Bandpass testing of Zeiss H-alpha filter to determine wavelength of transmission and optical characteristic

Neidig, D. F., Jr.

English

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"A REPORT OF THE TESTS PERFORMED ON THE ZEISS 
Ha, FlLTER" 
by 
Donald F . Neidig Jr. 
March 8 ,  1968 
1 
T H E  PENNSYLVANIA STATE UEIVERSITY 
ff 653 July65 
This report covers w o r k  carried on under National Aeronautics 
and Space Administration G r a n t  No. NGR 39 -009 -066. 
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https://ntrs.nasa.gov/search.jsp?R=19680011822 2020-03-12T08:35:22+00:00Z
A REPORT OF THE TESTS FERFORMED ON THE ZEISS 
Ha, FlLTER 
Introduction 
This report  contains the description and resul ts  of tes t s  performed on the Zeiss 
type "A" Ha, filter. N o  information on thr  theory of the fi l ter  is included here;  if the 
reader  is unfamiliar with the optics of the instrument the following references will be 
helpful. A chapter by Evans [ 2 ]  t reats  the basic theory of the filter. Another ar t ic le  
by Evans [ 11 gives a detailed discussion of t h e  filter and methods by which the wavelength 
of transmission can be tuned. The reference edited by Kuiper provides a bibliography 
with further references.  
1. Evans,  J .  W. Journ. Opt. SOL. A m e r .  39, 229. ----- 
2 .  Kuiper. ed. The -- Solar System Vol. 1 ,  see Birefringent F i l t e r s ,  by J .  W.  Evans. 
2 
Summary 
The tes ts  performed on the Ha f i l ter  were essentially a check on the more 
important optical properties stated in the specifications for the type "A" instrument, 
However, an additional value was obtained in that the fi l ter  will now be operated with 
an intimate knowledge of i ts  optical characterist ics,  a knowledge of the type which the 
specifications cannot supply. The mosr important of the results are listed below. 
0 0 
1. The ''i A" bandpass setting is actually slightly narrower than $ A, while the 
0 "3 A" bandpass is wider than 4 A and is squared-off at  the top of the profile. 
2. The wavelength of transmission is as denoted by the read-out on the fi l ter  
and is accurate to  within Limits smaller than the bandpass of the transmittance,  i .e. , 
the tuning is acc.urate. 
0 
3. Both desired responses and secondary responses (located 32 A from the 
desired response) have intensities whose magnitudes are determined by how far their 
wavelength positions a r e  located from 6564i A .  At fi l ter  settings of H Q  f 16 A ,  the 
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desired and secondary responses art. comparable in intensity. 
0 0 
4 .  The percentage transmittances of wavelength 6563 k .015 A for the $ A  
C 
and $ A bandpasses are 1 .6  and 1.2 %, respectively. 
5. The ratio of the intensit>y of che secondary response to  that of the desired 
response becomes objectionably large (for our purposes) for fi l ter  settings beyond about 
H a  f 8 A , and will limit the fi l ter 's  u s e  to within that range. 
0 
3 
Description of test, equipment 
The Ha monochromatic filter was tested for  those rransmission properties 
whish would pertain t o  i ts  usage in the study of so la r  flares with a solar  telescope. 
Rasically , these properties in(  lude: 
(1) transmission profiles and possible effects on these profiles by  tuning and 
angle of incident. light 
(2) wavelength accuracy of tuned settings 
( 3 )  measurement of position and relative intensity of the secondary transmittances 
(4) percent transmittance of light at wavelength 6563 A. 
0 
Tests for  these properties were carried out with a grating spectrograph at  the physics 
department of The Pennsylvania State University. 
