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Homework 1, assigned September 30th, 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.


NIRISS



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

1. What is the field of view and wavelength coverage of NIRISS? What is the pixel scale?
The FoV is 2.2’ x 2.2’. Wavelength coverage is between 0.60 and 5.0 microns. The pixel scale is 0.065” / pixel for all four observing modes: wide-field slitless spectroscopy, single-object slitless spectroscopy, aperture-masking interferometry, and imagine.



  1. What is the difference between the NIRISS readout patterns? Which should I choose for my science?


The JWST NIRISS detector allows two readout patterns—NIS and NISRAPID—that determine the sampling cadence for non-destructive readouts. NISRAPID is mostly used for bright targets, while NIS allows longer integration for fainter targets.

An integration consistents of Ngroups which each consist of Nframes. NISrapid has 1 frame per group, NIS has 4 frames per group.




  1. For which NIRISS observing modes do I have to use a target acquisition?
    TAs are supported for two NIRISS observing modes: single object slitless spectroscopy (SOSS) and aperture masking interferometry (AMI). While a TA is only required for those observing modes when a subarray is used, it is also strongly recommended for the full frame detector readout to ensure that the target is always placed on the same detector pixel.    

  2. Which NIRISS observing modes require dithering?
    Dithering with JWST NIRISS is required for the wide field slitless spectroscopy (WFSS) mode to improve point spread function sampling, as well as mitigate the effects of bad pixels and sensitivity variations across the detector.

  3. What are the four factors to consider when choosing a PSF reference (i.e., calibrator) star for an AMI observation?

    6. I want to observe a galaxy cluster field with NIRISS WFSS. Is there a good example of how to set up my observations? How do I remove contamination from overlapping spectra?

















NIRSpec



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

1. What is the wavelength coverage of NIRSpec? What is the pixel scale of NIRSpec?
The JWST Near Infrared Spectrograph (NIRSpec) enables 0.6–5.3 μm spectroscopy, always with 0.10”/pixel on all four modes: MSA spectroscopy, IFU spectroscopy, Fixed-slit spectroscopy, and bright object time series.


  1. What is the field of view of the NIRSpec Micro-Shutter Assembly? What is the field of view of the NIRSpec IFU?

For the MSA, 3.6’ x 3.4’, for the IFU, 3” x 3”.

3. What is the estimated best possible accuracy for target acquisition for the micro-shutter assembly shutters and which TA method will deliver it?

4. I have ground-based and Spitzer imaging of my field. Do I need NIRCam pre-imaging to ensure that my objects are precisely located in their MSA shutters?

Yes, either HST or JWST for pre-imaging.

5. There are bright stars in the MSA FOV that will cause leakage and will contaminate my IFU observations. What are the mitigation strategies that can be implemented when designing the observations?

6. What do I do if I need precise centering for a target that’s too bright for WATA?


  1. I want to use a 0.2" fixed slit to observe a source with an emission feature at 1.355 microns. Which slit should I use? Can I use both of the A slits for this?



Astronomer's Proposal Tool (APT)

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

1. When I enter an observation in APT, there is a box at far right labeled “ETC Wkbk.Calc
 ID”, but there is no context-sensitive help available. What am I supposed to put in that box, and is it a required input?

The ETC workbook calculation ID, and shows that you checked the validity of things in the ETC.


  1. The JWST Web site lists accepted Early Release Science programs:
    http://www.stsci.edu/jwst/observing-programs/approved-ers-programs
    I am interested in looking at program ID 1334, “The Resolved Stellar Populations Early Release Science Program” as an example, and I understand the APT files for the approved Early Release Science Programs can be loaded directly into APT for inspection. How do I do that?

File -> Retrieve from STScI -> Proposal ID


  1. If I am requesting a sequence of observations that need to be chained together in time (hence I put a special requirement in to make a non-interruptible sequence), is there a maximum time limit for such a sequence?

