GHRS FAQ for Proposal Writing

The following information may be of use to GOs submitting HST Phase II proposals. All comments, unless otherwise indicated, refer to Version 5.0 of the GHRS Instrument Handbook. These are provided here for archival purposes.

• Acquisiton Section
• Using V_0 to calculate count rates
• Using attenuated mirrors to acquire
• How to use line numbers in the ONBOARD ACQ FOR Special Requirement
• Wavelength Calibrations Section
• Scheduling wavecals during occultation
• SPYBALs appear to cause lots of confusion
• FOS-assisted ACQs and wavelength accuracy
• ACCUM Section
• Don't use EXPAND, END-ORBIT, etc. for GHRS
• FP-SPLITs
• Avoiding truncation of exposure time
• Resolving Power
• FLYLIM
• Phase I Questions

1. Acquisitions:
1. Please note that the wording on page 68 is confusing. V_0 is meant to refer to the dereddened magnitude of the object you are observing, and the table lists count rates for stars of magnitude zero simply to make the arithmetic easy.


2. If your object is very bright you may need to acquire with one of the attenuated mirrors, A1 or A2. This will take significantly longer because of the additional time needed to execute a DEFCAL (a deflection calibration to determine where the image of the aperture is falling on the photocathode) for A1 (especially) and A2. We have checked the times shown by RPS2 and they are accurate.


3. You should use the "ONBOARD ACQ FOR" Special Requirement to ensure that acquisitions are executed in the right order and in the right sense. One generally starts with an ACQ in the LSA (line 10, say), stating "ONBOARD ACQ FOR 20". Line 20 might then be an SSA ACQ/PEAKUP and that line would state "ONBOARD ACQ FOR 30 - 60", where in this case 60 is the last exposure for this visit or is the last exposure until another acquisition occurs. The initial acquisition passes control to a PEAKUP in this case, so the initial acquisition is FOR a single line but the PEAKUP is FOR several lines. If no PEAKUP were done, then the initial acquisition would apply to all lines and the Special Requirement would be something like "ONBOARD ACQ FOR 20 - 50". Example 5 in Chapter 5 shows how to use this Special Requirement.

If you leave out the "ONBOARD ACQ FOR" statement it may appear as though less time is used for the same operation, but what actually happens is that a key operation (the LOCATE phase) is skipped, leading to degraded pointing.


2. Wavelength Calibrations:
1. We cannot ordinarily schedule WAVE exposures during earth occultation. Doing so might appear to offer an efficiency improvement and would allow valuable time to be spent on your targets, but the cost is not negligible in complexity.
   The underlying reason has to do with a basic aspect of GHRS operations, namely interruptibility. GHRS exposures can be paused and then resumed without causing any degradation of the observations. This frequently occurs, for example, when HST passes through the South Atlantic Anomaly (SAA).
   PLEASE NOTE: What this means is very important when you view what RPS2 shows you because what you see there may not resemble what actually happens when your program is scheduled. WFPC2 exposures, for example, cannot be interrupted, so they must be scheduled to avoid the SAA. The overall efficiency of HST is significantly improved because the GHRS can easily be interrupted.
   Therefore what can seem simple initially can turn complex when seen in the light of real operations. You can try to plan WAVE exposures to occur at the beginning of an orbit, but it may or may not turn out that way. Alterations of these procedures have to be done by hand and are tedious, and are therefore reserved for special cases where the science to be done unambiguously requires special considerations. You may request such special consideration, of course, but we will ask for justification.


2. SPYBALs appear to cause lots of confusion. Please remember that they are added routinely in order to ensure good overall data quality, so that the observer in general need not concern him- or herself about it. If your wavelength calibration requirements are unusually stringent, please consult your PC or LS and we can advise you on an appropriate observing strategy.


3. FOS-assisted ACQs and wavelength accuracy
   Please read this if you need accurate wavelengths and an FOS-assisted acquisition.


3. ACCUMs:
1. An issue related to GHRS interruptibility has to do with using END ORBIT as a Special Requirement: PLEASE DO NOT use it, as the results may be very different than what you intend. Consult with your PC or LS on this, but we ordinarily recommend NO SPLIT or NON-INT if you wish to avoid having an exposure split over an earth occultation. END ORBIT is mainly useful when used with the EXPAND Special Requirement, but EXPAND may not be used with the GHRS (because the GHRS can be interrupted).


2. An FP-SPLIT can help to improve signal-to-noise in your final spectrum, but please remember that each individual exposure needs to have enough S/N to cross correlate with your first spectrum if complete restoration of the spectrum is to be done without degradation of resolution. The exact requirements will depend on the nature of the object being observed. We recommend that you calculate the count rate for your object and determine the total number of counts that will be achieved in a single exposure. You presumably have some idea of what the spectrum will look like and can then judge if a cross-correlation will work or not.
   At low S/N (roughly 30 or less), adding an FP-SPLIT will do no good and can even be counterproductive.


3. It is a good idea to specify your total exposure time as an integral multiple of the minimum cycling time listed in the last column of Table 8-5. While any valid time specification may be entered, the actual exposure time of the spectra obtained will be truncated down to an integer multiple of the minimum exposure time. Minimum exposure time is a function of STEP-PATT, FP-SPLIT, and COMB and can be determined by the following equation, taken from Section 8.3 of the V.6.0 instrument handbook:
   t_min = FP-SPLIT * STEP-TIME * COMB * [N_background + (N_spectrum * ratio)]
   
   where N_background, N_spectrum, and ratio come from Table 8-5 in the handbook. Please note that losses due to this truncation can be significant - so it is strongly recommended that exposure times be specified as multiples of the minimum exposure time for the options you have chosen. This truncation will not show up in your RPS2 output, so be careful.