T h e  lay-out of the major optic a1 components used in the tes t s  is illustrated 
In figure 1. The light sources  e i rhe r  a hydrogen dischdrge tube o r  d c oncentrated-arc 
lamp,  wds positioned abour the lv?  in( hes from a field lens of two-inch diameter wtiLLh 
focused the image of the source on the  spec trograph entrance slit ~ located about 60 
ihches from the lens. The ( one of converging hghr from this lens is s imi la r  to  that 
of a: lens of focal length 60 inche;.s uszd with a light source a t  infinity. The focal 
ra t io  of this system is 30 ,  where t.he angle of incidence to  the fi l ter  of the light near  
the edges of the cone is nearly one d t g r e t . .  The angle of incidence for light in the 
center  of the cone w a s  made n:ro by observing reflections from the entrance 
interference f i l ter ,  assuming it was perp~nditcular to  the optical axis. 
The spectrograph is of the Czerny -Turner type and utilizes a ten-inch plane 
g a t i n g  with 300 l inesjmm. used at 9th o rde r .  and a 6 amt-ra lens of focal length 
4 
8500 mm. The signal is detec-ted by a photomultiplier (whose gain is controlled by 
adjustment of the cathode vo1tagt~)and displayed on a chart  recorder  which records 
at the rate of 0.0301 A/cm oi ( harr at a wavelength of 6563 A (0.0765 A /inch of 
d 0 0 
chart). The chromatic resolving power, h / A h ,  of the instrument a s  used throughout 
t h e  tes t s  was about 220,000 where A h  = .03 A. This indicates that a purely 
L J  
monochromatic signal would appear 1 cm wide on the chart. Thus,  the spectral  
resolution of the specrrograph s o  far exceeds any features transmitted by the filter 
that one may neglect c onvolution products and assume that the t s s t s  were performed 
by a spect rog-raph Nith infinite resolvmg power. 
Throughout the tes ts  the wavelength scanning was performed by two methods. 
A slow, high -rt-solurion scan  was  a i  hievtd by mechanic ally scanning a small  portion 
of the spectrum ( -2 A) with an opticdl wedge. Scans made in this manner will here 
be called "wedge scans",  and \\ill  h;: ac L ompanied by d "scan numher", which denotes 
the advancement of a v o r m  gear which moves the wedge. The scan numbers vary from 
0 to 1600, with the wavelength inc rcasirig with the numbers. Reverse sLans (numbers 
decreasing. wavelength dec reasing also) may also be made, in which case attention 
has  been placsd on possible "balklash" in the gears.  Fo r  these tes t s ,  "backlash" was 
ne gli gible . 
n 
Rapid scans over larger  wiiv~lrngth ranges (100 A 1 can be made by scanning 
with the grating itself. These s c a n s  a r e  denoted "grating scans",  and are also 
accompanied by s i  a n  numbers? where t h e  wavelengrh decreases  as the number increases 
at a rate of 4 numbers / A .  Most of the graphs included here  will already have had a 
conversion from numbers to wavelength. 
Description of the tes t s  performed on the Ha! filter 
Eight distinctly different tests were performed on the Ha, f i l ter ,  using both 
0 0 
the  $ A and the A bandpass. A description of each of these tests is summarized 
below. 
0 
Test 1: Assignment of a wedge-scan number to  wavelength 6563 A 
A hydrogen discharge tube w a s  focused on the spectrograph slit without the 
fi l ter .  Wedge scans of the Ha! emission line were then made in order  to locate 
wavelength 6563 A. Number 770 was chosen t o  mark  6563 A. Due t o  a large line 
0 0 
width and an asymmetric profile, the uncertainty in the number assignment is on the 
order  of f .015 A (* . 5  c m  on the charts). 
0 
0 
Test 2: Transmission profile with the Ha! filter tuned at 6563 A 
0 
Using the white -light source and the Ha! filter set at 6563 A, the relative 
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transmittance was measured over a wedge -scanned range, 6562 A to 6,564 A .  The 
resu l t s  
Test 3: 
are plotted in figures 2 and 3. 
