UCL MSSL Swift


Introduction

The Swift UVOT grism calibration has been making use of he knowledge of the Zemax optical model which was used to design the grisms for the XMM/OM and the Swift/UVOT instruments. We have established that the model is valid and useful in various ways. The model has been compared the pre-launch ground calibration observations, to observations  at and away from the boresight of the instrument for the first order and occasionally for the second order or zeroth order data. However, since the Zemax model does not include the fibre taper optics between the MCP and the CCD, small corrections to the model were needed which were derived based on observations of the emission line spectra of WR stars.

The comparison between the original design model and the actual instrument  data suggests that the mounting of the grisms was a few degrees rotated away from the design as is evident of the angle of the spectra on the detector. The correction, based on the nominal and clocked angles of the grism spectra on the  detector, is 3.8  ± 0.2 degrees for the UV grism and 2.6 degrees for the V grism nominal mode, while none was needed for the clocked V grism mode.

Those have been the only corrections made to the original zemax grism design model. The original model for the UV grism was optimised for the 260nm wavelength in the first order; that for the V grism for 420nm in first order. These will be used (and refered to) as the anchor point of the spectra.  The spectra of on-axis sources give a boresight spectrum where these wavelengths in first order fall near the centre of the detector.

This is the second wavelength calibration. The first calibration was done during the commissioning of the UVOT instrument and used the zeroth order to anchor the wavelength scale. In this calibration, the wavelength scale is anchored to a point in the first order  and is determined from either the source position in an aspect-corrected lenticular image, or the aspect corrected grism image.

The instrument boresight is known to vary slightly for the different lenticular filters and is not a well-defined location on the detector for a spectrum. The boresight of the grisms in the first calibration was determined as a certain point in the zeroth order, whose location is defined by an algorithm based on the positions zeroth order peak emission for weak sources in the image. The values for the UVOT grism boresight in the CALDB TELDEF files are for the zero order.

Several drawbacks from the first wavelength calibration are now resolved.  The first is, that all first order spectra on the grism image can now be calibrated. More than 20% of the first order spectra on the detector do not have a corresponding zero order which was required for the first calibration's method. The second imaprovement is, that the wavelength scale accuracy has improved: in particular the offset of the whole wavelength scale has been reduced by using lenticular filter observations directly in sequence with the grism observation.  This solves a problem in the first calibration because the zeroth order in the UVOT is not a point, but extended. The zeroth order suffers for the brighter sources from a combination of modulo-8 noise and saturation. The dispersion of the zeroth order is very non-linear with most of the red part of the spectrum on a few pixels, and the blue part in a weak extended tail. Although using only weak sources in the image will preferenctially select the red peaks, some colour dependence may exist in the aspect correction of the grism image.  If there are no lenticular filter images taken with the grism observation in one mode, then the new wavelength calibration will still rely on the aspect correction derived from the grism image. The third improvement is a benefit of using the optical model: the zemax model predicts the dispersion and position of all orders over the detector, modulo some scale factor probably due to the lack of the fibre taper in the model. The multi-order data is essential for understanding these spectra, since the second and third order fall generally on top of the longer wavelengths in the first order.  Moreover, the angle of the spectral orders on the detector, and corrections for abbarrations in the optics are provided by the zemax model. Finally, the point spread function of incoming radiation as function of wavelength, order and position is predicted by the model, and the predicted PSFs are useful for these are not nice gaussians, but rather elongated donuts for the higher orders.

The wavelength calibration was done in two main steps by using a set of observations taken at different positions over the detector and using these to find scale factors to the zemax model predictions. 

In the first step the position of the reference wavelength in the spectrum's first order was determined and matched with the corresponding position predicted by the model.  The correction consists of a shift to line up the boresight in the model and that observed; and a scale factor which scales the distortion of the anchor point of the spectrum from the model to the observations.  The distortion correction is required for two reasons. The first is, that the calibration uses the 'detector image' which has been made by applying the distortion correction used for all filters to the raw grism image. This distortion was measured during the ground calibration using the lenticular filters, and is thought to be mainly due to the fibre taper with a small contribution from the lenticular filters themselves.  As a result, an additional distortion correction is needed to correct for the difference between the grism and lenticular filter distortions.

The second step is to scale the dispersion from the zemax model to the observed dispersion at various points over the detector. The positions of emission lines in the WR spectra were measured and used to find the scale factor. 

Relevant Documents

  1. description UVOT instrument: Roming et al. 2005, see also the XMM-OM description: Mason et al. 2001.

  2. SWIFT-UVOT-CALDB: Swift UVOT Grism Clocking, Alice Breeveld, 19th October 2005, Revision #01, Swift UVOT Calibration Documents Version 06-Apr-2006

  3. SWIFT-UVOT-CALDB: Teldef Files, Alice Breeveld, 19th October 2005, Revision #01, Swift UVOT Calibration Documents Version 06-Apr-2006

  4. Swift UVOT calibration report. Zemax optical models for the UV-Grism study: distortion.