timing for MegaCam

[ijiraq]

There was a change of the controllers for MP during the April run. Information below... we’ll need to adjust how the stepZjmp works.

Begin forwarded message:

From: Richard Wainscoat rjw@IfA.Hawaii.Edu Subject: Fwd: timing for MegaCam Date: May 21, 2015 at 10:13:58 AM EDT To: "Kavelaars, JJ" JJ.Kavelaars@nrc-cnrc.gc.ca

Here is our best knowledge of Megcam timing at this time. The change occurred in April 2015.

Richard

Begin forwarded message:

From: Richard Wainscoat rjw@ifa.hawaii.edu Subject: Fwd: timing for MegaCam Date: April 24, 2015 at 7:23:09 PM EDT To: David Tholen tholen@ifa.hawaii.edu

Looks like the midpoint of the exposure is now UTCEND - 0.73 sec - (exposure time / 2 ) Wobble on the 0.73 sec is +/- 0.15 sec.

Finally making progress.

Richard

[jmpetit]

Question: How is it currently done in stepZjmp ? Answer: uses MJD-OBS + EXPTIME/2

Question: Was that valid ? I don't know: 1751521 is from 2014-10-22 06:50:23.12 which gives an MJD that match MJD-OBS to a 0.0012 seconds.
But MJD-END - EXPTIME/24/3600.0 - MJD-OBS = 49s.76s But the END should not include the 40s overhead...

conclusion EXPEND - EXPSTART != EXPTIME and thus one of those 3 numbers is wrong.

Question: Is it still valid ? David Tholen is looking at NEOs so the time is important to him. I guess he finds that time at start is not the correct time but time-end is. Apparently (from Richard Wainscot) that has always been the case. So, our exptimes are all 49s ahead of the correct value? A good way to check this would be to look at measurements from GEMINI and CFHT and look for a systematic effect in the residuals... but in 40s our objects go about ~ 0.04'' so we would only just barely see that using Stephen's improve astrometric solutions. Which I don't think GEMINI ColOSSOS data is tied to?

[ijiraq]

Hi, I've double checked this.

mopheader contains the value:

MJD-OBSC = MJD-OBS + exptime/2.0

RW claims:

MJD-OBSC = DATE-OBS + UTCEND - 0.73s - exptime/2.0

those to approaches differ by ~50s for exposure 1785787

RW's value is based on NEO tracking. We could not notice that level of difference. likely they are correct.

[mtbannister]

So we are still 49 sec incorrect on the observation midpoint, but only in the mopheader, and so the pipeline does not need alteration for the purposes of TNOs?

[ijiraq]

I'm checking with RW about the pre-April computation that PS uses.

We should fix all our times as the occultation team thinks time is important.