0 
Relative transmittance of wavelength 6563 A with the Ha! filter tuned over 
its ranee.  Ha! f 16 A 
Using a white -light source (concentrated -arc lamp) and the spectrograph 
detector set on 6563 A, the filter w a s  tuned over i t s  entire range in 1/8 A increments,  
0 0 
according to  read -out numbers on the filter. The relative transmittance of wavelength 
6563 A for each filter setting was then recorded (see figure 4). The purpose of this 
test was to determine the amount of s t ray  6563 A light that is transmitted when the  
0 
0 
filter is tuned to  observe some other desired wavelength. 
6 
'1.cst 4: ChcLking t l w  Hcr filter fol tuiiing accuracy -..I--- I -- 
0 
Jsing the grating scan (6563 A located at number 80371) and a scale of four 
nlimhe-rs per A , i t  w d s  LwJlrmcd by inspection that the ti l tci  t t u s r r i i t t cd  liglri dt 
tlw wdvellmgth denoted by The read-out. There are no data included in the report  
for  this tes t ,  a s  it is merely a confirmation that the f i l ter  tuning is indeed acc.uratc:. 
Tuning accuracy may also be checked against the data in test  5, where wavelength 
posit ions of both desired transmittances and secondary transmittances have been 
0 
Test  5:  Grating scans over 6563 f 40 A with the Ha! filter tuned at various settings 
Separate scans over 80 A of the spectrum were made for each of the filter 
0 
settings listed below, us ing  a whitelight source.  
l u n e d  scir ing  hand pdSS 
b 
IlG! t OL4 
1 -+ 16 2 
1 
4 12 2 
.+- 6 2 
1 -
- 16 1 2 
1 - 12 2 
1 __ -t 16 4 
t 12 1 2 
figire 5 
figure 6 
figure 7 
figure 8 
figure 9 
1 -- 16 4 
The graphs representing these scans a r e  intended to measure only the 
relative intensities and positions of the transmittances. The grating scans too 
rapidly in wavelength for the profilzs to ht. shown. Profiles a r e  tested below (test 6) 
with the wedge scan. In addition, the relative intensities of the transmittances were 
determined in a scan separate from the one in which their  positions were located. A s  
a rule, the scans fo r  locating the wavelength position of a transmittance were made 
with high gain voltages, causing the recorder to  run off scale.  The purpose of the 
high gain was to  locate any possible, small  transmittances other than the ones expected. 
The results of certain scans have been corrected to some common gain voltage, using 
experimentally determined gain factors between two voltages. It should be mentioned 
that fluctuations in the level recorded for zero light were very small;  in addition, there  
was good reproduclbility of gain factors. This kind of check was made quite often during 
the tes ts .  
C 
Test  6 Transmission mofile with Ilcr filter tuned at Ha! - 16 A 
0 
Using a white-light source and the fi l ter  tuned at  H a  - 16 A , a wedge scan w a s  
C 0 
made of the transmission band at  both a Aand & A bandpass. The purpose of this test  
was to insure that the profile was not changed in shape and that the bandpass was not 
wjdened with the tuning of the fi l ter .  Since there w a s  no appreciable change observed, 
the graphs for t h i s  tes t  a r e  not included in the figures. 
Tes t  7: Transmission profiles with angle of incidence 
0 
With the Ha! filter tuned at 6563 A ,  transmission profiles were obtained for 
0 0 
both 4 A and 3 A bandpasses as in test 3 ,  except that in this t e s t ,  the angle of incidence 
for light at  the center of the convergmg f30 cone was made 1" by displacing the l e n s  
8 
0 0 
aperture  reflection 2 O .  A wedge scan of the i A bandpass and centered at  6563 A 
and with 0" angle of incidence w a s  taken immediately before inclining the filter 
in order  to insure the position of 6563 A with the scan numbers. This yielded a 
0 
check on any changes in the wavelength position of the profile that might possibly 
occur after introducing an angle of incidence. Since no appreciable changes were 
observed, the graphs of this test  are  not included in the figures. 
