A Test of Possible NICMOS Non-linearity
Observations of standard stars with NICMOS grism (G096), when compared with the same standards taken by STIS (G750L), show a disagreement over the wavelength range 0.8 to 1 micron. Further comparison with ACS F850LP data confirms thisnon-linearity to be due to NICMOS in the sense that fainter stars in NICMOS appear too faint relative to a brighter normalisation. We investigate this over a wide magnitude range by compiling samples of stars (over the range 8 < J < 17) and galaxies (spanning the range 17 < J < 25), with available data with both NICMOS (F110W and F160W) and ground-based JHbands. We find no difference between the NICMOS F160W and ground-based H-band magnitudes. However, in the case of NICMOS F110W and J-band filters, allowing for differences in the filter response functions, we find a trend at the bright-end (J < 17), confirming a non-linearity but smaller than that was seen in Bohlin effect by a factor of 2-3. No significant slope is seen at the faint-end. This non-linearity is interpreted as being due to trapping and the difference in exposure times between the bright and faint objects (Riess 2005). We conclude that most of the observations will not be strongly affected by this effect as the source count rates are sufficiently high so that the fraction of charge lost to traps is negligible. The most affected observations are for faint sources with relatively short exposures.
Count rate nonlinearity in near infrared detectors: inverse persistence
The count rate non-linearity of near-infrared devices was first found in the HST NICMOS. In this report we present a physical model of the cause of this effect, show how it is related to persistence, and compare the predictions of the model to other observations of anomalous detector behavior. This model is able to explain not only the count rate non-linearity but also several other effects. Overall, the excellent agreement between this model and the observations gives us strong confidence that we understand the underlying cause of the count rate non-linearity. This understanding should allow us to develop methods to accurately calibrate and remove the effect from JWST observations.
Long Ceramic Beam Tubes for Accelerator Magnets
The ceramic beam tubes for the fast and abort kickers and the bucker and pinger magnets used at Fermilab must meet a number of exacting requirements. The tubes must be long and sufficiently straight so as not to limit magnet design. They must have wall strength capable of withstanding the atmospheric and mechanical forces encountered during construction and operation. Vacuum tight ceramic to stainless steel transitions must be of reasonable cost; they must also withstand high temperature processing and remain vacuum tight to the 10/sup -9/ Torr range. To remove the possibility of static charge buildup each tube must have a surface coating of indium oxide applied to the inner wall. This thin coating is difficult to achieve and requires the most careful attention to detail. The methods used at Fermilab to achieve these goals are presented in this paper.