists in a 4k x 4k CCD deep-depleted E2V CCD231-84. This detector provides very low fringing and relatively good efficiency in the blue end of the optical window, specially when combined with the astro-2 coating provided by E2V, with the aim of enchancing the sensitivity of OSIRIS in this new configuration. The single pixel physical size is 15 µm, which corresponds to a scale of 0.127" in on the sky. However, OSIRIS+ standard observing modes use 0.254" binned pixels.

OSIRIS+ CCD is rotated 90 degrees with respect to the previous orientation in OSIRIS. Now, the vignetting is significant in the lower 250 pixels of the CCD and the last 250 pixels in the top right part of the image (though this is less prominent). Therefore one needs to be careful when doing photometry of stars near these areas. This vignetting defines a maximum unvignetted field of 7.8 x 7.8 arcmin

Below we show an image with the default pointing positions for all the OSIRIS+ observing modes: Broad Band imaging, Longslit Spectroscopy, and Milti-Object Spectroscopy. The pointing axis for Cassegrain focal station is also shown, now placed nearer to the standard pointings as in old OSIRIS configuration, increasing even more the global sensitivity of OSIRIS+.


OSIRIS+ standard pointing positions. Note that now the vignetting affects to the lower part of the FOV.

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icon Detector Setup

The CCDs control system offers a wide range of readout modes and gain settings, but for the time being the standard observing mode for OSIRIS+ is shown in the table below.

The slow and fast modes are only used for test images, and they are not offered for scientific cases. The following table gives an overview of the main characteristics of the OSIRIS CCD readout modes.

Imaging / Spectroscopy
Slow Fast
Readout configuration CCD_A CCD_A CCD_A
Readout velocity 233 kHz 150 kHz 800 kHz
Gain (e-/ADU) 1.9 1.5 2.3
Binning (X x Y) 2 x 2 2 x 2 2 x 2
Readout time ~24 sec ~38 sec ~7 sec
Actual readout noise ~4.3 e- ~3.6 e- ~7.6 e-

Note that if you wish to use readout mode other than the standard one (233 kHz), the time for taking the night-time calibrations will be charged to the project. Also note that for non-standard modes data quality is not guaranteed.

The standard readout mode for OSIRIS+ is nearly equivalent to the previously used in OSIRIS with respect to readout noise and dark current values, as well as for readout time (hence not affecting to the standard overheads considered for OSIRIS observations in general). However, gain value is about twice as before, meaning that there is a better sampling for bright targets, with a saturation level of ~58,000 ADUs.

Dark current level is about 5 e-/h/pix for this standard mode.


OSIRIS+ CCD Photon Transfer Curve for the standard readout mode.



OSIRIS+ CCDs Quantum Efficiency curve (red line), compared with the old detector (black line).



Spectrophotometric standard Feige110 observed with OSIRIS+/OSIRIS at Cassegrain, showing the sensitivity enhancement both in bluer and redder wavelengths. Additionally, at wavelenghts larger than 700 nm a small contribution of second order contamination is also present with the new CCD (see details here).


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icon OSIRIS+ Standard Pointing Positions

For the main observing modes of OSIRIS+ a reference position on the CCD has been adopted as the location where targets will be placed. These locations avoid blemished on the detector and they correspond to the coordinates introduced by the PI in the Phase-2. Alternative locations can be used, but this should be explicitly identified in the Phase-2 form. (Note that all the coordinates are referred to standard CCD pixels, that is, using 2 x 2 binning).

  • Broad Band Imaging: The standard pointing is at the CCD pixel (1050,925) to maximize the available FOV. The coordinates introduced by the PI in the Phase-2 will be positioned at this central pixel.
  • LongSlit Spectroscopy: Objects are centered on the slit at the coordinate Y=750 of the CCD, that is, pixel coordinates (1051,750). On this area, the distortion of the spectra is very low and it follows the same approach as in old detector, by placing the targets at approximately 1 arcmin away from the image center. When observing with OSIRIS+ in Long Slit Spectroscopy mode, an acquisition image and a throughslit image are provided to the user. During the observation, after the acquisition image is obtained with the target placed at the pixel Y=750 (that is assumed to be the standard pointing position for this observing mode) an iterative process for slit alignment is needed (usually a small offset to improve the target centering), and no intermediate products of this process are included in the data delivered. For this reason the coordinates for the target in the acquisition and troughslit images can be slightly different (never more than a pixel). It's recommended always to use the troughslit image for checking the proper centering of the target, and to use the acquisition image only for ensuring the correct field acquisition and target identification.

