Questions tagged [photometry]
Questions regarding the measurement of the amount of light coming from astronomical objects.
159 questions
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Why would surface flux include radiation pointing inward the target?
I have this integral limit question with how Sara Seager calculates the surface flux in her Exoplanet Atmospheres. As I understand it, the surface flux is the outgoing flux at a particular point on a ...
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Surface flux in a specific direction: why is specific intensity in other directions allowed to contribute?
I'm working with sections 2.3-2.4 in Sara Seager's Exoplanet Atmospheres. The book has these following definitions:
Specific Intensity $I(\vec x, \hat n, \nu, t)$: $dE(\nu, t) = I(\vec x, \hat n, \nu,...
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Vectorized definition of flux yields counter-intuitive result
I'm working on Section 2.3 from Sara Seager's Exoplanets Atmosphere, and went across this definition for flux:
The problem I have with this definition is: say we have a surface element radiating ...
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Converting HSC g, r, i, z to Gaia G-band Magnitude for Quasars
I am working on a project that requires converting magnitudes from the HSC (Hyper Suprime-Cam of Subaru) photometric system g, r, i, z to the Gaia G-band magnitude.
The HSC system uses filters based ...
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How to compute extinction for XP GAIA spectra?
I am working on the GAIA XP sampled spectral data and want to implement XP spectrum correction for extinction. I would like to check if my approach is correct as I am not an astronomer. I followed the ...
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conversion from magnitude to flux
I am using GalSim for creating mock galaxies in a magnitude range of 28-29. GalSim requires the input flux in units of photons/cm^2/s . I have used the halflight radii as per the value for this ...
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Flux unit conversion
I have a jwst nircam image of a galactic field. The science images are in units of MegaJansky per steradian (MJy/sr), and so the zeropoint is 28.08652 AB mag . When I ran sextractor on this image , I ...
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Luminosity function calculation
I have M_UV(absolute UV magnitudes) and redshifts of some high z sources and I want to fit UVLF for them.
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GAIN in Sextractor
I'm using SExtractor for source detection from JWST CEERS NIRCam images. I'm using the fully reduced mosaics provided in CEERS website (The mosaics are on pixel scales of 0.03"/pix and are pixel-...
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PSF Matching Images from different filters
I have a fits images corresponding to different filters of JWST. I want to make sure they are PSF matched for accurate photometric color measurements by using SExtractor. I wanted to match them to ...
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SNR (Signal to noise ratio) from image
I have JWST images across 7 filters. I extracted sources by using SExtractor. How can I estimate the SNR of each source in each filter. Should I use flux/flux_err or use flux/background_rms. (flux and ...
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Using EAZY as photometric redshift estimation tool
I am new to JWST data analysis and currently trying to extract redshift of high redshift sources from the catalogue I created from SExtractor. I'm using EAZY (Brammer et al. 2008) for this purpose.
I ...
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cross-matched SDSS-WISE catalog
Please where can I find cross-matched SDSS and WISE dataset for a machine learning project?
(Galaxy-star-qso classification task)
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BVR photometry and Luminiscense
I have the values from B, V and R filters. How do I obtain the total luminescence of a star?
My ultimate aim is to find the relative masses and luminescence of various stars/galaxies in a galactic ...
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What is the difference between albedo and Bond albedo?
Recently I came across Bond albedo, but I am not sure I grasped the difference (if there is any) between it and the "classical" albedo. Thanks in advance
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Why MIT's Benjamin Rackham says "nearby ultracool dwarf SPECULOOS-3" would look "purplish-red, spotted, and flaring" from an airless orbiting planet?
May 15, 2024 MIT News article Newly discovered Earth-sized planet may lack an atmosphere includes the following:
“We can say from our spectra and other observations that the star has a temperature of ...
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Errors in SDSS photometry
In plotting out the magnitude errors in SDSS photometry, I get the following pattern:
Why do the errors go up to about 0.5, then down? In other words, why does it look like a high-heeled shoe (or ...
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magnitude calculation if a blackbody's radius and temperature are provided
I know it is necessary to convolve its spectrum with the transmission curve of a filter.
Are there any online tools or codes available for calculating a BB's magnitude in any specific band (e.g., ...
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How do you calculate the reddening corrections for photometry?
I have photometric data from Gaia for a star and am trying to de-redden the colors. I need to find the reddening corrections for the photometry. I have photometry for G, BP, and RP.
To start I am ...
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How do you convert from the Gaia photometric system to Johnson-Cousin?
I have Gaia photometry (G, G_bp, and G_rp) and need to convert it to the Johnson Cousin system. In the Data release 3 (DR3) documentation they provide tables of how to do this but I’m very confused by ...
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Human Brightness perception and contrast
This is kind of an interdisciplinary question on human brightness perception and the ability to distinguish grayscales. To be honest, I wasn't sure if the Physics, Biology or Computer graphics SE site ...
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SExtractor parameters for identifying Star Forming regions
I am using SExtractor (Source Extractor) to identify Star-Forming regions in a galaxy. The problem is I am unable to choose the parameters required for it. SExtractor is just a tool to identify the ...
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Is it possible to detect lyman alpha blobs (LABs) with the lyman-break method?
LABs are luminous extended nebulae of hydrogen gas in the early universe (z ≥ 3) found in overdensities of LAEs (lyman-alpha emitters) and LBGs (lyman-break galaxies). I was wondering if LABs could be ...
