sbn Tsys 2017. Shutter problems

08may17

Intro

The sbn system temperature checks have occasionally shown a tsys much lower than the expected values. This was occurring in both polarizations.

The plots show tsys (using hcorcal) vs date for jan17 to may17 (.ps) (.pdf)

black is polA, red is PolB.
The green line is 22feb17. this looks like when the problem started.
The values between 10 and 12 K are wrong.

    To get a lower tsys, the value of the cal must increase, or the system temperature most really be lower.
    When the sband shutter is closed, the system temperature measures the reflected power from the cooled lna's. This is lower than the Tsys measured on sky.

    On 08may17 felix and I took system temperature measurements with the shutter open and closed. When open we got around 25 K. When the shutter was closed we got about 12K.

    We then found that:

closing the shutter and then opening it within a minute or 2 seemed to work ok
closing the shutter for a longer period of time : (we left it for about 45 minutes) and then trying to open it:

it took the shutter a minute or more before it opened.
(later carlos went up stairs and opened and closed the shutter many times.. he found that the failure to open would also occur when the shutter had only be closed for a short period of time).

This would explain why we had occasional measurements of 12 degs for sbn Tsys:

A radar experiment would end leaving the shutter closed.
When the tsys was run, the shutter was commanded to be open and then the tsys was measured.

It takes about 30 secs for the tsys cal firing.
During this time the shutter had remained closed.

Summary:

The sbn shutter has occasionally been slow to open.
This problem started on 22feb17.
The daily tsys meaurements showed this problem:

the shutter was commanded to be open

If a previous sb radar experiment had left the shutter closed, then it would have to open.

the cals were fired...
If the shutter hadn't completed its opening, then the tsys would measure around 12K (instead of 25K)

When we tested the shutter on 08may17 we saw that:

commanding the shutter to close and then open rapidly tended to work
leaving it closed for longer periods of time caused the problem to show up.

One of the relays used to close the shutter was found to be defective.

It was replaced on 08may17

This behavior may have been affecting the sband radar datataking scans.

processing: x101/170508/sbnshutter.pro

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