Frequently Asked Questions (FAQ) for observing with GPI.

Q: Can I observe a target fainter than I magnitude 9 by using a nearby bright star as the AO star?

A: The basic principle with the GPI AO system is that the science target IS the AO star, thus the science field is centered on the guide star.

 

Q: Can I observe close similar magnitude binaries?

A: No,  the AO system is designed such that any close similar magnitude binary would have a strong adverse impact on the AO performance, and possibly not even allow the closing of the AO loop.

 

Q: What is the airmass limit of the instrument as the OCDD (pg 39) says there's a 50-deg zenith limit?

A: The contrast curves should be generally good to about 30 degrees off zenith - there are no separate models for the 50 degree case. To first order, contrast would scale up as airmass ^12/5

 

Q: What is the AO Guide Star magnitude limit of the instrument?

Q: Can I observe targets that are Imag > 9.0 in better-than-median seeing?

A: Simulations suggest that the system will still work at I=10. However, it is unlikely that the performance would improve in IQ20 conditions, because GPI isn't designed to run slowly to take advantage of slow seeing. Further, performance at I=10 would be very sensitive to the read noise on the wavefront sensor. The latter is known to be a function of environment and we do not yet know what it will be on the telescope. A small number of targets at I>9 would be acceptable assuming they are the exception i.e. <<10% of the sample). If they are the majority, then we would be ill-advised to accept the proposal at this stage because it is formally outside the design specification and we will not really know the performance in that regime until we have a significant amount of experience on the sky. We do not think it would be sensible to include targets at I>10, in any case.

 

Q: Do I need PSF stars for Polarization observations?

A: In the polarization mode you don't need to observe PSF reference stars, because the two orthogonal polarizations essentially serve as each other's PSFs, though one can contemplate using some other form of PSF subtraction if you want to obtain a total intensity image as a followup characterization observation.