Day-2. Announcements Next week: Dark Night Observing on Tues. 9/8 & Thur. 9/10 Physics & Astronomy...

ASTR_4170Special Topics: Photometry

& Filter Systems

Day-2

Announcements•Next week: Dark Night Observing on Tues. 9/8 & Thur. 9/10

•Physics & Astronomy Department picnic Friday (8/28) in “the woods” by the Marks Building at 4:00pm

•Plan to re-start the “Astro-group” meetings.• When is good for everyone?• Right now I’m looking at 2:30-3:30pm on

Fridays, but can move it if needed.

Projects – Examples - 1

• Basic CCD reduction – Imaging. (IRAF, Sextractor, Astro-Image-J, MaxImDL, … IDL)

• Scale of a CCD – Calculate and then verify.

• Extinction measurement – basic step in reductions of photometric data. This also makes a good long term project to monitor the atmosphere over the observatory. Is there a usable “site average” value that can be assumed?

• Color Equations – standard transformations

Projects – Examples - 2

• Variable Stars – many possible types to observe.• We’ll try a naked-eye variable (Algol) if the

weather and calendar gods allow.

• Star Cluster CMDs – use 2, or 3 filters (or 5-8) to develop a CMD (and CCD) for a star cluster.

• Parallax and Proper Motion – can use the POSS-I or POSS-II as a base-line (or even SDSS)

• Astrometry of Asteroids – calculate an orbit. Derive surface colors.

• Asteroid Parallax – how far away is it?

On-Line Resources• Astrometry.net This is a VERY useful site for

finding the wcs of your images to identify objects. It will help you do asteroid orbits … among other things.

http://astrometry.net/

History of Photometry• The First Stellar Magnitudes• Visual Magnitude Estimates• Visual Photometric Measurements• Early Photographic Photometry• Early Photoelectric Photometry• Photometry at Longer Wavelengths• Photographic Photometry from 1922• Photometry & Birth of Astrophysics• PEP with a Photomultiplier• Large Area Detectors• The Measurement of Starlight; J.B. Hearnshaw; 1996; Cambridge

First Stellar MagnitudesThe first records go back to Ptolemy (c. 100-170

A.D.)Almagest, books VII & VIII list 1028 stars in 48

constellations from 1st – 6th magnitude along with ecliptic Lat. & Long. Good to about 1/6 to ¼ of a degree. Some are noted as brighter or fainter than the integral measure.

Nothing written as to HOW Ptolemy defined a magnitude. May have used Hipparchus’ work.

Mag. values probably just as a guide; mostly for navigation purposes.

First Stellar Magnitudesal-Sûfi’s Star Catalog – 10th-Cen. Persia (903-

986 A.D.)Retained the 48 constellations; 1151 stars.±0.38 mag. Uncertainty wrt Ptolemy. ~Same

mag. Scale, but independent measurements.Ulugh Berg (1394-1449): copied al-Sûfi’s mags.

New positions for Samarkand (epoch 1437)al-Battani (c. 858-929) –Mesopotamia included

500 stars, but took mags. from the Almagest.

First Stellar MagnitudesTycho Brahé (1546-1601): only the third major

observer of stellar magnitudes. Spurred by the 1572 SN.

1st catalog: 777 stars (epoch 1600); good positional data; poor magnitudes. Two intermediate grades (3, 3., 3:, and 4); probable errors of ±0.54 mags.

Later cat. of 1000 stars (published by Kepler, 1627) and another version in 1843 by Francis Baily.

In all of these, the magnitudes were of secondary interest. The positional data was critical for navigation.

First Stellar MagnitudesJohann Bayer (1572-1625) used Brahé’s

catalogs to construct the Uranometria of 1603, the first good star atlases. He added Greek and Roman letters to stars in order of descending brightness. Survives today.

Better than Piccolomini’s (1508-1579) charts De le Stelle Fisse of 1540 – the first printed star atlas.

First Stellar MagnitudesIn 1589 the Flemish monk Plancius (1552-1622)

drew a celestial globe showing non-Ptolemaic constellations in the deep south, based on observations from southern explorers.

Keyzer & de Houtman (Danish) both made observations and charts; de Houtman published 303 stars (196 were new – not in Almagest) plus a dictionary of Mayan and Madagascan native dialets.

