variable star differential photometry ccd. photometry is the measurement of a stars brightness early...

Variable Star

Differential Photometry

CCD

Photometry Is The Measurement Of A Stars Brightness

Early Measurement Systems Originated with Either Hipparchus or Ptolemy, Greek Philosophers (circa 150 BC-150AD)

Assigned “1” to the Brightest StarsAssigned “2” to the Next Brightest, etc., All the Way to “6”

In 1854 The English Astronomer, Pogson, Suggested that Magnitude Be Defined as a Brightness

Ratio of ~ 2.5 Between Successive Magnitudes Such that a Change in 5 Magnitudes Would Exactly Equal 100 Times Change in Brightness.

m1 – m2 = -2.5log(F1/F2)

What Are Variable Stars?Stars That Simply Vary Their Light Output Over Time

They May Be Part Of A Double Star System Or They May Be A Single Star

~ 2 % of All Stars Show a Measurable Change In Brightness

Visualization by Andy Beardmore

There Are Various Classifications forVariable Stars. A Few Examples:

Cepheids: Period ~ 1 - 70 Days, Vary ~ .1 -2.0 Magnitudes

RR Lyrae Stars: Period ~ .2 - 1 Day, Vary ~ .3 – 2.0 Magnitudes

Long Period Variables: Period ~ 80-1000 Days, Vary ~ 2.5 – 5 Magnitudes

V368 Peg Super OutburstWith Humping10/05/09

Su Ursae Majoris Types of CataclysmicVariables: Frequent & Short OutburstsLasting ~ 1 - 2 Days. Occasionally SuperOutbursts Lasting ~ 10 – 20 Days WithSmall Periodic Modulations CalledSuperhumps.

Why Observe Variable Stars?

The Data Is Very Important to Astronomers & Astrophysicists

Variable Star Data, Dependent Upon Type, Can Be Used To Determine: Luminosity, Temperature, Radius, Mass Composition, Rotational Period & Distance

There Is Also a Critical Need

For Us To Understand & Monitor The More Nasty High Energy Eruptive Variables Such As GRB’s (Gamma Ray Bursts), Supernovae & BL Lac Objects (Blazars).

Our Survival May Depend On These Observations!

The American Association Of Variable Star Observers

With It’s Paid Staff of ~12 Folks & Many Volunteers , LocatedIn Cambridge, MA, isThe Main Repository For Submitted Variable Star Data

You Do Not Have to Be a MemberTo Submit Observations.

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World Wide There are About 260 “Main” Observatories (Infrared,Optical & Radio)

The ~ 260 Big “Scopes” Are Over Whelmed With Requests For Their Limited Time

Then There Is The Humongous Size Of Our Universe With Zillions of Stars

Therefore, Amateur’s Are Very Important to The Data GatheringProcess & Play a Critical Role Observing Variable Stars.

The AAVSO Receives FrequentRequests From Professional Astronomers For Photometry Data as Well asEducators,Students,& OtherAmateurs

AAVSO Members & Observers Are Often Asked To Support Scientific Projects By

Professional Astronomers, Whether Using Land or Space Based Equipment, To Make Observations Of Specific Targets

Three Request Examples From Jan. 2010

Monitor HMXB’s In SupportOf 1.85 Meter Dominion Ob.(Victoria, BC) for Phd @ Univ.of Saskatchewan, Canada

Monitor KT Eri/Nova Eri 2009 In Support Of Swift Satellite X-Ray Campaign for Phd @ Univ of Leicester, England

Monitor Recurrent Nova UScorpii For Outburst to TriggerObservations by Hubble, Swift& Spitzer for Phd@ LSU, USA

Observing Requires A CCD Camera - Preferably OneDesigned For Photometry, i.e.NAB (non Anti-Blooming),Monochrome & Cooling Ability

ST-7/8/9/10XMEST-402ME

A Lot Of Potential Variable Star Observers Already Own AB (anti-

Blooming) CCD’s – ‘taint no thangWe Can Work With That: Either Turn AB Off, Keep Exposures to

½ Full Well Capacity Or Plot the Linearity

A[pg

CCD Linearity - NABG vs ABG

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Time (Seconds)

AD

U's Series1

Series2Series3

Notice How Much More Random Noise Is In The Hotter Image

+ 25C Dark Image - 25C Dark Image

CCD’s Need To Be Cooled

While Mentioning Darks

Photometry Requires That Images Be Calibrated: Subtract Darks & Flats!

Flats Contain Light Path“Artifacts” Within The Optical “Train.”

