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B Overlapping Orders

  We all remember from our college optics classes that at a given angle from a diffraction grating one will see wavelengths corresponding to the extra path distance:

where d is the spacing between groves on the grating and m is the order. Thus at a given angle from the grating you might see light corresponding to 10000Å (m=1), 5000Å (m=2), 3333Å (m=3), and so on. You can count on the atmosphere cutting off any light bluewards of 3200Å, and in the CCD having no sensitivity beyond 12000Å. But, if you are planning to observe from 4000 Å to 5000Å in second order, you need to worry about overlapping first order (8000-10000 Å) and possibly even overlapping third order (2700Å to 3333Å, so in practice just 3200-3333 Å, say); the latter could be a problem if your objects or standards have appreciable blue flux.

Looking at the order-blocking filters in Fig. 4 we find that either a 4-96, BG-38 or BG-39 would do a good job both at blocking longer wavelength light (overlapping first order) and even squashing the possible overlapping first-order UV light in this example.

For convenience we show the effect of overlapping orders in Fig. 14.

 


Figure 14:   The effects of overlapping orders. The atmosphere cuts off orders on the blue side at approximately 3200Å, while the CCD is effectively dead at 12000A.


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Previous: A.3 Aligning the Spectrum on the Chip
Updated: 02Sep1996