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FAQs: IRAC |
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See also IRAC web pages.
Table of Contents: Q: I want to create a cluster target, but I don't see this as an option in the MIPS/IRS/IRAC AOT window. Q: 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? Q: How should I decide which dithering pattern to use for a given IRAC observation? Q: What is the best observing strategy for an ultradeep observation with IRAC? Q: What is the accuracy of measuring the relative separation between two sources in an IRAC observation? Q: What is the first-frame effect and why should I worry about it? Q: Why should I use dithers rather than repeats? Repeats take so much less telescope time! Q: I see weird image artifacts in my IRAC data, including horizontal and vertical wings around bright sources, and some unreal looking fuzzy light patches. What are these and how can I get rid of them? Q: 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? Q: 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? Q: What is the absolute and relative calibration accuracy in IRAC data? Q: Do you have IRAC zero magnitude flux density values posted? Q: 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? Q: Is there a document or internal memo describing transformations of the IRAC fluxes to those in similar, but better known bandpasses, such as L, M, and N? Q: 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? Q: I want to create a cluster target, but I don't see this as an option in the MIPS/IRS/IRAC AOT window.A: Cluster targets are a TARGET definition, not an AOT definition. First, create the target in Spot, then create the AOT for that target. Be careful when using map center offset or cluster array coordinate offsets for an asymmetric pattern about the map or cluster center, because it may yield differing results in different visibility windows. (see how this carefully-planned map changes 6 months later -- the small map on the far right has a separate target, but the others are controlled by map center offset. The AORs are exactly the same in each figure.
Q: 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?A: 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 the Spitzer Observer's Manual for the frame times and number of frames taken in HDR mode). The first frame HDR mode observation is executed to help us to mitigate the first frame effect, also documented in Spitzer Observer's Manual. The subsequent frames will be taken at your selected frame time only.
Q: How should I decide which dithering pattern to use for a given IRAC observation?A: 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 dither pattern to enable you to separate these variations from the flat-field/sky dark variations of the instrument. If you plan to attempt superresolution, use the appropriate subsampling size (1/2, 1/3 or 1/4 pixel) together with a medium scale size dither pattern. The medium cycling pattern works well for shallow surveys. For sources with a size similar to array size, use a small map with a large or medium-scale dither pattern. The large-scale 36-point Reuleaux and 16-point spiral patterns are well-suited to the Arendt, Fixsen & Moseley self-calibration technique. See examples in the SOM and read the IRAC dither section in the SOM carefully.
Q: What is the best observing strategy for an ultradeep observation with IRAC?A: 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.
Q: What is the accuracy of measuring the relative separation between two sources in an IRAC observation?A: 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 the knowledge of the distortion, which we think is known to 0.1" or better. If the two sources are in different BCDs, and the position difference is being measured on the mosaic, then the error will be dominated by the error in the pointing refinement. We've measured this to be ~0.3" for frames where the pointing refinement has worked well. For a difference this should be multiplied by sqrt(2), but that's probably a little conservative as the errors in any patch of sky are probably correlated.
Q: What is the first-frame effect and why should I worry about it?A: 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. Also, the effect is most noticeable in sensitive, long frames, and not of much concern in 2-second frames. Therefore we recommend that for the most accurate image reduction and calibration of your data you should avoid frame repeats. We have a first-frame effect corrector in the IRAC pipeline, but the corrections are not perfect for the worst first-frame effect cases. The worst first-frame effect is in channel 3.
Q: Why should I use dithers rather than repeats? Repeats take so much less telescope time!A: 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-place repeats are specified in the AOT window under For Each Pointing/ Number of Frames; dithers are specified lower in the AOT window under Dither Pattern. ) Dithering is used to eliminate array-dependent or transient artifacts from the true celestial map. A well-dithered map will mitigate the effect of pixel-to-pixel gain differences, which will average down when a celestial-coordinate mosaic is generated. Cosmic ray rejection is greatly facilitated by taking highly-redundant observations. And, bad pixels on the array are filled in when dithered images are combined. Scattered light from bright sources near the edge of the field of view sometimes cannot be avoided, but it can be prevented from contaminating multiple frames by using a dither pattern larger than the characteristic size of the regions that produce stray light, e.g., medium or large scale dithers. We discourage using in-place repeats (successive frames taken at the same position), especially in observations that have less than ten different dither positions, both for the reasons mentioned above and also because the residual first-frame effect will lead to a bias pattern difference between the repeats and the first frames of a repeat set. Taking in-place repeats is recommended only in the case of time series measurements with stringent requirements for stability. In general, the better handling of pixel-to-pixel variations using dithers will more than compensate for the reduced amount of integration time; that is, the realized signal-to-noise level will be higher using N-1 dithered observations than N observations with repeats.