  • Multi-Object Spectroscopy: Pointing coordinates will refer to pixel (1050,1005) in CCD coordinates, but in this case the file coming from the Mask Designer Tool will include that information and no further action within Phase-2 tool is needed.

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icon OSIRIS+ Image Orientation

Images obtained with OSIRIS+ show some changes with respect to those from the old OSIRIS configurations regarding to the image orientation. Sky orientation in Cassegrain means that for P.A.=0 deg, North is up and East is right in the images , with a flip in X coordinates with respect to the old OSIRIS observations at Nasmyth focus that has to be taken into account when defining offsets in RA/DEC coordinates.


Crab Nebula image obtained with OSIRIS+ (right) compared with the previosuly obtained with OSIRIS at Cassegrain with the old CCD mosaic (left). Both images show North up - East right orientation, but vignetting is now at the lower part of the FOV due to the 90 deg. turn.


Instrument Position Angle (IPA) for OSIRIS+ is 127.215 deg. This is the pysical angle in Cassegrain rotator which produces that for a P.A.=0 deg, North is aligned vertically in the images. Different P.A. are defined as usual for OSIRIS+, with P.A.>0 when measured from North to East direction, and P.A.<0 when measured from North to West direction. In order to help in the image identfication, as a rule of thumb, when P.A.>0, upper image is oriented to North-East direction, while for P.A.<0, upper image is oriented to North-West direction.


Instrument Position Angle for OSIRIS+, and the different rotation angles when defining a P.A.in the images.


In spectroscopy mode, the slits appear vertical in the images, so now the spectral direction is horizontal, with the bluer wavelengths on the right of the CCD. Despite there is no longer a gap in the CCD, slit masks still include a reinforcing joint that produces an obscuration in the spectra in the central position of the FOV. A new set of slit masks are still being manufactured, so the user must be aware of this effect to be avoided in the case of requesting extended sources observations by doing offsets along the slit.


Feige 110 spectrum obtained with OSIRIS+, showing the change in the dispersion direction with respect to OSIRIS. Spectra are now horizontal, and the obscuration observed in the middle of the images is caused by the slit masks.


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icon Observing Modes

icon Broad Band Imaging

OSIRIS+ allows broadband imaging over a FOV of 7.8 x 8.5 arcmin (7.8 x 7.8 arcmin unvignetted) covering the full spectral range from λ=3650A to λ=10000A, with a high transmission coefficient in particular at longer wavelengths. The full spectral range is covered by the Sloan system broadband filters: u' (λ3500 A), g' (λ4750 A), r' (λ6300 A), i' (λ7800 A), z' (λ9250 A).

The broad-band photometric zero-points for OSIRIS+ are given in the following table. In order to compare with previous values from OSIRIS, zeropoints are also transformed to e-/s units. The sentivity gain at bluer wavelenghts (0.5-1.2 mags) is noticeable.

Filter λc(0º) [Δα(0º)] nm Zeropoint (ADU/s) Zeropoint (e-/s) Zeropoint OSIRIS (old) Transmission (0º)
u' 350.0 [60.0] 26.2 ± 0.07 26.9 25.7 --- / ---
g' 481.5 [153] 28.6 ± 0.05 29.3 28.85 Image / Table
r' 641.0 [176] 28.7 ± 0.03 29.4 29.3 Image / Table
i' 770.5 [151] 28.3 ± 0.05 29.0 28.85 Image / Table
z' 969.5 [261] 27.6 ± 0.07 28.3 28.15 Image / Table

Standard extinction coefficients for the ORM can be found here

Daily monitorizing of the OSIRIS/OSIRIS+ broadband zeropoints yields to a decrease of 0.5-0.7 mag due to presence of thick cirrus (spectroscopic nights), that shows the excellent sensitivity of the instrument even in bad observing conditions. Unusual presence of dust in the atmosphere above the observatory also produces a slightly decrease of this sensibility, but typically no larger than 0.2-0.3 mag respect to the standard zeropoint values. Here can be found the complete table with the OSIRIS zeropoints values, including the night conditions, for the full dataset of OSIRIS/OSIRIS+ observations. The table is expressed in e-/s units to continue the monitorizing started with old OSIRIS data. The table with OSIRIS+ zeropoints in ADU/s is also available here.

