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Accumulation Mode

GHRS Instrument Handbook


The previous chapter provided the information needed to estimate an exposure time to achieve a given level of signal-to-noise. We reiterate several factors having to do with the detectors that must be taken into account to achieve the best data quality. Note that it is not necessary to explicitly specify these parameters (except for FP-SPLIT) because the defaults that apply to each mode of operation will automatically invoke them. Moreover, you should not deviate from the defaults without good reason.

The Digicon detectors have faceplates with some granularity (uneven response). The diodes onto which the faceplate is imaged also have response irregularities and some of them have been turned off because of misbehavior. Both of these effects are relatively small but enough to prevent you from obtaining a spectrum with S/N much in excess of about 30. They can also produce "glitches" that can mimic spectrum features. The FP-SPLIT parameter causes the carrousel to move slightly between each of the two or four separate subexposures (depending on the value you choose). The COMB parameter suppresses diode-to-diode gain variations and allows one to work around the dead diodes. Both features should normally be used, especially since they cost little or nothing in exposure time and improve data quality.

The Digicon diodes also undersample the spectrum by about a factor of two. The parameter STEP-PATT causes electronic motions of the spectrum so as to sample the spectrum fully. It is possible to STEP-PATT at two samples per diode width, but we recommend using four samples per diode to yield optimum results, and again at no net cost. You can always rebin a quarter-stepped spectrum into a half-stepped one during your data analysis, but the process cannot make a quarter-stepped spectrum out of a half-stepped one. Deconvolution has worked best with quarter-stepped spectra (the default); see Gilliland et al. (1992). STEP-PATT also determines the way in which the background is measured (see Section on page 112).

We also remind you to break up long exposures into subexposures that are no longer than about 5 to 10 minutes each, so as to defeat the effects of geomagnetically-induced image motion. Bear in mind that a 20 minute exposure, for example, specified with FP-SPLIT=4 will result in four 5-minute exposures.

Wavelength Calibrations

Remember to have wavelength calibration exposures precede the ACCUM to which they apply. This minimizes the time interval between these two exposures, so that wavelength drifts do not occur. Use SEQ <exp list> NON-INT as a Special Requirement to make sure the two lines as a group are not split.

Wavelength calibration exposures cannot ordinarily be scheduled during Earth occultation. The reasons have to do again with the interruptibility of GHRS exposures and the fact that the sequence of exposures that actually executes bears only a casual resemblance to what RPS2 shows you due to SAA passages. In a few cases we have executed WAVE exposures during occultation, but the manual effort needed is substantial, meaning that the science requirements have to be unusual and demanding.

There are calibration exposures made called SPYBALs that can often substitute for a WAVE exposure. A SPYBAL is an exposure of the wavelength calibration lamp, but it is made at a fixed setting for each grating as a means of centering the spectrum on the diodes in the direction perpendicular to dispersion (SPectrum Y BALance). Your WAVE exposure, on the other hand, would be made at the specific wavelength you're observing at. Clearly the latter is superior, but in many cases the SPYBAL exposure contains enough information to correct the wavelength zero point of your spectrum very nearly as well as the WAVE exposure would have.

SPYBALs are executed every time a new grating is used and about once every other orbit thereafter (the details of the thereafter depend on how you write your exposure lines and are impossible to generalize about). Sometimes an observer wishes to ensure that the grating carrousel is not moved at all until they have done all their exposures at a particular setting. That can be achieved with SPYBAL=NO. However, the defaults almost always provide satisfactory results.

Memory Usage

A problem can arise when a science program specifies a large number of separate GHRS exposures. This problem is caused by the relatively small amount of memory available on HST in which to store GHRS commands. It is usually possible to break up such a program into several visits so that the separate exposures are not all together, but occasionally the science goals cannot allow that and some other compromise must be made. Roughly speaking, about 40 total spectra can be scheduled in a single visit (a WSCAN with n set-points counts as n exposures and an FP-SPLIT counts as 2 or 4). Once that number is exceeded the remaining observations must be scheduled in a new visit, and that means a new target acquisition will be needed, with the resultant overhead time. Using a large number of iterations in an ACCUM does not cause a memory problem, but what does is lots of different instrument settings.

Summary of Accumulation Mode Parameters

WSCAN and OSCAN modes

Use of WSCAN can result in a spectrum covering a broader total bandpass than is possible with a single exposure. All the parameters listed above for an ACCUM exposure are available in WSCAN mode. The most important parameter to specify is WAVE-STEP, which is the spacing (in Ångstroms) between each subexposure. If WAVE-STEP=DEF is specified, the central wavelengths of the separate exposures will be equally spaced so as to cover the range of wavelengths that you specify, with at least 20% overlap from one subspectrum to the next.

You may also explicitly give a WAVE-STEP value. If lmin is the central wavelength of the shortest-wavelength exposure, and lmax is the central wavelength of the longest-wavelength exposure, then choose these values in concert with WAVE-STEP so as to yield an integral number of WAVE-STEPs between lmin and lmax.

OSCAN mode makes it possible to scan across echelle orders at a fixed value of ml, where m is the order number and l is the wavelength. It is rare that adjacent orders both have features of astrophysical interest and so this mode is primarily used for calibrations and not for science observations. If you do use this mode, all the parameters of an ACCUM observation are available.

Wavelength Calibrations
Memory Usage
Summary of Accumulation Mode Parameters
WSCAN and OSCAN modes