December 03, 2019J. Debes, J. Anderson, M. Wenz, J. Stock
Between 21-April 2018 and 5-October 2018, the combination of gyroscopes 2,4, and 6 resulted in a higher than normal spacecraft jitter that exceeded 15 mas in amplitude, with larger excursions possible. This amplitude of jitter was predicted to present noticeable degradation to the contrast performance of the STIS coronagraphic apertures and thus a special calibration program was executed on 23-September 2018 to assess the impact. We find that higher levels of jitter increase a systematic component to the noise within a set of coronagraphic images, impacting contrast performance. We review our current recommendations on when jitter reaches a level that could impact specific scientific cases.
ISR 2019-03: Pushing the Limits of the Coronagraphic Occulters on STIS
July 02, 2019J. Debes, B. Ren, G. Schneider
The Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) contains the only currently operating coronagraph in space that is not trained on the Sun. In an era of extreme-adaptive-optics-fed coronagraphs, and with the possibility of future space-based coronagraphs, we re-evaluate the contrast performance of the STIS CCD camera. The 50CORON aperture consists of a series of occulting wedges and bars, including the recently commissioned BAR5 occulter. We discuss the latest procedures in obtaining high-contrast imaging of circumstellar disks and faint point sources with STIS. For the first time, we develop a noise model for the coronagraph, including systematic noise due to speckles, which can be used to predict the performance of future coronagraphic observations. Further, we present results from a recent calibration program that demonstrates better than 1e−6 point-source contrast at 0.6′′, ranging to 3e−5 point-source contrast at 0.25′′. These results are obtained by a combination of subpixel grid dithers, multiple spacecraft orientations, and postprocessing techniques. Some of these same techniques will be employed by future space-based corona- graphic missions. We discuss the unique aspects of STIS coronagraphy relative to ground-based adaptive-optics-fed coronagraphs.
ISR 2019-02: Identifying Jitter Induced CCD CR-SPLIT Combination Errors
June 21, 2019Joleen K. Carlberg
By default, all STIS CCD exposures are split into a minimum of two subexposures to allow for cosmic ray removal. An underlying assumption of the cosmic ray removal algorithm is that the subexposures are well-registered. This assumption breaks down if the target wanders too much in the spectroscopic slit during the observation due to telescope jitter, particularly if the jitter occurs predominantly in one subexposure. In such cases, the algorithm may reject large fractions of valid data, leading to systematically underestimated flux, even for wide slit widths where slit losses from the jitter are negligible. Such datasets may or may not present with unusual line profiles. In this work, we present a technique for using the cosmic ray data quality flags to identify potentially problematic spectra and demonstrate its effectiveness in flagging observations of standard stars with apparent 5-15% flux losses.
ISR 2019-01: A New Method to Monitor the HST/STIS Focus
May 24, 2019M. Maclay
We present results from a new, empirical, Normalized Halo Method of STIS/CCD focus measurement relying on direct measurements of real and simulated point spread functions (PSFs). We evaluate the eligibility of this method to be employed as a focus monitor by comparison to phase retrieval results from STIS/CCD and WFC3/UVIS. Through the application and comparison of phase retrieval and the Normalized Halo Method to STIS/CCD F28X50OII observations, we find an absolute constant offset of approximately 3 μm secondary mirror despace and relative precision and consistency between the two methods. We determine the Normalized Halo Method to be effective, suitable for implementation as a STIS focus monitor, and a precise proxy of historical and current STIS phase retrieval measurements.
ISR 2018-07: STIS Bright Object Protection for Available-But-Unsupported Modes
December 08, 2018J. Carlberg
In this Instrument Science Report, we describe techniques for determining the safety of proposed observations using the STIS FUV- or NUV-MAMA for spectral settings or apertures that are available, but unsupported. Such modes may not be selectable in APT using the BOT tool or in the exposure time calculator. These observations can be cleared if a supported mode can be identified that is guaranteed to estimate comparable or higher count rates than the proposed setup. Observations clearing the BOT with the related supported mode would thus be safe in the proposed unsupported mode. A practical example of the described techniques are presented for clearing a planetary nebula observation (the topic of a recent a Cycle 25 SNAP program), which also lead to the inclusion of a new planetary nebula template for the exposure time calculator.