The graph below show the zeropoint values for different observing nights with clear and/or photometric conditions.


Evolution of OSIRIS/OSIRIS+ broadband zeropoints with time (only considering clear/photometric nights) since the beginning of the GTC operation. Each year covers the interval between consecutive dashed lines. Vertical solid line shows the change to Cassegrain focal station, while vertical thick solid line shows the CCD change to OSIRIS+ configuration.


Detailed evolution of OSIRIS/OSIRIS+ broadband zeropoints in the period 2020 - 2023, to show the sensitivity enhancement obtained with OSIRIS+. Vertical solid line shows the change to Cassegrain focal station, while vertical thick solid line shows the CCD change to OSIRIS+ configuration.


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icon Broad Band Detection Efficiency

The graph below shows the overall photon detection efficiency of GTC and OSIRIS+ in each of the Sloan filters.

BB efficiency

OSIRIS detection efficiency with Sloan filters.


Also, the following plots shows the limiting magnitudes with OSIRIS Sloan filters for getting S/N=3 as a function of the exposure time, assuming dark conditions, seeing = 1.0 arcsec, and airmass =1.2. A detailed view for exposure times lower than 1.0 h is shown in the lower panel.

BB efficiency BB efficiency

Limiting magnitudes achievable in OSIRIS Broad Band Imaging mode as a function of time. The lower panel shows a zoomed view of the upper panel for exposure times lower than 1 h. OSIRIS+ sensitivity in Sloan u band (not shown) resembles the same values achieved in Sloan i filter shown in the plots above.


Next table show current detection limits for OSIRIS+ derived directly from scientific data obained in 1.5 h of integration time under photometric and 0.9-1.0 arcsec seeing conditions (courtesy by Dr. Trujillo-Cabrera).

Filter Surface mag limits
(3sigma; 10"x10" boxes)
Limiting magnitude
(5sigma; r=1")
Sloan u 30.3 26.0
Sloan g 31.5 27.3
Sloan r 31.0 26.6


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icon Broad Band Sky Flats

OSIRIS+ Sky Flats are obtained observing in a selected set of Blank fields containing few (bright) stars, that have been produced specifically for GTC (the selected list of GTC blank fields is given here). This list was compiled using the TESELA tool developed under the umbrella of the Virtual Observatory and where you can also make your own searches. Full details about the search tool for GTC blank fields can be found in Jiménez-Esteban et al. (2012, MNRAS, 427, 679).

The flat fielding homogeneity in each of the Sloan filters is better than 2.5% over the full unvignetted FOV of the instrument, except in Sloan u' and Sloan g', where fluctuations up to 5-6% and 2% respect to the mean value (caused by the filter coating) are found as measures from many twilight flat fields show. Sloan g' features are more noticeably in OSIRIS+ than in OSIRIS configuration, probably due to the enhanced blue sensitivity for OSIRIS+. Day to day fluctuations in the flatfields are less than 0.05% , and less than 0.1% week to week. Hence, Sky Flat fields obtained with OSIRIS+ are well usable up to within a week before or after the observations.

Comparisons with SuperFlats derived from GTC scientific observations during bright time (Sky background values around 15,000 - 20,000 ADUs on average) show no variations with respect to the standard Sky Flats up to such a low level as 0.01%, hence they can be considered practically identical for scientific purposes. These percentage variations are measured globally, while of course locally, due to dust particles that can come and go, the variations may be larger. Moreover, differences between the night-sky and the twilight spectrum may result in suble flat fielding differences.

By combining large series of Sky Flats in each filter, obtained during the GTC scientific operations, we constructed a series of MasterFlats frames that can be retrieved here. Flat fields were all obtained by using the GTC automatic sequence for SkyFlats, that allows to get a series of flats with exposure times always larger than 1 s (to minimize possible photometric effects due to OSIRIS shutter) and a maximum exposure time of about 20 s (where the detection of stars is notable), with an average of 25,000-40,000 ADUs in each individual image. Rejection parameters were chosen accordingly to as much images as possible in each filter.


