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  • Everything You Always Wanted to Know About Narrowband Filters, But Were

    Afraid to Ask

    Don GoldmanAstrodon Imaging

    Advanced Imaging Conference

    October, 2007

  • Steve Mandel (what it looks like)

    Re-decorating Steves and Sandy Barnes SRO Observatory (we

    use it a lot!)

    If that doesnt work, drastic action may be needed. Keith Quatrocchi,

    M.D. helping Mike Mayda.

  • Narrowband Filter (NB)Topics

    Benefits

    Terminology

    Definition

    Bandwidth

    Contrast, light pollution, moon, angular velocity, NII/H-

    Manufacturing variability

    FWHM myth

    Spectral (angular) blue shift

    Faster optics

    One filter works for all?

    Practical implications

  • Benefits of NB Imaging

    Enhance contrast

    Extend Imaging Time when the moon is up

    Use in light polluted areas

    Emphasize different structures H-a, OIII, SII..

  • Contrast Improvement

    Wide Red (Orange)

    Narrow Red H-

    9 nm H-

    6 nm H-

    4 nm

    3x5min exposures, RCOS 12.5 Ritchey-Crtien, SBIG STL11000XM CCD, Bisque Paramount ME

    Crescent Nebula, Wolf-Rayet Bubble in Cygnus, NGC 6888

  • Enhanced Structure at Hi-Res

    Conventional RRGB(courtesy Rob Gendler)

    Red = H- Blue=Green=OIII

    (Don Goldman)

    Crescent Nebula in Cygnus, NGC 6888

  • Narrowband Terminology

    Wavelength

    Transmission Plot

    OD, or Blocking Plot

    S/N or IB, Integrated Blocking

  • Filters and WavelengthPhotometric Red

    Imaging RedOIII H-alpha SII

    Photometric Red Imaging Red H-alpha(UVBRI) (RGB)

    H-alpha

  • H-a Filter Transmission Plot

    0102030405060708090

    100

    640 645 650 655 660 665 670Wavelength, nm

    % T

    rans

    mitt

    ance

    (I/Io

    )

    Narrowband Terminology

    FWHM = Full Width at Half Maximum Transmittance

    (aka BW = Bandwidth)

    Peak Transmittance, Tpk

    CWL, 0 Center Wavelength

    Out-of-Band In-Band Out-of-Band

    Photometric RedImaging Red

    OIII H-alpha SII

  • Out-of-Band Blocking

    O.D. = Optical Density = Absorbance

    OD = -log(Transmittance) = -log (%T/100)

    Larger OD means less out-of band light leakage

    OD Transmittance %T

    0.3 0.50 50

    1.0 0.10 10

    2.0 0.01 1

    3.0 0.001 0.1

    4.0 0.0001 0.01

    5.0 0.00001 0.001

    An OD 4 filter blocks 99.99% of the light outside the bandpass

    Must use OD scan, not %T scan

  • Blocking Scan

    Blocking covers the sensitivity range of the CCD

    Light leakage outside of the bandpass

    will be detected

    Comparing significant light coming through the narrow bandpass

    to little light coming in over a huge wavelength range outside the bandpass

    This filter could be specified as > OD4 (good)

    Quantified by S/N or IB (Integrated Blocking)

    H-a Filter Blocking

    0

    1

    2

    3

    4

    5

    6

    7

    8

    350 450 550 650 750 850 950 1050

    Wavelength, nm

    O.D

    QE KAF16803E

  • What is a Narrowband Filter?

    Bessell Approximation to UVBRI

    0

    20

    40

    60

    80

    100

    120

    200 400 600 800 1000

    Wavelength, nm

    %Tr

    ansm

    ittan

    ce

    Stromgren Passbands

    0

    20

    40

    60

    80

    100

    120

    300 350 400 450 500 550 600

    Wavelength, nm

    %Tr

    ansm

    ittan

    ce

    Use photometric filters as a guide

    Johnson-Cousins (Bessell) UVBRI as broadband

    Stromgren* uvby

    as intermediate filters

    U B V R I

    u v b y

    * Originally designed for fast surface gravity and metallicity studies of stars

  • What is a Narrowband FilterFilter Bandwidth Terminology

    0

    20

    40

    60

    80

    100

    120

    550 600 650 700 750 800 850

    Wavelength, nm

    %Tr

    ansm

    ittan

    ce

    Kitt Peak H-

    #1468 FWHM 8.5 nmNARROWBAND

    Stromgren

    b FWHM ~20 nmINTERMEDIATE BAND

    Bessell

    V FWHM 85 nmBROADBAND

    http://spiff.rit.edu/classes/phys440/lectures/filters/filters.html (M. Richmond)

    Melles

    Griot

    Glossary

    http://www.mellesgriot.com/glossary/w

    ordlist/glossarydetails.asp?wID=100

    Broadband FWHM >60 nm

    Intermediate FWHM 20-60 nm

    Narrowband FWHM

  • Proposed Narrowband Definition

    Broadband > 60 nm (FWHM)

    Intermediate Band 11

    60 nm

    Narrowband

    10 nm

    Ultra-narrowband < 4 nm (split H-a, NII)

  • How NB Filters Are Made

    Substrate

    Blocking Layer

    %T

    BandPass

    Layer

    %T

    Result%T

  • Close Up of Narrowband Filter

    H

    L

    H

    L

    H

    Ta2

    O5 tantalum pentoxide

    SiO2 silicon dioxide

    Ta2

    O5

    SiO2

    Ta2

    O5

    0.2 microns0.0002 mm

    Human hair 50 mic.