0 
Test 8: Percent transmittance of waveleneth 6563 A 
0 
Using a white -light source and the H a  f i l ter  set  at 6563 A , recorded values 
0 0 
for the intensity of transmitted light of wavelength 6563 A were taken for a A and 
4 A bandpasses. With a spectrograph bandpass of about . 03  A centered on 6563 A , 0 0 0 
the relative intensity of the two filter bandpasses is just the relative heights of the 
transmission profiles at 6563 A ,  which is about 1.3: 1 (see figures 2 and 3). 
0 
In principle, the fi l ter  would then he removed from the bench and the intensity 
of the light again recorded. This was achieved by t a k h g  several measurements,  
reducing the gain voltage in s teps  for  each of which gain factors were determined. 
Results and conclusions 
Test 1: Assignment of a wedge-scan number t o  wavelength 6563 A 
0 
0 
Number 770 was chosen t o  mark the center  of the H a  emission l ine,  6563 A. 
Tes t  2: Transmission profiles 
0 
Transmission profiles for both 4 A and 3 A bandpasses over the spectrum 
0 0 
6562 to  6564 A are displayed in figures 2 and 3.  Grating scans (h 40 A) a r e  shown 
in figures 5 and 8. The results indicate that the "i A" bandpass is slightly narrower 
than 4 A at the half -power level. The ''4 A" bandpass, on che other hand, is 
0 
0 0 
9 
0 0 
slightly wider than 3 A .  In addition, the 3 A response is "squared-off'' at  the top 
of the profile and has an asymmetry with greater  transmission on the shorter  
wavelength side (this is probably not a ser ious effect). The high-gain grating scan 
(h 40 A )  revaled the presence of very slight responses ( and not objectionable) at 
Ha! * 4 A for  the 
0 
0 0 
A bandpass. 
0 
Test 3: Transmittance of wavelength 6563 A for various tuned filter settings 
0 
With the spectrograph set to observe 6563 f .015 A , the resul ts  seen in 
figure 4 are equivalent to scanning the transmission profile over a narrow emission 
line. Other than the light transmitted by the central  portions (* 1 A )  of the profiles, 
there  were no responses detected beyond f 1 A from 6563 A for either bandpass during 
the tuning over the ent i re  range (Ha * 16 A ) .  
0 
0 0 
0 
Test  4: Tuning accuracy 
At all filter settings a response was detected at the wavelength denoted by the 
0 
filter read-out dial. The secondary responses were always located 32 A f rom the 
desired response for all filter settings. One may conclude that the tuning is accurate 
for  all settings to  within limits smaller than the bandpass. 
0 
Test 5: Grating scans over 6563 f 40 A 
The relationship between positions and intensities for both desired and 
secondary responses at both bandpasses a r e  displayed in figures 5, 6, 7, 8 ,  and 9.  
It should be noted that no secondary responses were seen for  a filter setting of Ha!. 
Most important of all, it was observed that when the desired and secondary responses 
are each positioned 16 A from 6563 A ,  as in the case of setting the fi l ter  at H a  - 16 A 
0 0 0 
10 
0 0 
(with the secondary response at Hat 16 A) o r  with the filter set at H a  + 16 4 (with 
the secondary responses at  Ha! - 16 A ) ,  the intensity of that response at H a  + 16 A 0 0 
(regardless of whether o r  not i t  was the desired o r  secondary response) was the 
brighter by a factor of 1 .4  t o  1.5. This was found to  be t rue  for both bandpasses. 
0 
Test 6: Transmission profiles at  Ha! - 16 A 
Results (not presented in the figures) show that the transmission profiles 
0 
for  both bandpasses are not significantly changed when the fi l ter  is tuned to Ha! - 16 A 
One may probably assume that the profiles are also unchanged for  other fi l ter  
settings. 
Test 7: Transmission profiles with angle of incidence 
Results (not presented in the graphs) show that the profiles for both bandpasses 
are not significantly changed when the angle of incident light is made about 1" for the 
center of the f30 cone. Since the f30 field lens already introduces an angle of 
incidence of 1' for  light at the edge of the cone the filter was therefore being used 
at an angle of incidence of about 2" for at  least some portion of the incident light. 