Q: I see weird image artifacts in my IRAC data, including horizontal and vertical wings around bright sources, and some unreal looking fuzzy light patches. What are these and how can I get rid of them?A: Please take a look at our IRAC Data Handbook which explains what these features are and shows example images. We are currently working on tools that will help to get rid of most of these artifacts. If your data were well dithered, and you have sufficient redundancy, at least some of these artifacts should go away when mosaicing your data together.
Q: 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?A: Unfortunately, we do not have a TinyTim-like tool available. Empirical PSFs are being derived from in-flight data, using 25 positions on the array and two different-colored sources. The preliminary PSF (really a PRF) is on our website now. Please check the best observing strategy that we recommend for such cases (see SOM IRAC Chapter section on Best Observing practices).
Q: 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?A: The relationship between Y and Z and IRAC x and y is the following: +Y = IRAC +x If you look at Figure 2.1 in Spitzer Observer's Manual, the +Z axis is pointing to the right (Sun direction), and this is also the direction of the IRAC +y axis. +Y axis of the telescope points to the top of the page, and this is also the direction of the +x axis of IRAC. In this figure, the 1A box would be to the left of the bottom left corner of the IRAC 3.6/5.8 field of view, the 1B box would be to the top-left of the top left corner of the IRAC 3.6/5.8 field of view and the 1C box would be to the top right of the of the top right corner of the IRAC 3.6/5.8 field of view. Similarly, the 2A scatter light box would be to the left of the bottom left corner of the IRAC 4.5/8 field of view, and 2B box to the top-left of the top left corner of the 4.5/8 field of view. So IRAC x coordinate increases from 1A to 1B, and IRAC y coordinate increases from 1B to 1C. The zero point for the IRAC detectors is the bottom left corner of the arrays in these diagrams. The IRAC BCD images are mirror images (y-axis mirrored about the x-axis) with respect to what you see in Figure 2.1.
Q: What is the absolute and relative calibration accuracy in IRAC data?A: The absolute calibration accuracy for IRAC is as given by Reach et al. (2005) and is better than 3% for all channels. The repeatability is 1.5% for all channels. To obtain the stated absolute calibration accuracy several effects need to be accounted for. If these effects are not taken into consideration, then the uncertainty in photometry can be up to 10%.
Q: Do you have IRAC zero magnitude flux density values posted?A: Yes, they are posted in our website under the IRAC section here.
Q: 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?A: 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 arcseconds squared, whereas in the pipeline mosaic the pixelsize by default is 1.2 arcseconds squared exactly. So, for example, for the pipeline mosaic a pixel value needs to be multiplied by (1E12 micro-Jy)/(4.254517E10 arcsec**2) x 1.2 arcsec x 1.2 arcsec= 33.84638 to obtain micro-Jy/pixel flux densities. Remember to make all the appropriate corrections to your measurements. These include
Once you have measured the flux density of your source,
you can convert from Jy into magnitudes using the
zero-magnitude flux densities
posted here.
If F(i) is your measured flux density and F(0) is the
zero magnitude flux density in the corresponding channel,
then the magnitude corresponding to F(i) is
Q: Is there a document or internal memo describing transformations of the IRAC fluxes to those in similar, but better known bandpasses, such as L, M, and N?A: The spectral responses of IRAC are posted on our website. 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.
Q: 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?
A:
S13.0+ pipelines:
S11.0 through pre-S13.0 pipelines:
Pre-S11.0 pipelines: Go back to FAQ page.
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help@spitzer.caltech.edu http://ssc.spitzer.caltech.edu/faq/irac.html
