Instruments of 3.6M

 1.   TIRCAM2 (TIFR Near Infrared Imaging Camera – II) 

TIRCAM2 (TIFR Near Infrared Imaging Camera – II) is a closed cycle cooled imager that has been developed by the Infrared Astronomy Group at Tata Institute of Fundamental Research for observations in the near infrared (NIR) band of 1 to 3.7 μm. Figure 1 shows the TIRCAM2 system mounted at the main port of the DOT.

TIRCAM2 is sensitive between 1 and 5 μm and contains selectable standard NIR filters J, H, K, Kcont, Br-Gamma, Polycyclic Aromatic Hydrocarbon (PAH) and narrow-band L (nbL) for imaging. Table 1 lists the details of the imaging filters and Figure 2 shows the transmission curves of the filters. TIRCAM2 uses a 512 x 512 InSb Aladdin III Quadrant focal plane array. It is cooled to an operating temperature of 35 deg K by a closed cycle Helium cryo-cooler. TIRCAM2 is currently the only NIR imaging camera in the country which can observe up to L band in NIR. The dark current measured was ~12 electrons/sec and the readout noise was ~30 electrons for the FPA. The median gain of the detector was found to be ~10 electrons/ADU.

                           Table 1: TIRCAM2 Filter Characteristics

Filter λcen (μm) ∆λ (μm)
J 1.20 0.36
H 1.60 0.30
Br-Gamma 2.16 0.03
K 2.19 0.40
Kcont 2.17 0.03
PAH 3.27 0.06
nbL 3.59 0.07

 

The Field-of-View (FoV) of TIRCAM2 on DOT is 86.5 x 86.5 arcsec2. Pixel Scale of TIRCAM2 on DOT is 0.169 +/- 0.002 arcsec/pixel. With typical 1.2 arcsec seeing conditions, TIRCAM2 heavily oversamples the star profile. This pixel sampling is ideal for high accuracy photometry of bright NIR sources. For details of the TIRCAM2 instrument and calibration see the following paper:Naik, M. B., Ojha, D. K., Ghosh, S. K., et al. ‘TIRCAM2: The TIFR Near Infrared Imaging Camera’. Bull. Astr. Soc. India (BASI) 40 (2012): 531–545.

                                            Figure 1: TIRCAM2 mounted on the DOT on 1 June 2016.

Optimal Observation Strategy

The observation strategy for TIRCAM2 is largely standard NIR observation strategy which involves taking flats in the morning and evening twilight. Dithered short exposure observations of the target star as well as a nearby NIR photometric standard star with 5-point dithered pattern is sufficient. The only additional step needed in TIRCAM2 observations is additional blank sky images of identical exposures. This is to remove a non-uniform additive illumination in TIRCAM2 images. During data reduction they are to be removed by subtracting these blank sky images before the flat-fielding step.

 
  Figure 2: Filter transmission curves of TIRCAM2 (at present H2 narrow band filter is not available in TIRCAM2 due to limited slots in the filter wheel).
 

Performance of TIRCAM2 with the 3.6m DOT during the Early Science Cycle (11 – 23 May 2017)

TIRCAM2 had its early science runs with the 3.6-m DOT telescope during 11 – 23 May 2017. The weather conditions during these nights were generally clear but with high relative humidity of typically more than 60%. It is expected to achieve better numbers during the winter cycle than what are being reported below.

  • Typical seeing during May 2017 nights was < 1.0 arcsec in the NIR bands.
  • Best seeing achieved during these nights was ~6 arcsec in K-band.

Figure 3 shows an example image with seeing (FWHM) of ~0.6 arcsec.

 

Figure 3: Upper panel shows cut-out of a stellar image observed with TIRCAM2 in K-band on 22 May 2017 towards the Serpens OB2 association. Typical seeing on this night was ~0.6 arcsec. The outer green contour shows the stellar FWHM of the image. The lower panel shows the radial profile of the image shown in the upper panel.

Limiting magnitudes in different NIR bands:

The limiting magnitudes in JHKL bands obtained from the analysis of several target fields (viz. globular clusters, star-forming regions, etc.) are the following:

  • J-band : 0 mag (S/N ~ 10); for a total exposure time of 550s
  • H-band : 8 mag (S/N ~ 10); for a total exposure time of 550s
  • K-band: 0 mag (S/N ~ 10); for a total exposure time of 1000s
  • L-band: 2 mag  (detection limit); for a total exposure time of 20s

Figure 4 shows the typical DAOPHOT errors in magnitude as a function of JHK magnitudes.

