ANDICAM Neutral Density Filters

The ANDICAM instrument has two optical neutral density filters, V+ND4 and I+ND4, and two infrared neutral density filters, H+ND4 and 10830+ND3. (The 10830+ND3 filter is a narrowband He filter.) The optical ND filters are 5-arcminute diameter round filters, and the IR ND filters are 2.4-arcmin square filters. According to the CTIO SMARTS web page, the V+ND filter has an attenuation of 4.6 (a factor of 39,000, or 11.5 magnitudes) over that of the V filter, and the I+ND combination has an attenuation of 3.0 (1100, or 7.6 magnitudes).

From observations of Eta Car in the ND4 filters done by Alan Whiting, the FWHM generally ranges from 2.5 to 4.9 pixels in V. He also notes that flat fields taken through the attenuated filters have not been successful in calibrating the data. However, the flats from the unattenuated V, I, and H filters seem to work fine with ND filter observations.