[daip] [!15373]: AIPS - AIPS APCAL opacity correction

[NRAO Helpdesk]

Data Analysts updated #15373
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       Staff (Owner): Eric Greisen (was: -- Unassigned --)

AIPS APCAL opacity correction
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           Ticket ID: 15373
                 URL: https://help.nrao.edu/staff/index.php?/Tickets/Ticket/View/15373
                Name: JongHo Park
       Email address: jhpark at astro.snu.ac.kr
             Creator: User
          Department: AIPS Data Reduction
       Staff (Owner): Eric Greisen
                Type: Issue
              Status: Response Overdue
            Priority: Default
                 SLA: NRAO E2E
      Template group: Default
             Created: 29 December 2019 02:34 AM
             Updated: 29 December 2019 02:34 AM
           Reply due: 03 January 2020 05:00 PM (1h 37m 10s)
      Resolution due: 26 September 2022 05:00 PM (997d 1h 37m)
Dear colleagues,


How does AIPS APCAL treat different Tsys values for different baseband channels in opacity correction?

I may understand how APCAL does an opacity correction. It fits a linearized function for Tsys (Tsys = T_rx + T_atm * tau_0 * secz, or the full non-linear function?) to the "lower envelope" of Tsys as a function of secz, assuming that this envelope represents the same tau_0. Then, it derives opacity as a function of time by using the observed Tsys, T_atm (in WX tables), and the fitted receiver temperature (T_rx, e.g., equation (9.24) in Moran & Dhawan (1995), ASPC, 82, 161).

I know that in most cases this is not an issue but in some cases, I see a significant difference in Tsys for different baseband channels (or IFs). Please see the attached plot as an example. The Tsys values for different baseband channels for LCP are significantly different offset from each other. It appears that there are constant offsets, while the slopes are almost the same.

Based on the above approach, it indicates that the receiver temperatures for different baseband channels are significantly different. However, it seems that AIPS APCAL obtains a single value of T_rx and tau_0 and applies it to all different IFs. On one hand, this makes sense because the receiver temperature should be ideally the same for different baseband channels. On the other hand, I'm worried if this assumption, which is apparently different from what we see in some data (as attached figure shows), can introduce some spurious results in amplitude calibration.

Thank you very much for your support in advance and I look forward to hearing from you at your earliest convenience.


Best Regards,

Jongho Park


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