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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.

Homework 2

Understanding JWST detectors questionnaire

(some questions have JDox links you might want to check).

1. What is the difference between the JWST NIR and MIR detectors?

The near-infrared detectors are made of mercury-cadmium-telluride (HgCdTe), and are used for wavelengths of up to 5 microns, beginning at around 0.70 microns. The mid-infrared detectors are made of arsenic doped-silicon, and are made for the wavelength range 5-28 microns.

1. What is the fundamental difference between a CCD and a JWST detector readout?

From the Jdox: “The infrared-sensitive detectors in JWST science instruments operate very differently from the CCDs that many astronomers are familiar with from ground-based work or HST's ACS and WFC3/UVIS. These IR detectors, similar to those in the Spitzer Space Telescope instruments and HST WFC3/IR, are read out using a non-destructive up-the-ramp readout technique that provides a number of advantages.”

These advantages enable a greater dynamic range as bright stars can be read before saturation, a reduction of read-out noise, and a reduction of sensitivity to cosmic rays.

1. What is a frame?

A single read of all pixels in the detector array or sub-array.

1. What is a group?

On-board average of the multiple frames, which is used to reduce the readout pattern.

1. What’s the meaning of “group gap” or “dropframes” in the JWST NIR detectors?

Drop-frames are between groups, they are sampled, but are not included in the average that gets downloaded to the ground.  

1. What is a reset?

Closing the detector readout, and reduces all charge counts to the bias level.

1. What is an integration?

An integration is a set of groups between a reset.

1. What is an exposure?

Set of identical integrations, separated by a constant set of resets.

1. Suppose you have data from a CCD and from a JWST detector. They both reach the

saturation level in one-half of the total integration time. Can you describe what the

main difference is? Can you recover information in the saturated pixels?

For the CCD, you’ll have to do saturated star photometry.

For the JWST detector, you may be able to get the photometry from the individual groups, if you have more than two groups per exposure.

1. What will be the impact of a cosmic ray in a JWST integration? Can information be

recovered?

A cosmic ray will cause a large jump, like a theta function, between two frames of a group. It can be accounted for as long as there are multiple groups in the exposure.

1. a) What is the practical difference between a MIRI SLOW mode exposure and a NIR

exposure with NFRAMES=8? b) Which has a higher data rate: a single MIRI Si:As

detector running in SLOW mode or a single NIRCAM H2RG detector using the

MEDIUM8 readout pattern?

Nsample=9 in SLOW mode (first frame is lost) and cannot be altered, observers can alter Ngroups and Nintegrations.

 Nframes=8 refers to MEDIUM8 and DEEP8, which actually have more skipped frames, they have 10 and 20 total frames, so they technically go “deeper”.

NIRCam requires ~80 megabytes of download per group. For Medium8 and Deep8, the maximum comes out to 34 and 68 gigabytes per day.

MIRI takes up ~2 megabytes of data per group per detector, presumably you’re using one at a time. I don’t know what the minimum exposure time is …

1. Given a certain readout pattern, why is the group time different for full and subarray

mode?

https://jwst-docs.stsci.edu/near-infrared-camera/nircam-instrumentation/nircamdetectors/

nircam-detector-subarrays

Subarrays are read out more quickly than the full detector, allowing for shorter integration times. Shorter integration times can allow brighter objects to be observed without saturating the detector. Each pixel takes up 10 microseconds of readout time.

1. If a user defines a single NIRCam exposure (i.e. no dithers) with all modules in FULL

array and BRIGHT1 readout pattern, that uses 10 groups and 1 integration, the

exposure time is 203.99 second. 10 groups and 2 integrations result in 418.73 seconds.

Why the total time of 2 integrations is not twice as long as one? Can you guess why

that would not be the case for MIRI?

https://jwst-docs.stsci.edu/jppom/visit-overheads-timing-model/instrument-specificoverheads/

nircam-overheads

Full Array, 2048x2048 array, t_frame = ((N_columns / N_outputs + 12) × (N_rows + 1) + 1) × 10.00 µsec = 10.737 seconds per readout.

Bright1 means 2 samples per group.

