Astronomical Spectroscopy - Physics - University of Cincinnati

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$1 Ch. 22 â Reflection and Refraction of Light 22.1 The ... - Physics$

– 8 – 2.1.1. Selecting a Blocking Filter There is often confusion over the use of order separation filters. Figure 2 shows the potential problem. Imagine trying to observe from 6000Å to 8000Å in first order. At this particular angle, one will encounter overlapping light from 3000Å to 4000Å in second order, and, in principle, 2000Å to 2666Å in third order, etc. Since the atmosphere transmits very little light below 3000Å, there is no need to worry about third or higher orders. However, light from 3000-4000Å does have to be filtered out. There are usually a wide variety of blue cut-off filters to choose among; these cut off the light in the blue but pass all the light longer than a particular wavelength. In this example, any cut-off filter that passed 6000Å and higher would be fine. The transmission curves of some typical order blocking filters are shown in Figure 3. The reader will see that there are a number of good choices, and that either a GG455, GG475, GG495, OG530, or an OG570 filter could be used. The GG420 might work, but it looks as if it is still passing some light at 4000Å, so why take the chance What if instead one wanted to observe from 4000Å to 5000Å in second order Then there is an issue about first order red light contaminating the spectrum, from 8000Å on. Third order light might not be a problem—at most there would be a little 3333Å light at 5000Å, but one could trust the source to be faint there and for the atmosphere to take its toll. So, a good choice for a blocking filter would seem be a CuSO 4 filter. However, one should be relatively cautious though in counting on the atmosphere to exclude light. Even though many astronomers would argue that the atmosphere doesn’t transmit “much” in the near- UV, it is worth noting that actual extinction at 3200Å is typically only about 1 magnitude per airmass, and is 0.7 mag/airmass at 3400Å. So, in this example if one were using a very blue spectrophotometric standard to flux calibrate the data, one could only count on its second-order flux at 3333Å being attenuated by a factor of 2 (from the atmosphere) and another factor of 1.5 (from the higher dispersion of third order). One might be better off using a BG-39 filter (Figure 3). One can certainly find situations for which no available filter will do the job. If instead one had wanted to observe from 4000Å to 6000Å in second order, one would have such a problem: not only does the astronomer now have to worry about >8000Å light from first order, but also about
– 9 – Fig. 2.— The overlap of various orders is shown.
Page 1 and 2: Astronomical Spectroscopy Philip Ma
Page 3 and 4: - 3 - Similarly the emphasis here w
Page 5 and 6: - 5 - could see 8000Å light from f
Page 7: - 7 - ratio of the focal lengths of
Page 11 and 12: - 11 - 2.1.2. Choosing a grating Wh
Page 13 and 14: - 13 - 4-Meter Telescope - RC Spect
Page 15 and 16: - 15 - gratings blazed at the red.
Page 17 and 18: - 17 - each order (Equation 9). Thi
Page 19 and 20: - 19 - wants can be a real challeng
Page 21 and 22: - 21 - Fig. 7.— Multi-object mask
Page 23 and 24: - 23 - Fig. 8.— Optical layout of
Page 25 and 26: - 25 - Fig. 9.— View of the focal
Page 27 and 28: - 27 - Fig. 10.— Optical design o
Page 29 and 30: - 29 - When built twenty years ago,
Page 31 and 32: - 31 - corrected” image cube is c
Page 33 and 34: - 33 - • The data frames themselv
Page 35 and 36: - 35 - Fig. 11.— Three examples o
Page 37 and 38: - 37 - bumps and wiggles are presen
Page 39 and 40: - 39 - Fig. 13.— The spatial prof
Page 41 and 42: - 41 - one might want to do this on
Page 43 and 44: - 43 - The steps involved in this r
Page 45 and 46: - 45 - be obtained for each grating
Page 47 and 48: - 47 - the flat-field division. Alt
Page 49 and 50: - 49 - night. More about this is di
Page 51 and 52: - 51 - can be answered given a comp
Page 53 and 54: - 53 - that it degraded the signal-
Page 55 and 56: - 55 - 3.2.6. Long-Slit Flat-Fieldi
Page 57 and 58: - 57 - over the “perfect” flat-
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- 59 - 3.2.7. Radial velocities and
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- 61 - A good general line list (id
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- 63 - the only overlap would be wi
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- 65 - a far better job at matching
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- 67 - dispersion. Going to the par
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- 69 - red (including at least 7770
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- 71 - Exposures of the dome flat q
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- 73 - A bright star was seen just
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- 75 - 500 stars. Figure 20 shows t
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- 77 - So, attempt to select a tell
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- 79 - Fig. 21.— Output from the
Page 81 and 82:
- 81 - Wrap-Up and Acknowledgements
Page 83 and 84:
- 83 - Massey, P., DeVeny, J., Jann
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Astronomical Spectroscopy - Physics - University of Cincinnati

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