Dekkers

For ISIS, individual Dekker masks are mounted in a frame (Fig. 11) which in turn is inserted by hand into the motorised slide. For polarimetric observations, specialised Dekkers are required. A simple mask to your own special design can usually be made up on site (give advance notice of weeks, preferably months). Enquire with your support astronomer what the current situation on polarization Dekkers is. To be able to see a source on the Dekker, the Dekker must be polished and/or aluminised, which cannot be done on site. Dekkers run up on site will appear dark on the acquisition TV and you will have to slide the Dekker masks out when acquiring your object.

 

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Figure 11: Dekker frame with Dekkers used during commissioning. Actual suite of Dekkers will be different from this. See text.

We hope to provide soon the following Dekkers for general use:

• point source + 2 sky apertures.
• long-slit: combs of duty fraction 1 : 3.5 (4 exposures) and 1 : 2.5 (3 exposures). Scattered light may be a problem with 1 : 2.5.

Note that one position in the frame is usually left empty for finding faint objects on TV. It takes far less time to move to the last Dekker position than to move the entire Dekker slide out of the beam; however, to see the entire field, the latter course may be necessary.