Day-4. Announcements Next week: Dark Night Observing on Tues. 9/8 & Thur. 9/10 Del Square Psi...

ASTR_4170Special Topics: Photometry

& Filter Systems

Day-4

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

•Del Square Psi meeting at 4:30 today, right here.

•First “Astro-group” meeting.• Next Friday, 9/11; 2:30-3:30 E-109

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

Photographic Photometry from 19221922 – 1st GA meeting of the IAU (Rome).Frederick Sears (Mt. Wilson) presented a new set of NPS

stars, both photographic (UV-B) and photovisual (G-Y).These were designated as the new International standard.Contained corrections from his earlier works.Extended to mpg = 20; mpv = 17.5; p.e. ±0.024

These new standards allowed extensive faint surveys.1. Pickering – Kapteyn collaboration using the 24-inch

Bruce telescope in Arequipa (obs. started in 1907). Generally 1-hour exposures: 16th (to 17th mag); p.e. ±0.11206 Selected Areas; 251,000 stars

Photographic Photometry from 19222. The Astrographic Catalogue continued, but was not tied

to the NPS. By 1925, 1.4 X 106 stars; generally low quality, often tied to visual scale of the BD.

3. Mt. Wilson Catalogue of Photographic Magnitudes of Selected Areas 1 – 139; Seares, Kapteyn, & van Rhijn.Involved finding pg magnitudes for about 67941 stars to 18.5

in the 139 SA’s north of -15°; typical precision ±0.15.All magnitude values were corrected to the zenith using

0.324 mags/airmass

Photographic Photometry from 19221910, E. Pickering suggested adding a calibration source to

each plate.1911, Harlan Stetson at Dartmouth, constructed a thermopile

photometer to try measuring magnitudes – effectively, this was the first scanning densitometer. Tie to a galvanometer and deflection gave value proportional to magnitude.

Independently, Jan Schilt at Groningen designed a nearly identical instrument. Working over a useful range of 4-5 magnitudes, he was able to achieve ±0.007 p.e.Yale and Columbia built on this to develop the Yale Catalog in

1931.After WW II, move to iris photometers.

Photographic Photometry from 1922The Leander-McCormick Observatory (U.Va.); van Rhijn & B.

Bok; used Cramer Isochromic plates to obtain 509 stars in 7 Kapteyn SA’s north of +75 ±0.12. 1st epoch pg (1914-24) and 2nd epoch pv (1924-32). Main goal was to study PM, so magnitudes were about ±0.12.

Plates sensitive in red (beyond H_alpha) became available in early 1920’s; into the IR in the 1930’s.Ed King (1923-24, Harvard) did a survey to find red stars;Ruth Ingalls (1929-30, Radcliff) continued the work and;Combined both to develop a red-visual color index system tied to

A0 stars; correlated to spectral types.Began a new “photo-red” magnitude (pr) scale; tied to 3 A0 NPS

stars; typical errors were ±0.15 per plate.

Photographic Photometry from 1922Wilhelm Becker appointed as assistant at Potsdam, 1933.

Began experimenting with red photographyBegan exploring ISR using color-color diagrams.After WW II, proposed a new RGU system to explore ISR;

Only carried out with aluminized reflectors; B0 stars as ZP. Studied area around M52 and verified the 1/l IS absorption law

found by Stebbins & Whitford in their photoelectric work.This effort compounded a growing problem: thousands of

nights and generations were spent trying to keep track of all the systems and how they related.

Photographic Photometry from 1922UV photometry, including red-leak studies were taking place;Photographic spectrophotometry began in earnest

Studies of the Balmer jump began.

Two major catalogs of the 1940’s and 1950’s:1. The Mt. Wilson Polar Catalog (Seares, Ross, & Joyner)

2271 stars within 10° of the pole gave pg and pv magnitudes on the International system to pg=11.7 and pv=11.0 Suffered from poor spectral type availability.

2. Cape Photographic Catalog for 1950.0 Last major photographic catalog undertaken without PEP sequences.

Photographic Photometry from 1922The NPS as a definitive system of standards goes back to

Pickering in about 1907. Published in preliminary form in 1912 and final form by Ms.

Leavitt in 1917.Extended by Seares at Mt. Wilson to fainter magnitudes (1922)Used with most every magnitude system devised.By 1955, had essentially become obsolete.

Beginning in the 1930s, began to think about the full SED; Color-color diagrams became important; Spectrophotometry was gaining in importance. Al coating techniques were changing, affecting the reflectance of the

mirrors (no longer refracting lenses). Balmer absorption was an issue for the blue-sensitive plates.

