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 | Why, in the IRAC overlay in Spot, does it have two identical frames taken at my first observing position? OR, why do I have two BCD files for the first observing position? |  |
| Every IRAC full array AOR with a frame time equal to or greater to 12 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 |
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| How should I decide which dithering pattern to use for a given IRAC observation? | |
| 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 |
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| What is the best observing strategy for an ultradeep observation with IRAC? |  |
| 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. |
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| What is the accuracy of measuring the relative separation between two sources in an IRAC observation? | |
| 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 |
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| What is the first-frame effect and why should I worry about it? | |
| 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 |
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| Why should I use dithers rather than repeats? Repeats take so much less telescope time! | |
| 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 |
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| I need the most accurate PSF available to do image deconvolution and search for faint objects near a bright star in my image. Are such PSFs available? | |
| 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 |
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| I am trying to figure out the relationship between the observatory axes Y and Z and the IRAC image x and y axis. Where exactly are the scattered light boxes? | |
| 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 |
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| What is the absolute and relative calibration accuracy in IRAC data? | |
| The absolute calibration accuracy for IRAC is discussed in Chapter 5 of the IRAC Data Handbook and is 3% for all
channels. The repeatability is better than 1% for all channels. To obtain t |
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| Do you have IRAC zero magnitude flux density values posted? | |
| Yes, they are posted in our website under the IRAC calibration page. |
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| How do I convert my IRAC images into flux density units? What corrections do I need to worry about? How do I convert the flux densities into magnitudes? | |
| 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 |
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| What were the IRAC FLUXCONV values (the BCD header value showing the conversion from MJy/sr to DN/sec) in the different IRAC pipeline processing versions? | |
| S13.0+ pipelines:
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ch1 FLUXCONV = 0.1088
ch2 FLUXCONV = 0.1388
ch3 FLUXCONV = 0.5952
ch4 FLUXCONV = 0.2021
S11.0 through pre-S13.0 pipelines:
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ch1 FLUXCONV = 0.1104
ch2 FLUXCONV = 0.1390
ch3 FLUXCONV = 0.6024
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| How do I modify my FIF.tbl so that MOPEX makes my output mosaic the size of my choice? | |
| 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_ |
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| Have the pipeline-processed data been corrected for muxbleed, column pulldown and banding? What about saturation? | |
| 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 |
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| What is the pixel phase effect and do I need to worry about it? | |
| 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 |
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| Almost all the background pixels in my image are negative. What is wrong? | |
| 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 2.2 in the IRAC Data Handbook. You will n |
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| I see large gradients and background variations in my channel 3 (5.8 micron) mosaics. What is wrong? | |
| 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 |
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| I want to make sure the pointing refinement ran without problems for my data so that the coordinates in my images will be accurate to 0.2 arcsec or less. | |
| 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 |
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| How do I search for Spitzer calibration star data observed in the same campaign as my target? | |
| 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. |
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| How is the dark/sky subtraction actually performed in the pipeline. Can you give a mathematical representation of it? | |
| 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 |
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| How do I convert the measured IRAC flux densities to IRAC fluxes so that I can compare them to theoretical models? | |
| We discourage the conversion of IRAC flux densities to IRAC fluxes by the multplication with the effective bandwidth since the fluxes depend on the (unknown) absolute normalization of the instrument throughput. Instead, you should calculate the flux dens |
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| I have heard of a non-standard observing mode (not an AOR).
How do I tell if the data were taken using this non-standard mode? What are some of the characteristics of the BCDs obtained with the non-standard mode? | |
| 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 |
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| What is the best way to do photometry on slightly extended objects (size a few arcsec to about 5-6 arcsec)? | |
| 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 |
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| I need to operate with electron units on the fluxes coming from my target. How do I convert the images (in MJy/sr)
to do my analysis? | |
| 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 |
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| How important is the array-location-dependent photometric correction for my data? | |
| 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 |
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| How important is the photometric color correction for my data? | |
| 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 mor |
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| How do I know if sources in my IRAC data are saturated? | |
| 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 |
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| Can you please explain how the IRAC zmags were derived? | |
| 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 w |
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