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
ittan
ce
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
(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
ittan
ce
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
ittan
ce
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
ittan
ce
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
%Tr
ansm
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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
ittan
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
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
ittan
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