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What could these LMC objects in this color-color diagram be?
I have performed a color-color and colour-magnitude plot of various sources detected in an LMC image (JWST NIRCAM F200W, F150W and F115W filters). I then also queried SIMBAD to collect relevant ...
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Understanding PSF models and PFSEx output
I understand that PSFs are models for spreading of light. When I visit Hubble website (https://www.stsci.edu/hst/instrumentation/wfc3/data-analysis/psf) there is PSF model for me to download. I could ...
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How do you estimate standard deviation of sky from (Hubble) image
I'm working on a simulation project with Hubble data and I need the standard deviation of the sky in ADU per pixel. Sometimes I can obtain a fits file (e.g. hdfs image), open this with astropy, and ...
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Rest-frame magnitudes and fluxes
I have a question concerning rest-frame vs observed frame. In the observer frame, I can define an apparent magnitude and an absolute magnitude (as the apparent magnitude if the object were at d=10pc). ...
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Calculation of average FWHM by PSF fitting of all the detected stars in the fits image
I need to calculate the FWHM of all the stars detected via the DaoStarFinder package/any other package by performing PSF fitting in Python. Here is the code for my detection.
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Photometry in SExtractor vs Python
I wanted to find stars in the dwarf Spheroidal (dSph) galaxy. I performed photometry using SExtractor and in Python using DaoStarFinder and other astropy packages. However, while extracting the ...
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Split Multi-Extension fits file - fits.fz format
I have CFHT/MegaCam fits.fz file on which I want to do photometry. I want to split these large fits.fz file into separate CCD detector fits and then run Source Extractor on them for photometric ...
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Effective gain when stacking N frame images
I am trying to understand how to derive the effective gain in [electrons / ADU] when stacking N frame images in different (summed, averaged, or medianed) ways. I found the effective gains for the ...
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Magnitude cuts in different bands/surveys
I have data on around 142,000 galaxies. Some of these galaxies have apparent magnitude data in different bands. For example, many of my galaxies have SDSS PSF data in ugriz bands,GALEX FUV, GALEX NUV, ...
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How does one actually fold a light curve?
I'm studying the variable stars (periodic variables with brightness changes repeating over time), and to examine the shape of the light curve it is useful to fold the data points at this period and ...
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Surface Brightness Definition
I know the surface brightness is calculated by taking the apparent magnitude of an object and then divide by the size of the thing. But this is more of "observed" surface brightness, how do ...
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How do I calculate the flux density of a substellar object?
I am currently trying to generate photometric passband colors for brown dwarfs and planets generated through MESA (Modules for Experiments in Stellar Astrophysics). I believe I understand the ...
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Calculating background photons from flux levels
For a project, I'm trying to calculate the photon flux from the zenith galactic background that a sensor would see. From ScienceDirect I get a value of $2.6 \times 10^{12}$ photons sr$^{−1}$ s $^{−1}$ ...
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Fitting a line or polynomial to optical and radio SEDs
I've been trying to fit functions to plots showing photometric data for AGN from both optical and radio measurements, such as:
but my supervisor says that this shouldn't be done; the radio and ...
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How do astronomers calibrate the intensity scale of their spectrometers?
Discussion on Strange bump in solar spectrum taken with home-made spectrograph made me wonder: How do astronomers calibrate the intensity scale of their spectrometers? I mean, how to take in ...
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Contamination problem data
I do not have a source for this statement. A teacher just said it. Could you tell me what 'a contamination problem' mean? He explained that 'stellar images from a space-based photometry data have ...
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General questions about SDSS
I'm considering doing research on SDSS spectral classification (STAR, GALAXY, QSO) with machine learning for my school dissertation. Not sure if this is an appropriate place for these kinds of ...
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creating asteroid light curves and analysing them
I came across the video Tutorial: Creating a Lightcurve of a Minor Planet linked below by Tycho Tracker, which is software that builds an asteroid light curve from raw images and analyses it to obtain ...
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Querying VizieR using SQL interface on their website
I'm trying to query http://tapvizier.u-strasbg.fr/adql/?%20V/147/sdss12 using the following query, built by clicking Construct your query and ...
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How do I phase fold the light curve for a variable star?
I have some observational data for a star where I've done aperture photometry to get a partial period. I understand that you need to use other techniques to estimate a period for stars whose period is ...
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How does SDSS estimate photometric redshift of sources?
I'm looking through the data in SDSS DR 12:
...
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Where can I find data on SDSS filter profiles?
I've seen lots of filter throughput curves for the SDSS, such as this one:
from the SDSS camera page, but I'd really like to produce my own from the data. Is the filter response data per wavelength ...
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Should I drop source shot noise from the SNR calculation when all I care about is detection (not photometric precision)?
All of the calculations for signal-to-noise ratio (SNR) of sources in astronomical images include various noise terms of relevance: read noise, quantization noise, and shot noise from each of the ...
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Angle to the line of sight
I don't understand this angle used in galaxy clustering. There is the line of sight and this angle to the line of sight which is not clear to me. Here is a transcript of a description from 1705.05442:
...
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Errors in photometric magnitudes from SDSS DR16
I'm playing around with the data in the SDSS DR16, but the magnitudes have huge uncertainties:
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