I think that there’s one extra readout time …

Answer …  Readout mode: BRIGHT1, 1 integration of 10 groups takes 203.99 seconds, two integrations take 418.73 seconds. The clock time of two integrations is more thantwice one integration, because there is a reset in between. Thiswouldnot be the case for MIRI because aread-reset (rather than a reset) is executed between integrations, adding no extra time.

Done.

Homework 3, ETC

Exercise 1 exploration:

o What do you think the “out-of-date” field in the workbook list means?

Probably workbooks made with an older version of the ETC.

o Try to find out how you can share a workbook with a collaborator. Experiment

with read and write permissions.

Highlite the workbook on the available workbooks page, that enables a set of options at the bottom under “User Access permissions”.

Exercise 2: Explore an ETC workbook

https://youtube.com/watch?v=2IpSHsPda2A

1. Workbooks open in the “Calculations” tab. You will see three other tabs are also

available: “Scenes and Sources,” “Upload Spectra,” and “Caveats and Limitations.”

1. A workbook always includes three boxes at the top of the page.
2. One contains a unique workbook ID, which cannot be edited.
3. The second is a box for the title of the workbook. Name your workbook.
4. The third is a box for you to provide additional information on what is contained

in the workbook. Add a description.

1. Move to the “Available Workbooks” browser page. You will see that the title and

description have automatically updated for this workbook.

1. Now, return to the workbook. Take note of the “Help” dropdown menu at the top right.

Click on each of the options in the dropdown to see what they link to.

1. Scroll to the bottom of the page and view the “Notes” section. Typing in this section will

automatically save notes you leave for yourself or your collaborators. Type something in

the Notes.

• Exercise 2 exploration:

o Find the known issues under the help menu. Which one, if any, do you think

could most affect your favorite science case?

The ability to deal with a large load of users could be a problem.

Exercise 3: Using the Calculations Tab

1. Create a NIRCam SW Imaging calculation. Remember the first step in the “Quick Start”

walkthrough.

Done.

1. Highlight the calculation by clicking on it. This populates the “Calculation Editor” pane in

the top right with additional options for customizing the calculation.

Done.

1. A new calculation always begins with the “Instrument Setup” tab selected. For NIRCam

SW Imaging, you will see a dropdown providing a list of all available short-wavelength

filters. Change the filter.

OK, F200W

1. Now move to the “Detector Setup” tab. Change the “Readout pattern” from DEEP2 to

RAPID. Click calculate.

Done, SNR dropped from 97 to 31.

Note: The ability to change other parameters is not available while the calculation is running.

1. Locate the “Expand” dropdown menu in the top bar. This dropdown lets you easily copy

calculations with one parameter altered. Select the “Expand Groups” option.

In the dialog box, fill in 11 for the “Start Value,” 1 for the “Step Size,” and 10 for the

“Iterations.” Click “Submit” to create 10 new NIRCam SW Imaging calculations covering

the range from 11 to 20 groups. Observe that the exposure time increases with the

number of groups.

Done,  gradually goes up to SNR=55 in 215 seconds.

1. Highlight the original calculation and repeat the process for the “Expand Integrations”

option (Start Value=2, Step Size=1, Iterations=10).

Done,  gradually goes up to SNR=100 in 1288 seconds seconds.

1. You can view the results of all the calculations at once in the “Plots” pane by clicking on

the checkmark icon above the rows of calculations and selecting “All.” Then scroll down

to view the “Plots” pane.

Done.

1. Since all of these calculations were made using the same filter, the default “SNR” tab is

not particularly illuminating. Switch to the “SNR (time)” tab.

SNR increases with time very rapidly with more groups, relative to more integrations.

• Exercise 3 exploration:

o Note the two different slopes in the signal-to-noise ratio (SNR) for increasing the

number of groups and increasing the number of integrations. What do you think

causes this behavior?

The additional integrations lead to the addition of more readout time and more bias noise.

o Experiment with other readout patterns (under the Detector Setup tab) to find

the highest SNR for a total exposure time of about 1000 seconds. What is it?

I’m assuming DEEP8, which reaches SNR=125 in 944 seconds.