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GPI Non redundant Mask (NRM) FAQ

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Frequently Asked Questions (FAQ) for observing with the Non Redundant Mask in GPI.

Q: Is the NRM commissioned and available?

A: The mode has been partially commisioned. It has been determined that the mode works operationally but there are a few open issues, listed below. This means that the mode is only offered in shared risk mode.  

  • There is no pipeline reduction of the NRM mode. Observations will only deliver the raw data and the extracted data cubes. Any post-processing is up to the user. 
  • Due to commisioning being affected by weather, we have no clear performance data, and it was found that in 2015 that the internal vibrations smeared significantly the fringes. Since then we have installed the active dampers on the GPI CCR's, but we have not had the opportunity to use the mode after the active damper install, due to issue with the PPM mask. See "November 12th, 2015" entry on the Status page. We will update the web pages with performance data as soon as we have obtained more observations. 

Q: What is Non Redundant Masking?

A: The GPI Non Redundant Mask is a specially designed mask with several apertures (10 hole mask giving 45 baselines, or 36 independent closure-phases) placed in one of the apodizer wheel positions, allowing the use of Aperture Mask Interferometry. Each of the pair apertures is thus forming an optical interferometer. The design of the mask is such that it gives the maximum number of non redundant baseline pairs that each forms fringes with unique spatial frequency in the image plane.

Q: What is the expected performance?

A: Note that all the performance and constraints are based extrapolations of performances achieved at other telescopes, with less advanced AO systems. Currently at the design team for the NRM mask on GPI are expected to soon achieve 5-sigma K-band contrasts of 7.5 magnitudes (1 hour per target). It is estimated that by correcting the dominant systematics this should improve to at least 10 magnitudes with GPI (1 sigma contrast of 2 * 10-5), and a magnitude limit identical to any other mode for the science object, see the wave front sensor limits page, for AOWFS I-band limits.

Q: What is the Seeing constraints when using the NRM? A: The NRM will be used with the GPI AO working, but is less demanding on the Strehl and it is expected that up to IQ70 seeing conditions can be used for K and H band imaging, in J-band it is expected that median seeing (IQ50) is needed to reach the desired performance.

Q: What are the overheads when using the NRM?

A: The method requires larger overheads in the observing as each science observation of one hour must be bracketted by one hour calibration observations, thus a complete sequence is expected to last three hours. Ideally this is done as a Calibrator-Science-Calibrator cycle repeated over the three hours.

Q: Will the NRM have the Cassegrain in fixed or follow mode?

A: Observations will be using the fixed Cassegrain method so that the sky will rotate during the observations.

Q: What is the expected Field of View?

A: With a single observation (no sky rotation), the field of view of NRM is approximately matched to the coronograph size, i.e. a 3 lambda/D radius. i.e. there is a clear boundary between where NRM and standard GPI operate. Beyond 3 lambda/D, the standard GPI observing configuration should do better - the requirement for performance is at 4 lambda/D and although ADI/SDI will barely work at 3 lambda/D, more than 10 magnitudes of contrast should be achievable at that separation. Companions beyond 3 lambda/D will alias back somewhat, but with a little sky rotation (e.g. multiple observations) the masking field of view is increased to ~6 lambda/D.

Q: With the SAM/NRM being inserted into the instrument, will it be more tuned for point source detections or will it also be able to detect extended emission (disks/rings)? i.e what structure sizes will it be most sensitive to?

A: The method is most sensitive to 0.5 to 3 lambda/D structures. Systematics are always higher for visibility amplitudes than closure-phases, so point-symmetric structures (e.g. a disk/ring) are much harder to detect.

Q: What is the product after processing the raw data taken with the IFS?

A: The raw data file is processed to create OIFITS (Optical Interferometer FITS files) and input files for the BSMEM package. There are plans to produce radio-interferometer FITS (UVFITS), as well as a map and a map of binary solutions to model fitting.

Q: What Software can be used to model and/or reconstruct the image?

A: The processed files can be processed by MACIM/BSMEM/MIRA and the model fitting is usually done with custom code f.ex mpfit IDL library.

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