Page History

...

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.