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Homework 1, assigned September 30^th, 2019

Cross Instrument

In addition to the required questions in bold-faced, answer an additional 1 question from this list.

1. What's the first thing I should do when preparing my proposal?

Become familiar with JWST capabilities and terminology – in other words, take a proposal writing class ! Be sure to read through the Call for Proposals and familiarize yourself with JWST Science policies.

1. What observing methods does JWST support?

Imaging, slitless spectroscopy, high-contrast imaging, integral-field spectroscopy, MOS (multi-object on NIRSpec micro-shutter assembly) spectroscopy, time-series observations, moving-target observations, parallel observations, and targets-of-opportunity observations.

1. How do I know when a given target is visible to JWST?

The APT (astronomer’s proposal tool) visit planner tool performs a detailed check of the schedulability of observations specified in APT, including visibility, constraints checking, and whether guide stars are available to support the observation. Diagnostic information is provided when scheduling checks fail.

1. When should I propose for NIRISS Wide Field Slitless Spectroscopy (WFSS) instead of NIRCam WFSS?

NIRISS covers a shorter wavelength range (0.60-2.80 or 0.80-2.20 microns rather than 2.40-5.00 microns), at lower resolution (700 or 150 rather than 1,500). It is also a good option when is designing a parallel observing program to NIRcam imaging.

1. If I want to observe the spectra of transiting exoplanets, what spectroscopic JWST observing modes are available to me?

1. Which JWST instruments offer standard imaging? What is the wavelength coverage of the imaging modes?

NIRCam, 0.60-5.00 microns, primary or parallel, 9.7 arcmin^, 2 simultaneous wavelength channels

MIRI, 5.60-25.50 microns, primary or parallel, 3.1 arcmin^2

NIRISS, 0.80-5.00 microns, parallel only, 4.84 arcmin^2, NIRISS can be used in parallel with NIRCam to increase sky coverage

1. I would like to obtain spatially resolved 2-D spectroscopy with JWST. Is that possible? If so, what observing modes support this, and what wavelengths are covered?

1. What JWST observing modes will allow me to observe faint companions near bright host objects?

NIRCam coronographic imaging, MIRI coronographic imaging, NIRISS aperture masking interferometry.

MIRI

In addition to the required questions in bold-faced, answer an additional 2 questions from this list.

1. What is the wavelength coverage of MIRI? What are the pixel scales for the various observing modes?

The JWST mid-infrared instrument (MIRI) provides imaging and spectroscopic observing modes from 4.90 to 28.80 microns.

Observing Mode, Wavelength (microns), Pixel Scale

Imaging, 5-28, 0.11

4QPM (4-quadrant phase mask), 10.65/11.40/15.50, 0.11

Lyot coronographic image, 23, 0.11

Low-resolution spectroscopy, 5-12, 0.11

Medium-resolution spectroscopy, 4.90-28.80

1. For what MIRI observing modes should I dither? Is there a limit for the amount of time I should spend in a given dither position?

All of them apparently. However, no coronograph imaging dithers are available in APT. I think that there’s no dithering for time-series observations.

Note: no dithering on slitless spectroscopy in the low-resolution spectrograph … as above.

1. What is the field-of-view & wavelength range for the MIRI IFU (medium-resolution spectroscopy) channels?

The wavelength range is 4.90-28.80, and the field of view of up to 7.2" × 7.9".

1. What separations between a faint companion and bright host can I achieve with the MIRI coronagraphic masks? What are the central wavelength coverages of these masks?

1. When observing with the low-resolution spectrometer (LRS), should I choose slit or slitless spectroscopy?

The slit provides better sensitivity, and slitless is dedicated to time-series observations. The latter uses a smaller subarray for faster read times. It has a brighter saturation limit, by several magnitudes. The slit is expected to give better performance for faint targets, as it blocks out other light.

1. When using MIRI MRS Simultaneous Imaging, will I get imaging observations of my target "for free"? What is this mode used for? When should I choose to not use this option?

Simultaneous use of the JWST MIRI imager and the medium resolution spectrometer (MRS) modes is offered for all observations where the MRS is the primary observing mode. Astrometry of stars in the imager field will result in more accurate data cube construction, as it is a larger field.

It is highly recommended, in part as MRS cannot be part of coordinated parallel observations with other instruments. There are no data volume issues when using both simultaneously. Therefore, the only instance where simultaneous imaging should be turned off is if saturation of the imager would occur due to a long MRS observation or if a bright target is in the imager field of view. There is a small cost due to overheads.

1. When should I take a dedicated background observation?

NIRCam

NIRCam has five observing modes: Imaging, coronographic imaging, wide-field slitless spectroscopy, time-series imaging, Grism time-series.

In addition to the required question in bold-faced, answer an additional 2 questions from this list.

1. What is the wavelength coverage, field of view, and pixel scale for NIRCam's shortwavelength and long-wavelength detectors?

Short-wavelength channel, 8 detectors, 0.60-2.30 microns, 0.031”/pixel, 2.2’ x 2.2’, with 4” gaps (64” x 64” per detector)

Long-wavelength channel, 2 detectors, 2.40-5.00 microns, 0.063”/pixel, 2.2’ x 2.2’, (129” x 129” per detector).

Note: NIRCam images are, by default, obtained simultaneously with both modules using all detectors at short and long wavelengths.

1. I would like to observe the gaps in between NIRCam's A & B module when using imaging. What dither pattern should I use? What dither pattern should I use for NIRCam Wide Field Slitless Spectroscopy?

The gap between the modules is 44 arcseconds. For imaging, JDox suggests mosaics are best obtained in combination with primary dithers. Primary dithers are telescope pointing maneuvers (4"–100") to fill gaps in sky coverage between detectors and mitigate flat field uncertainties. FULL and FULLBOX cover the 43” gap between modules A & B. FULLBOX covers a smaller area, but is more efficient.

Only the "INTRAMODULE" variants are available for wide field slitless spectroscopy. It fills the ~4” gap between the short-wavelength detectors.

1. What NIRCam observing modes support mosaicking? When I should a mosaic and when should I dither?

MOSAICing  (large pointing shifts) typically emerges from a combination of large dithers, which allows a larger field of view, at the expense of more observing time. They are primarily for imaging and for wide-field slitless spectroscopy. Small grid dithers are very small pointing shifts (<0.06") performed quickly with the fine steering mirror primarily to improve reference PSF subtraction in coronagraphy.

No dithering is allowed for NIRcam time-series observing mode.

1. Which NIRCam readout patterns have skipped frames?

1. What coronagraphic masks are offered by NIRCam, and what wavelength ranges do they cover?

1. Should I dither for grism time-series imaging observations?

No, it’s not allowed.