Inaccurate anchor position

Hi Paul,

I am using uvotpy-2.4.1 to extract uvot uv grism spectra. For some observations, if one uses the sky coordinate (ICRS) of the target as the input parameter of getSpec(), the wavelength of the extracted spectrum is incorrect.

The green circle in Fig. 1 shows the loacation of GSPC P 177-D on the detector image with the WCS-S keyword, and if one uses this coordinate to extract the spectrum, the absorption lines do not match with the reference spectrum, shown in Fig. 2. However, if one uses the coordinate of the red circle as the input parameter, the extracted spectrum matches the reference spectrum well, shown in Fig. 3. For spectra in Fig. 2 and Fig. 3, update_pnt=True and uvotgraspcorr_on=False. g-raw_r-offset Fig. 1 tf_icrs_raw_g Fig. 2. The extracted spectrum with the coordinate of the green circle as the input parameter. update_pnt=True, uvotgraspcorr_on=False tf_icrs_offset_g Fig. 3. The extracted spectrum with the coordinate of the red circle as the input parameter. update_pnt=True, uvotgraspcorr_on=False

I tried to set uvotgraspcorr_on=True to check if there could be some improvement, but the result becomes worse :(. tt_icrs_raw_g Fig. 4. The extracted spectrum with the coordinate of the green circle as the input parameter. update_pnt=True, uvotgraspcorr_on=True tt_icrs_offset_g Fig. 5. The extracted spectrum with the coordinate of the red circle as the input parameter. update_pnt=True, uvotgraspcorr_on=True

obsid = 00056760016, ext = 1 So, what can I do about this, since the inaccurate anchor position is common? For souces without reference spectrum, you cant know how large should the offset to the sky coordinate of the target be to get the accurate anchor position (wavelength calibration).

PaulKuin / uvotpy

Inaccurate anchor position #16