12 new southern constellations. Probably ±0.54 mag errors to Almagest.

First Stellar MagnitudesFirst telescopic estimates not really done until 19th

cen.Early estimates suffered from Purkinje effect (1825):

describes the apparent brightness of of blue and red objects of equal intensity shifting to the bluer one appearing brighter as the eye shifts from photopic (cones) to scotopic (rods) vision sensing.

18th Cen. Meridian-circle observations. James Bradley (1693-1762) the 3rd Astronomer Royal, worked at Greenwich and helped develop the meridian-circle.

Critical to navigation. Again, positions were more important than brightnesses. 3222 stars in PM cat.

First Stellar MagnitudesThe most extensive transit-circle catalogues

were published by de Lalande (1732-1807), using 2.67-inch telescope at École Militaire in Paris: ~50,000 stars; reductions yielded 47,390 stars with equatorial positions to mag=9.5 in half-magnitude intervals (epoch 1800).

In the southern hemisphere, de Lacaille (1713-1762): 9766 stars and magnitudes (epoch 1750). This remained the largest southern hemisphere work until the Cape Photographic Durchmusterung in 1900.

First Stellar MagnitudesFrancis Baily (1774-1844) produced two notable

catalogs in the early 19th century. Both contained magnitude information, but positional accuracy was the primary importance.Astronomical Society Catalogue of 1826: 2881 stars

to mag. 5 over the entire sky; to mag 6 within 30° of the equator; to mag 7 within 10° of the ecliptic.

Catalogue of Stars of the British Association encompassed 8377 stars from 32 separate sources.

Some stars to 9.5

History of Photometry• The First Stellar Magnitudes• Visual Magnitude Estimates• Visual Photometric Measurements• Early Photographic Photometry• Early Photoelectric Photometry• Photometry at Longer Wavelengths• Photographic Photometry from 1922• Photometry & Birth of Astrophysics• PEP with a Photomultiplier• Large Area Detectors

Visual Magnitude EstimatesWilliam Herschel (1738-1822) is frequently called “the

father of stellar astronomy”. His instrumentation was better than anyone else’s at the time.First telescope in 1773 while living in Bath with sister

Caroline.Moved to Datchet (near Windsor) in 1782 and completed

the 20-foot reflector (18.8-inch) in 1783.2 catalogs of double stars: 269 and 434 objectsStudied stellar proper motions to determine Sun’s place.Began the first methodical magnitude estimates to

eliminate the discrepancies of Flamsteed and others. Sequences of stars within a field. 6 comparison notations (6 divisions)

Visual Magnitude EstimatesCharles Pierce, Harvard College (1839-1914) put Wlm.

Herschel’s values onto a quantitative scale, 5 sub-divisions. Still had uncertainties which grew with faintness.

Pickering tied these scales to the Harvard meridian photometry which was set to Pogson’s scale with a zero-point chosen to agree with the estimates of Argelander.

Later, using the 4-inch transit circle, these were tied to the 722 “standard stars” by Gould and assistants from Córdoba, Argentina.

Ernst Zinner (1886-1970) at Bamberg later recalibrated Herschel’s work to account for color differences.

Visual Magnitude EstimatesJohn Herschel (1792-1871) extended his father’s

work of sequences into the southern hemisphere when he visited the Cape of Good Hope, 1834-1838. No corrections for extinctions, smooth sequences which were tied field-to-field by other works.

He was aware of the shortcoming and his writing foresaw the need for spectrophotometry.

Wilhelm Struve (1793-1864) double star estimates using 9-inch refractor in Estonia: 2640 pairs with magnitude estimates.

Argelander began the process of reconciling all the magnitude differences (Bonn, 1836).

Visual Magnitude EstimatesBonner Durchmusterung (BD): Argelander undertook

effort to record the many thousands of telescopic stars that exist. Magnitudes and positions to 9th magnitude and down to declination -2°. This was chosen to provide overlap for a planned southern extension. 324,198 stars recorded; epoch 1855. Still errors in magnitude.

The Southern BD (SD): Schönfeld (1821-1891) extended to -23° and goal of magnitude 10. Still errors, mostly the 10th magnitude stars were progressively too bright.