Tes t  8: Percent transmittance 
0 0 
Using a spectrograph bandpass of . 0 3  A centered on 6563 A ,  the percent 
0 0 
transmittance at the peak of the band of the 3 A and a A bandpasses were 1 .6% 
and 1.2%, respectively. However, the total power transmitted by the entire profiles 
fo r  the two bandpasses must be different by a factor of about 3 . 2 ,  which is evident 
f rom the area under the transmission profile curves 
11 
0 
Plotting the intensity of the responses for the $ A bandpass (desired and - 
secondary) against their  wavelength position yields the points in figure 10. To an 
approximation, these points lie on a cos 8 curve whose peak is centered at about 
Ha! -t 1 4  A .  This asymmetric effect may be caused by the dielectric entrance 
filter having a transmission centered not on Ha! but on Ha! + 14 A. The ratio of 
2 
0 
0 
the intensity of the secondary response to that of the desired response may be 
estimated by the following procedure: to  obtain the intensity of the desired response,  
take the value of the curve above the fi l ter  setting; the intensity of the secondary 
response is then represented by the value of the curve above a wavelength position 
f 32 A from the desired response. Plotting the above rat io  against i ts  corresponding 
0 
f i l ter  setting yields the curve shown in figure 11. At wavelength positions where 
actual experimental intensities were measured the corresponding points have been 
denoted "experimental" in figure 11. Other points obtained by using the values of the 
cos 8 curve have been denoted "theoretical". Figure 11 represents  the most 2 
important aspect of the results - - -  namely a determination of the wavelength range 
over  which the filter can be used. FOP solar  photographic work it is probably 
essential  that the rat io  of the sec,ondary response to  the desired response be no 
larger than 0. 1. This criterion allows tuning the filter over a range no larger than 
0 0 
H a - 8 A  to  H a ! + l O A .  
5 . . - .  + ,  
Figure 1 
7- 
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t 
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i 
I 
-1 
f 
r 
1 
i . .. . .  
c 
Figure 2 
i 
I I 1 I I I 
(i, d c\J 
Figure 4 
t 
Figure 5 
8 -  
F- 
IC %C j i  40 
cr 
w n 
13 
8 
LiJ  
CK 
12 
8 
4 
0 
Figure 6 
f i 1 te r  set t i rig : 
Ha +I2 ; 1/28 b.p. 
-1.5 
1 I I L 
- 8.0 
b I I 
40 30 20 10 - H a +  IO 20 30 40 
WAVELENGTH, (angstroms) 
12 
8 
4 
0 
f i l te r  sett ing: 
Ha + 6; 1/2A b.p. 
0.2 
I A- I I I 
- 10.7 
L l  I I 
10 2 0  30 40 
WAVE LENGTH, (angstroms j 
I 
12 
8 
4 
f 
f i  I ter se t t ing :  
Ha - 16; 1/2& b p .  
- 4.6 
I 
20 30 -+Q 
WAVELENGTH, (angstroms) 
i 4 
Ha-12, '1!2 E 
5. '
I 2 
b. p. 
4 
a 
f 
WAVELENGTH, (angstrovs) 
Ly w 
cr n 
0 
0 w 
CK 
Figure 8 
12 I 
L 
I I I 
f i l ter setting; 
Ha +O ; 1/4 8, b.p. 
- 8.1 
c 
- 
Figure 9 
'3i 12 - f i It e r set t I n g : 
Ha+12; 1/4 b.p. 
0 
-c 
( J  
c 
- 4.8 
I r 
- 
- - 2.1 
- 
G. I I I I 1 
40 30 20 IO - Ha+ 10 20 30 40 
W A d t  LENGTH, (angstroms) 
i 
Figure 10 
say3u !> Niav 
0 
d 
0 
M 
0 
c\i 
0 
T-- 
+ 
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I 
0 
r- 
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(\I 
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t 
Figure 11 
06 
w 
I I  I I I I  I I I I  I l l  
0 
r 
0 
7 
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A report of the tests performed on the Zeiss H alpha filter

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