We advertise the opportunity for proposals that would push STIS to the limiting resolving power in the FUV. We find that the FUV-MAMA has improved resolving power (at the cost of sensitivity) when the repeller-wire high-voltage is turned off. Application is recommended shortward of 1400A, only for the FUV echelle modes E140H, and possibly for E140M, when applied with the 0.1x0.03 aperture, the so-called "Jenkins slit."
The repeller wire establishes a field above the FUV-MAMA microchannel plate that reflects forward-ejected photoelectrons back into the microchannel pores. This increases the detective quantum efficiency (DQE). However, these reflected electrons form a halo that makes broad wings on the detector point spread function (PSF). Turning the repeller wire voltage off gets rid of the halo and provides a cleaner FUV-MAMA PSF, but at the expense of 35% DQE. Prelaunch measurements, done with an external monoisotopic platinum lamp, demonstrated resolving power~220,000 (~2 HiRes pixels). The repeller-generated halo broadens with increasing initial photon energy. Hence, the most significant PSF improvement is obtained shortward of 1400A.
We specifically point out that the FWHM does not change significantly, but rather the halo disappears when the repeller wire voltage is turned off. The shape of the PSF and the integrated energy changes and indeed the profile of interstellar lines would be affected.
In Figure 1, we show the average profile of calibration lamp line at 1234A with the repeller wire on and off. This data was obtained during thermal vacuum testing with a monoisotopic platinum lamp.
The normal observing mode is with the repeller voltage turned on. Interstellar spectral line measurements with the 0.1x0.03 aperture have been accomplished on orbit with R~200,000 (Jenkins, BAAS 32, 731, 2000). Likely resolving powers approaching 220,000 should be accomplished longward of 1400A with the normal repeller voltage on mode. To achieve the highest resolving power shortward of 1400A, the observer may want to consider the repeller-off mode for bright stellar sources that would provide good photon statistics within five orbits or less, yet would not approach the global or local count rate limit.
The HST telescope PSF in the FUV is significantly larger than the limit of the STIS FUV PSF. At Lyman alpha, it is 3.5 LoRes pixels. No advantage is to be gained by turning the repeller voltage off for direct imagery.
An additional caution must be included: in ACCUM mode, the photon event is stored in the nearest HiRes bin with an orbital velocity correction changing in time. Some centroid shifting and smearing will occur in the accumulation mode, potentially leading to a small loss in resolving power, depending on the details of the orbital interval of integration time. It is recommended that five to ten minute integration times, or intervals small compared to the velocity shifting of the spectrum due to the HST orbit, should be used for accumulation mode. Short ACCUM intervals are also recommended for limiting resolving power as tests on orbit demonstrate thermal shifting in the dispersion direction up to 0.25 LoRes pixels per orbit. Timetag, as the photon arrival time and x,y coordinates are stored in memory, is not affected, but the full detector count rate is limited to 30,000/second.
A limited number of proposals (of order 1-2) will be accepted for this mode. Users must make a scientific case for the use of this limited resource, which requires special commanding to control the MAMA high voltage. Use of this mode should be specifically noted in the "Special Requirements" section of the Phase I proposal. Note that the Exposure Time Calculator does not take the additional 35% loss into account, so this correction must be applied by hand. A Contact Scientist will automatically be assigned to any successful proposal using this mode to ensure the health and safety of the FUV-MAMA.
Note that the MAMA detector data comes down as a 2048x2048 array of HiRes pixels, but the standard pipeline process utilizes 1024x1024 LoRes pixels. To fully realize the increase in resolution provided by this mode, users will have to reprocess the data on their own using the CALSTIS package in STSDAS with parameters set for HiRes pixels.