We present complimentary techniques to find emission-line targets and measure their properties in a semi-automated fashion from grism observations obtained with the Advanced Camera for Surveys aboard the Hubble Space Telescope. The first technique is to find all likely sources in a direct image, extract their spectra and search them for emission lines. The second method is to look for emission-line sources as compact structures in an unsharp masked version of the grism image. Using these methods we identify 46 emission-line targets in the Hubble Deep Field North using a modest (3 orbit) expenditure of HST observing time. Grism spectroscopy is a powerful tool for efficiently identifying interesting low luminosity, moderate redshift emission-line field galaxies. The sources found here have a median i band flux 1.5 mag fainter than the spectroscopic redshift catalog of Cohen et al. They have redshift z 100{\AA}), and are usually less luminous than the characteristic luminosity at the same redshift. The chief obstacle in interpreting the results is line identification, since the majority of sources have a single emission line and the spectral resolution is low. Photometric redshifts are useful for providing a first guess redshift. However, even at the depth of the state-of-the-art data used here, photometric errors can result in uncertainties in line identifications, especially for sources with i > ~24.5 ABmag. Reliable line identification for the faintest emission-line galaxies requires additional ground-based spectroscopy for confirmation. Of particular concern are the faint high EW [OII] emitters which could represent a strongly evolving galaxy population if the possibility that they are mis-identified lower redshift interlopers can be ruled out. (Slightly abridged)

Automated selection and characterization of emission-line sources in ACS WFC grism data

The low-frequency content of ACS flat fields cannot be accurately determined from ground-based calibration data or internal lamp exposures. It must therefore be determined from on-orbit science data. Photometry of a stellar field that is imaged multiple times with different pointing or roll is ideal for this purpose. I describe a mathematical algorithm to use such data to determine the residual low-frequency flat-field structure (L-flat). The L-flat is expanded as a linear sum of two-dimensional basis functions. Magnitude differences between measurements for the same star at different positions on the detector constrain the coefficients of the linear sum. This reduces mathematically to an over-determined linear least-squares problem that can be solved through singular-value decomposition. This yields both the best-fitting L-flat and its formal error. A numerical FORTRAN implementation of the algorithm is presented and its accuracy verified using tests with artificially generated data sets. The software was used by Mack et al. (ACS ISR 02-08) to analyze early ACS WFC and HRC data of the globular cluster 47 Tuc. The inferred fourth-order polynomial L-flat corrections to the ground-based calibration flats were implemented in the ACS pipeline in late 2002. It is shown that newly implemented features in the software should allow further improvements in these L-flats. The methods presented here are generally applicable to other imaging instruments, and are not restricted to use on ACS data. Copyright c © 2003 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.

Determination of Low-Frequency Flat-Field Structure from Photometry of Stellar Fields

Following recovery of ACS with Side 2 electronics the temperature setpoint was lowered from a nominal value of -77 C to -81 C for the WFC in order to minimize impacts of the continuously growing hot pixel contamination. The CCDs are expected to have changes of 1-3% in overall sensitivity, with some spatial dependence (Sirianni et al. 2006) – the latter is calibrated via changes to the L-flats. We find that required L-flat changes range from about 0.6% peak-to-peak at F435W to 0.15% at F814W. Separate ISRs will review evidence for possible pixel-to-pixel flat field and overall sensitivity changes post cooldown. L-flat changes for F435W, F625W and F814W are derived by comparison of internal tungsten exposures taken before and after cooldown. Flat field updates for other filters within this wavelength range follow from linear interpolation. New LP-flats with a separate useafter date have been delivered.

WFC L-Flats Post Cooldown

pixel-to-pixel flat field changes noted by Bohlin and Mack (2005) for the WFC are further quantified. During each period between anneals, a population of pixels with lowered sensitivity develops which is largely reset by the next anneal. The sensitivity deficits are twice as large in the blue as in the red. The pixels with lowered sensitivity appear to be a unique set each anneal cycle, rather than a subset that 'telegraph' on and off. The low QE pixels recover �90% of their losses on a time scale of a few monthly anneals, but never return fully. Some evidence for spontaneous recovery of the low QE pixels between anneal cycles is developed, but is not conclusive. The number of low pixels would become a large source of error in the absence of performing anneals on a frequent basis. Prior to cooldown in July 2006, the flat field changes that arise continuously within anneal cycles are larger than cumulative persistent changes in the pixel-to-pixel flats. The pre-cooldown reference flat field remained excellent. Post-cooldown, the number of persistent deviant pixels, although still modest in number, may have reached a level justifying delivery of new pixel-to-pixel flats, although providing such will require acquisition of further data.

Pixel-to-pixel Flat Field Changes on the WFC

We present complementary techniques to find emission-line targets and measure their properties in a semi-automated fashion from grism observations obtained with the Advanced Camera for Surveys aboard the Hubble Space Telescope (HST). The first technique is to find all likely sources in a direct image, extract their spectra, and search them for emission lines. The second method is to look for emission-line sources as compact structures in an unsharp masked version of the grism image. Using these methods we identify 46 emission-line targets in the Hubble Deep Field-North using a modest (three-orbit) expenditure of HST observing time. Grism spectroscopy is a powerful tool for efficiently identifying interesting low-luminosity, moderate-redshift emission-line field galaxies. The sources found here have a median i-band (F775W) flux 1.5 mag fainter than the spectroscopic redshift catalog of Cohen et al. They have redshift z ≤ 1.42, and high equivalent widths (typically >100 A) and are usually less luminous than the characteristic luminosity at the same redshift. The chief obstacle in interpreting the results is line identification, since the majority of sources have a single emission line and the spectral resolution is low. Photometric redshifts are useful for providing a first-guess redshift. However, even at the depth of the state-of-the-art ground-based and HST data used here, photometric errors can result in uncertainties in line identifications, especially for sources with i magnitudes fainter than 24.5 ABmag. Reliable line identification for the faintest emission-line galaxies requires additional ground-based spectroscopy for confirmation. Of particular concern are the faint high-EW [O II] emitters, which could represent a strongly evolving galaxy population if the possibility that they are misidentified lower redshift interlopers can be ruled out.

/pdf/automated-selection-and-characterization-of-emission-line-x22uid6ojl.pdf

Automated selection and characterization of emission-line sources in advanced camera for surveys wide field camera grism data

Laboratory flats with simulated sky illumination of the CCD cameras have been obtained for supported and many unsupported ACS modes. The ACS HRC flight detector shows a typical sensitivity variation from center to edge of ~10%, while the large format WFC minimum/maximum sensitivity variation is ~15%. The CEI specification of +-10% uniformity is met for WFC and HRC. The intrinsic rms variation of sensitivity in the pixel-topixel P-flat is 0.5-0.9%, so that observed flats with ~100,000 electrons/px and an rms of ~0.3% are required to improve the intrinsic S/N that is possible without a P-flat field reference file These high S/N flats are the P-flats combined with the low frequency L-flat sensitivity variation over the field. The 142 measured LP-flats include 13 and 17 single filter flats for WFC and HRC, respectively, 78 polarizer mode flats, and 34 flats of all types for the HRC coronographic mode. In addition, estimates are made for 15 supported modes that were not measured directly. All 157 flats are now available as reference files for the pipeline processing of ACS observations from Cycle 11.

HRC and WFC Flat Fields: Standard Filters, Polarizers, and Coronograph

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HRC and WFC Flat Fields: Standard Filters, Polarizers, and Coronograph