DPCC Time stamp from FITS header

[Mark Salisbury]

Hello

I am trying to find out if it is possible to change the FITS header field used to determine the UTC-based start of exposure time in DPCC?  One of the telescopes I am using has recently been upgraded with a GPS time unit and shutter trigger to provide very accurate timing but the new GPS determined start time is being added to the FITS header with a different FITS keyword - "GPSSTART".  I would like to use this time stamp to determine BJD_TDB rather than the usual "DATE-OBS" keyword.

Many thanks
Mark Salisbury

[karenacollins]

Hi Mark,

It is not currently possible to select the timing information keyword. Doing so would be somewhat complicated because the various keywords and keyword combinations tend to have different value formats. 

AIJ currently searches through a list of well known keywords (or combinations of keywords) to find what it thinks is the best timing information available in the header. 

Is the format of the  "GPSSTART" keyword the same as the standard "DATE-OBS" keyword? If so, I should be able to add this keyword to the sequence that is checked when I return to the office where my development computer is later in the coming week. 

Also, do you know what AIJ is currently selecting as a timing source keyword? The GPSSTART keyword would need to have higher priority in the search list.

Karen

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From: Mark Salisbury [via AstroImageJ] <ml+[hidden email]>
Sent: Sunday, August 5, 2018 1:52 AM
To: karenacollins
Subject: DPCC Time stamp from FITS header

 

Hello

I am trying to find out if it is possible to change the FITS header field used to determine the UTC-based start of exposure time in DPCC?  One of the telescopes I am using has recently been upgraded with a GPS time unit and shutter trigger to provide very accurate timing but the new GPS determined start time is being added to the FITS header with a different FITS keyword - "GPSSTART".  I would like to use this time stamp to determine BJD_TDB rather than the usual "DATE-OBS" keyword.

Many thanks
Mark Salisbury

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AstroImageJ - DPCC Time stamp from FITS header

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DPCC Time stamp from FITS header. Hello I am trying to find out if it is possible to change the FITS header field used to determine the UTC-based start of exposure time in DPCC? One of the...

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NAML

[Mark Salisbury]

Many thanks for the very quick response Karen.

I would just want to check with the software developers for the telescope that the FITS keywords are not going to change at all before you do any work.

Currently the FITS headers use DATE-OBS for the date and time.  The GPSSTART field uses the same format, example below.

32 DATE-OBS '2018-08-02T00:54:43.0112279' S
36 GPSEND '2018-08-02T00:56:45.9334801' S
37 GPSSTART '2018-08-02T00:54:45.9325047' S

Regards

Mark

[karenacollins]

Hi Mark,

If the GPSSTART time is expected to be more precise than the source currently used for DATE-OBS, I would recommend copying the GPSSTART time into the DATE-OBS time in the header for maximum compatibility with all tools, if that is possible for your software developers to implement. If not, I can look at adding the GPSSTART time as a keyword used by AIJ, but at the moment I will not be able to implement a user selection, so I would need to understand the order that GPSSTART should be searched relative to DATE-OBS (for in the sequential search of several keywords by AIJ).

Karen

On 8/5/2018 4:05 AM, Mark Salisbury [via AstroImageJ] wrote:

Many thanks for the very quick response Karen.

I would just want to check with the software developers for the telescope that the FITS keywords are not going to change at all before you do any work.

Currently the FITS headers use DATE-OBS for the date and time.  The GPSSTART field uses the same format, example below.

32 DATE-OBS '2018-08-02T00:54:43.0112279' S
36 GPSEND '2018-08-02T00:56:45.9334801' S
37 GPSSTART '2018-08-02T00:54:45.9325047' S

Regards

Mark

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NAML

[Mark Salisbury]

Hi Karen

Apologies for the delay in getting back to you on this, due to holiday etc.  It's been decided that the observatory control software will be updated next month to write the GPS derived time stamp into the DATE-OBS field.  In the meantime I'm just using a small script to overwrite the DATE-OBS time in the headers.

Many thanks for your support on this question.

Regards

Mark