Master Flats
MasterFlat u' MasterFlat g' MasterFlat r' MasterFlat i' MasterFlat z'


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icon Illumination effects in OSIRIS+ / Ghosts

OSIRIS instrument has an off-axis design, with the rotation center of the instrument displaced from the CCD center and located at pixel -binned- (1049,408). Due to the Cassegrain configuration, this implies that there is a slight vignetting to the edge of the FOV with a 5-10% flux decrease at distances larger than 5-6 arcmin from the center. This effect is noticeably at the upper part of the CCD, but has no impact in the photometry. However, this should be taken into account in MOS observations when a target is placed at those extreme positions in the FOV, or when inspecting the apparent inhomogeneities in the flat field images that are fully consistent with this effect.

OSIRIS+ configuration, with the new standard pointing positions nearer to the rotation centre of the instrument both in imaging and spectroscopy is intended for further improving the sensitivity of the instrument.


Sky Flat image taken with OSIRIS+ showing the flux gradient in CCD due to Cassegrain focus design. Flux decreases about 5-10% at the most extreme egdes of the field.

As in previous OSIRIS configuration, the high sensitivity of GTC can produce some ghosts in OSIRIS+ when bright targets (or at least those too bright for a 10m telescope), fall within OSIRIS+ FOV. Some of these ghosting effects as saturated spikes are easy to identify, however there are others that need to be carefully inspected in order to not be identified as a real detection of a source. The most spectacular one is the so-called "OSIRIS parachute", produced by obtaining a deformed pupil image of the primary mirror, located symmetrically to the optical center (located at the center of CCD) with respect to the position of the bright parent source. In this case, to avoid any effect in our scientific targets the field orientation might be changed in order to displace this reflection accordingly. The higher gain that OSIRIS+ CCD uses prevent saturation effects, and produces that ghosting is less severe than in previous OSIRIS observations, but in any case user should be aware of this effect when observing fields including bright sources within OSIRIS+ FOV.


An example of the "OSIRIS parachute" ghost, produced in this case by a bright star placed at the bottom of the CCD.

Sloan u' filter in OSIRIS+ has shown a red leakage at approximately 670-750 nm ,producing that contamination coming from these wavelengths would affect the photometry obtained in u' band. This was previously unnoticed in OSIRIS as the lower sensitivity with Sloan u' in OSIRIS prevented long integration exposures, needed to detect this effect.


Transmission curve of OSIRIS+ Sloan u' filter, showing the red leakage at 670-750 nm. Despite of being relatively small, contamination can be important depending of the emission of the source at these wavelenghts.

The effect is more noticeable when emission of the target at redder wavelenghts is higher, so extra-emission through the leackage also becomes more intense. Even more, when observing at lower airmasses the atmospheric dispersion produces a 'double-star' effect due to spreading the blue and red emission of the source at different positions in the FOV.


"Double-star" effect shown in a long integration with Sloan u' in OSIRIS+. Some extreme examples are marked with red circles.

Summarizing, this effect makes that photometric measurements with Sloan u' in OSIRIS+ are unreliable. To solve this problem a new Sloan u' filter is being requested to substitute the current filter in OSIRIS+. Meanwhile, potential users interested in getting deep photometry observations in u' band must request the use of HiPERCAM instead of OSIRIS+ at GTC.



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icon Medium Band imaging with SHARDS filters

From June 2012, 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 has been executed on the GTC (for further details on this program, see http://guaix.fis.ucm.es/~pgperez/SHARDS/).

This set consists of 25 filters spanning the wavelength range from 500 to 940 nm with bandwidths from 14 to 34 nm. Interested parties who would like to use of any of these filters should contact Dr Pérez González and GTC to request their use, and write the appropriate credits in any paper that may result from the use of these filters. The main characteristics of these filters are summarized in the following table:

Filter λ (nm) FWHM (nm)
U500/17 500 15


As in the rest of filters used in OSIRIS, the SHARDS filters are placed in the collimated beam and close to the pupil of the instrument, at an angle of 10.5º with respect to the optical axis of the instrument. This causes that the central wavelength depend on the position in the field, with a center of symmetry corresponding to the center of rotation of the instrument, located on CCD towards the lower part of the field in OSIRIS+ configuration.

