PUPPI Observing and Support Guide

Observing Modes

PUPPI has two main modes of operation, incoherent filterbank mode, and coherent dedispersion. Each has a large amount of flexibility in terms of number of channels, etc. Baseband recording is also possible but is still under testing.

Incoherent Mode

Sometimes this is also called "search mode" or "filterbank mode". This mode is just like a normal spectrometer except the integration times are very short (typically tens of us). In this mode, the spectra are accumulated in the FPGA hardware, then output over the network and recorded to disk on puppimaster. Observing instructions for incoherent mode. The available parameters are:

• Total bandwidth: 100, 200, or 800 MHz
• Number of channels: Any power of two from 32 to 4096.
• Output: 8 bits only.
• Polarizations: Total intensity (summed) or full-Stokes. 4096-channel mode is total intensity only.
• Minimum spectrum integration times:
  • For 100 and 200 MHz bandwidth the minimum integration is given by 4 * (# channels) / BW.
    • This is a FPGA hardware limit.
    • e.g., for 200 MHz, 2048 channels, minimum integration time is 40.96 us.
    • In "Fast4K" mode the minimum is 2 * 4096 / BW.
  • For 800 MHz, the minimum is 16 * (# channels) / BW. This is a disk write speed limit (max 200 MB/s to the RAIDs on puppimaster).
• Maximum spectrum integration time:
  • The accumulation length (T_int * BW / N_chan) should not exceed ~1000 (typically ~ms).
  • The 8-bit output values do not have enough dynamic range for this.
• Data format: Search mode PSRFITS.
• There is a real-time folding mode available, but it is hardly ever used now due to the availability of coherent dedispersion mode. An exception is when observing the pulsed noise cal, the data are often folded in real time at the 25 Hz standard cal period.
• Note that in this mode, the true achievable time resolution depends not just on PUPPI setup but also on dispersion smearing. This in turn depends on the pulsar's DM and the observing frequency.

Coherent Mode

For observing known pulsars, a dispersion measure-specific filter can be applied to completely remove dispersion smearing.This is often done for MSP timing. In this mode, voltage data are output from the FPGA hardware to the gpu cluster computers. The GPUs apply the dedispersion filter and either fold or integrate in real time. Observing instructions for coherent mode.

• Total bandwidth: 100, 200, or 800 MHz
• Number of channels: Powers of two from 32 to 2048.
• Polarization: Total intensity or full-Stokes.
• Fold mode:
  • Folds the data modulo the current pulse period in real time.
  • Outputs time-integrated profiles, minimum dump time is ~1 second.
  • Output is floating point profiles in PSRFITS format.
  • The time (or pulse phase) resolution is given by N_chan / BW.
• Cal mode: Special case of fold mode, folds at a constant 25 Hz period for the cal signal.
• Coherent search mode:
  • Coherently-dedispersed spectra are accumulated in time rather than folded.
  • Spectrum integration time is subject to the same total data rate limit (200 MB/s max) as incoherent mode.
  • Data are re-quantized to 8 bits and output in search-mode PSRFITS format
• Note, for all types of coherent dedispersion observations, each gpu node processes and independently records 1/8 of the total BW. The 8 sub-bands are assembled into a single output file after the observation is over.

Baseband Recording Mode

For some applications, e.g. cyclical spectroscopy of pulsars, raw voltage can be recorded with PUPPI. In practice, the desirable data rate is too high for only one GPU to do the data recording, so data taking uses a relatively small number of very wide channels that get split between GPUs. The user then recombines them to recover the raw voltage data stream. Observing instructions for baseband mode.

• Total bandwidth: 100 or 200 MHz
• Number of channels: 32 or 64

Typical Setups

While there are a potentially large number of combinations of BW, # channels, etc available, a few setups get used much more than others:

• 800 MHz BW, 2048 channels, 40.96 us, incoherent. This is used for searches at higher frequencies (L-band, S-band).
• "Fast4k" - 100 MHz BW, 4096 channels, 81.92 us, total intensity, incoherent. This is used for low-freq searches (350 MHz; GBNCC, etc).
• 800 MHz BW, 512 channel, coherent. For MSP timing at L-band and higher.
  • Fold mode is usually used for single pulsars (eg NANOGrav timing).
  • Search mode is used for multiple pulsars in the same beam (double pulsar, globular clusters).
• 200 MHz BW, 128 channel, coherent. For MSP timing at 820 MHz.

There are also a set of standard frequencies and bandwidths for the commonly used pulsar receivers:

• 327 MHz receiver: 327 MHz center, 100 MHz BW.
• 430 MHz Gregorian receiver: 430 MHz center, 100 MHz BW.
• L-wide receiver: 1380 MHz center, 800 MHz BW.
• S-low receiver, bottom half of band: 1700-1880, 2040-2400 MHz filters, 2030 MHz center, 800 MHz BW.
• S-low receiver, top half of band: 2600-3100 MHz filter, 2030 MHz center, 800 MHz BW.