2WoRLDS Reduction Page

This page will document the steps and intermediate results for the PISCES narrow-band line data taken with 2WoRLDS.


Live coverage and photometric plots:

Click HERE.


Coverage (as of Oct, 2006)

Fully reduced coverage:


Field Selection

The fields were selected using the full 2MASS PSC within +-10 degrees of the Galactic plane. A composite J-H, H-K CCD using only high photometric quality 2MASS stars, overplotted with various 2MASS sub-catalogs, including dwarfs and giants, Carbon stars, Planetary Nebulae, Young Stellar Objects, etc., along with known and recently discovered WN/WC stars, is here. Our selection criteria are in blue, which is apart from the main body of plane stars, and follows the slightly steeper than reddening WC track.

Applying this selection in 5 armin bins produces the following maps:

Two areas were selected for the July, 2004, covering approximately 2 square degrees. They are illustrated in the following binned map of the galactic plane from l=20-120 in magenta, with known WRs in red (WC) and blue (WN). Lines of contant declination are also indicated.

Region Map


Runs

So far we've had one run with the new filter set (manufactured April, 2004).

July, 2004

This run was the first for the new WR line filters. They were installed late June by C Kulesa and D. McCarthy, but the standard H2 line filter which was intended to serve as our red-side continuum could not be located, so a broader K continuum filter was used instead. The weather was surprisingly clear for Kitt Peak in July, and only the last two nights saw any cloud cover. Night 6 was called at 2 am for increasing cloud cover. See the observing NOTES for more info.

The log sheets:

The observing method was switched starting night 2 to a method which makes 45 frames per filter per tile. See the NOTES for more. The standard 3x3, 384" stepped tiles look like:

Tile Map


Reduction Steps

Still being expanded... see also Rose Finn's Pisces Reduction Guide.

A raw Pisces frame:

Raw frame

Pre-photometry

  1. Use corquad to remove quadrant-mirrored artifacts (dark comets). From the corquad README:
           This routine corrects PISCES images for the negative "shadows" on
           all four quadrants due to bright sources. It actually assumes that
           any flux in a pixel has a negative effect on all 4 corresponding
           pixels in the 4 quadrants at about a 0.002 level, plus a "shadow"
           at a much lower level.
           
    Jackie Monkiewicz has updated corquad with new coupling parameters, and fitting functions for the most recent Pisces data. Her version is available here, and inludes a README. It can be used standalone, or through IRAF. An example before and after image. Note also that
  2. Generate updated bad pixel masks using flat field data (look for local outliers), including the usable circular field, or use the ones provided by Rose. The masks will be used for subsequent image combinations, etc. Essentially, these pixels will be ignored in all subsequent steps.
  3. Make flats in each filter for each night using the 7 pairs of dome flats made with lamp on and off, i.e. lamp_on-lamp_off, using a trimmed mean, e.g. with IRAF imcombine, normalized to the images mean.
  4. Flat-field data with appropriate night+filter flat field image.
  5. Create sky frames by combining, with an aggressively trimmed mean, all 45 frames in a single 3x3 tile. Monitor for sky variations.
  6. Sky subtract data. In crowded regions, we must be very careful of the sky computed in situ, to ensure it doesn't retain signal from stars. If necessary we'll need to adjust the trimming threshold of the trimmed sky mean.
  7. Reject individual frames which suffered from pointing jitter (e.g. this frame) or bad relative seeing/focus. Poor global seeing will have to be dealt with.
  8. Combine all 5 frames for each filter, which have been dithered randomly by 10" typically, into a single coadded image. This can be accomplished with, e.g., IRAF daofind together with geomap and geotran. Another, perhaps easier option would be gross cross-correllation of the 5 images to find the maximum, most easily accomplished in IDL. Shift and add should achieve acceptable image quality for further photometry.

Photometry and Astrometry

  1. Use DAOPHOT, either through IRAF, or as a stand-alone application, to locate and compute photometry for the stars in our crowded fields. This is a complicated package: much of our reduction efforts will be directed to tuning its operation. Correcting for or at least identifying saturated or non-linearly exposed stars will be critical in this step, keeping in mind that stars may saturate in one filter and not another.
  2. Select robust photometry from the DAOPHOT lists. Depending on the frequency of spurious or compromised detections (e.g. incorrect dark comment subtraction, saturation bloom from a nearby saturated star, etc.), we may need to be clever here: e.g. reject all stars located in a certain region with respect to saturated stars. This selection will be a function of filter (e.g. different bloom patterns, like this WRCont2 pattern).
  3. Match the positions computed by DAOPHOT to an astrometric reference using 2MASS PSC stars of good quality, and suitably clipped in K magnitude. The goal here is to match positions from one step in the tile to the next, and from one filter to the next filter. This may be complicated by some distortions in the Pisces field (2-3 pixels).
  4. Compute continuum subtracted line indices and select candidates. Since the sky transmission is quasi-variable, possibly even on the timescale of the 3x3 tiles, we may need to make use of fact that most stars in the frame will have feature-free spectra, such that deviations from locally averaged line-continuum values are a robust indicator of broad emission line sources.

Last modified: Thu Aug 9 16:43:22 MST 2007