NASA's James Webb Space Telescope

March 26th Social Media Q&A Session on Twitter

News Feature April 2, 2018

Promotional image for March 26, 2018 Q&A session.

STScI held a Question & Answer session on our JWST Observer Twitter account on Monday, March 26, 2018. Below are the questions and answers from that session.

Q. I want to observe a fairly faint point source with NIRCam. I know I need to dither. Any advice on choosing groups per integration and integrations per exposure (other than guess-and-check) to see calculated S/N and exposure time on the ETC?

A. ETC mimics dithers through number of exposures. It’s recommended to keep the number of exposures fixed at the number of dithers in your program. The NIRCam documentation recommends increasing the number of groups/integration rather than number of integrations/exposure. Do keep in mind the limit on the number of groups in an integration for your chosen readout pattern.

Q. Are 2 groups per integration too little? Or does that depend on the readout pattern?

A. It’s a balancing act between minimizing overheads and achieving optimal signal-to-noise. 2 groups per integration is an allowed option. This JDox article illustrates the signal-to-noise for different readout patterns as a function of exposure time and includes a table of recommended readout specifications.

Q. The proposal prep web page says for surveys, just an example target list is sufficient. Any guidelines on how many targets should be included in that list?

A. We don't have firm guidance on that, as it depends very much on the nature of the program. If you'd like more specific information, please submit a JWST Help Desk ticket so we can better advise you.

Q. In the APT, what goes in the "Observing Description" versus what goes into the "Technical Justification" portion of the PDF? Is this like an abstract of the Technical Justification?

A. The "Observing Description" is a condensed summary of your proposal's "Technical Justification," meant to give reviewers a high-level understanding of your observing plans. You can review the approved ERS programs for examples.