This effect, small for the standard Broad Band Sloan filters but has a stronger impact in the operation of the SHARDS filters, as the filter widths are notably narrower than in the case of Sloan filters. Hence this is not a defect of the filters, but due to the design of the OSIRIS instrument that becomes more prominent as filter pass band gets narrower. The central wavelength variation effect is more relevant in the case of medium-band filters such as SHARDS' (and also for the order sorter filters) given that the central wavelength shift from edge to edge of the OSIRIS FoV is of the same order of the width of the filter. Note that the typical shift (14-15 nm) would be 10%-20% for broad-band filters. Summarizing, potentially users of the SHARDS filters set have to take two main aspects into account when using these filters:

  • The central wavelength in the rotation center of the instrument, (approximately pixel 1049,408 in CCD), is sligthly different from the central wavelength at the standard pointing position for OSIRIS Broad Band imaging. This latter can be as much as 2 nm bluer than the central wavelength at CCD, reaching differences up to 12-14 nm at the extremes of the FOV. Taking into account that most SHARDS filters have a bandwidth as narrow as 17 nm (except in two cases), this produces that the wavelength range observed in lower part of OSIRIS+ CCD can be notably different than the one observed in the upper part of CCD. Hence, for instance when making use of the whole FOV of OSIRIS+ for mapping a single emission line probably more than one filter has to be used.

    An example of this effect is shown in the following Figure for U840/17 filter. The central wavelength is 843 nm, while the nominal wavelength for an angle of incidence of 10.5º is 840 nm (approximately at the centrer of the FOV). At the upper part of the CCD, the central wavelength changes drastically, from 840 nm to 830 nm at the edge of OSIRIS FOV, a value that is 13 nm bluer than the central wavelength.



Effect of the dependence of the central wavelength variation with the position in OSIRIS FOV.


  • The wavelength variation over the FOV results in that the sky background is inhomogeneous. This is in particular pronounced when strong sky lines fall within the band. This makes that the sky background subtraction is a critical step in the reduction of data taken using SHARDS filters.
    The following Figure demonstrates this effect. In the left panel, we have the wavelength variation for U687/17 filter, in combination with a sky emission spectrum. As it can be seen, the stronger emission lines fall at the reddest wavelengths, -the ones that are observed in the upper part of OSIRIS+ CCD-, while these same lines are nearly undetected at the bluer wavelengths, that are the ones observed at the lower part of CCD, This produces that the sky background will be notable different from one region to other in the CCD.

    This effect can be clearly seen in the right panel, where a sky image with U687/17 filter is shown. Note the strong gradient observed in the background level, and how this follows a radial geometry from bottom to top of the OSIRIS CCD+. This effect should be taken into account when using the sky flat frames that can show some variability as sky emission itself changes (in any case, this can be properly corrected during the data reduction process).



Effect of the wavelength variation over the FOV in the background level when using SHARDS filters.


For a detailed description on the wavelength change along the FOV when using the SHARDS filters, see Table 1 in Perez-Gonzalez et al. 2013 (http://adsabs.harvard.edu/abs/2013ApJ...762...46P), as well as other details on the data reduction, etc. Also, a complete updated table on the SHARDS filters' characteristics can be found in:


Note that for OSIRIS+ configuration X and Y coordinates must be swapped, as now there is a 90 deg turn with respect to previous OSIRIS configuration.

The next Figure shows the overall photon detection efficiency of GTC and OSIRIS with the SHARDS filters. The full table with the SHARDS filters' efficiencies can be retrieved here for flux calibration purposes (see details on this contribution).

shards filters

SHARDS filters efficiency curve.


SHARDS filters are available for its public use without any restrictions, but users must be aware the all the science publications based on the use of SHARDS filters should include the following acknowledgement: "This work is (partly) based on data obtained with the SHARDS filter set, purchased by Universidad Complutense de Madrid (UCM). SHARDS was funded by the Spanish Government through grant AYA2012-31277."


