Changes in the upcoming LWS OLP Version 8

SPL updates

• Improvements to glitch checking. SPL currently checks the detector ramps for glitches. Before the glitch checking is performed, "bad" points are discarded from the ramps. However, many of these points, while not suitable for processing into SPD data, are good enough to use for checking for glitches. In fact, in some cases they are required in order to detect the glitch in the first place. For example, a large glitch may cause the detector voltage to "hit the rails" for the remainder of the ramp. If these points are discarded then the ramps appears normal and no glitch is detected.


The solution implemented allows the set of points checked for glitches to be different from the set of points used for the determination of the photocurrent for the ramp. The set of points used for glitch checking will be the raw data points with only the portion of the ramp affected by the reset removed. This update should result in more glitches being correctly identified. SCREW-396

• Addition of data following a glitch. SPL currently discards a fixed number of ramps following each glitch, since these ramps are often affected by the glitch. However, it would be useful to have access to this discarded data for "expert" users to be able to examine.


The solution implemented is to include the discarded data in the LSPD file, but to flag the data as "invalid" within the status word. An additional flag within the status word will indicate that the data has been discarded because it is following a glitch. These data will NOT be copied into the LSAN file to avoid any observer using them as though they are valid data. SCREW-396
 

• Addition of mini-scan counter. For L03 observations the line number fields in the LSPD and LSAN files do not contain useful information. For OLP 8 the "mini-scan" number is written in place of the line number. The mini-scan number is a counter which is incremented for each different grating position used. This is effectively a "line" number, so it is logical to write this into the line number field. SCREW-396

AAL Updates

• Fit to problem with drift correction. The correction for the drift in responsivity is currently being performed at the wrong stage in AAL. The drift correction is being performed at the end of AAL, while the theory behind the drift correction requires it to be done at the start. In practice this will only make a difference for observations with very low signal levels, comparable to the dark current.


As part of the solution to this problem, the LSNR file will no longer be generated by AAL. This file contained intermediate results, before the application of the responsivity drift correction and absolute responsivity correction. With the drift correction being performed at the start of the pipeline there is little point in the generation of this file. SPR-3037.
 

• Use of standard dark currents for FP observations. For FP observations it has been found more reliable to used standard dark current values, rather than the values measured during the surrounding illuminator flashes. The solution involves the creation of a new calibration file named LCDK. The LCDK file will be a binary extension FITS file containing the dark current values for each detector. In case of future need, the file can contain different dark currents for different ranges of revolution numbers. SCREW-396

 
• Updated flux uncertainty calculation. The uncertainty value inserted into the LSANFLX field of the LSAN file has been updated to make it a more meaningful value. The uncertainty value is now calculated using a new algorithm, as specified in the note "Error calculation in LWS offline processing" by Bruce Swinyard. SCREW-396

 
• Improved FP processing. In previous OLP versions the LWS Fabry-Perot data were converted from photocurrent to flux by using the grating position to identify a particular calibration curve in a look up table. Owing to the inaccuracy of the grating position sometimes the wrong FP position was used and no calibration curve was found. This problem has proved intractable and errors continued to occur in OLP V7. An interactive analysis routine has been written to cure the problem but this is not sufficiently robust to be reliably automated. It has therefore been decided to stop attempting to remove the signature of the grating resolution element from the pipeline products and leave this aspect of the processing to IA.


In addition to this change, a new method of calibrating the total throughput of the FP's has been devised using Mars as the calibration standard. This method uses a fitted polynomial between the wavelength and the FP throughput and allows the FP LSAN products to be put into W cm-2 um-1 for the first time. All L04 and L03 AA products will be in these flux units. The spectra will still have the relative change in flux over the grating resolution element present for the reasons given above - this can be removed by the use of LIA.

This update uses a new calibration file name LCTP. This consists of a FITS header only. The following calibration files will no longer be used: LCLRZ, LCSRZ, LCLR_n (n=0..9), LCSR_n (n=0..9).

SCREW-396. This update also fixes SPRs 2853 and 2855, which were caused by the problems described above.
 

• Update to GR spectral responsivity warning flag. The status word in the LSAN file contains a flag which warns that the grating wavelength is outside of the nominal calibration range in the LCGR file. L03 and L04 observations may use such grating positions. For L03 observations and for the active detector on L04 observations this warning flag is not appropriate and will no longer be set.


SCREW-396. This update also fixes SPR 2856, where all of the data for detector 8 had the warning flag set.
 

• Updated AAL calibration files. The LCIR and LCGR files have been updated using an improved model of Uranus. SCREW-396