Darks Contain The Random ElectronicNoise Inherent Within The CCD.

Different Models of CCD Chips Have Different Spectral

Responses

Each Of The Three PopularChips Shown Has A Different Spectral Response In The Region Of A Star’s Light.

If Unfiltered ObservationsWere Made With Each Then The Reported Magnitudes Would Be Wildly Different

One Of The Strengths Of CCD’s Is That When We Use The “Right” Filter We Can Equalize The Passbands Of The Various Types Of Ccd’s.

Observe With “V” Filter!

The Johnson-Cousins FilterStandard System

We Need To Find A Chart For TheFOV We Are Going To Observe: 3C 66A

WWW.AAVSO.ORG

Enter Target Name Here To Find The VSP

VariableStar ChartPlotterEntry Screen

aka VSP

Target Name

FOV Scale

FOV Scale In Arc Minutes

Magnitude Limit

FOV Orientation

Use DSS Image

Next Click OnThe “ComparisonStars” At Top Of Plotted Chart To Acquire Comp Data

Magnitudes ShownAre For Visual Use

Comp Star Data For 3C 66A

“V” Filter Values

m1 – m2 = -2.5log(F1/F2)

Instrumental Magnitudes

Mag(ins) = ZPmag-2.5log(StarFlux)

Mag Formula:

Can Be Rewritten ForInstrumental Magnitudes

Where ZP Is A Somewhat Arbitrary Zero Point Used By Software

Differential Photometry

The Differential Magnitude (Vmag) Is Computed By Taking The (difference) Between The Variable (unknown) Stars Instrumental Magnitude And A Known Stars Instrumental Magnitude Such That:

Vmag = (v-c) + Cmag

Where v & c are instrumental magnitudes

Cmag is the known Comp Star Magnitude

The Differential PhotometryEquation Can Also Be Expressed As:

Vstd = (Vins – Cins) + Cstd

Now It’s Time To Do A Real World Example

AGN

NASA Drawing

3C 66A Original Image

3C 66A

3C 66A Is A Blazer , a TypeOf “AGN”Which Is a Catch All For Galaxies That Have Bright & ConcentratedEmissionsFrom TheirCentral Regions

3C 66A Images by Tim Crawford

Comp

3C 66A

Check

Comp = 142 Star & Check = 148 Star

Compute IM’s

Above Is The AIP4WIN Log Output Showing The Instrumental Mags Of The Variable Star, The Comp Star & The Check Star.

Comp & Check Star Data For 3C 66A

“V” Filter Values

Vmag = (v-c) + CmagWhere v & c are instrumental magnitudesCmag is the known Comp Star Magnitude

Vmag = (14.321-14.525) + 14.182 = -.203 + 14.182 = 13.979 With An Error =.014

Check Star

In An Ideal World: K-C = k-cThe Difference Between the Known Check Star Mag and The Comp Star Mag = the Difference Between Their Instrumental Mags (IMO, Any Difference up to ~ .06 Is probably OK). K C k c

14.771 – 14.182 = .589 & 15.112-14.525 = .587

In this Case K-C ~ = k-c (Difference of .002)

WWW.AAVSO.ORG

You Do Not Have To Be A Member Of The AAVSO To Report Observations

However, You Do Have To Secure Observer Initials From The AAVSO To Make Reports.

This Is Easy To Do . Choose the New Observers Link From The Main Page Then Choose the FAQ’s

After Sign In To Blue & Gold

WWW.AAVSO.Org

Filled Out Observation Form With The Information From The ComparisonStars Data & What We Generated

Chart “name” & Comp Label From Comp Star Page

The Julian Date (JD) Is The Interval Of Time In Days And Fractions Of A Day Since January 1, 4713 BC Greenwich Noon

Many Of The Photometry Programs, Including AIP4WIN (above), Now Allow You To Simply Enter the Comp Star Value, Or Multiple Comp Star Values, Directly Into The Program So That You Do Not Have To Do Any Math As The Vmag Will Be Automatically Output.

This Is A Beta But Similar To Current Pgm

Above Is An Example Of AN AAVSO Up-Load Ready File AS Output By The “Measurement Magnitude Tool” Of AIP4WIN.

Note That No Calculations Are Required & The Air Mass Has Been Calculated

Best Kept Secret OFVariable

Star Observing

You Can Do CCD Observing In Spite Of Mr. Moon!

LogoContributeTo Science By ObservingVariable Stars!

Observatory

PresentationTim R Crawford - CTX

Oregon

tcarchcape@yahoo.com