[158] | 1 | \secA{Notes and hints on the use of \duchamp} |
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| 2 | \label{sec-notes} |
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| 3 | |
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| 4 | In using \duchamp, the user has to make a number of decisions about |
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| 5 | the way the program runs. This section is designed to give the user |
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| 6 | some idea about what to choose. |
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| 7 | |
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| 8 | The main choice is whether or not to use the wavelet |
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| 9 | reconstruction. The main benefits of this are the marked reduction in |
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| 10 | the noise level, leading to regularly-shaped detections, and good |
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| 11 | reliability for faint sources. The main drawback with its use is the |
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| 12 | long execution time: to reconstruct a $170\times160\times1024$ |
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| 13 | (\hipass) cube often requires three iterations and takes about 20-25 |
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[162] | 14 | minutes to run completely. Note that this is for the more complete |
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| 15 | three-dimensional reconstruction: using \texttt{reconDim=1} makes the |
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| 16 | reconstruction quicker (the full program then takes about 6 minutes), |
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| 17 | but it is still the largest part of the time. |
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[158] | 18 | |
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| 19 | The searching part of the procedure is much quicker: searching an |
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| 20 | un-reconstructed cube leads to execution times of only a couple of |
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| 21 | minutes. Alternatively, using the ability to read in previously-saved |
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| 22 | reconstructed arrays makes running the reconstruction more than once a |
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| 23 | more feasible prospect. |
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| 24 | |
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| 25 | On the positive side, the shape of the detections in a cube that has |
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| 26 | been reconstructed will be much more regular and smooth -- the ragged |
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| 27 | edges that objects in the raw cube possess are smoothed by the removal |
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| 28 | of most of the noise. This enables better determination of the shapes |
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| 29 | and characteristics of objects. |
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| 30 | |
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| 31 | A further point to consider when using the reconstruction is that if |
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| 32 | the two-dimensional reconstruction is chosen (\texttt{reconDim=2}), it |
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| 33 | can be susceptible to edge effects. If the valid area in the cube (\ie |
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| 34 | the part that is not BLANK) has non-rectangular edges, the convolution |
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| 35 | can produce artefacts in the reconstruction that mimic the edges and |
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| 36 | can lead (depending on the selection threshold) to some spurious |
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| 37 | sources. Caution is advised with such data -- the user is advised to |
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| 38 | check carefully the reconstructed cube for the presence of such |
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| 39 | artefacts. Note, however, that the 1- and 3-dimensional |
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| 40 | reconstructions are \emph{not} susceptible in the same way, since the |
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| 41 | spectral direction does not generally exhibit these BLANK edges, and |
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| 42 | so we recommend the use of either of these. |
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| 43 | |
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| 44 | If one chooses the reconstruction method, a further decision is |
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| 45 | required on the signal-to-noise cutoff used in determining acceptable |
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| 46 | wavelet coefficients. A larger value will remove more noise from the |
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| 47 | cube, at the expense of losing fainter sources, while a smaller value |
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| 48 | will include more noise, which may produce spurious detections, but |
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| 49 | will be more sensitive to faint sources. Values of less than about |
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| 50 | $3\sigma$ tend to not reduce the noise a great deal and can lead to |
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[160] | 51 | many spurious sources (this depends, of course on the cube itself). |
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[158] | 52 | |
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| 53 | When it comes to searching, the FDR method produces more reliable |
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| 54 | results than simple sigma-clipping, particularly in the absence of |
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| 55 | reconstruction. However, it does not work in exactly the way one |
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| 56 | would expect for a given value of \texttt{alpha}. For instance, |
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| 57 | setting fairly liberal values of \texttt{alpha} (say, 0.1) will often |
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| 58 | lead to a much smaller fraction of false detections (\ie much less |
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| 59 | than 10\%). This is the effect of the merging algorithms, that combine |
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| 60 | the sources after the detection stage, and reject detections not |
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| 61 | meeting the minimum pixel or channel requirements. It is thus better |
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| 62 | to aim for larger \texttt{alpha} values than those derived from a |
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| 63 | straight conversion of the desired false detection rate. |
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| 64 | |
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| 65 | Finally, as \duchamp\ is still undergoing development, there are some |
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| 66 | elements that are not fully developed. In particular, it is not as |
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| 67 | clever as I would like at avoiding interference. The ability to place |
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| 68 | requirements on the minimum number of channels and pixels partially |
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| 69 | circumvents this problem, but work is being done to make \duchamp\ |
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| 70 | smarter at rejecting signals that are clearly (to a human eye at |
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| 71 | least) interference. See the following section for further |
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| 72 | improvements that are planned. |
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