VDS_DUMMY.PRO NOTES ------------------- Richard A. Harrison, RAL, 19 March 1993 --------------------------------------- The code VDS_DUMMY produces CDS dummy data for the VDS (normal incidence) system. The full VDS CCD array consists of 1024x1024 pixels. Two normal incidence spectra are focused onto the CCD, offset on either side of the horizontal centre-line. The longest slits are 240 arcseconds in length which, at a plate scale of 12.5 micron/arcsec, translates to 3mm on the detector. Thus, only the central 1024x256 pixel area (approximately) is used, as shown in the figure. <-- 1024 pixels --> +++++++++++++++++++++++++++++++++++++++++++++++++ + + + + 1 + + 0 + + 2 + + 4 +===============================================+ + + p + Area covered by 310-380A spectrum + i + (assumed 1024x128 pixels) + x + + e +-----------------------------------------------+ centre line l + + s + Area covered by 517-633A spectrum + + (assumed 1024x128 pixels) + + + +===============================================+ + + + + + + + + + + + + +++++++++++++++++++++++++++++++++++++++++++++++++ The horizontal direction is the direction of spectral dispersion. The vertical direction is the spatial dimension along the slit. The code, VDS_DUMMY, produces an IDL array of 1024x256 words, which can be written to a data file (default IMAGE.DAT). VDS_DUMMY is run by typing: IDL> vds_dummy [,slit=slit,expose=expose etc...] The keyword inputs are the following: SLIT slit size (2, 4, 8 and 90 arcsec) EXPOSE accumulation time (sec) WLIST line (window) list option (1, 2 or 3) WIN_WIDTH individual line window width (pixels) IMAGE name of the data array returned FILE name of data file created (default IMAGE.DAT) NO_FILE inhibit saving of data on disk Example: IDL> vds_dummy, slit=4,expose=10,wlist=1,file='vds1.dat',image=vds1 If not supplied as key parameters, the slit width, accumulation time and window option are prompted for. If written to disc, the output data file is unformatted. To retrieve the data use the following commands: IDL> a = intarr(1024,256) IDL> openr,lun,filename,/get_lun IDL> readu,lun,a IDL> free_lun,lun In practice, it will be rare that CDS can return full VDS images; we will be selecting data, i.e. windowing. In the output data array/file the data outside the selected windows are set to zero. VDS_DUMMY requires access to the files NI1.DAT and NI2.DAT, which give the solar intensities and wavelengths for the two VDS bands, and PIXELS.DAT which provides a pixel address/wavelength calibration. The code finishes by typing - VDS_Dummy - Normal End. The total duration of the calculation is of order 30-40s and the output file requires 1024 blocks. Please report any errors to RAH on 19592 or UK.AC.RUTHERFORD.ASTRO. The calulation has the following features: - The lines seen in the solar EUV spectrum by Vernazza and Reeves (1978) and Malinovsky and Heroux (1973) are included. Line positions are consistent with their spectra. - Line intensities are calculated from the Vernazza and Reeves, and Malinovsky and Heroux data folding in CDS efficiencies and geometrical features. Intensities are given in counts per pixel. (see CDS Science Report for method). - The influence of the telescope point spread function is included as a Gaussian spread in the spatial dimension (along slit), with standard deviation 1.5 arcsec (i.e. width 3 arcsec). - A choice of slits of width 2, 4, 8 and 90 arcsec is included. (The 90 arcsec selection is included as an option, though it is not catered for well in this current routine - no spatial structure is given in the direction of wavelength dispersion). - The shape of the slit image is given as a Gaussian function with standard deviation equal to half of the slit width. This function spreads pixel intensities in the spectral dimension. - Any accumulation time can be included. Count rates saturate at 4095 per pixel (in keeping with the VDS operation). - Three window options are included. These include (i) a full image, (ii) a set of narrow windows based on the L12 line list of the CDS Science Report, and (iii) two wide windows. The L12 option covers 15 lines with N pixels across each in the spectral dimension - where N is input. Some of these will overlap, thus producing variable data windows on return to the Earth. The two wide windows cover the ranges 345-360 and 550-565A. In the final data file, a 1024x256 array is always returned, with the non-window data set to zero. - Solar structure is added along the slit by varying the intensities in line with four periodic structures. This is constant from image to image. - Background and continuum intensities are assumed constant - 1ct/s is added to each pixel. See the CDS Science Report. - Noise is folded into each pixel intensity by calling a random number between -1 and +1 and adding its product with the square root of the pixel intensity with the original intensity. This mimics rather crudely, but adequately, the level of noise expected. Noise will differ from image to image. - Doppler shifts are included. At 10 locations there are shifts of spatial width 1 pixel - either positive or negative. At 20 locations, 3 pixel groups are shifted, again, either positive or negative. The locations are chosen at random - thus, varying between images. The degree of shift is fixed within the code. - Some minor brightenings are included at spatial locations consistent with the locations of some Doppler shifts. These are seen across the whole wavelength range. - The code neglects pixel sampling, which is assumed to be a small effect relative to those considered. Similarly, scatter from the grating (i.e. scatter in wavelength space) is neglected with the assumption that the slit image spread dominates this effect.