Figure 5 shows the exposure times required for different magnitude stars to achieve a signal-to-noise ratio of 20.

A colour composite image of M92 globular cluster generated using TIRCAM2 J (blue), H (green) and K (red) images, is presented in Figure 6. A 2MASS image of the same region is also presented for comparison.

Figure 7 shows a mosaic image made using four TIRCAM2 J-band images (each with 550 sec exposure) of NGC 4567 & NGC 4568 twin galaxies observed on 15 May 2017.

A mask for bad pixels in TIRCAM2 array is also constructed and presented in Figure 8.

Typical Sky brightness at Devasthal (May 2017):

The typical values of sky brightness values obtained in good night conditions (during May 2017) are the following:

  • J-band : 4 mag / arcsec2
  • H-band : 0 mag / arcsec2
  • K-band:  12.2 mag  / arcsec2

Figure 4: J, H and K magnitudes versus magnitude errors observed with effective exposure times of 550, 550 and 1000s, respectively. The photometry was carried out with an aperture of radius 1 FWHM.

Figure 5: Estimated exposure times required for photometry in TIRCAM2 JHK bands to achieve S/N∼20 on a typical DOT night. The top panel shows sensitivity for all bands with 75% M1 mirror reflectivity, while other three panels show sensitivity for 75%, 50%, and 25%, separately for each band, respectively.

Figure 6: RGB colour composite image (red: K, green: H, blue: J) of M92, a Galactic globular cluster, generated using TIRCAM2 with the 3.6m DOT (upper), and 2MASS (lower).

Figure 7: Mosaic of four TIRCAM2 J-band images (each with 550 sec exposure) of NGC 4567 and NGC 4568 twin galaxies observed on 15 May 2017.

                                                                  Figure 8: Bad pixels mask of TIRCAM2 array.

 nbL-band detection and observing possibilities:

An array of nbL-band (λcen ~ 3.59 µm) images (100×100 pixels cut-outs) for different magnitude stars is shown in Figure 9. The sources up to nbL magnitude of 6.0 are aligned and combined as they are visible even in the short exposure frames of 0.001s or 0.05s, which finally helps us to achieve a better signal-to-noise ratio in the combined image. However, sources having nbL magnitudes fainter than 6 are co-added blindly. A substantial difference in signal-to-noise ratio can be observed depending up on if they are aligned and combined or co-added blindly (see Figure 10). Hence, a source fainter than 8.0 mag in the nbL band is possible to observe if a bright source is present in the frame for alignment.

Figure 9: Mosaic of nbL band images (100×100 pixels cut-outs) observed during 13-14 May 2017. Sources brighter than 6 nbL mag are aligned and combined and the remaining sources are co-added blindly.

Figure 10: Similar to Figure 9, frames (100×100 pixels cut-outs) co-added after alignment (left) and blindly (right). The signal-to-noise ratio improves if they are aligned and then combined.

Figure 11 shows the plot of instrumental magnitudes calculated using log of ADUs/sec versus standard L-band magnitudes (W1 band; 3.4 µm) from the WISE. The plot shows that our array is linear in the L magnitude range from 3.0 to 8.0 in spite of scatter in the data in the fainter magnitude regime. The scatter is possibly seen because of variable sky conditions with high humidity of more than 60%.

Figure 11: Count rates (in ADUs/sec) versus actual L-band magnitudes from WISE (W1 band), showing linearity of TIRCAM2 in the nbL-band. Scatter towards the fainter magnitude regime is seen possibly because of variable sky background due to high humidity.

Detection of PAH emission:

Emission in the PAH band (3.27 µm) towards Sh2-61 centre region is detected with an effective exposure time of 6.6 sec. We have also observed the region in the nbL-band for same exposure time for continuum subtraction. The left panel in Figure 12 shows the continuum-subtracted PAH band image of 30 x 30 arcsec2 area towards the Sh2-61 centre region. PAH emission is detected with a signal-to-noise ratio of 6.

Figure 12: The left panel shows the continuum-subtracted PAH band image of 30 x 30 arcsec2 area towards the Sh2-61 region. Contours are overlaid for clarity. Spitzer 3.6 μm image for the same area is also presented for comparison in the right panel.

