! Hubble Space Telescope Cycle 5 (1995) Phase II Proposal Template ! $Id: 5945,v 13.1 1995/11/07 21:11:34 pepsa Exp $ ! ! Refer to the HST Phase II Proposal Instructions to fill this out ! ! Anything after a "!" is ignored, and may be deleted ! ! All keywords with multiple entries are comma delimited except the ! Visit_Requirements and Special_Requirements keywords which can be ! delimited with carriage returns or semi-colons, but not commas ! ! For help call your Program Coordinator: Debra Wallace ! Phone: 410 338-4506 , E-mail: wallace@stsci.edu ! ! This partially completed template was generated from a Phase I proposal. ! Date generated: Sat Dec 17 11:07:09 EST 1994 ! Proposal_Information ! Section 4 Title: Diffuse Helium Absorption as a Probe of Quasar History and Intergalactic Gas Proposal_Category: GO Scientific_Category: Quasars Cycle: 5 Investigators PI_name: Craig Hogan PI_Institution: University of Washington CoI_Name: Scott Anderson CoI_Institution: University of Washington Contact: Y ! Y or N (designate at most one contact) Abstract: ! Free format text (please update) We propose to reobserve the region of the possible helium Gunn -Peterson edge found by Jakobsen et al., using GHRS G140L (with FLYLIM commanding) to achieve significantly better wavelength calibration, resolution and signal to noise. Together with already extant ground-based data on the quasar and its hydrogen Lyman Alpha absorbers, these proposed GHRS data will be used to confirm that the edge is indeed produced by HeII 304 Angstrom\ absorption, to measure the Stromgren region of ionized helium near the quasar and compare with the corresponding void in the LyAlpha forest, to search for possible emission well beyond the edge corresponding to another void in the LyAlpha forest, and to set combined constraints on quasar luminosity history and IGM density. We will waive all proprietary rights to the data. Questions ! Free format text (please update) Observing_Description: Description of the observations. QSO Q0302-003 will be observed with the now available GHRS side 1 and the G140L grating; wavelength coverage of 1235-1520 Angstrom\ is chosen to include the spectral region of interest, while avoiding geocoronal LyAlpha (see discussion below on FLYLIM). The large science aperture (LSA) will be used to maximize throughput, and permit FOS-assisted target acquisition (TA). The spectral region of most interest occurs near 1303 Angstrom\ (He II 304 Angstrom\ redshifted by z=3.286), which is nearly coincident with OI airglow at 1304 Angstrom; thus the observations require ``dark time" when OI 1304 Angstrom\ emission may be neglected. The low flux of Q0302 in this region (~ 10^-16 erg/s/cm^2/Angstrom) necessitates the use of special FLYLIM commanding (page 97 of the GHRS Handbook), in order to reduce the background count rate to an acceptable level. A significant fraction of the average background noise (0.007-0.008 cts/s/diode) arises in short Cerenkov radiation bursts (induced on the digicon faceplate by cosmic rays); such burst events can be rejected using sufficiently short ``STEP-TIME" integrations. With FLYLIM commanding a 0.2s STEP -TIME (the default anyway) integration is performed. At the faint flux of Q0302, this short STEP-TIME will yield <1 count summed over the entire 500 diodes of GHRS. FLYLIM checks each STEP-TIME integration, and if 2 or more counts are detected, that individual STEP-TIME integration is assumed to be dominated by noise and discarded (on-board the spacecraft). ! This approach has been successfully tested on the GHRS D2 ! detector (e.g., Beaver et al. 1993, in Calibrating the Hubble ! Space Telescope, p. 304), as well as with the FOS blue side ! (where it is called REJLIM; e.g., Beaver and Lyons, CAL/FOS- ! 076); for both very similar detectors the mean background ! count rate was decreased from 0.007-0.01 cts/s/diode down to ! 0.002 cts/s/diode. Although FLYLIM has not yet been tested ! for side D1 of GHRS, the very consistent successes with ! GHRS/D2 and FOS/BL suggest that a mean background of 0.002 ! cts/s/diode may be reasonably expected with a rejection of ! only about 25\ (page 97 of GHRS Instrument Handbook; and ! confirmed to us by GHRS Instrument Scientist C. Leitherer). ! That is, effectively the integration time is reduced by only ! 