4. The resolving power of the GHRS which is illustrated in Figure 8.1 of the V.6.0 Handbook refers to a point source observed with the SSA. If a point source is observed with the LSA, a degradation of the spectral resolution by about 20-25% occurs. If an extended source is observed which fills the LSA uniformly, the spectral resolution will be worse by about a factor of 8 in comparison with the case shown in Figure 8.1. This is because the LSA samples 8 diodes, in contrast to the SSA whose size equals 1 science diode. Those 3 cases a briefly discussed in GHRS ISR #066.


5. The FLYLIM parameter may be useful for certain programs, but it must be used with great care. Please consult with us to verify all the details before your Phase II proposal is submitted.

• SPYBAL: Spectrum Y-Balance
• Geocoronal Lyman-alpha through the SSA
• Spectrum bandpasses and dispersions
• Acquisition counts
• Achieving high signal-to-noise on faintish objects
• Overhead times in special cases
• The memory problem

1. SPYBAL Spectrum Y-Balance:

A SPYBAL is executed to balance the light of the spectrum in the GHRS "y"-direction (cross-dispersion) so that light does not fall off the top or bottom of the diode array. A SPYBAL consists of a wavelength calibration exposure. The wavelength used is different for each grating and was chosen to give an even distribution of light over the bandpass sampled.

In general, a new SPYBAL is inserted each time a different grating is used. This means that as long as the grating does not change, no new SPYBAL will be inserted, unless one has several exposures and the total alignment time (not real orbital time) exceeds 90 minutes. If one has several short exposures but does not change grating, a SPYBAL will be inserted when one reaches the 90-minute mark, but only after the completion of the executing exposure and before the next.

For science exposures (IMAGE, ACCUM, WSCAN, OSCAN), any use of Number_of_Iterations > 1 will be treated as a single exposure (i.e., all iterations will have the same exposure_id).

For example, if you have a single exposure run 6 hours, a SPYBAL will only be inserted at the beginning of the observation. The 90-minute rule does apply, but an exposure will not be interrupted in order to insert a SPYBAL. However, we discourage this latter type of setup since thermal drifts could result in having the spectrum miss the diodes even though the object itself is still well centered in the aperture. We do, of course, have the new Optional Parameter SPYBAL which will allow the PI to control the usage; however, we recommend the default usage of SPYBAL except in unusual circumstances.

2. Geocoronal Lyman-alpha through the SSA:

If an object is observed in Dark Time, the relative quietness of the sky and the small size of the SSA mean that geocoronal Ly-a is coming through at a rate of only 0.02 to 0.03 counts per second, most of which is falling on one diode. This is only a factor of 2 or 3 above detector dark noise.

3. Spectrum bandpasses and dispersions:

The GHRS Instrument Handbook provides typical values for these quantities for the various gratings. However, in certain situations one needs to know a precise value. We will include an algorithm in the next version to allow you to do this, but for the present if you need precise information on what wavelengths can be centered on or the bandpass achieved at a particular wavelength, please contact us.

4. Acquisition counts:

The GHRS Instrument Handbook now recommends that you achieve 1,000 to 10,000 counts in the peak pixel during an acquisition. Those values were based on the old Point Spread Function. The high contrast of the post-COSTAR PSF means that that condition can be relaxed. It is our opinion that satisfactory acquisitions should be achieved with about 100 counts in the peak pixel, that value allowing for some error as well when estimating. This may be of particular value in acquiring faint objects if you are confident of the UV flux (perhaps from an IUE observation) and so can make a good prediction.

5. Achieving high signal-to-noise on faintish objects:

A recurrent problem is applying FP-SPLIT to not-so-bright objects to get the best possible S/N. The objects are bright enough to get the needed overall S/N, but that needs to be broken down into segments that are only 5 to 10 minutes long, and within one of those there may not be enough counts to do a good cross-correlation to register the spectra. The carrousel positioning is, unfortunately, not reliable enough to do the registering without the ability to do the cross-correlation.

There is no easy cure for this. One possibility is to specify discrete grating positions at which to integrate instead of using FP-SPLIT. One would then obtain a WAVECAL for each position. If the separate exposures were pushed to 10 minutes each, the fraction of time spent on WAVECALs is not too excessive.

6. Overhead times in special cases:

The nominal overheads listed on the worksheets that come with the Phase I Proposal Instructions are adequate in most cases, but occasionally one may wish to obtain a large number of repeated exposures on an object and to get them with better S/N than RAPID mode allows.

For example, if Number_of_Iterations were specified for a WSCAN, the full set of repeats would be done first at the initial wavelength, then the carrousel would be moved and a second series obtained at the second wavelength. In other words, the wavelengths would not be cycled through repeatedly. In this situation, the true overhead time per repeat is about 5 seconds. As a concrete example:

Suppose we use STEP-PATT=5, which has a minimum cycle time of 27.2 seconds, and also specify Number_of_Iterations: 1000 exposures). The total exposure time is then 1000 x 27.2 = 27,200 seconds. During this operation the overhead would be 4 minutes plus 1000 x 5 seconds, or 5,240 seconds. The overall duty cycle is then about 79% (84% from these numbers less a 6% correction for the time spent looking at background). Other STEP-PATTs could be used to get shorter exposures, but with less on-target efficiency (high percentage of overhead time).

7. The memory problem:

As noted in the Instrument Handbook, observers can run into a problem if their program specifies so many instructions that too much onboard memory is used. The typical limit is about 40 separate ACCUMs (each taking its own Exposure Logsheet Line) or, equivalently, 10 FP-SPLITs that each have four set points. A WSCAN with 40 individual wavelength set points would also cause a problem. However, the Number_of_Iterations does not create a memory problem. In the above example, an ACCUM with Number_of_Iterations: 999 would not cause a memory problem.