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 2.        4K x 4K CCD Imager for the 3.6m DOT

As an in-house developmental activity, the 4KX4K CCD Imager is designed and assembled as a first light instrument for the axial port of the 3.6m DOT using the f/9 beam directly for a plate scale of 6.4 arc-sec/mm. The pixel size of the blue enhanced liquid Nitrogen cooled STA4150 CCD chip is 15 micron with options to choose gain and speed to utilize the dynamic range. It is possible to acquire multi-band images of the central 6.5×6.5 arc-min of FoV of the telescope for variety of scientific goals getting a deeper photometry in several broad-band filters for point sources and objects with low-surface brightness. The median seeing of Devasthal site is 1.1 arc-sec and occasionally seeing as low as 0.7 arc-sec (~ 10% of the observing time) as seen during ground level seeing measurements.    

Camera Specifications:

      Port/f-ratio                           : Axial port / direct use of the f/9 beam     

      CCD Chip                              : 15 micron/pixel, 4096×4096 pixels, back-   illuminated, 16-bit A-D

      Full-well capacity                : 250k — 265k electrons, MPP/non-MPP modes

      Gain                                       : 1,2,3,5,10 electrons/ADU (selectable)

      Read-out Noise (speed)     : 7-9 electrons (@1 MHz) or  4-6 electrons (@500 KHz) or

                                                      2-3 electrons (@100 KHz) 

      Binning                                 : 2×2, 3×3 or 4×4, selectable as per the requirement

      Dark Current                       : 0.0005 electrons at -120 C

      Dewar, Operating Temp    : Liquid Nitrogen based, -121.3 C

For this CCD camera, images could be saved in .FITS format with an option to save bias and dark levels of each frame individually by adding certain number of over-scan pixels both in lines and columns. It is also possible to read-out the chip in a single more using only one amplifier or one could choose a ”quad” mode reading all the four quadrants separately. However, while opting for a ”quad” mode for some of the science cases, one should be careful about the pre-processing of the .FITS files before further processing/calibrations.

Shutter and Filters:

The camera has software controlled (ARCHON controller, OFC-based) mechanism to open and close 125mm Bonn shutter (shortest exposure of 10 msec) along with an automated motorized filter wheel movement for two sets of wheels. Each wheel contains 5 sets of Bessel or SDSS filters with one clear space to select one filter at a time. From one filter to the next one, minimum time taken is 15 sec.

Presently, with this instrument, CCD imaging could be done in following sets of 10 broad-band filters.

  

Filters       Central wavelength/Band-width      Filters          Central Wavelength/Band-width

(Set 1)      (Angstrom units)                                    (Set 2)          (Angstrom units)

Bessel U    3663 / 650                                                 SDSS u                3596 / 570

Bessel B    4361 / 890                                                 SDSS g                4639 / 1280

Bessel V    5448 / 840                                                 SDSS r                6122 / 1150

Bessel R    6407 / 1580                                               SDSS i                 7439 / 1230

Bessel I     7980 / 1540                                               SDSS z                8896 / 1070

 

Simulations and Calibrations:
Throughput of the CCD Imager was calculated using the above parameters, the standard specifications of the 3.6m DOT and using the published values of the extinction coefficients, sky-brightness for an assumed value of 1 arc-sec FWHM at zenith. In the figure below, simulated results of the signal to noise ratio at different brightness levels for a CCD read-out speed of 1 MHz and for a 300s exposure time in UBVRI filters are given for reference.

Figure 1: Simulated throughput in terms of S/N for the combined set-up of the 4Kx4K CCD Camera and the 3.6m DOT for a given exposure time of 300 sec in set of Bessel UBVRI filters for reference. 

The fully assembled CCD camera was tested and characterized at the axial port of the 3.6m DOT in early 2016. Using sky flats in several filters and bias frames, photon transfer curves were generated to verify gain and read-out noise values at different speeds. Parameters like linearly, bias stability were verified in single output mode.  Extinction coefficients for all 10 filters were also recently measured using Landolt standard fields between airmass range of 1.1 to 3.5 during median seeing conditions.

Figure 2:  The 4Kx4K CCD camera along with the automated filter wheel as mounted at the axial port of the 3.6m DOT. Exposure time in a given filter and other CCD parameters could be set as per the science requirements. The Telescope could be operated both in open and closed loop modes using the auto-guiding unit through the Telescope Control Software. The details about making the telescope ready for observations and using the auto guiding units etc. will be provided through a separate set of documents/manuals. On site observing assistance will be provided. The user manuals of the CCD camera will also be provided separately   to   the   observers.   The   instrument   will   be   mounted/un-mounted by the observing assistants/staff members on duty in consultation with the instrument team.

