SiFAP2: Silicon Fast Astronomical Photometer and Polarimeter
SiFAP2 is an extremely fast photometer based on Hamamatsu MPPC bi-dimensional silicon photo-multiplier, with a custom readout electronic system built in Università La Sapienza (Rome) capable to tag single photons with a time resolution of 8 nanoseconds. The absolute timing is provided by a commercial GPS system with an accuracy better than 60 microseconds with respect to the UTC (Universal Time Coordinated). SiFAP2 is the opto-mechanically improved version of SiFAP (html; pdf) with added polarimetric capabilities.
SiFAP2 – High Speed Photometer at TNG
Figure 1: SiFAP2 is mounted at the F/11 direct focus on the Nasmyth Interface.
For the next semester of April/September 2021 SiFAP2 will be offered as PI instrument.
Please refer to the Call for Observing Time if you wish to submit a proposal. [link to the call]
For more information about the instrument please refer to the Reference.
For the scientific results obtained with SiFAP and SiFAP2 please refer to the References.
The Detectors
SiFAP2 uses HAMAMATSU MPPCs (Multi Pixel Photon Counters) detectors, composed of a bidimensional matrix of SPADs
(Single Photon Avalanche Diodes) with overall linear dimension of ~1.3 × 1.3 mm².
Photon counting capability rather than imaging.
Plate scale at F/11 focal plane: 0.187 mm/arcsec → ~7 × 7 arcsec² FoV.
Target detector mounted on-axis (center of FoV).
Reference detector displaced from Target depending on on-sky separation.
Photon Efficiency
Figure 3: Typical Photon efficiency of a MPPC
Expected Counts
Figure 4: Best fitting curve for net count rates of standard K0V-K5V stars observed with SiFAP2 in white light (no filter applied) as a function of catalogue V mag.
Observations at small Zenith distances (air mass correction negligible).
Red point (G0V star) not included in fitting procedure.
Resume of Main Characteristics of SiFAP2
Acquiring mode: Time Tag, Gated mode
Absolute timing accuracy: < 60 µs
Relative time resolution*: 8 ns (Time Tag), 1 ms (Gated Mode)
Field of View: 7 × 7 arcsec
Bandpass: 320 – 900 nm (peak @450 nm: 40% eff. ; @900 nm: 5% eff.)§
Linearity: < 10 Mcps (detector saturation @15 Mcps)
Count rate/Vmag: 10-(0.942 × Vmag – 18.67) 100^0.2 kcps
Maximum storing count rate:
< 2 Mcps for Time Tag (throughput saturation)
< 15 Mcps for Gated Mode (detector saturation)
* Relative time accuracy suffers from clock drift, corrected via linear function of time. Thermal drift negligible (five orders of magnitude smaller than clock period).
Observing Modes
1. Photometry
Simultaneous light collection by Target + Reference detectors.
Broadband filter optional.
Reference star required to check for: sky transparency, clouds, dust, airmass, tracking/guiding accuracy.
2. Linear Polarimetry
Polarizing cube beam splitter inserted before Target Detector.
Splits two polarized states between Target and Folded detector.
Lambda/2 retarder rotates at [0°, 22.5°, 45°, 67.5°].
Focus shift compensated by TNG secondary mirror.
3. Circular Polarimetry
Achieved with lambda/4 retarder.
System requires re-calibration.
NOTE: Not offered in next call.
Reference Position Constraints
Separation range: 210 – 480 arcsec
Min: due to detector dimensions
Max: limited by FoV
Position Angle: calculated N → E, added as offset from SiFAP2 PA.
PA with 0 offset → Reference north of Target.
Example:
Target: Crab pulsar
Reference: 2MASS J05340719+2200294
Separation: 344.88 arcsec (64.49 mm)
Derotator PA: SiFAP2 + 266.203
DOLORES Filter Set
A set of 8 broad-band filters is available. Narrow-band filters are also available on request (requires pre-arrangement with PI). Focus shift is corrected by the secondary mirror.
| Filter Name | FW [deg] | Notes |
|---|---|---|
| U Johnson | 125 | Broad-band |
| V Johnson | 197 | Broad-band |
| R Johnson | 233 | Broad-band |
| B Johnson | 269 | Broad-band |
| r Sloan | 17 | Broad-band |
| i' Sloan | 53 | Broad-band |
| u Sloan | 89 | Broad-band |
| g Sloan | 305 | Broad-band |
| CLEAR | 161 | White light photometry |
Preparing Observations
User must provide an ASCII .txt catalogue with format:
Targ_name, Targ Alpha, Targ Delta, Targ Vmag, Ref Alpha, Ref Delta, Ref Vmag, Obs Mode
Example:
Crab_PSR,05:34:31.94,+22:00:52.2,M_Vt,05:34:07.194,+22:00:29.48,M_Vr,PHO
Crab_PSR,05:34:31.94,+22:00:52.2,M_Vt,0,0,0,0,POL
If reference not necessary (e.g. Polarimetry), values can be replaced with 0.
Data Output
Pre-reduced data delivered as FITS files.
Contain: time tag of detected photons and multiplicity.
References
- SiFAP: A New Fast Astronomical Photometer, Acta Polytechnica CTU Proceedings 1, 2014. [DOI: 10.14311/APP.2014.01.0311]
- The Latest Version of SiFAP: Beyond Microsecond Time Scale Photometry of Variable Objects, JAI Vol. 05, No. 03, 2016. [DOI: 10.1142/S2251171716500057]
- Fast multichannel astronomical photometer based on silicon photo multipliers mounted at the TNG, Proc. SPIE 9147, 2014. [DOI: 10.1117/12.2064649]
- SiFAP: High Speed Photometry for variable sources in the Optical band, COSPAR Assembly, 2018.
- SIFAP2: a new versatile configuration at the TNG for the MPPC based photometer, Proc. SPIE 10702, 2018. [DOI: 10.1117/12.2316348]
- Optical pulsations from a transitional millisecond pulsar, Nat. Astr., Vol. 1, p.854, 2017. [DOI: 10.1038/s41550-017-0266-2]
- Pulsating in Unison at Optical and X-Ray Energies: Simultaneous High Time Resolution Observations of the Transitional Millisecond Pulsar PSR J1023+0038, ApJ, Vol. 882, 2019. [DOI: 10.3847/1538-4357/ab2fdf]