    H = High refractive index layer (e.g. 2.3)

    L = Low refractive index layer (e.g. 1.45)

    Ne

    = effective index of layer = sqrt

    ( H * L) = 1.8 (affects blue shift)

    Micrograph ref. ThermoCorion

  • Are Astronomy NB Filters Special?

    Company Part CWL FWHM OD Tpk (%) 25mm 50mmThermo Corion S10-656- 656 2 11 2 4 50 $106 $256

    Edmund Optics M43-138 656 2 10 2 3 50 $79.50 $153.70

    Melles

    Griot 03FIR06 656.3 +2/-0 10 2 4 45 $111 -

    Off-the-Shelf

    Astronomy NB CCD Filters

    Smaller FWHM -

    typically 4.5

    7 nm

    Higher Tpk

    -

    70-95%

    OD -

    often not specified (important?)

    Need better than 2 nm CWL for 6 nm FWHM

    Lets see what effect 1 nm CWL has

  • Manufacturing -

    1 nm CWL 6 nm H-

    6nm FWHM H-a Filter Transmission Plot +/- 1 nm CWL

    0102030405060708090

    100

    648 650 652 654 656 658 660 662 664 666 668Wavelength, nm

    % T

    rans

    mitt

    ance

    (I/Io

    )

    H-656.3 nm

    No practical change in %T

    6nm FWHM H-a Filter Transmission Plot +/- 1 nm CWL

    0102030405060708090

    100

    648 650 652 654 656 658 660 662 664 666 668Wavelength, nm

    % T

    rans

    mitt

    ance

    (I/Io

    )

    -1 nm

    H-656.3 nm

    6nm FWHM H-a Filter Transmission Plot +/- 1 nm CWL

    0102030405060708090

    100

    648 650 652 654 656 658 660 662 664 666 668Wavelength, nm

    % T

    rans

    mitt

    ance

    (I/Io

    )

    +1 nm

    H-656.3 nm

  • Manufacturing 1 nm CWL 3 nm OIII

    No change for -1 nm shift (~94%T)

    94% to 72% for +1 nm shift at OIII

    If Tpk

    at 0

    is 80%, +1 nm shift

  • Tpk

    and FWHM Myth

    Idealized case

    same Tpk

    , different FWHM

    Can compare effect of just FWHM on S/N

    Very difficult to CONSISTENTLY keep Tpk

    large with narrower filters

    6 and 3 nm FWHM H-a Filter

    0102030405060708090

    100110

    645 647 649 651 653 655 657 659 661 663 665 667 669

    Wavelength, nm

    % T

    rans

    mitt

    ance

    (I/Io

    ) Goal

    6 and 3 nm FWHM H-a Filter

    0102030405060708090

    100

    645 647 649 651 653 655 657 659 661 663 665 667 669

    Wavelength, nm

    % T

    rans

    mitt

    ance

    (I/Io

    )

    Lower graph is typical

    Causes confusion (two parameters are changing)

    Smaller FWHM lowers N in S/N (moon, light poll)N

    S

    Smaller T lowers S in S/N

    ~Cancels the S/N perceived advantage of narrower filter

  • OIII FWHM Image Comparison6 nm, Tpk

    = 90% 3 nm, Tpk

    = 93%

    Apogee U16M (KAF16803) at -25C (gain = 1.2)

    Takahashi FSQ106N at f/5

    3 x 10 min mean combined with darks and flats

    City backyard (18.3 mag/sqarcsec, no moon)

    S = (Sig-Bkg)g

    N = SQRT(S g + BKG STD2)

    Compare regions with no stars

    Sig

    = 325 ADUBkg

    = 257 ADUBkg

    STD = 16.84S = 81.6N = 19.1S/N = 4.27

    Sig

    = 187 ADUBkg

    = 118 ADUBkg

    STD = 13.04S = 82.8N = 15.9S/N = 5.21

    S/N improvement of 22% with similar Tpk but half the FWHM

  • OIII Test Implications

    Halving the FWHM halved the bkg

    ADU

    Signals are comparable due to similar Tpk

    OIII is more sensitive to moonlight and light pollution

    May mix narrower OIII filter with wider H- and SII filters

    Need Tpk

    to be similar to achieve advantage

  • H-

    - FWHM vs. Background

    H-a Background

    y = 3.6393x + 5.2R2 = 0.9944

    0

    5

    10

    15

    20

    25

    30

    2 3 4 5 6 7

    FWHM

    Mea

    n A

    DU

    12.5 RCOS RC

    Apogee U16M at -20 C

    3 x 10 min unbinned exp.

    Darks, flats, bias calibrated

    Mean combined

    Taken in same region w/o stars in CCDStack

    Tpk

    ~ 92-95%T

    Regular relationship

    Lower FWHM leads to lower background

    Need to keep high Tpk

    for good S/N

  • Moonlight & Light PollutionPhotometric Red

    Imaging RedOIII H-alpha SII

    5750 Blackbody

    0.00

    0.20

    0.40

    0.60

    0.80

    1.00

    1.20

    400 500

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