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icon Overall efficiency in OSIRIS imaging modes

The following diagram summarizes the overal system efficiency of OSIRIS in imaging mode, as a function of wavelength. Note that in the curves for the tunable filter order sorters, the efficiency of the tunable filter itself is not included in the curve.

overall efficiency

OSIRIS detection efficiency in the different imaging modes.

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icon LongSlit Spectroscopy

OSIRIS+ allows longslit spectroscopic observations by using slits of 7.4' in the spatial direction and with different choices for the widths: 0.4", 0.6", 0.8", 1.0", 1.2", 1.5", 2.5", 12" and 40". Note that in OSIRIS+ now the spectral direction coincides with the horizontal direction in the detector.

Due to the obscuration present in one of the edges of OSIRIS+ FOV, and the manufacture process in producing the slits, the maximum distance allowed for placing two targets in the same slit configuration is 7.4 arcmin. For a proper sky subtraction, however, no distances larger than 7.0 arcmin are recommended in order to get enoguh pixels for the background estimation on both sides of the targets. Likewise, if offseting is required during the observation, the maximum distance to the targets has to be estimated accordingly (for example, a maximum distance of 6.5-6-7 arcmin between the targets ia a good approximation for this kind of observations).

OSIRIS+ has a wide variety of grisms and volume-phased holographic gratings (VPHs) covering low to intermediate resolutions, from R=300 up to R=2500. The following table summarises the resolutions and spectral ranges available. Resolutions and dispersions are measured at λc(A) for a slit with of 0.6". Dispersions correspond to binned pixels, that is the standard operation mode, while the physical pixels (unbinned) dispersions are half of those listed in the table.


ID λc(A) λ range (A) D (A/pix) Resolution Peak Efficiency Type Efficiency
R300B 4405 3600 -7000 4.96 360 70% Grism graph
R300R 6635 4800 - 10000 7.74 348 70% Grism graph
R500B 4745 3600 - 7000 3.54 537 68% Grism graph
R500R 7165 4800 - 10000 4.88 587 67% Grism graph
R1000B 5455 3630 - 7000 2.12 1018 65% Grism graph
R1000R 7430 5100 - 10000 2.62 1122 65% Grism graph
R2000B 4755 3950 - 5700 0.86 2165 87% VPH graph
R2500U 3975 3440 - 4610 0.62 2555 70% VPH graph
R2500V 5185 4500 - 6000 0.80 2515 80% VPH graph
R2500R 6560 5575 - 7685 1.04 2475 80% VPH graph
R2500I 8650 7330 - 10000 1.36 2503 80% VPH graph


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icon Arc Line maps

A complete arc linelist for all the lamps available at the OSIRIS+ Instrument Calibration Module (ICM) can be retrieved here. As addtional help in the wavelength calibration, some sky spectra at the resolutions covered by OSIRIS+ are available: R300, R500, R1000, R2000, and R2500.

OSIRIS+ Spectral Calibration ARCs
(HgAr, Ar, Ne, Xe)
(HgAr, Ar, Ne)
(HgAr, Ar, Ne)
(HgAr, Ar, Ne, Xe)
(HgAr, Ar, Ne, Xe)
(HgAr, Ar, Ne, Xe)




All the OSIRIS grisms show some minor ghost effects in the arc images. Those ghosts are due to internal reflections within the grisms, and can be clearly identified as the curvature in these spectra clearly differ than that shown by the arc lines. The intensity of those ghosts is negligible, and do not affect neither to the line identification nor to the science images. In OSIRIS+ these ghosts are even less intense than in previous configuration..



OSIRIS+ grism ghost effect shown in the arc images.

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icon VPHs R2000/2500 ghosting

OSIRIS R2000 and R2500 VPHs suffered from a quite strong ghost image of the spectrograph slit, that was 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 is now 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 ghost is negligible in the R2500I and R2500R VPHs, while in R2000B, R2500U, and R2500V the ghost was only noted in the spectral flat-field images and arc lamp frames, where a very faint slit image can be observed superimposed on the spectral flat / lamp arcs. In the new OSIRIS+ configuration, that uses lower exposure times for acquiring calibration frames, these ghosts are now negligible.

