GT_DPE extracts what DP exposure (DPE) level was used. The DPE is a function
of the shutter exposure level (MBE) and the neutral density filter use.
It is possible to return the effective exposure duration in milliseconds by using the /conv
IDL> dpe = gt_dpe(index)
IDL> msec = gt_dpe(roadmap, /conv)
The effective exposure for each of the DPE values are shown in the following table. For DPE values above 22, the effective exposure is
exposure = (DPE-21) * 0.25 seconds