Filters Measured ‘k’ values

(17 Apr 2017)

Published ‘k’ value for

Devasthal  (Mean value)

Bessel U 0.64+-0.03 0.49+-0.09
Bessel B 0.39+-0.02 0.32+-0.06
Bessel V 0.29+-0.02 0.21+-0.05
Bessel R 0.22+-0.01 0.13+-0.04
Bessel I 0.17+-0.02 0.08+-0.04
SDSS u 0.70+-0.02  —
SDSS g 0.30+-0.04
SDSS r 0.20+-0.02
SDSS i 0.14+-0.02
SDSS z 0.09+-0.03

Table 1: Measured values of the extinction coefficients in all 10 filters as measured using the 3.6m DOT and the CCD Imager. The values of extinction coefficients in UBVRI filters are compared with those measured at Devasthal long back.

 

 

 

 

 

 

 

 

Figure 3: Colour composite RGB images of NGC 488 (left, 3*200 sec) and NGC 613 (right, 3*200 sec) as observed with the 4Kx4K CCD Imager mounted at the axial port of the 3.6m DOT.

Several low-surface brightness objects (see Figure 3) along with deep photometry of point sources were performed during the testing phase of the camera during early 2017.

The CCD Imager was used to calibrate some of the Landolt standard fields (PG and SA standards) along with known open cluster fields in various filters. For the assumed values of extinction coefficients published for the Devasthal site, multi-band photometry of the observed Landolt standard fields were used to derive the transformation coefficients. Once applied to the observed science frame of an open cluster field NGC 4147 (Figure 4), several point sources in B-band with a brightness of fainter than 24 mag with an accuracy of < 0.1 mag were detected in an effective exposure time of 2x600sec. These observations were performed during a dark/gray night and during average seeking conditions of around 1.6 arc-sec.

 

Figure 4: An example demonstrating capability of the 4Kx4K CCD imager getting deeper image for photometry of  point sources in a Globular cluster NGC 4147.

The pre-processing of the images were done using the bias and flat frames observed in respective filters were used to pre-process the images using standard IRAF routine called ”ccdproc” after trimming the images to remove the over-scan area. We used ”cosmicrays” routine to remove the cosmic hits in various exposures and images were stacked wherever required.

After the pre-processing is done, we used the DAOPHOT-II FORTRAN subroutines in sequential standard orders (ie. Daogrow, ndaomatch, ndaomster, nccdstd, nccdlib, nfinal…etc) to calculate the transformation coefficients determined using the observed Landolt standard fields. These transformations were then applied to the observed globular cluster field NGC 4147 to calibrate the field. The results obtained through this procedure are described below in Figure 5 as color-magnitude diagram (CMD) of the NGC 4147.

Figure 5: The color-magnitude diagram (CMD) of the Globular cluster NGC 4147 as obtained using the present calibration data taken using the 4KX4K CCD mounted at the axial port of the 3.6m DOT. The total number of common stars plotted (detected in both the filters) are around 3500 with a photometric accuracy of < 0.2 mag. The number of stars having B < 24 mag are around 150. There are many more detections for which error could not be established using the present data set. The scattering in the CMD could be due to membership issue which has not been accounted for at this stage of calibration. The main branch and other features typical for this cluster are clearly identified using our data set observed in B and R filters. If we go deeper in good seeing conditions, even fainter point sources could be detected with an improved photometric accuracy

Longer monitoring of the same cluster field also produced lightcurves of several variable stars as expected in the main sequence and horizontal branch of the colour magnitude diagram.

Figure 6: Examples of lightcurves of several variable stars as observed in the field of the Globular cluster field NGC 4147. The lightcurves have a range of periodicities and brightness as expected in case of such older clusters. The data and calibration results presented in this report will be published soon.

Another example, showing the capability of the imager demonstrating to observe features of low surface brightness are shown below in Figure 7.

Figure 7: One of the nearby galaxy NGC 5486 along with neighbouring galaxy NGC 5487 (right panel) is compared with the similar field as observed by the SDSS survey (left panel). The figure shown in right panel is an equivalent exposure time of 5600 sec taken in r-band using the 4Kx4K CCD camera mounted at the axial port of the 3.6m DOT under a joint Belgian-Indian proposal accepted during the last cycle. It is clear that in the 3.6m DOT image (right panel), we are able to show the low-surface brightness features very clearly. The contour levels are 22, 23, 24, 25 and 26 mag/arcsec2. The interaction features between the two galaxies are clearly seen as an extended low-surface brightness feature between the two galaxies. The detailed analysis of the data is ongoing.

