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Form sheet to determine mag vs. DN around Alkyone
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AMM-FO-002/D.0, 01 Jan 2026, dvk
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This Excel sheet allows you to use the area around the star Alkyone (in the Pleiades, M45) to get a rough estimate of a magnitude of a star-like object.
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- Gray fields are those that can be filled in - don't change anything else
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- The picture at the right shows stars down to about 10.3 mag around Alkyone (eta Tau, in M45)
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- Use a tool like AstroImageJ to measure the pixel values (Digital Number = DN) for the stars that you see in your field of view
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- Make sure you use background-subtracted values (a procedure on how to use AstroImageJ for this will be given separately)
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- Enter the DN for the respective star in the sheet 'magnitude calibration' in fields E11 to E32. If you don't see all stars, just leave the field empty.
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- Since you are at it - also measure the Point Spread Function, i.e. the Full-Width Half Maximum (FWHM) of the stars and enter in fields G11 to G32 (not used for the magnitude calibration).
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- The plot will show you a fit between the logarithm (base 10) of the visual (V) magnitude versus your DN.
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- The slope should be close to -2.5 - anything between -2.2 and -2.9 is ok.
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- Just for info: The 'intercept' is the V magnitude a star with 1 DN would have.
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- Now to estimate the magnitude of an impact flash - measure it with the same aperture as you measured the stars, e.g. using AstroImageJ.
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- Type in the DN in field I8. You will see the estimated magnitude to the right - both with and without color correction. Note: The impact flash color is set such that a temperature of 4500 K is assumed.
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- In addition, measure the FWHM using e.g. AstroImageJ. It should be close to the average of your stars - see
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Using AstroImageJ to measure brightness intensities in DN
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- Install AstroImageJ from https://astroimagej.com.
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- Open it and load a single frame (if possible, flat and dark corrected - if not, don't worry).
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- In the histogram below the image, set the right (upper) limit such that you can see several stars. Typically by sliding it all the way to the left.
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- Find a 'plate solution', i.e. the direction and orientation of your field: (a) Click on 'WCS / plate solve using Astrometry.net with options'. (b) Type in your user key (*). (c) Set 'constrain plate scale' to 'enable'. (d) Type in your plate scale in arcseconds per pixel (roughly the field of view in arcsec divided by the number of pixels). (e) Set 'annotate' to 'enable'. (f) Press 'start'.
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- After a while, it should say 'Plate solve succeeded' or similar, and display 'HD xxxxx' next to the identified stars. The brightest one should be eta Tau.
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- 'Press 'Analyze / Multi-Aperture'.
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- Go to 'aperture settings'. Make sure the apertures you use are not too small or too large (details to come). Note the 'aperture' in the sheet 'magnitude calibration'.
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- Alt-Click (or Option-Click) on the star to get the so-called 'seeing profile' showing the FWHM. Put it in the cell corresponding to the matching star in the 'magnitude calibration' sheet. Do the same for the FWHM.Figure: An area of about 45' x 30' around the star eta Tau (Alkyone) in the Pleiades, M45. North is up. Stars to about 10.3 mag are shown. The gray text gives the designation of the star, either in the 'HD' system, or from the UCAC catalog ('3UC…'). The green text gives a consecutive number, corresponding to the lines 11 to 32 in the sheet 'magnitude calibration'. After that, the V magnitude of the star is given.
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- That's it!
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(*) Getting a user key for Astrometry.net: Go to astrometry.net. Create a user account following their instructions. In your profile, find the user key.
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