Photographic Photometry from 1922By 1955, development of a Pogson scale anywhere in the

sky became possible, down to 18-19 mag with ±0.02 s.e.Use of plates (wide angular field, vs PMT);Thousands of simultaneous images;Excellent long-term storage;made large studies possible & moved use of photographic

photometry to forefront of research efforts.Still, there were issues over defining systems. As late as

1958, still some work using the old International System, but it was becoming clear that the Johnson UBV (PEP system) and Becker’s photographic RGU system would prevail.

Photographic Photometry from 1922The UBV system was devised by Johnson & Morgan in

1953 as a photoelectric system. But, Johnson made a prescription available to apply this to photographic work.

This system was put to use for cluster studies.The UBV system quickly became dominant over the RGU

system due to the development of UBV PEP standard stars.

Passband Filter Plate

Ultraviolet Corning 9863 or Schott UG 2 (2mm)

Kodak 103aO

Blue Schott GG 13 (2mm) Kodak 103aO

Visual Schott GG 11 (2mm) Kodak 103aD

Photometry & Birth of ApDuring most of the 19th and first decade of 20th century,

astronomical photometry (visual or photographic) was a major activity.

This began to change rapidly:1908, Wilsing & Schneiner were deriving the first stellar

temperatures from spectrophotometry using Planck BB curves.

1918, Shapley successfully concluded the size of our Galaxy and the Sun’s place in it.

Shift of the lead in Ap research from Europe to America.Mt. Wilson 60-inch (1908) and 100-inch (1918).

Photometry & Birth of ApSolon Bailey discovered “cluster variables” in M3 and M5

in 1895. This opened a new chapter in galactic research.Search for these stars continued, plates actually are an

advantage here! Follow-up with PEP.Henrietta Leavitt (1903) found Cepheids in the two

Magellanic Clouds; in large numbers. Followed with Bruce telescope. 16 SMC stars gave periods, quickly led to distances.

Hertzsprung (Potsdam) used these data to calibrate the P-L relation, and tied it to 13 galactic Cepheids. Lots of work to show these were pulsators, not eclipsing systems.

Baade was able to show this conclusively.

Photometry & Birth of ApStructure of the MW began to come into view.Shapley’s GC research (Ph.D. at Princeton, under Russell,

1913) took off when he moved to Mt. Wilson to work with Hale. (Seares, his U. Mo. Professor recommended him).Culminated in deriving distances to several GCs, all of which

had pulsators. This gave distances that did not agree with Curtis’ view of the size/shape of MW.

Still some errors, but enough to show Curtis was wrong.Trumpler did detailed studies of galactic clusters and

determined ISR was a major concern which must be accounted for. It effected distance determinations.

Photometry & Birth of ApFinally, Hertzsprung and Russell –independently- devised

the HR diagram which showed clearly the main sequence of stars and evolved populations. Based on trigonometric parallaxes and photometry.

Lots of work to obtain: stellar temperaturesStellar metallicitiesGiant –vs- dwarf color differencesStellar population models

Finally, fundamental stellar parameters from EB systems

PEP with a PMTPMTs revolutionized astronomy after 1945.

1918 – Albert Hall propose amplifying thermionic currents1935 – RCA and Phillips labs proposed using secondary

emission to amplify weak photoelectric currents.1st photocathode and secondary surfaces were Cs-O-Ag (S1)Gains of 6-7 with 1kV or 0.5 kV between anode & cathode.

Only slightly better than gas-filled diode tubes.Weiss (German Post Office) developed a 7-stage tube with

wire-mesh screen dynodes.Zworykin (RCA) devised the iconoscope television camera in

1931, but in 1936 he made 9- and 12-stage PMTs, magnetically focused (S1s) and electrically focused 2-stage tubes. Noise dominated by shot noise.

PEP with a PMTGörlich (Dresden) described a new blue-sensitive

photocathode using layers of Cesium and Antimony. Gave a peak QE close to 13% near its maximum at 400nm.Far superior to the KH surfaces being used. Extended the red side to about 630nm.

CsSb photocathode that RCA developed became the S4 and was incorporated into the RCA 929 vacuum photocell in 1940. Almost immediately this led to the RCA 931 PMT; a compact 9-stage side-illuminated electrostatically focused circular tube. All surfaces were S4 in nature.Gains of 2 million were achieved at 1kVLinear response

PEP with a PMT

G.E. Kron (Lick) got one in 1946 – reliable photometry for the first time.

Whitford: “the advent of the 1P21 also broke a psychological barrier. PEP was no longer the province of a small band of specialists.”

