Expected performance · Time Domain Telescope

Sensitivity

The TDT numbers on this page come from version 1.0 of the TDT exposure time calculator, for 2 m unit telescopes with adaptive-optics correction to the diffraction limit, detector read noise of 0.1 e⁻ per pixel, and 15% efficiency from the top of the atmosphere to the detector. The full list of assumptions is in the calculator.

With all 100 units on one target, the TDT reaches g ≈ 25 in 30 minutes at a signal-to-noise ratio of about 10 per resolution element at R = 5000: spectroscopy that matches the imaging depth of the Vera C. Rubin Observatory, Euclid and Roman.
In a 10-minute exposure at R = 1000, a signal-to-noise ratio of 10 is reached at g ≈ 20.5, 23.0 and 25.3 with 1, 10 and 100 units.
At R = 10⁵, the full array reaches a signal-to-noise ratio of about 200 per resolution element in one hour on a g = 16 star.

Compared with the VLT and the ELT

Exposure time to reach a signal-to-noise ratio of 10 at R = 5000 (or the closest available configuration) on a g_AB = 23 star at zenith, with no Moon and, where applicable, 0.8″ seeing.
Quantity	VLT + X-Shooter	ELT, seeing-limited¹	ELT, GLAO¹	ELT, LTAO²	TDT-1	TDT-10	TDT-100
Telescope diameter (m)	8.2	39	39	39	1 × 2	10 × 2	100 × 2
Spatial resolution (″)	1	0.8	0.12	0.12	0.1	0.1	0.1
Exposure time (s)	46 000	450	2 340	1 170	31 000	3 115	318

¹ From the ESO exposure time calculator. ² The ELT mode closest to the TDT; ELT adaptive optics will not be available in the optical blue.

A single TDT unit performs comparably to VLT + X-Shooter in the sky-background-limited regime, so the TDT outperforms the VLT as soon as its second unit is operational. The full 100-unit TDT performs similarly to the ELT in seeing-limited mode, which uses the ELT as a light bucket with no adaptive-optics correction. With laser tomography adaptive optics (LTAO), the ELT performs like a 30-unit TDT, so the point-source survey speed of the full TDT is three times that of the ELT. The ELT is not optimised for the optical blue, where no high-Strehl adaptive-optics correction is foreseen for it.

Compared with the WST

Exposure time to reach a signal-to-noise ratio of 10 at zenith with no Moon, for the low- and high-resolution multi-object modes (MOS-LR and MOS-HR) of the Wide-field Spectroscopic Telescope, from the WST V1.0 exposure time calculator.
Quantity	WST MOS-LR	TDT-1	WST MOS-HR	TDT-1
Telescope diameter (m)	12	1 × 2	12	1 × 2
Spatial resolution (″)	1	0.1	1	0.1
Spectral resolution R	5 000	5 000	40 000	40 000
Magnitude g_AB	23	23	21	21
Exposure time (s)	19 800	31 000	112 500	38 720

In the low-resolution mode a single WST fibre performs like about 1.5 TDT units. With over 30 000 fibres against the TDT’s 100 units, the WST surveys about 450 times faster in this mode. The TDT, on the other hand, can place its units anywhere on the sky rather than within one field of view, covers the near-infrared from 1 to 1.8 µm, which is not currently foreseen for the WST, and is built for time-critical observations.

At high resolution a single TDT unit outperforms the WST by a factor of 2.9 per target in the sky-background-limited regime, because of the WST’s relatively large 1″ fibres. This reduces the survey-speed advantage of the WST’s 2000 high-resolution fibres over the 100 TDT units from a factor of 20 to about 7. The integral-field mode of the WST, for mapping extended structures, has no TDT equivalent.