Dust Filter Search

AUTHORS : A.R. Martel, G. Hartig, J. McCann.

Purpose :

The main goal is to identify the location and number of dust grains on the filters by comparing offset flat fields. The results will serve as a baseline to evaluate the deterioration of the filter cleanliness after integration of the flight build detectors and over the upcoming calibration campaings, when this dust search. test will be repeated. The impact of the dust grains on the flat-fielding will then be assessed and it will be decided if any filters require further cleaning before launch. The optimal filter wheel position for each filter can also be refined from these data by eliminating any vignetting.

DATA :

The entire data set is listed in the following tables. All the images were acquired with SMS procedures JGCW27A-E (internal tungsten lamp T2 and WFC Build#3, Amps BC) and JGCH27F (lamp T4, deuterium lamp D₂, and HRC Build#1, Amp C) and include all the filters. For each SMS sequence of broad-, medium-, and narrow- band filters (JGCW27A-B), a bias frame was first acquired, followed by pairs of flat fields through each filter. The first flat-field of a given pair is obtained at the nominal filter position and the second with a filter offset of three steps. The F892N and polarizing filters were acquired in the same fashion (JGCW27E), but at the wheel position appropriate for these small filters. Their flats were read out as 1/4-field subarrays on Chip 2. For the ramp filters (JGCW27C-E), six consecutive images were acquired at three wheel positions with 3-step offsets to completely map the segments. Flat fields through the 'blue' filters and the prism (F220W, F250W, F330W, F344N, and PR200L) were acquired with HRC in SMS JGCH27F. The D₂ lamp illuminated the F220W and F250W filters.

METHOD :

The method simply consists of subtracting the bias frame from each flat field, removing the average residual counts in the physical overscans as described in the Bias Subtraction and Overscan Analysis for WFC and HRC, and then dividing the first image of each pair, taken at the nominal filter position, by its offset counterpart. Only the signature of features on the filter, such as dust motes, will remain in the ratioed image; all other features will flatten out.

RESULTS :

In Tables 1 and 2, we list the number and location of the identified dust features for the WFC Chips 1 and 2, and in Table 3 for the HRC chip. Dust motes have a very distinctive, elongated "half-moon" pattern because of the internal illumination of the lamps (see the F660N (Chip 1) image for a good example). Typically, one dust grain covers a 900x600 pixel region on the chip. Other notable features are also present in a few filters. For example, the F435W Chip 1 shows a partial streak or chain of rings crossing the chip, from the upper, right corner to the middle of the left edge. This could be due to a scratch on the filter or a residual of the liquid cleaning process. Also, bright streaks cross the central segment of some ramp filters (for example, see FR388N and FR505N). These features are most obvious in the offset positions (2 and 3), where the filter edges are visible, and are even seen in the observed flat fields (see WFC and HRC Internal Lamp Count Rates). The streaks are attributed to scatter from the filter mounting. They are not seen with external OTA-like illumination such as RAS/HOMS.

No obvious dust features are detected on the filters imaged with HRC (Table 3). Both PR200L images are completely saturated and are therefore of no use. Also, the central area of the POL filters are near saturation (see Internal Lamp Count Rates for details), resulting in "flat" ratioed images which make it difficult to detect any dust feature.

CEI SPECIFICATIONS :

None.

CONCLUSION :

The location of the dust grains on the filters was determined.