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Webb Office Hours Session 2:  February 22, 2024

Q&A's: 

Q1: Where are the Webb Office Hours procedures and guidelines?

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A2: Consider an integration with three groups up the ramp, each group combining four frames on board before downlink, i.e., 1 1 1 1 2 2 2 2 3 3 3 3. The time associated with each downlinked group is the average of the times of the four individual frames that were combined, e.g., 1, 1, 1, 1. The total time spent gathering photons is 12 frame times. The effective integration time is the time interval between the first (1) and last group (3), which is only 8 frame times. To calculate flux, divide the flux difference by effective integration time, not by the photon gathering time.Consider an integration with three groups up the ramp, each group consisting of a single frame. Assume the time to read all pixels in a single frame is 10 seconds. Photons are gathered for a total of 30 seconds, starting with the first pixel in the first frame and ending with the last pixel in the last frame. The first pixel is read at t=0, t=10, and t=20, so the effective integration time is only 20 seconds. The last pixel is read at t=10, t=20, and t=30, so the effective integration time is again only 20 seconds. When multiple frames are combined into a group on board before downlink, the downlinked 

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To conserve downlink bandwidth, some detector readout patterns average measurements for a pixel from multiple frames. In this case, the effective time for a downlinked measurement of a pixel is the average of the measurement times in the frames that were combined on board. In this scenario, the effective integration time for a pixel is the time associated with the last group in an integration minus the time associated with the first group.  

Effective integration time is provided in the JWST Header keyword: EFFINTTM "Effective integration Time in units of seconds.

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