Lab 4 updates Spring 2024
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Maxwell Millar-Blanchaer
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"# <p style=\"text-align: center;\">PHYS 134L Spring 2022 Lab 4</p>"
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"# <p style=\"text-align: center;\">PHYS 134L Spring 2024 Lab 4</p>"
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"<div class=\"alert alert-block alert-danger\"><b>Due date:</b> Sunday, May 1st, 2022 by 11:59pm, submitted through Gradescope.</div>"
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"<div class=\"alert alert-block alert-danger\"><b>Due date:</b> Sunday, May 5th, 2024 by 11:59pm, submitted through Gradescope.</div>"
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@ -93,7 +93,7 @@
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Read the photutils background estimation [documentation](https://photutils.readthedocs.io/en/stable/background.html) to determine the best method to use on this data. Play with image scaling and colorbars to help determine if a 2D background estimation is needed. Describe below your final decision on the best method here and why"
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"Read the photutils background estimation [documentation](https://photutils.readthedocs.io/en/stable/background.html) and test out several methods of background subtraction. Determine the best method to use on this data. Play with image scaling and colorbars to help you decide the best method. Show your work below. Describe below your final decision on the best method here and why. "
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Now we're moving on to extracting the photometry. Here we'll be carrying out photometry using a technique called \"aperture photometry\", where we simply add up all the counts within a circular aperture. See the ```photutils``` documentation [here](https://photutils.readthedocs.io/en/stable/aperture.html). **Read up on the documentation and try it out yourself.** There are other types of photometry, such as \"weighted PSF\" photometry, that may be useful in your final project. \n",
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"Now we're moving on to extracting the photometry. Here we'll be carrying out photometry using a technique called \"aperture photometry\", where we simply add up all the counts within a circular aperture. There are other types of photometry, such as \"weighted PSF\" photometry, that may be useful in your final project. Another type of photometry is \"Isophotal photometry\", which we encountered earlier in the course. You can read about that in the Sextractor documentation or elsewhere online. \n",
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"\n",
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"See the ```photutils``` documentation [here](https://photutils.readthedocs.io/en/stable/aperture.html) on aperture photometry. **Read up on the documentation and try it out yourself.**\n",
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"\n",
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"To get started quickly you can ignore some of these sections in the documentation: sky apertures, pixel masking, aperture masks, aperture photometry using Sky Coordinates. \n",
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"\n",
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Play with the various input parameters to the photometric extraction funtion until you get the two catalogs to match as well as you can. One key parameter you can change is the aperture radius, especially if the background isn't sufficiently subtracted. What radius did you have to choose to match the two catalogs? "
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"Play with the various input parameters to the photometric extraction funtion until you get the two catalogs to match as well as you can. One key parameter you can change is the aperture radius, which can make a difference especially if the background isn't sufficiently subtracted. What radius did you have to choose to match the two catalogs? "
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]
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@ -299,7 +301,7 @@
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Using that equation create an error map for the whole data array and pass that into the ```aperture_photometry``` function so that you can get an estimate of the flux error on each star. "
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"Using that equation to create an error map for the whole data array and pass that into the ```aperture_photometry``` function so that you can get an estimate of the flux error on each star. "
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Use the website [sky-map](http://www.sky-map.org/) to determine the magnitudes of the same 4 stars that you used for the image-scale calculation in the previous lab. Make a table below that shows the star number from the Figure in Lab 3, its magnitude from *sky-map*, and (by matching $\\{x,y\\}$ coordinates) the magnitude that you calculated with ```photutils```. Calculate the average difference between your magnitudes and those from *sky-map*, and the standard deviation of this difference:"
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"Use the [Aladin Lite](https://aladin.cds.unistra.fr/AladinLite/) online tool to determine the magnitudes of the same 4 stars that you used for the image-scale calculation in the previous lab. Find your stars in Aladin, and then use the \"SIMBAD\" catalog (the little box on the right-hand side) to find the magnitude of your targets. You'll have to pick the appropriate filter (either the same one as the data, or not that covers similar wavelengths). Make a table below that shows the star number from the Figure in Lab 3, its magnitude from *Aladin*, and (by matching $\\{x,y\\}$ coordinates) the magnitude that you calculated with ```photutils```. Calculate the average difference between your magnitudes and those from *Aladin*, and the standard deviation of this difference:"
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]
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Star| sky-map Mag | ```photutils``` Mag | difference \n",
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"Star| Aladin Mag | ```photutils``` Mag | difference \n",
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"---|---|---|---\n",
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"1 |---|---|---\n",
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"2 |---|---|---\n",
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"**Write an expression for magnitudes in the *sky-map* system (assumed here to be the true on-sky magnitudes) in terms of your derived magnitudes.** This expression will convert your measured magnitudes (based on image counts) into on-sky magnitudes. "
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"**Write an expression for magnitudes in the *Aladin/SIMBAD* catalog (assumed here to be the true on-sky magnitudes) in terms of your derived magnitudes.** This expression will convert your measured magnitudes (based on image counts) into on-sky magnitudes. "
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]
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Assume the brightest of the stars in your list of 4 has an **absolute** magnitude of 2.75. What is its distance (in parsecs) based on its apparent magnitude? What uncertainty do you assign to this distance estimate, given only the uncertainty that you just calculated in estimating *sky-map* magnitudes from the magnitudes you calculated? Justify your answers."
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"Assume the brightest of the stars in your list of 4 has an **absolute** magnitude of 2.75. What is its distance (in parsecs) based on its apparent magnitude? What uncertainty do you assign to this distance estimate, given only the uncertainty that you just calculated in estimating *Aladin* magnitudes from the magnitudes you calculated? Justify your answers."
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]
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