    4. Why does my observation have "Implicit" special requirements in APT, and why can't I edit them?

    5. Which APT observation templates fall into the category of mini-mosaics?

    6. I see an option for "Module" in the NIRCam APT template. What do these two options refer to?

Both modules or B modules. I think that one might just use “B” module, for example, if they want to save on bandwidth, or if they only need long-wavelength data.



Exposure Time Calculator (ETC)

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

1. The ETC will give me a warning if I start inputting parameters that are not supported by APT, right?
APT warnings and errors are not always reflected in ETC: The ETC needs to be able to support engineering users as well as astronomers. Hence, it was developed to allow a user to choose values for various parameters that are not available by default, but can be accessed for engineering purposes. APT contains numerous warnings and errors that alert users when they are attempting to select options that are not available by default. Unfortunately, those errors and warnings are not always reflected directly in the ETC. That is, the ETC may let you select options that will be considered invalid when the information is transferred to the appropriate APT template.


  1. Can I upload a custom spectrum for my source for ETC calculations? What information should I provide, if so?
    Column 1 should be the wavelength in µm and column 2 should be the flux density in mJy.  

  2. What options do I have for defining the flux distribution for an extended source?

. Choosing extended allows for flux distribution, normalization, and axial extent options. The flux normalization for extended sources may be done in integrated magnitudes or surface brightness units, and the selection made in the Shape tab will be reflected in the units used in the Renorm tab.


  1. In the Hubble ETC, I can input my desired SNR and receive as output the necessary integration time. Why can't the JWST ETC optimize the number of groups and integrations for my signal-to-noise goal? Running a whole bunch of calculations is tedious... Is there a way to speed this up?

    5. When should I use the IFU Nod off Scene strategy?

    6. I would like to do additional analysis beyond what the ETC reports. Is there a way to access the output data products so I can use my own software tools for further analysis?



JWST Help Desk Homework Questions

Please answer all questions below.

  1. Announcements such as new APT and ETC releases/downtimes, Call for Proposals downtimes, Call for Proposals, etc. are posted on the Help Desk homepage (jwsthelp.stsci.edu, see screenshot below). What is the latest announcement posted on the homepage?


JWST Video Tutorials now available! !!!



  1. From the Help Desk homepage (jwsthelp.stsci.edu) you can search for answers to your questions. Search results will give Knowledge Base articles first and then JDox results. You can use the sidebar tree to filter for specific types of articles. See the screenshot below for the search results for “APT MIRI”. Use the Help Desk search bar to find out why you may be having trouble connecting to the APT server. Explore using the sidebar tree to filter types of articles.

Done.


  1. If you can’t find an answer to your question using the search function, you may submit

a question (otherwise known as a “ticket”) for a member of the Help Desk staff to

answer. You submit a question by clicking on the “Get Help” icon on the

homepage. This will send you to a page of “catalogs” to choose from to get an answer

quicker (See example screenshot below). If you can’t determine what catalog to

choose you may also choose “General”. What catalog would you submit to if you have

a question about adding an investigator to your proposal?

(You may have noticed while answering Question 2 that your search results may also

send you directly to the page to submit your question, that is also fine.)

APT support.


  1. Practice using the Help Desk by submitting a practice ticket to the catalog “JWST

Master Class”. Request that your ticket be proprietary, use the subject “My Test

Ticket”, and the description “Hello, I am practicing using the Help Desk for the JWST

Masterclass, please send me a response. Thank you.”

Done.


  1. After submitting your question you will see your ticket under “My Open Tickets” in the

top menu bar. You can view and update your tickets here. You may add an

attachment or a collaborator to the “watchlist” (the collaborator will then receive all

future updates to the ticket in an email. Note: you can add also someone to watchlist

when you first submit your ticket). Re-open the practice ticket you submitted in

Question 4 and either add someone to the watchlist or send an attachment image of

your favorite astronomical target. See example screenshot below:


Done.

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