Princeton, New Jersey
January 12, 1976
|FROM:||D. G. York
Princeton University Observatory
|SUBJECT:||Possibility of spurious emission line detection|
Some of you have been trying to detect weak emission lines using U1 or U2. This is to make you aware of the fact that it is possible to produce spurious emission lines, at the level of 5 cts/14 sec at arbitrary wavelengths. For a given program, the features are likely to occur at identical wavelengths for repeated scans, so that in stacked data, the features will appear to be statistically significant.
The cause of the spurious features is that U1, when slewed, will produce pickup in the U2 count registers (or associated electronics), and vice versa. We generally do not try to obtain U2 data or U1 data while either carriage is slewing. However, tube U1, when retracing, slews 2 steps further than the start position of a scan, and takes two antibacklash steps. The main retrace slew (-17 steps), occurs during the 2 second period used to store data, and hence any pickup produced never occurs in the count registers. However, the two antibacklash steps (2 sequential +1 steps) occur during a valid integration period, and thus will be stored in memory. In the process of reducing data, this quarter minute of data is discarded for U1 and never appears in any plots or on disc records of the U1 data. However, if U2 is moving, this frame is regarded as valid data for that tube and it is plotted and stored on disc. Thus, if U1 and U2 standard routines are started simultaneously, the first point of the U2 scan will include some pickup from U1. If a U2 standard routine is started, and a U1 standard routine is started 1 minute later, the 5th U2 point will include pickup, etc. Each subsequent U1 retrace will lead to a corresponding blip on U2. The effect would go unnoticed except in the case where there is very low continuum signal, several scans have been stacked together, and U1 is always retracing when U2 reaches a particular wavelength, as for instance occurs when one line is being looked for on U1, another on U2, and U2 is reversed after a fixed number of U1 standard routines. In this case the blip will be reinforced on all negative U2 scans at certain wavelengths, and at a different set of wavelengths for positive U2 scans. The effect can be easily recognized by stacking all positive scans only, and then all negative scans separately.
I thank Bill McClintock (Johns Hopkins) for calling this problem to my attention. I urge you to check that this effect has not compromised any published data, and to take this subtle pitfall into account in any further data reduction with which you are involved. In my experience, in testing and in orbital operations, the amplitude of the pickup caused by carriage slews is variable, and therefore may be weaker or stronger than the 5 cts/14 sec quoted here.