AstroNotes 1980 July Vol: 19 issue 07

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Astrone ISSN 0048-8682
The Newsletter Magazine of the Ottawa Centre of the RASC
Vol. 19, No. 7 $2.00 a year July, 1980
Editor........Rolf Meier.......77 Meadowlands Dr.W...224-1200
Addresses Jacqui Tapping...61 Oval Dr., Aylmer...684-1186
Circulation.. .Barry Matthews...2237 Iris St........225-6600
OBSERVER'S GROUP MEETING - JUNE 6
Mary Geekie and Renee Meyer
Chairman Robt Dick opened the meeting at 8:15 pm
with 46 people in attendance, 10 of whom were non-members.
Rob McCallum reminded the group of the deadline for
display entries for the General Assembly, and of the upcoming
Stellafane convention near Springfield, Vermont, August 9.
Rob Dick informed the group that those members interested in the re-issue of the Beginner's Package should
contact Brian Burke.
The IRO update by Robin Molson proceeded; the drive
has been repaired, the screen door has been installed, the
mosquitos are very bad, and there is a chip in the 16-mm
Konig eyepiece. Robin stressed that any damage should be
reported immediately.
Instrumentation coordinator Ted Bean reported that he
has held two telescope-making workshops with 13 people in
attendance. Another workshop will be held in the near
future. Ted advised the group to take advantage of the
good astronomical equipment deals that are sometimes
found in Astronotes.
A Star Night at IRO will be held on June 13/14 at
21:00. The group was also informed of the Celestron
telescope sale sponsored by the Museum of Man and Nature
in Winnipeg.
Frank Roy displayed his home-built oscillator for a
telescope drive. It took him 2 weeks to design and 2
months to construct. The one now at IRO is adequate but
Frank's has the advantage of a self-starting heater that
at low temperatures prevents the apparatus from malfunctioning. The oscillator will be installed by the
first week of July.Brian Burke informed the group that the planets are
getting low in the vest and are setting A hours after
sunset. This month's major astronomical event is the
daytime occultation of Saturn by the moon. Saturn
(magnitude 1.3) will have 4 contacts that can be recorded.
Brian has also attempted a photometric observation of the
occultation of a 3rd magnitude visual double star. Doug
George and Brian will be submitting a display of photometric observation at the General Assembly. Brian
also told the group of a conference to be held in Daytona,
Ohio, concerning photoelectric photometry.
Mike Roney explained how to determine the maximum
of a Cepheid variable. Using data collected by Rob
McCallum in 1975 and 1979, Mike explained how to place
all the observations on one light curve. His talk will
be the basis of a paper to be given at the General Assembly
Rolf Meier, deep sky and astrophotography coordinator,
discussed guided photography and the equatorial mount.
Rolf also displayed various slides of Messier objects
including the North American Nebula.
Rob Dick displayed slides he had taken of the sun
using his refractor and filters. Around the 21st of May,
Rob observed a huge group of sunspots.
Barry Matthews relayed to the group Dr. Helen Hogg's
search for anybody with astronomical books before 1900
in order to compile a list. Anybody with such material
is urged to contact Barry Matthews at 225-6600.
Rob Dick adjourned the meeting at 9:39 pm.
#########
FOR SALE: Tasco 4½-inch telescope, equatorial mount.
Cost $389 from Focus, but will take best offer.
Call Lucille Gravel; 233-3390; 5 to 7 pm.
