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Knowledgebase : IRAC
You need to multiply the values of the pixels by GAIN*EXPTIME/FLUXCONV to get electrons. These keywords are given in the BCD header. You should also add a term (-ZODY_EST+SKYDRKZB)/f_ext, where ZODY_EST is the estimated zodiacal background light at your o...
Most likely the skydark that was subtracted from your data over-subtracted the background in your images. You can add back the subtracted skydark background value using the BCD header keyword SKYDRKZB. See section 6.2 in the IRAC Instrument Handbook. You ...
Yes. Electronic (ASCII) versions of the transmission curves for the IRAC filters can be found from the IRAC pages.
To understand where the IRAC zmag comes from, you can start with the fundamental equation between magnitudes and flux densities. In one incarnation, it becomes m - M0 = -2.5*log(F/F0) Here m is the magnitude of the source you want to measure, M0 is ...
They can be found in Table 4.1 of the IRAC Instrument Handbook.. They are 280.9±4.1 (ch 1), 179.7±2.6 (ch 2), 115.0±1.7 (ch 3) and 64.9±0.9 (ch 4).
Yes, we are correcting the data for muxbleed and column pulldown in the pipeline. Sometimes these corrections do not produce a perfect mitigation of these effects. Therefore, you should always check your cBCDs and try to perform additional mitigation if n...
IRAC images are in units of MJy/sr. If you want to convert them into flux density/pixel units, you can convert steradians into arcsecconds squared, and then multiply by the area of the pixel. Remember that in BCDs the pixel area is approximately 1.22 arcs...
We discourage the conversion of IRAC flux densities to IRAC fluxes by the multiplication with the effective bandwidth since the fluxes depend on the (unknown) absolute normalization of the instrument throughput. Instead, you should calculate the flux den...
There are several things you can do to check for saturation. If you have HDR data, the saturation flag in the imask file of the long frames (bit 13) will be set. Similarly the same bit will be set in the imask if saturation is indicated during linearizati...
Here is an example FIF.tbl file, which is by default placed in your output directory by the MOPEX module Fiducial Image Frame. ******************************************************************************** \char comment = Output from fiducial_image_...
You can look up primary calibrators from the Reach et al. (2005) IRAC calibrator paper and then search for observations of these stars, constraining your search to the relevant time interval or campaign.
The pixel phase effect will go down as sqrt(number of dithers), so if it is a 4% effect to start with in channel 1, after combining 5 dithers into a mosaic it should be less than a 2% effect. Of course you can get even better results by applying the pixel...
Absolute calibration assumes source spectrum F(ν) is proportional to ν-1. For any other source spectrum, a color correction must be applied. Corrections are typically a few percent for stellar and blackbody sources. Corrections can be more significa...
A so-called labdark is subtracted from each frame, including skydark frames. So, in simple mathematical terms, (obj-lab) - (sky-lab) = obj-sky = BCD where "obj" is a science object frame, "lab" is the appropriate labdark, and "sky" is an appropriate...
You want to select the dither pattern in such a way that every part of your target that is of interest to you will be imaged at least three times. If your object has spatial variations on a certain large scale, you will want to select a large enough dithe...
The relationship between Y and Z and IRAC x and y is the following: +Y = IRAC +x +Z = IRAC -y Please see Figure 6.6 in the Spitzer Observer's Manual. It explains the orientation of the x and y axes and the scattered light boxes with respect...
The first frame correction in the pipeline has not worked well for such data. This was sometimes true for channel 3 data in the cryogenic mission and this is currently always true for warm IRAC data. The first-order mitigation is fairly simple. Take a ...
Technically challenging observations (e.g., long-term monitoring) may be converted to Instrument Engineering Requests, IERs, from the submitted Astronomical Observation Requests, or AORs. To tell whether your data are from an AOR or IER, look at the BCD h...
The STinyTim PSFs do not work well for IRAC. IRAC pixels are large relative to the spatial frequencies found in the PSF, and the response of an IRAC pixel to light varies with position on the pixel. Instead we recommend the use of the PRFs available on th...
Channel 3 suffers from a large first frame effect. Depending on the observation strategy, this may cause severe variations in the background level of the individual BCDs. The effect is worst in frame repeats (as opposed to frame dithers). To mitigate, you...
Please take a look at the Data Features page and the IRAC Instrument Handbook which explain what these features are and shows example images. The pipeline tries to mitigate some of these artifacts, but the correction is not always perfect. The artifact-co...
You can check the BCD header keyword USEDBPHF. If pointing refinement ran without problems, it should be set to "T". To see whether the superboresight pointing refinement ran successfully, you can look at the value of the BCD header keyword "BPHFNAME." I...
The spectral responses of IRAC are posted on our web pages. You can use those to figure out the conversions. The STAR-PET also gives conversions to the K band in different photometric systems for stars of various types.
The pbcd products available from the archive were originally intended as quick-look images of your data. Post-BCD products have been produced with generic namelists, and the parameters have not been optimized for your data set. Post-BCD products and Level...
As part of the closeout for the Spitzer cryogenic mission custody of Spitzer documentation was transferred from the Spitzer Science Center to the Infrared Science Archive (IRSA) in April of 2011. However some information needed to run the warm mission was...
Subarray mode is useful for observing very bright sources and/or observing with high temporal resolution. In subarray mode, only one corner, 32×32 pixels offset by 8 pixels from the edges, is read out from one array. Pixels (9:40, 9:40) of the array ar...
The absolute calibration accuracy for IRAC is discussed in Section 4.3 of the IRAC Instrument Handbook and is 3% for all channels in the cryogenic mission data, and currently 5% in the warm mission data. The repeatability is better than 1% for all channe...
The relative accuracy of measuring the separation of two sources in IRAC images is probably better than 0.3" for source with signal-to-noise ratio greater than 10 on the same BCD image. One of the main factors limiting the accuracy is the uncertainty in t...
We recommend that you use 100 second frames, and a large or medium-scale dither pattern, and as many dithers from the AOT dither patterns as needed to reach the sensitivity goal.
You could use the SExtractor and the Kron flux from its output. This was done by the SWIRE Legacy team. Or, you could take a circular aperture, with a size just larger than the outermost isophote of your target, and do aperture photometry and apply the ap...
The bias level varies depending on the type of the previous observation and the time delay between the current and the previous observation. The variation is highest for very short delays. Therefore, in-place frame repeats suffer most from this effect. Al...
The pixel phase effect is due to the varying sensitivity of a pixel across its area. The sensitivity is usually highest in the center of the pixel. This effect is seen most clearly in channel 1 data, but appears to exist at a detectable level in channel 2...
In general, users should trust the FLUXCONV values in the headers of the IRAC data FITS files that they have. Here is a brief summary of the changes in the FLUXCONV values throughout the Spitzer mission. Warm Mission S18.18 and S19 pipelines: ch1 ...
Pipeline products after S18.5 no longer provide dmasks as they contain misleading and incomplete information. They have been superceded by the information contained in the the imask files. The imasks are more robust, include flagging of various artifacts ...
In-place repeats are successive frames taken at the same position, and dithers move the telescope between pointings. When you move the telescope, it takes time both for the slew and the subsequent settle before astronomical observations can begin. (In-pla...
Every IRAC full array AOR with a frame time equal to or greater to 6 seconds will have its first frame taken in the high dynamic range (HDR) mode. Therefore, you will get one or two short frames before the long exposure at exactly the same position (see t...