LETGS 0th Order Blocking

The HRC MCP performance degrades at integrated doses exceeding 500 events per pore. Using dither and occasionally adjusting the position of the detector behind the aim-point of the HRMA help to spread the dose over many pores. Because the available area on the HRC-S for the placement of the zeroth order is limited, the total dose that will occur for the planned observations is a greater concern. Some observations, particularly those whose 0th order rates are high, may be performed using one of the HRC shutter blades to block the 0th order.

The figure below shows how one of the HRC-S shutters can be rotated to block the entire 0th order beam. The locations of each of the MCPs (one for the HRC-I and the three segments of the HRC-S) are shown. The three circles represent the size of the converging cone of x-rays from the HRMA at the plane of the shutter blade (a radius of ~15.77 mm at a distance from the focal plane of 257.86 mm). One circle is drawn for the 0th order, the other two are shifted 53 mm off the optic axis.

Full view of HRC focal
plane with 0th order blocked

Below is a zoomed-in view on the zeroth order. Essentially all of the central MCP segment is at least partially vignetted, but the outer segments are for the most part un-vignetted.

Zoomed-in view of HRC focal
plane with 0th order blocked

In the best-case alignment, a diffracted x-ray cone that passes the shutters farther out than 53 mm will not be vignetted. This corresponds to a wavelength of about 62.7 Å (198 eV). Raytrace simulations will be required to estimate the amount of vignetting for wavelengths shorter than this.

Page created: 1998 October 5

Dr. Michael Juda
Harvard-Smithsonian Center for Astrophysics
60 Garden Street, Mail Stop 70
Cambridge, MA 02138, USA
Ph.: (617) 495-7062
Fax: (617) 495-7356
E-mail: mjuda@cfa.harvard.edu