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GPI Frequently Asked Questions

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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: No, 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, but the performance is NOT guaranteed for Zenith distances larger than 50 degrees. Any proposal applying for Zenith distances larger than 50 degrees should take contact with the instrument scientist previous to submitting the proposal. The ADC is specified to work up to ZD=50 and thus both OIWFS performance is not guaranteed AND the centering of the science object on the coronographic mask will be adversely affected for large Zenith angles. 


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: Testing has shown that with slower camera rates it will work at decreased performance at I=9+. 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=9+ would be very sensitive to the read noise on the wavefront sensor, and is thus a complex undertaking. One should always refer to the "Instrument Performance pages" for the latest updates to the limiting magnitudes. 


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. 


Q: Can I do dithering?

A: The only dithering possible is going off to take sky frames (for K1 and K2 bands) in open loop. There is NO capability to dither on the detector with closed OIWFS loops.  


Q:Can GPI keep the sky fixed on the detector?

A:NO, the masks in the instrument are fixed orientation and optimized with respect to a fixed Cassegrain angle. With the Cassegrain angle always fixed (and not adjustable) it means that the sky will always rotate around the detector optical axis determined by the OIWFS axis.   


Q:Can I mix different observing modes in the same observation?

A:NO, each observing mode requires its own set of internal calibrations and thus each observing mode setup requires its own acquisition. Even a change of filter is its own observing mode so no mixing of filters in the same observation.   

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