The MSSSO Wide Field CCD H Imaging Survey

Data Reduction

The images of stars are distorted at the edge of our fields due to the camera optics and filters. As the fields will overlap by , the worst sections will be removed when combining images of adjacent fields.

As our field of view is so large (), the sky gradient becomes significant, particularly in sky flat fields. We have based our techniques of obtaining sky flats on Chromey & Hasselbacher's method (1996). The telescope is moved slightly over from zenith in the opposite direction to the sun. Dome flats have also been obtained with good results and we prefer to use these flats in the reduction of our images. A screen has been set up within the telescope dome and is illuminated by light scattered from two flood lights shining on the opposite side of the dome. The screen is positioned so the telescope is facing almost perpendicular to it.

Reduction of the survey data and median filtering of images will be done within the software package IRAF. IRAF scripts have been produced by Watson and Buxton for the survey. It is planned to make this procedure as automated as possible.

Figure 2 shows an example of a reduced and combined image in H.

  
Figure: Field 210 taken with H(6563/55 Å), 1800s exposure, field size is centred on RA 08 30 Dec -50 00

Images of different filters shall be combined to produce colour pictures using Adobe Photoshop.

The emission line standards of Dopita & Hua (1997) will be used for standardization to absolute fluxes. Preliminary analysis shows a limiting magnitude in H of 3R, however, we are confident of obtaining lower values. The B, V and I images will be calibrated using Landolt standards (Landolt 1992). There are up to 30 Landolt standards in the equatorial selected areas in our field of view. The photometric precision from one exposure with its undersampled images (FWHM 1 pixel) using aperture photometry and a radius of 2 pixels is 0.04 mag for each star.

© Copyright Astronomical Society of Australia 1997