25\ of 4! We will avoid the spectral region including ! geocoronal LyAlpha, but airglow OI 1304 Angstrom\ cannot be ! avoided because it falls within the spectral region of most ! scientific interest. As OI may yield a count rate (over ~10 ! diodes) in excess of 1 cts/s/diode at HST noon, we are again ! driven to ``dark time" in order that FLYLIM rejection does ! not discard all or most of the individual integrations. ! Exposure time and orbit estimates. From Jakobsen et al. 1994, ! we know that the Q0302 flux averaged over a 50 Angstrom\ bin ! to the redward of the likely HeII break (1303 Angstrom) is ! F_Lambda=1.25* 10^-16 erg/s/cm^2/Angstrom. In a fortuitous ! coincidence, the peak of the F_Lambda-to-cnt-rate conversion ! factor occurs very near 1300 Angstrom\ with GHRS+G140L. From ! the information in Table 8-2 of the GHRS Handbook, we expect ! a count rate per 0.6 Angstrom\ diode of 168 * 10^11 * 1.25* ! 10^-16=0.0021 cts/s/diode. As discussed in the science ! justification section, it would not be too surprising to see ! an unabsorbed continuation of this flux in at least a few ! diodes blueward of the HeII break; even down to 1240 ! Angstrom, the throughput of G140L+GHRS/D1 remains very high, ! with an expectation of 0.0019 cts/s/diode for such an ! unabsorbed flux. As D1 is ``solar-blind" and sensitive only ! to UV light at <1800 Angstrom, contamination from scattered ! visible light need not be considered further. UV scattered ! light sources should also be negligible (scattered fraction ! <10^-3). Then in ``dark time", when the OI airglow may also ! be neglected, the dominant (non-source) noise will be just ! the background count rate, i.e., 0.002 cts/s/diode using ! FLYLIM commanding. Thus at 1300 Angstrom\ of the HeII break, ! we should achieve the following S/N ratios in 19,000 sec of ! useful integration (e.g., before FLYLIM considerations): ! S/N=4.5 at the full 0.6 Angstrom\ per diode resolution, ! S/N=6.4 with 2-diode (1.1 Angstrom) bins, S/N=10.1 in 5-diode ! (2.9 Angstrom) bins, and S/N=14.3 in 10-diode (5.7 Angstrom) ! bins. (The diode binning occurs in data reduction, of course, ! as needed). ``Dark time" can constitute only about 25 min per ! orbit, and we expect 25\ the individual 0.2s STEP-TIME ! integrations to be rejected on-board with FLYLIM commanding, ! so we need 18 orbits in order to obtain these S/N levels. ! Target acquisition will require an additional one HST orbit ! per visit (see below). We assume that the observations would ! thus be divided into three visits (as 9 HST orbits per day ! is the max possible while avoiding the SAA), and thus request ! a total of 21 HST orbits, including time for target ! acquistion. It is interesting to note that this compares ! quite favorably to the 10 ``dark time" orbits required to ! take the discovery FOC prism spectrum with its S/N~ 5 in ~ 10 ! Angstrom\ bins. A typical 52 minute target visibility is ! available per HST orbit. Other overheads (6 min for guide- ! star reacquisition and 4 min for instrument overhead) will ! easily fit into the ``non-dark time." As the continuum flux ! of Q0302 throughout the UV/optical is measured to be a few * ! 10^-16 erg/s/cm^2/Angstrom standard GHRS TA approaches are ! unlikely to work. Target acquisition into the LSA on each ! visit will thus be accomplished using FOS-assisted TA, if ! available; C. Leitherer informs us that the calibration ! program to test FOS-assisted TA was submitted for Director's ! approval in July 1994 and is currently under review. With ! V=17.6, initial TA of Q0302 is well within the capability of ! FOS, and should easily be accomplished in one HST visibility ! period (including overheads), for example: guide-star ! acquisition (12 minutes), ACQ/BINARY (9 minutes), followed ! perhaps by an additional ACQ/PEAK into the 0.3 or 0.5'' FOS ! apertures (12-25 min). The switch to GHRS could occur during ! earth occultation. If FOS-assisted TA is not available, we ! might require WFPC2-assisted interactive acquisitions. Why ! GHRS rather than FOS? It should be noted that using ! FOS/BL+G160L (and REJLIM) we could accomplish similar goals ! in similar exposure times, but at the expense of both lower ! spectral resolution and lower S/N. We have opted to propose ! for GHRS because its combination of higher S/N and better ! spectral resolution will provide more information and allow a ! greater flexibility at the data reduction stage (e.g., choice ! of diode binning while still maintaining good spectral ! resolution). Our more quantitative reasoning is as follows. ! Consider the count rate achievable with FOS/BL+G160L versus ! GHRS+G140L. Because of differing numbers of Angstrom\ per ! diode, it is convenient to normalize to a common wavelength ! bin of 6.9 Angstrom which is one diode with FOS+G160L and 12 ! diodes with GHRS+G140L. With FOS/BL+G160L (and 3.7'' ! aperture), we expect (using corrected version of formula 1 ! from Table 1.2.1 of FOS Handbook) a count rate of 0.0065 ! cts/s in the 6.9 Angstrom\ bin at the relevant wavelength of ! 1300 Angstrom, for the Jakobsen et al. quoted flux of ! F_Lambda=1.25* 10^-16 erg/s/cm^2/Angstrom. Note from the ! above, that in the equivalent 12-diode bin with GHRS+G140L, ! the count rate is nearly 4* higher at 0.025 cts/s. This, of ! course, is somewhat offset by the higher GHRS background ! count rate when normalized to the same 6.9 Angstrom\ bin ! (0.002 cts/s/diode for FOS+G160L versus 0.024 cts/s/bin for ! GHRS). However, the resultant S/N ratio is still considerably ! higher with GHRS than FOS: in the proposed 19,000s of useful ! integration time, we expect at S/N of 15.7 with GHRS in a 6.9 ! Angstrom\ bin, compared to S/N=9.7 with FOS/BL+G160L in the ! same 6.9 Angstrom\ bin. Hence, we have opted for GHRS. Real_Time_Justification: Dark time is required. In order to maximize throughput and make use of FOS-assisted TA, it is necessary to use the LSA. We will choose wavelength coverage with GHRS+G140L to avoid geocoronal LyAlpha (see discussion above). However, the spectral region of most interest occurs at 1303 Angstrom\ (He II 304 Angstrom\ redshifted by z=3.286), which is unfortunately nearly coincident with contaminating airglow OI at 1304 Angstrom. Thus, the most fundamental data will need to be taken during ``dark time" when OI 1304 Angstrom\ emission is negligible. (Also note that OI may conceivably yield a count rate in excess of 1 cts/s spread over ~ 10 diodes in non-dark time with the LSA, and so we are also driven to ``dark time" in order that FLYLIM rejection does not discard all or most of the individual integrations.) The most detailed level of modeling will make use of published ground-based data for the hydrogen LyAlpha forest lines and voids (e.g., Dobrzycki & Bechtold, ApJ, 377, L69, 1991; also, Sargent, Steidel, & Boksenberg, ApJS, 69, 703, 1989), and the systemic redshift of Q0302 has recently been measured by B. Carswell (private communication) from the OIII 5007 emission. Thus, no additional supporting observations are essential. (Although not a part of this proposal, we also point out that extremely deep WFPC2 or FOC parallel imaging data of a field nearby may be acquired simultaneously with the proposed GHRS data on Q0302. Such deep parallel data should prove quite useful, for example, to one or more of the various approved parallel deep survey imaging programs.) Calibration_Justification: ! Move appropriate text from Real_Time_Justification Additional_Comments: This is a rather unusual set of observations, with FLYLIM commanding, FOS-assisted TA into GHRS Fixed_Targets ! Section 5.1 Target_Number: 1 Target_Name: Q0302-0019 Alternate_Names: Q0302-003 Description: GALAXY,QSO Position: RA=03H 04M 49.82S +/- 0.03S, ! Most common specification format is DEC=-00D 08' 13.6" +/- 0.5", PLATE-ID=03U0 Equinox: J2000 RV_or_Z: RA_PM: ! Units are seconds of time per year Dec_PM: ! Units are seconds of arc per year Epoch: 1982.940 Annual_Parallax: Flux: V=18.4 ! * Include at least V and B-V Comments: Coordinates updated from inventory GASP measurement ! This is a template for a single visit containing a single exposure ! Repeat exposure and visit blocks as needed Visits ! Section 6 Visit_Number: 1 Visit_Requirements: BETWEEN 29-OCT-1995 AND 5-NOV-1995 ! Section 7.1 ! Uncomment or copy visit level special requirements needed ! Most of these requirements (including ORIENT) will limit scheduling ! PCS MODE [Fine | Gyro] ! GUIDing TOLerance ! ORIENTation TO ! ORIENTation TO FROM ! ORIENTation TO FROM NOMINAL ! SAME ORIENTation AS ! CVZ ! PARallel ! AFTER [BY [TO ]] ! AFTER ! BEFORE ! BETWEEN AND ! GROUP WITHIN