The Córdoba Durchmusterung (CD): epoch 1875 by Thome (1843-1908) extended to -62°. It was the last great catalog based on visual magnitude estimates.

Visual Color Estimates19th and early 20th century. Both Herschels, Struve,

Smyth, Argelander, Sestini and others.Devised a color scale (similar to magnitude scale) in

steps with subdivisions. Generally a 6-point scale from purple, red, orange, yellow, white, bluish-white with up to 3 divisions per step.

Hermann Klein (1844-1914) adopted a 5-point scale;0 (yellow to 5 (deep fiery red). His interest was in

orange-red stars. Started a 50+ year tradition of color estimates, mostly by German astronomers.

More refinements with time …

Visual Photometric Measurements

Early experiments started in the 1600’s by Huyghens.

Started to get serious in the 1700 with Celsius and Tulenius, the Herschels, and Humboldt began investigating use of a standard comparator. Second half of the 19th century saw the birth of astrophysics. Visual photometers came to be.Reduction of brightness of an image by a known

factor to render it invisible (extinction photometers);Comparison of the brightness of two images to

achieve equality in brightness (comparison photometers).


4 primary variants saw extensive use:Wedge photometer (extinction);

Used a smoked wedge of glass that slid into the eyepiece to dim the star light. The distance was proportional to a known decrease in light.

Steinheil, Zöllner, and Pickering (comparison photometers). 1/r2 relationship was known at this point in time. All had a comparison source and series of

diaphrams and intensity changing devices. Pickering’s also used polarizers.

Zöllner’s astrophotometer, 1861

From: J.B. Hearnshaw; The Measurement of Starlight

2” Harvard meridian photometer

From: J.B. Hearnshaw; The Measurement of Starlight


What didn’t exist yet was a uniformly defined scale, with zero-point which everyone could reference.

Early work and writing, notably Halley in 1720 stated that sixth magnitude stars should be one hundred times fainter than first magnitude, when he considered the effect of placing a star 10 times farther away. (The thinking at this point was still all stars are uniform in brightness.)

Both Herschels did work in this area but lacked good quality observations.


Steinheil, 1836, was the first person to deduce the logarithmic form for the magnitude-intensity relation.

Fechner (1850s) and Weber (brother of that Weber) explained the psychological nature of the visual sensitivity effect.

Norman Pogson (1829-1891) proposed adopting a light ratio of 2.512 for two stars that differ in brightness by one magnitude. Based on his work on variable stars. Others (sort of) ignored it until Pickering used it some. The biggest support came from Müller at Potsdam, a prominent observatory at the time:m = -2.5 log I + cThe “-” sign signifies that higher intensities will have a smaller

magnitude.Debate continued until about 1920 over revising the system.


Zero point: Pickering chose Polaris at m0 = 2.0

Later revised to 2.15, then 2.12 Found to be a variable in 1852. In practice, the Havard scale was tied to 100

circumpolar 5th magnitude stars.Giving (m – m0) = -k log (I/I0) where the

subscript refers to the standard.Müller chose an average of 144 “6th

magnitude” stars in the BD – to protect against variability.


Three great photometric catalogs of late 19th century.Pickering’s Harvard Photometry (HP)

4260 stars, all naked-eye north of -30°, all compared directly with Polaris. Atmospheric extinction was taken into consideration.

Potsdamer Durchmusterung (PD) 14,199 stars from the BD – most precise (and largest) of

the three.Revised Harvard Photometry using the meridian

polarimeter telescope.Some work by Zöllner in Russia also took place.


The advent of photography, and improved uniformity of the plates, led to the end of visual photometry.

This was foreseen by Pickering in 1906 who realized that the photographic plates were detecting stars too faint to be reliably seen by eye.

Early efforts in visual colorimetry, filter photometry, and spectrophotometry also started in this timeframe; all using the eye as the detector.


The main achievements of the age of visual photometry:The universal adoption of the Pogson scale.Extension of photometric measurements to

faint telescopic stars.Correction of photometric magnitudes for

atmospheric extinction.Photometric cataloging of all stars to

magnitude 7.5.Discovery of new variable stars.

Day-2. Announcements Next week: Dark Night Observing on Tues. 9/8 & Thur. 9/10 Physics & Astronomy...

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