ISR 2018-06: Impacts of focus on aspects of STIS UV Spectroscopy
November 05, 2018A. Riley, T. Monroe, S. Lockwood
The STIS Instrument focus has been drifting more and more positive over time since the Optical Telescope Assembly (OTA) was shifted in 2011. Unfortunately, the STIS best focus was found to be at more negative focus values relative to HST observatory focus. However, this slight upward trend in focus is still smaller than the orbital variation in focus that is seen due to the thermal evolution of the telescope. Both the upward trend and the change in focus due to the orbital variation come into play and this is why sometimes the absolute focus is closer to STIS best focus than at other times. When the absolute focus is not close to STIS best focus, some spectroscopic data has been observed to have larger FWHM values and more spatially dispersed profiles, causing flux anomalies in data. This document discusses these flux anomalies and outlines analysis done to determine whether the Enclosed Energy (EE) tables contained in the HST Exposure Time Calculator (ETC) or the Photometric Conversion Table (PCTAB) have changed. We also discuss what the user community might expect to see in their data due to the changing focus of STIS.
ISR 2018-05: Performance of the STIS CCD Dark Rate Temperature Correction
October 08, 2018D. Branton
Since July 2001, STIS has operated on its Side-2 electronics due to a failure in the primary Side-1 electronics. While nearly identical, Side-2 lacks a functioning temperature sensor for the CCD, introducing a variability in the CCD operating temperature. Previous analysis utilized the CCD housing temperature telemetry to characterize the relationship between the housing temperature and the dark rate. It was found that a first-order 7%/�C uniform dark correction demonstrated a considerable improvement in the quality of dark subtraction on Side-2 era CCD data, and that value has been used on all Side-2 CCD darks since. In this report, we show how this temperature correction has performed historically and explore other, more complex, temperature correction methods in order to re-evaluate the optimal temperature correction for dark subtraction on STIS CCD data.
ISR 2018-04: Monitoring the STIS Wavelength Calibration: MAMA and CCD First-Order Modes
August 28, 2018D. Welty
We discuss a uniform analysis of the Pt/Cr-Ne lamp spectra used to monitor the accuracy of the STIS wavelength calibration during cycles 7-11 and 17-25, focusing on the first-order MAMA and CCD settings. We find that both the mean wavelength offsets and the scatter in the individual wavelength residuals have remained within the desired accuracies (0.2-0.3 pix, 1σ) for all of the monitored first-order settings. We note, however, that there has been some reduction in the number and strength of the lines measurable for some of the settings (due to the fading of the lamps), there are slight systematic trends (versus wavelength) in the wavelength residuals for some of the settings, and there was a slight (~0.1 pix) shift in the mean offsets for most of the CCD settings following a change in the lamp in cycle 11. We recommend that a re-calibration of the wavelengths be performed for the final archive of STIS observations, in order to correct the observed systematic deviations in the current calibration.
ISR 2018-03: Investigating the Binary Offset Effect in the STIS CCD
May 22, 2018J. Debes
Recently, Boone et al., (2018) presented the “Binary Offset Effect” for the SNIFS instrument, which uses a CCD detector. The source of this uncertainty is related to the analog-to-digital readout process, which converts the analog electronic signal of the detector into a digital number as represented by binary bits. The Binary Offset Effect is due to cross-talk between the digital conversion process for a source or driver pixel and pixels read out after the driver. In the course of Boone et al.’s experimentation with this effect they identified a similar effect with the STIS CCD. The STIS team has independently investigated the Binary Offset Effect for a range of bias images currently used for scientific observations, broadly confirming that the effect exists. However, our preliminary investigation suggests that the impact is smaller than reported in Boone et al. (2018) for biases taken with Amplifier=D and GAIN=1, and a lesser effect exists for Amplifier=D and GAIN=4. There is a hint that the effect is time variable for the detector. We broadly assess the potential impact of this effect and make recommendations both for users and future directions of investigation.
