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 Steve’s and Sandy Barnes’ SRO Observatory (we

use it a lot!)

If that doesn’t 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-α

•

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-α

9 nm H-α

6 nm H-α

4 nm

3x5min exposures, RCOS 12.5” Ritchey-Crétien, 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-α 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, λ0 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

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Kitt Peak H-α

#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 <20 nm

•

Michael Bessell–

http://www.mso.anu.edu.au/~bessell/araapaper

.pdf

(Annu. Rev. Astron. Astrophys, 2005, 43)–

Broadband FWHM < ~100 nm–

Intermediate FWHM ~7 –

40 nm–

Narrowband FWHM < ~7 nm

50 nm100 nm

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

•

Let’s see what effect ±1 nm CWL has

Manufacturing -

±1 nm CWL 6 nm H-α

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-α656.3 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-α656.3 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-α656.3 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 λ0

is 80%, +1 nm shift →

<62%T at OIII

3 nm FWHM OIII Filters Transmission Plot +/-1 nm

0102030405060708090

100

490 492 494 496 498 500 502 504 506 508 510

Wavelength, nm

%Tr

ansm

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OIII500.7 nm

3 nm FWHM OIII Filters Transmission Plot +/-1 nm

0102030405060708090

100

490 492 494 496 498 500 502 504 506 508 510

Wavelength, nm

%Tr

ansm

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OIII500.7 nm

-1 nm

3 nm FWHM OIII Filters Transmission Plot +/-1 nm

0102030405060708090

100

490 492 494 496 498 500 502 504 506 508 510

Wavelength, nm

%Tr

ansm

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OIII500.7 nm

+1 nm94%T

to72%T

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-α and SII filters

•

Need Tpk

to be similar to achieve advantage

H-α

- 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

400 500 600 700 800

Wavelength, nm

Norm

aliz

ed U

nits

_

OIII

H-α SII

Fluorescent, Hg

Sodium Street Lamp Incandescent, Outdoor Flood Lamp

H-α SII

OIII

Blue Shift With Faster Optics

•

Simple tilts in collimated light will cause λ0

to shift toward shorter (blue) wavelengths

•

λΘ

= λ0

·√

(ne2

– Sin2 Θ) / ne

•

ne

effective index of the coating (~1.8)•

λΘ

is the shifted wavelength•

Faster optics may shift the passband

off of the

emission line

Θ

501nm Filter, 6nm BW, from 0° to 10° AOI

0

10

20

30

40

50

60

70

80

90

100

490 492 494 496 498 500 502 504 506 508 510

Wavelength (nm)

%T

_

A-6nm-10° %T

A-6nm-9° %T

A-6nm-8° %T

A-6nm-7° %T

A-6nm-6° %T

A-6nm-5° %T

A-6nm-4° %T

A-6nm-3° %T

A-6nm-2° %T

A-6nm-1° %T

Angular Measurement 6 nm OIII

f/10

f/7

f/5

f/4

f/3.5

Θ

Θ

0°10°

OIII Emission Line

f/3

•

Not much effect at 6 nm FWHM

•

Because emission line at low wavelength side

•

If at high side, will see lower T at OIII

Angular Measurement 3 nm OIIIOIII Filter 3 nm FWHM (from 0° to 8°)

0

10

20

30

40

50

60

70

80

90

100

490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510

Wavelength (nm)

%T

0°

1°

2°

3°

4°

5°

6°

7°

8°

f/10

f/7

f/5

f/4

f/3.5

ΘΘ

0°

8°

OIII Emission Line

Decrease

Compare Theory & Experiment

•

Measurements show less shift than theory•

3 nm filter is reasonable to f/3.5

•

Not significant effect for >4 nm FWHM•

Not the whole story –

Telescope optics

OIII Spectral Shift at 500.7 nm 3 nm FWHM

0102030405060708090

100

0 1 2 3 4 5 6 7 8 9 10

Degrees Off Normal

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Meas.

Theory

f/10 f/9 f /7 f /5 f /4 f /3

3 nm FWHM OIII on f/5 530 mm Refractor

•

Peak distribution at f/5 ~ 5.7°•

Many other angles striking the CCD

•

Multiply the frequency of the angle (lower graph) * %T at 500.7 nm in the upper graph

•

Effective %T for 3 nm FWHM OIII is 92%

•

Not much loss from 94% •

Slower telescopes, including RCs, SCTs

will not be affected.

•

Wider NB filters will not be affected

Half-Angle Distribution f/5 Refractor100 mm Aperture 88mm Field

00.10.20.30.40.50.60.70.80.9

1

0 2 4 6 8 10

Angle Off Normal

Dis

trib

utio

nn

%T at 500.7 nm 3 nm FWHM OIII Filter

0

20

40

60

80

100

0 1 2 3 4 5 6 7 8 9 10

Degrees Off Normal

%Tr

ansm

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f /10 f/9 f /7 f /5 f /4 f /3

f/5 5.7º

Ray trace results courtesy of Mike Jones, Optical Designer, Lockheed Martin

3 nm FWHM OIII on 10” f/3.5 mm Astrograph

•

20% obstruction (area)•

50 mm square detector area•

Peak distribution at f/3.5 ~ 8°•

Many other angles striking the CCD

•

Multiply the frequency of the angle (lower graph) * %T at 500.7 nm in the upper graph

•

Effective %T for 3 nm FWHM OIII is 80%

•

Still useable at f/3.5? •

Slower telescopes, including RCs, SCTs

will not be affected.