   In this report, we present status of the 4Kx4K CCD imager used during first cycle of observations. As described, one could go deeper during best seeing conditions in rather longer exposures. During first cycle of observations many other objects like AGNs, Supernovae, White dwarfs, galaxies etc have also been observed under various proposals. We are yet to know about those results observed under various proposals. Noticed issues related to the CCD Imager like filter wheel slippage, light leakage, CCD temperature sensor related problems are expected to be resolved during the next cycle of observing with help of an improved version of the Imager.


 

 

 


Upcoming Instruments

 

 1. TANSPEC

 TIFR-ARIES Near Infrared Spectrometer (TANSPEC) is being built in collaboration with MKIR, Hawaii for the 3.6 meter Devasthal Optical Telescope (DOT). It will be a unique spectrograph which provides simultaneous wavelength coverage from 550 nm to 2540 nm, and a resolving power of R ~ 2750. Spectrograph operates in two modes which images the spectrum on to a 2k x 2k H2RG array. In cross-dispersed (XD) mode combination of a grating and two prisms are used to pack all the orders on to the H2RG array at a resolution of R ~ 2750. It also has a low resolution prism mode (R ~ 150) for high throughput observations. TANSPEC consists of an independent imaging camera with a 1k x 1k H1RG detector. The reflected beam from slit (built-in slit viewer) is imaged to this camera through a filter wheel which consists of broad band r’, i’, Y, J, H, Ks and narrow band H2 & BrG filters. This camera has a field of view of 1 x 1 arcmin 2 , and is used for guiding the telescope (IR guider) as well as imaging field for photometry. It also functions as a pupil viewer for instrument alignment on the telescope. For calibration, a uniform flat field from an integrating sphere outside the dewar as an identical f/9 beam from telescope will be imaged. Wavelength calibration will be done by Argon and Neon lamps. Spectroscopy sensitivity (100-σ in 1 hour, 1’’ seeing) is expected to be 15.4 mag (R ~ 2750), whereas in prism mode (R ~ 100) it would be 17.3 mag in the J-band. TANSPEC will be used for a wide range of studies from local star formation to extra-galactic astronomy. Simultaneous coverage of wavelength from 550 nm to 2540 nm makes TANSPEC a unique instrument and ideal for studies which require simultaneous measurement of lines in optical and near-infrared. TANSPEC is expected to be ready for tests on telescope by February, 2018.

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2.    Faint Object Spectrograph & Camera (FOSC)

Faint Object Spectrograph & Camera (FOSC) is a versatile instrument, which enables one to do spectroscopy, imaging, and also polarimetric observations of faint celestial objects.

FOSC is based on proven ESO design concepts, and uses a collimator and a focal reducer.Optical filters and grisms (a grating ruled on prism) are inserted in the collimated beam. Slit can be placed at the focal plane of telescope.FOSC for the 3.6 meter Devasthal Optical Telescope (DOT) is designed, developed and assembled by ARIES with inputs from various organizations like ISRO, IUCAA, IIA, and several industries.ARIES-Devasthal FOSC is indigenously designed and developed, for the first time in India.

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3. HIGH RESOLUTION SPECTOGRAPH

 In order to learn the nature of the celestial sources in-depth, spectroscopy is considered as the major tool for astronomers. This immediately demands to equip the astronomical telescope with the accurate and efficient spectroscopic instrumentation. Moreover higher the spectral resolution of the instrument, the more detailed information about the observed source becomes available to the astronomer. A High-Resolution Spectrograph is envisaged as a key instrument to meet the spectroscopic requirements of the telescope. The design of the spectrograph will be based on the concept of white pupil beam folding for high-resolution spectrograph. The instrument is planned to be a side port instrument with an optical fiber carrying the light from the telescope focal plane to the spectrograph room. The scientific requirements are framed to meet the interests of the Indian astronomers. The technical specifications have been generated based on the following the science drivers.
–> Asteroseismology
–> Abundances Studies
–> Doppler imaging of spotted stars
–> Peculiar eruptive young stellar objects and Winds from T-Tauri stars
–> Spectroscopy and Ground-based Follow-up of Exoplanets
–> Supernovae
Proposed instruments will be designed for use with the 3.6-m Devasthal Optical Telescope of ARIES with the average seeing of 1 arc seconds at V band. It will consist of the Main spectrograph unit, Interface unit with the telescope, Exposure meter, guiding unit, calibration unit, and Atmospheric dispersion corrector. The instrument will cover the entire optical band from 380 to 900 nm. It will have two resolutions mode: the high-resolution mode (80k) and the low-resolution mode (20/40k).
 
 
 
 
 
 
 
 

 

3D view of Preliminary design layout of Spectrograph                                                                                             Preliminary design layout of Spectrograph 

 

 

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