A summary of instruments proposed for observing pulsating variables from the Mt. Abu Observatory

TL;DR

The paper presents instrument concepts to enhance follow-up observations of pulsating variables, particularly Type-II Cepheids, from Mt. Abu Observatory using small to mid-sized telescopes. It outlines a high-throughput two-channel spectrograph, a SWIR imager based on InGaAs detectors, and imaging cameras for the 50 cm telescope, emphasizing simplicity, off-the-shelf components, and reliability to enable phase-resolved spectroscopy and infrared photometry. The designs aim to exploit the observatory’s existing facilities to probe pulsation physics, mass loss, and line-profile variability, while enabling efficient follow-up on discoveries from surveys like OGLE and TESS. The work also includes practical considerations and an exposure-calculation framework to guide implementation and observing strategies, with the potential broader impact on variable-star science and related fields.

Abstract

Pulsating variables play a significant role in shaping modern astronomy. Presently it is an exciting era in observational study of variable stars owing to surveys like OGLE and TESS. The vast number of sources being discovered by these surveys is also creating opportunities for 1-2m class telescopes to provide follow-up observations to characterize these. We present some initial observations of type-II cepheids from the Mt. Abu observatory and highlight the need for dedicated observing runs of pulsating variables. We also present optical designs for several suggested instruments for the Mt. Abu observatory that will contribute towards this goal. We present designs that are fairly simple and yet take due benefit of the unique telescopes and facilities present at the observatory.

Figures (13)

• Figure 1: An attempted Period luminosity plot of OGLE type-II cepheids. OGLE amplitudes are also shown in the colour map (see text description). A sample of stars is seen to deviate from the linear P-L law in a fork like pattern. A better picture of this plot with lower scatter is likely to emerge with newer GAIA data releases and from surveys such as VVV.
• Figure 2: KTCOM lightcurve collected from TESS archive: KT COM has been listed in the GCVS catalogue as a suspected Type-II cepheid. The lightcurve from TESS data shows a characteristic secondary bump in the lightcurve, however, the secondary peak seems to have a variable amplitude between cycles.
• Figure 3: Variation in emission lines of type-II cepheid W VIR : spectra of W VIR collected over 4 epochs is shown. Variability of the emission line is seen as well as a reddening of the spectra is observed coincident with a reduction in emission line strength.
• Figure 4: The proposed high throughput two channel spectrograph: The block diagram of the instrument is shown in (a). The spectrograph is split into two separate channels for blue and red wavelength range so that in each channel the components are optimised for high throughput. The slit for the spectrograph is at the cassegrain focus of the 2.5 M telescope. Standard collimator camera type design with a grating in the collimated beam section is followed for both channels. The collimator optic is common for both the channels; consisting of a Hastings triplet. The splitting between the channels is done after the collimator using a dichroic beamsplitter such as Newport 20CMS-45 or Edmund optics 64-451 centered at around 700nm. The camera is implemented using off-the-shelf doublets from Edmund optics and Thorlabs. The layout of the blue channel is shown in (b) and that of the red channel is shown in (c). The complete optical chain including the VPH gratings (described in figure \ref{['fig:efficiency']}) is implemented using off-the-shelf components.
• Figure 5: Matrix spot diagram of the spectrograph: The spot of the blue and the red channel are presented in (a) and (b) respectively. During optimisation of the optical performance, the spot was allowed to be larger in the direction normal to the dispersion axis (i.e. along the slit) so that the best spectral resolution is achieved along the direction of dispersion. The "spot width" is aimed to be within 1 pixel for the blue channel and within 2 pixels for the red channel.(details of the CCD arrays are provided in table \ref{['tab:spect']}) The resolution of the spectrograph is expected to be pixel size limited for a total field of +/- 6 arcseconds and is shown to be reasonably uniform along the slit.