Electrical FOV/FOV CrossTalk



Note: the following information is intended for the IRAC instrument team only. If you are not such a person, you should not be reading this. No guarantees are made as to the applicability of this information to the real IRAC.

Since there has been a lot of confusion over the crosstalk issue, I thought I would put together this web page to explain it better.

First, we have been using "crosstalk" to mean at least three different things:
This is about the last one.

Crosstalk was first noticed during April TTACS testing. During these tests we had a GSE source placed on the Channels 1 & 3 FOVs. We saw it in the same place on Channels 2 & 4. Since the two fields of view have separate light paths, this is probably the result of crosstalk in the electrical system. It's not a latent because we never had the source on these two arrays. There was some additional evidence pointing to electrical crosstalk, and not optical crosstalk in the GSE or something, in that when IRAC was screwing up and not taking the data concurrently, the ghost seemed to disappear. We couldn't do any sophistaicated tests at the time because we had no real time feedback and no control of the GSE. At it's worst, the crosstalk is about 0.5% (comparing channels 1 & 2, the weakest "real" source vs. the strongest ghost). Comparing this ghost to the strongest real source it is about 0.07% of the peak value.

April 08 FOV Crosstalk (central 128x128 of array)
Channel 1 (A.2000.099.15.19.26.01.08.0000) Channel 2 (A.2000.099.15.19.28.02.08.0000)
Channel 3 (A.2000.099.15.19.30.03.08.0000) Channel 4 (A.2000.099.15.19.32.04.08.0000)


What was slightly confusing was that later Matt ran a very good test to try and characterize this, and as he summarized in his other report, he found nothing. It is undeniable that there was crosstalk, hence something changed. The only real difference is that he had a different set of cables when he ran his (the "usual" cables we use were being baked out for thermal vac), hence the going explanation is that the crosstalk is a function of what cables we are using.