PEP with a PMTLallemand (Paris) was another PMT pioneer. Developed 7-,

12-, 17-, and 19- stage tubes using Ag-Mg surfaces which also gave large multiplying factors. Several Lallemand tubes were used for research beginning in 1950, mostly throughout Europe.

First to produce successful IR PMT.Others followed this work: RCA 7102 (S1 surface) was one

of the most successful.

PEP with a PMTPulse counting techniques were important when it was

realized that the stellar flux is a stream of photons in early 20th century. This led to full statistical understanding – on the basis of Poisson statistics.

Sinclair Smith (1932) calculated that a 60-inch telescope equipped with a 10% QE PMT and Blue filter would require 21 seconds to reach 1% photometry on a 14th magnitude star – provide photon statistics were to dominate the noise.

Even as early as 1933, Bengt Strömgren had tried to record pulses from individual photons.

PEP with a PMTThe first successful pulse- (photon-) counting stellar

photometer was built by William Blitzstein at Cook Observatory (U. Penn.) and Gilbert Yates at Cambridge, both in 1948. Required development of discriminator to reject thermal noise photons.

Tube development and refinement continued into the 1980s. Hamamatsu is the only major company still involved in this work.

PEP with a PMTPhotometric systems generally started with B and Y filters

to try to reproduce the old IS photographic system of Seares. But, no agreement on filters construction.

1951, Gerry Kron proposed the P,V designations. Still had issues depending on which NPS stars were used as standards. Stebbins, Whitford, and Johnson chose just 9 stars, and Kron and J.L. Smith used the same.

6 July 1950 an informal meeting in Pasadena; the 9 leading photometrists in US agreed to use these 9 stars for all future work. The Stebbins-Whitford-Johnson magnitudes would define the (P,V) system. Issues arose almost instantly.

PEP with a PMTIn the winter of 1950-51, Johnson (McDonald Obs.) had

begun on a UBY program (363, 426, 529 nm) using the 13- and 82-inch telescopes. Used a combination of Corning and Schott filters. UV light (l < 380nm) was excluded from the B-band. B and Y could transform to the (P,V) system, but still disagreed with data from Eggen.

Johnson did a lot of work to determine why. Came down to the inclusion of UV light in the B(P) band.

Once UBV system was defined, the (P,V) system essentially died out … but slowly as Eggen continued to use it.

The UBV System3-color system devised by Johnson and Morgan (Yale &

McDonald Obs.) in 1950. Observations began in winter of 1950-51 with UBY filters. Experiments led to the UBV introduction in 1953.Johnson, H.L. and Morgan, W.W. 1953 ApJ, 117, 313

Specified passbands and filter manufacture; 1P21 PMT, aluminized reflecting telescope, altitude of 7000 feet.

10 bright primary standards: O9 V to K4 III were the initial defining stars. 1954 added 98 mainly equatorial secondary standards.Johnson, H.L. and Harris, D.L. III 1954 ApJ,

120, 196

The UBV SystemFurther standards led to the definitive paper with the full

account of the UBV system.Johnson, H.L. 1955 Ann. d’Astrophys., 18, 292

Created a reddening-free parameter Q=(U-B) – 0.72(B-V)Allowed to distinguish between reddened early-type stars

and unreddened stars of later spectral types. Johnson did all the observing, Morgan did the spectral

classifications of the stars.WWM says he devised the U filter and HLJ did the observing

and created Q.Multicolor photometry proved to be a valuable tool for

solving astrophysics problems.

The UBV SystemIn the south, the UCBV system was developed at Royal Obs.

in Cape Town. The Cape U filter had a different passband.Extension to the red followed. Johnson did a lot of the

initial development of the UBVRIJKL system. The UBVRI work was done at Catalina Station of the LPL and Tonantzintla Observatory in Mexico.

In spite of all the work, there were still deficiencies with the UBV system.

Kron developed a dry-ice cold box (to cut thermal noise) and incorporated a Fabry lens to provide more uniform illumination of the photocathode.

PEP with a PMTWork continued and specialized systems developed from

1950-1970.Narrow-band systems: (Strömgren – uvby)

David Crawford added the Hb filter later.Intermediate-band systems:

Walraven – WULBV Geneva – UB1BB2V1G

Red extension work continued: Alan Cousins VRI system was a successfully defined system and is now combined with the Johnson-Morgan UBV system to get the modern UBVRCIC system.

Definitions & Terms -1• Orbit:• Solar System:• Year:• Second:• Kilometer:• Light-Year:• Astronomical Unit:

Day-4. Announcements Next week: Dark Night Observing on Tues. 9/8 & Thur. 9/10 Del Square Psi...

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