Introducing Weizmann Fast Astronomical Survey Telescope (W-FAST) (Astronomy)

Eran Ofek and colleagues in their recent paper presented an overview, performance metrics, science objectives, and some first results of the Weizmann Fast Astronomical Survey Telescope (W-FAST). Their study recently appeared in Arxiv.

The Weizmann Fast Astronomical Survey Telescope is a 55-cm Schmidt telescopes. The telescope is located at the Wise Observatory near Mizpe-Ramon in the Negev desert in Israel and is expected to be relocated to a new site in Neot-Smadar, Israel, during 2021.

The Telescopes were designed and built in the Weizmann Institute. The telescope design provides a 23 deg² corrected field of view on a 9×9 cm in the focal plane.

It is equipped with a fast readout sCMOS-camera, having a 7 deg² field of view and is capable of taking 100 images per second with low read noise.

In addition, it is equipped with a single filter at any given time. There are two available filters that can be manually exchanged. The F505W from Asahi, has a tophat transmission curve between 400 to 610 nm. This filter was chosen to maximize throughput at the blue side of the visible spectrum (where the Fresnel scale is smaller). A second filter, F600W, covers the range 500–700 nm.

Fig 1: Transmission curves for the F505W and F600W filters © Eran Ofek et al.

What are its goals?

W-FAST is built to explore the periodic and variable sources, as well as short duration transients.

One of the science goals of W-FAST is to use occultations of background stars to detect Solar System objects that are too faint to detect in reflected light. For these purposes, W-FAST produces lightcurves and raw-data cutouts for a few thousand of the brightest stars in the field at a cadence of 10–25 Hz, as well as full-frame coadded images with a cadence of 4–10 seconds. The coadded images provide measurements of dimmer, lower-time-scale objects. The first objects they expect to detect using this method are Kuiper Belt Objects, which are estimated to be detectable at a rate of ≈ 1 per month. Another class of objects are Oort cloud objects for which the detection rates are uncertain.

W-FAST will have the advantage of high-cadence, continuous coverage when searching for short-time periodic signals. Even low amplitude periodic variations could be detected when added coherently on a long enough time-span. Thus, it can also be used to detect or follow-up Mdwarf flares, close binaries, variability of accreting compact objects, FRB optical counterparts, Near Earth Objects (NEOs) etc.

First results

1) Search for fast transients

During 2020 July and August E. Ofek and colleagues conducted a blind search for sub-second transients using a custom pipeline running that search for transients in the 25 Hz, full-frame imaging data. They have detected a high rate (30–40 events per day per deg²) of short-duration glints (on the order of 0.2 s), with magnitude in the 9–11 range, coming from geosynchronous satellites. These glints would be an important foreground to any searches for astrophysical transients.

2) Cataclysmic Variables: DQ Her

DQ Her is an intermediate polar, with a white dwarf rotating on a shorter period than the orbital period. The orbital period is 4.65 hr while the rotation period of the white dwarf is 71s, and the B-band magnitude of the system is around 14.5. E. Ofek and colleagues obtained an hour of observations at 2020 May 26, of a field centered on DQ Herculis. They used slow-mode (native 3s exposure times) to observe this target. They showed the resulting lightcurves for DQ Her and a few other stars of similar magnitudes in Figure below. The lightcurves of the other stars, shown in fig. for comparison, have a relative RMS of about 2-3%. The lower plot, for DQ Her, shows variability of 6%, with obvious structure, but no apparent periodic signal. When plotting the power spectrum they saw a clear peak at a period of 71.4s. This is consistent with the white dwarf rotation period. The power at lower frequencies is due to systematic red-noise and shot-noise from the object itself.

Fig 2: Lightcurves of DQ Herculis and three other stars in the field, with similar magnitudes, for comparison. Data was taken at cadence of about 3s. The lower, purple plot is for DQ Her. The relative RMS of the other stars is around 2-3%, while the DQ Her lightcurve shows variability of 6%. The apparent variations are not periodic, but a 71s period is buried under the noise © Ofek et al.

3) Targeted KBO occultation

In this, they observed an occultation by a known KBO, 38628 Huya, which is a ≈ 400 km object in the Kuiper belt and set a lower bound (of at least 364 km) on the diameter of Huya.

Findings

They showed that, the system can reach a photometric precision of better than 1% for bright stars (Bp < 10) and 2% for faint stars (Bp < 13) when summing data from multiple exposures at ∼minute time-scales. They also found that, with a large sample of stars in each field, self-calibration could help reduce some of the systematics involved with co-adding images over longer time-scales, reaching down to few milli-mag precision.

“In the near future, we intend to conduct a galactic plane survey, to characterize stellar variability on second time-scales and to detect short period binaries. We are also actively searching for astrophysical sub-second transients inside the Earth’s shadow, where flashes. from high-orbit satellites should not be visible.”

— concluded authors of the study

Featured image: The W-FAST dome in Mizpe-Ramon, Israel. © Weizmann Institute of Science


Reference: Guy Nir, Eran O. Ofek, Sagi Ben-Ami, Noam Segev, David Polishook, Ofir Hershko, Oz Diner, Ilan Manulis, Barak Zackay, Avishay Gal-Yam, Ofer Yaron, “The Weizmann Fast Astronomical Survey Telescope (W-FAST): System Overview”, Arxiv, pp. 1-13, 2021. https://arxiv.org/abs/2105.03436


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