Actionable statements are italicized below.

We will be scheduling a second meeting later in the year, probably in Pasadena, to discuss further refinements to the routine calibration.

Agenda

9:00 AM

Opening Remarks - description of goals for meeting. See above. (Surace)

9:10 AM

Channel 1 Pixel Phasing (Glaccum)

• Joe presented his measurements, which can be seen in the first few slides of this presentation.
• Bill G. presented an explanation of what causes this effect, which is 4% p-p in Ch.1.
• John Stauffer presented evidence from FEPS confirming the observed relation in Ch.1, but also showing evidence for a similar (perhaps 2%) effect in Ch.2. Examination of Joe's plots shows a similar result.
• John also presented a FEPS result that seemed to show consistent brightening as a function of time in Ch.4. Could this be a latent issue?
• We debated whether or not more test data was needed. In all likelihood the best datasets are already in hand, particularly from FEPS. It was suggested that the Dark Field is probably an ideal dataset for probing this, since it has hundreds of the same stars viewed over and over. It would require someone, like Jason, to write the software to sort through it all.
• It might be possible to refine the accuracy of the correction.
• The SSC Apex software could be modified to take this effect into account automatically.
• The effect should be fully documented (actually, it is in the data handbook now).

9:30 AM

IRAC Color Terms (Hora)

• Joe presented his results, showing the nature of all the corrections and the potential photometric accuracy if all factors are taken into account.
• Joe finds it is difficult to explain the observed results using the color variation and spatial distortion.
• Joe suggests we supply maps of the transmission curves on a pixelwise basis. He can supply these.
• Jason showed results from SWIRE. Mean correction is 1% or so for a half-array offset mapping strategy, but specific areas can still have large corrections of 5% or so.
• We discussed what would be good ways to address this for the community. APEX could be modified to use the gain correction images. MOPEX could produce as a user-product a map of the gain correction factors corresponding to the mosaic image product. MOPEX could also apply a BCD-wise correction to each image prior to mosaicing, in the manner that SWIRE does.
• It was suggested Rick might be able to come up with an idealized dither pattern that minimizes these corrections.

10:30 AM

Break

10:45 AM

Instrument Cross-Calibration (Surace)

• Jason presented Lin Yan's slides on the instrument cross-calibration effort at the SSC. Calibration between all instruments appears good to within the 10% requirement drafted by the IPT. Differences appear attributable to the underlying models for the calibrators.
• It was suggested we should consider observing Sirius and Vega to get measurements of fundamental calibrators.
• It was agreed we should continue to pursue an understanding of why these measurable differences exist.

11:00 AM

Point-Spread Function (Marengo)

• Massimo presented results on his construction of a very high dynamic range PSF from saturated stars. He showed results from using this for PSF subtractions in the context of finding faint objects around bright stars. He also showed that it was possible to reach milli-mag photometric accuracy.
• Bill Hoffman presented results which showed a sizeable variation in noise pixels for the PSF from the center to the edges of the array.
• Joe showed a small movie showing the expansion and contraction of the PSF over the array.
• We debated whether this had a sizeable effect on small-aperture aperture corrections. We considered whether this may be part of the effect that Joe is mapping in the position-dependent gain correction. We also worried that this may be the source of some of the non-reducible scatter some groups (like SWIRE) are seeing. Tom Megeath refers us back to an early IOC experiment which would seem to indicate that the variation in aperture correction for a 2-pixel aperture is only about 1-2% for channels 1,2,4 and perhaps 4% for channel 3.
• It was felt that this should be better documented with a recommendation to observers that certain parts of the array are effectively more sensitive due to the sharper PSF.

Noon

Lunch

1:30 PM

Scattering/Charge Diffusion (Glaccum)

• Glaccum presented a discussion of the mechanism of charge diffusion.
• We concluded this has little or no effect on photometry.

2:30 PM

Muxbleed/Banding (Carey)

• Sean presented a discussion of the IDL artifact corrector.
• Sean requested that someone provide an analysis of the power spectrum of cosmic ray hits.

3:00 PM

Linearization (Surace)

• Jason presented results from the stellar linearity test and from SWIRE. The conclusion is that channels 1, 2 and 4 are properly linearized, but channel 3 is not.
• Jason also presented a discussion (in the above ppt file) of the muxbleed-like behaviour of the Si:As arrays. This appears 4 pixels past bright objects, only appears above 1/3-1/2 full-well, and is an increasingly large fraction of the original source peak. This has affected all linearity measurements. It also affects aperture photometry. It should be better documented.
• Bill G. presents an explanation of the effect, which is a limited bandwith issue in the mux, not the cables, and how it arose based on decisions about how the array was to be operated prior to flight. He points out that changes in the array voltages and currents could significantly ameliorate this effect. Changes in handling the channel 3 array prior to flight probably account for the confusingly contradictory linearization tests.
• Jason will further examine the SWIRE results to decide if linearization of channel 3 changed after routine annealing began. A new ch.3 linearity curve will then be created.
• We agreed we should perform a test to see if we could ameliorate the channel 3 bandwitdh issue.

3:30

Extended Source Measurements (Pahre, Jarrett)

• Mike presented his approach for determining the extended source calibration from E-galaxies. He pointed out that the SOM and two published papers all disagree on what the factor for extended sources should be.
• Tom presented his analysis of the extended source calibration, as offsets vs. source extension as derived from comparison between 2MASS and IRAC images, and an assumed model spectrum. There is a surprising amount of scatter, and similarly there were difficulties matching a red point source (Mrk 231). There are also consistent offsets in all channels between 2MASS and IRAC (closest in ch.2).
• We arrived at no real consensus between methods looked at to better than the 10% or so level.
• We discussed the effect of the assumed models used to determine "true" fluxes in the IRAC channels. It is aparrent from other work that offsets between simple predictions from 2MASS and IRAC are often between 5-10%. We discussed a need to understand these differences generically (for stars, for example) before we are likely to get much further for extended sources.
• We discussed the idea of performing convolution/deconvolution experiments with the extended PSF to see if we could approach a solution that way.