ISR 2018-02: A New Geometric-distortion Solution for STIS FUV-MAMA
April 18, 2018T. Sohn
We derived a new geometric distortion solution for the STIS FUV-MAMA detector. To do this, positions of stars in 89 FUV-MAMA observations of NGC 6681 were compared to an astrometric standard catalog created using WFC3/UVIS imaging data to derive a fourth-order polynomial solution that transforms raw (x, y) positions to geometrically- corrected (x, y) positions. When compared to astrometric catalog positions, the FUV- MAMA position measurements based on the IDCTAB showed residuals with an RMS of ∼ 30 mas in each coordinate. Using the new IDCTAB, the RMS is reduced to ∼ 4 mas, or 0.16 FUV-MAMA pixels, in each coordinate. The updated IDCTAB is now being used in the HST STIS pipeline to process all STIS FUV-MAMA images.
ISR 2018-01: A Python Script for Aligning the STIS Echelle Blaze Function
January 15, 2018M. Baer
Accurate flux calibration for the STIS echelle modes is heavily dependent on the proper alignment of the blaze function for each spectral order. However, due to changes in the instrument alignment over time and between exposures, the blaze function can shift in wavelength. This may result in flux calibration inconsistencies of up to 10%. We present the stisblazefix Python module as a tool for STIS users to correct their echelle spectra. The stisblazefix module assumes that the error in the blaze alignment is a linear function of spectral order, and finds the set of shifts that minimizes the flux inconsistencies in the overlap between spectral orders. We discuss the uses and limitations of this tool, and show that its use can provide significant improvements to the default pipeline flux calibration for many observations.
ISR 2017-02: Bright Object Protection Considerations for M Dwarf Flare Events
January 02, 2018R. Osten
We provide clear and concise guidance for Guest Observers and Contact Scientists to evaluate the health and safety of the instrument while observing M dwarfs, taking into consideration current scientific research about the frequency with which large flare events occur, and a risk tolerance level for causing an inadvertent detector shutdown due to overlight conditions from a large flare during an observation.
ISR 2017-06: Updated Time Dependent Sensitivity Corrections for STIS Spectral Modes
September 27, 2017J. Carlberg
The sensitivities of the STIS CCD and MAMA detectors have been slowly changing over time, and these changes vary with wavelength. The STIS time dependent sensitivity monitors track these changes, which must be corrected to allow accurate flux calibration and exposure time calculation. Here, we present updated calculations of the time dependent relationships using data obtained through mid-Cycle 24. We also retested the validity of applying the relationships derived with the lower resolution spectra to the medium resolution first order modes and echelle modes, for which we find reasonable agreement in most of the modes.
ISR 2017-05: STIS CCD Performance through Cycle 24
September 27, 2017A. Riley
This report summarizes the overall performance of the STIS CCD since Servicing Mission 4 (SM4). Overall, the dark current and hot pixels have remained stable. The gain values using all three available amplifiers were remeasured and they have also continued to be stable. The read noise for amplifiers C and D have both exhibited substantial increases within the last few years. The charge transfer efficiency (CTE) was indirectly measured using the Extended Pixel Edge Response (EPER) Test, which shows both the parallel and serial CTE has continued degrading over time.
ISR 2017-04: On the Fading of the STIS Ultraviolet Calibration Lamps
September 27, 2017M. Peeples
The Space Telescope Imaging Spectrograph (STIS) has been in orbit for over 20 years, so its calibration lamps are fading, risking the precision of future science opportunities. The report documents analysis on the fading of these lamps in the ultraviolet and suggests possible mitigation strategies. A discussion of the strategy of alternating years to obtain deep NUV and FUV flats appears to have slowed the fading of these lamps, but the modes in which the flat fields are taken may have to be changed again in future cycles to obtain enough counts for robust flat fields.