•

Wider NB filters will not be affected

%T at 500.7 nm 3 nm FWHM OIII Filter

0

20

40

60

80

100

0 1 2 3 4 5 6 7 8 9 10

Degrees Off Normal

%Tr

ansm

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ce

f /10 f/9 f /7 f /5 f /4 f /3

f/5 5.7º

Ray trace results courtesy of Mike Jones, Optical Designer, Lockheed Martin

Half-Angle Distribution 10" f/3.5 Astrograph 20% Central Obstruction

50x50mm Detector Area

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14

Angle Off Normal

Dis

trib

utio

n

H-a Comparison on E-180 at f/35nm FWHM >92%T 3nm FWHM > 92%T

SBIG STL11000XM, -25C, 1 10 min unbinned exposure, calibrated (dark, flat, bias), equalize screen stretch in MaximDL, same S/N

93.1%92.7%91.7%90.1%88.0%

40 30 20 10 0

KAF16803, 9000

KAI11000

3 nm FWHM OIII Spatial Sensitivityf/5 100mm System, 88 mm Field Angle Histograms in Each “Strip”

mm

Effective %T

Ray trace results courtesy of Mike Jones, Optical Designer, Lockheed Martin

FWHM, H-α

and Nitrogen

•

NII doublet occurs close to H-α

on either side•

NII can be 20% to >100% of H-α

in PN and some galaxies•

Most H-α

filters will include H-α

AND

NII•

Need ≤3 nm FWHM to exclude most NII in H-α•

Imagers want narrower filter to exclude light pollution & moonlight •

Imagers want highest signal, even if it comes from NII + H-α•

3 nm H-α

may not have S/N of 4.5 –

7 nm H-α

on some objects (e.g. planetary nebula, WR bubbles, etc.) due to minimizing NII signal

6 and 3 nm FWHM H-a Filter with NII Lines

0102030405060708090

100110

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

% T

rans

mitt

ance

(I/Io

)

NII 658.4 nm

NII 653.8 nm

H-α

656.3 nm

M76 H-a S/N Comparison•

Apogee U16M at -20C

•

Gain 1.2

•

RCOS 12.5” RC

•

5 x 10 min 1x1

•

Dark, Flat, Bias calibrated in CCDStack

•

S = (Sig-Bkg)*g

•

N = SQRT (S * g + BKG STD^2)

•

Compare regions with no stars

Sig

= 208 ADUBkg

= 16.1 ADUBkg

STD = 1.68S = 230N = 15.3S/N = 15.1

Sig

= 257 ADUBkg

= 30.4 ADUBkg

STD = 1.61S = 271N = 16.6S/N = 16.4

3 nm, 94% Tpk 6 nm, 92% Tpk

Unlike OIII, narrower H-α

may not improve S/N perhaps due to loss of NII

FWHM & Radial Velocities

•

AAO/UKST H-α

survey of the galactic plane –

Q.A. Parker and J. Bland-Hawthorn, “Technical Aspects of the New AAO/UKST Hα

Interference Filter”, Publ. Astron. Soc. Aust. 1998, 15, 33-7

•

-400km/s to +600 km/s •

-0.9 nm to +1.3 nm•

655.4 nm to 657.6 nm•

No significant effect for most H-α

filters

6 and 3 nm FWHM H-a Filter with NII Lines

0102030405060708090

100110

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

% T

rans

mitt

ance

(I/Io

)

+600 km/s-400 km/s

H-α

656.3 nm

Red Continuum Filter•

Subtract stars from H-a, SII data

Conclusions

•

Narrowband Filters–

FWHM ≤

10 nm

–

Tuned to specific emission lines–

Increase contrast by eliminating continuum

–

Show different structures (OIII, SII, NII, etc.)–

Extend imaging time with moonlight

–

Allow us to image in light pollution–

Astronomical NB filters are special

Conclusions (2)Narrower Filters

–

Want lowest FWHM with highest Tpk–

More difficult to make CONSISTENTLY

–

More expensive–

Cannot achieve S/N advantage unless high Tpk

is maintained.

–

Wavelength blue shift is not as significant as previously thought

–

Off-the-shelf appropriate for ≥

f/3.5–

Specially shifted filter for <f/3.5?

–

May not achieve large advantage for H-α

due to loss of NII in PN and galaxies

Thank You!!

Clear Skies

How Do the Pros Do It –

AAO/UKST

•

“Technical Aspects of the New AAO/UKST Hα

Interference Filter”, Pub. Astron. Soc. Aust, 1998, V15, pp 33-37

•

Consortium for Hα

survey of the Galactic plane •

Cover radial velocities of galaxies of -400 to +600 km/s (shifts of -0.9 nm and +1.2 nm from 656.3 nm)

•

Placed in the f/2.48 convergent beam•

Coated 305 mm Hα

diameter (~10”) on a 356 x 356 mm plate (12”)•

7 nm FWHM•

CWL 659 nm ±2.5 nm to account for blue shift at f/2.48•

Blocking 4 OD from 200 to 699 nm•

Tpk

>85%•

¼

wave transmitted wavefront•

0.004 nm/C blue shift with temperature.•

Narrow enough for use under a bright moon