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icon Second Order Contamination

All the OSIRIS+ red grisms/VPHs (R300R, R500R, R1000R, R2500R, and R2500I) are used in combination with an spectral order sorter filter (GR), which cuts out the light blue-ward from ∼495 nm. However, there is a slight contamination in the spectrum due to the second order, as the spectral order sorter filter doesn't block completely the contribution for wavelengths lower that the defined cut level (see Figure below). Hence, there is a distinguishable contribution for wavelengths at 480 – 490 nm, whose second order may contribute somewhat at 960 nm – 980 nm, depending of the source spectral distribution.

Below is an example spectra of the flux standard star PG1545+035 taken with a 2.5 arcsec slit and 300 secs exposure time. The low-resolution spectrum with the R300R grism shows the first order of dispersion well centered on the CCD. Also visible is the zeroth order on the left. Although the spectral order sorting filter was used, which cuts out the light blueward from ~495nm, the second order spectrum is visible on the right-hand side of the graph.


Example of a OSIRIS+ spectrum taken with a red grism, showing the contribution of the second order contamination in the reddest wavelengths.

The higher sensitivity in the blue of OSIRIS+ compared with OSIRIS, produces that the blue grisms (R300B, R500B, R1000B) also shown some contamination in the spectrum due to the second order, as the emission for wavelenghts at 330 - 360 nm are now contributing to the spectra at wavelenghts larger than 650 nm.


Sensitivity curves for OSIRIS and OSIRIS+ with R1000B grism, where the change in slope due to the second order contamination is seen at wavelength larger than 650-700 nm (producing an increase of the sensitivity that is not real).

The contribution of the second oder contamination is dependent of the source spectral distribution with values ranging from 3% at 650 nm to 7 % at 700 nm, but as large as 20-25% at 750 nm. For this reason, spectral coverage of blue grisms in OSIRIS+ should be restricted to 360-700 nm in all the cases (R300B, R500B, R1000B). Additionaly for users combining data from a blue and a red grism is recommended to use the data coming from the red grism for wavelenght > 650 nm.


Example of a spectrum obtained by combining R1000B and R1000R data, the effect of the second order contamination is slightly visible for wavelengths redder than 700 nm.

This second order contamination only appears always at lower resolutions in OSIRIS, so VPHs for R=2000-2500 are completely unaffected.


Sensitivity curves for OSIRIS+ with all R=2000-2500 VPHs compared with those for R=1000.

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icon Flexures

OSIRIS+ allows a very stable spectral calibration, with no significant drifts with rotator position or elevation changes at Cassegrain focus station thanks to its active collimator. Therefore, the calibrations for each observation can be taken either at the beginning or at the end of the night regardless of the orientation of the instrument when the science observation is carried out. Next figure shows an example of the wavelength shift as a function of both rotator angle and elevation, for different spectral lines, with R1000R grism.


Shift in the spectral direction (X) for the OSIRIS+ arcs emission lines with respect to Cassegrain rotator position (left) and Elevation (right). The more extreme variations are less than +-0.1 pix (binned), even better than in Nasmyth focus.

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icon Spectroscopic Photon Detection Efficiency

The overall photon detection efficiency in spectroscopic mode has been measured using a spectrophotometric standard star through a wide slit, as a function of wavelength and for different grisms. The results are displayed in the following graphs showing the end-to-end overall percentage detection efficiency, and the limiting magnitudes (AB) for obtaining S/N=5 in 1 h of integration time with OSIRIS@GTC for the complete set of grisms/VPH available (assuming 1 arcsec seeing, dark night and airmass=1.2).

spectral efficiency

OSIRIS detection efficiency in spectroscopic mode.


spectral efficiency

Limiting magnitudes for obtaining S/N=5 in 1 h of integration time with OSIRIS+.

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icon OSIRIS+ Overall Efficiency

The following diagram summarizes the overall photon detection efficiency in both imaging and spectroscopic modes of OSIRIS (for the higher resolution grisms/VPHs)

spectral efficiency

Overall detection efficiency in OSIRIS+.

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icon OSIRIS+ Data Reduction Tools

Reduction of OSIRIS+ longslit and MOS data is included in the PypeIt python package.

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icon OSIRIS+ contact persons at GTC


contact email @ gtc.iac.es
Daniel Reverte - main contact daniel.reverte
Gabriel Gómez Velarde gabriel.gomez

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icon Useful Documents

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Last modified: 31 May 2024

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