Filters and bandpasses

In this chapter I will analyse observational data taken through two different filters, the I-band filter from the HiRac instrument at the NOT, and a $780$ $nm$ long pass filter (called HiRac I and $780$ $nm$ edge respectively). The bandpass of the HiRac I filter is approximately top-hat shaped with a centre wavelength of $810$ $nm$ and a width of $125$ $nm$. The long wavelength limit was set by the sensitivity curve of the front-illuminated CCD for observations using the $780$ edge filter. Figure 5.5 shows simple models for the responses of the filters (based on the data available from the manufacturers). Also shown are estimates of the combined photon-detection efficiency of the telescope, filter and instrument based on estimates of the quantum efficiency of the front-illuminated CCD, the reflection coefficients for the dewar window and lens, and the reflectivity of the telescope and instrument mirrors.

Figure 5.5: Bandpasses for two filters used at the NOT. All the observational results presented in this chapter were taken through one of these two filters. Curve A shows the approximate response for the HiRac I filter, and curve C shows the estimated bandpass for the $780$ $nm$ edge filter. These curves are based on data provided by the manufacturers. Also shown are the combined responses of these filters with the estimated quantum efficiency of the front-illuminated CCD, the estimated mirror reflectivities and estimates for reflection coefficients of the dewar window and lens. These are labelled B and D for the HiRac I and $780$ $nm$ edge filters respectively.