Articles for the August issue of Astronotes are due
by July 18.H O W T O U S E F O U C A U L T R E A D I N G S T O F I N D T H E S U R F A C E
A C C U R A C Y O F A M I R R O R
D o u g W e l c h
T h e F o u c a u l t t e s t , w h i l e b e i n g i n v a l u a b l e a s a n a i d
d u r i n g f i g u r i n g , d o e s n o t t e l l t h e m i r r o r - m a k e r t h e s u r ­
f a c e a c c u r a c y o f h i s m i r r o r . A s t h e u l t i m a t e a i m o f
m i r r o r - m a k i n g i s t o u s e t h e m i r r o r , i t w o u l d b e u s e f u l t o
k n o w w h e n t h e w a v e f r o n t e r r o r a t t h e f o c a l p l a n e h a s b e e n
r e d u c e d b e l o w 1 / 4 o r 1 / 8 w a v e s o t h a t t h e m i r r o r c a n t h e n
b e a l u m i n i s e d a n d a p p r e c i a t e d . U n f o r t u n a t e l y , t h e r e i s
l i t t l e i n t h e a m a t e u r l i t e r a t u r e t o a d v i s e t h e m i r r o r -
m a k e r o n t h i s p o i n t . I n f a c t , m o s t d i s c u s s i o n s o n t h i s
t o p i c i n m i r r o r - m a k i n g b o o k s i s i n c o r r e c t ! T w o a c c u r a t e
s o u r c e s a r e l i s t e d a t t h e e n d o f t h i s a r t i c l e . I w i l l
d e v e l o p i n t h i s a r t i c l e a m e t h o d b y w h i c h t h e m a k e r o f a
l a r g e m i r r o r c a n d e t e r m i n e t h e a c c u r a c y a n d s t a t e o f t h e
m i r r o r ' s s u r f a c e a n d w a v e f r o n t .
T h e f o l l o w i n g a b r e v i a t i o n s a r e u s e d i n t h i s a r t i c l e :
R r a d i u s o f c u r v a t u r e
r i r a d i u s o f t h e i t h z o n e o n t h e m a s k
s t h e f u l l s e m i - d i a m e t e r o f t h e d i s c
t h e f r a c t i o n o f t h e s e m i d i a m e t e r a t w h i c h t h e
c e n t e r o f t h e i t h z o n e i s l o c a t e d ( x i = r i / s )
a w a v e l e n g t h o f l i g h t ( 0 . 0 0 0 0 2 2 i n c h e s )
Theory
F o r a r e a d i n g i n t h e i t h z o n e , t h e t r a n s v e r s e a b e r a -
t i o n a t t h e f o c u s w i l l b e :
w h e r e λ i s a u s e f u l s c a l e f a c t o r . T h e s l o p e o f t h e s u r f a c e
r e l a t i v e t o a p a r a b o l o i d i s t h e n :
N o w , i f w e a c c e p t t h a t t h e s l o p e e r r o r a t t h e
c e n t e r i s z e r o - a s i t h a d b e t t e r b e - t h e n t h e s e t s
o f ( ( r a d i u s ) i , ( s l o p e e r r o r ) i ) d e f i n e a c u b i c e q u a t i o n :
T h e i n t e g r a l o f t h i s e q u a t i o n g i v e s t h e s u r f a c e
e r r o r p r o f i l e r e l a t i v e t o t h e p a r a b o l o i d t h a t f i t s i t b e s t .This equation is:
We are at liberty to adjust C and so we will make the
reasonable assumption that the paraboloid that fits the
figure best will touch the figure at both the center and
the edge. This is accurate as long as the edge is not
turned! If we call
C = -s(sA-B)
thenIt is obvious that once the mask is made and the
fraction of the semidiameter for the center of each
zone is found, most of the above quantities are
determined. So let
Then we need find the Ai's and only once. They
are, then:
Practice
This all looks very complicated, but let's see how
easy it is to use. I have a 12.5-inch mirror that is in
the final figuring stages. We'll use the above to determine the state of the mirror at this writing. First:
The mask has 4 zones, so i = 1, 2, 3, 4. The center
zone will be excluded to shorten the article!
R = 112.5
s = 6.25and hence D= -1.6131816.10-4
Next
-8.7554
B2= 4.0296
B3= 6.8381
B4= -4.5711
A1 = 4.0993
A2= -2.4617
A3= -3.5791
A4= 2.8389So we have
A= (4 .0993f1 - 2 .4617f2 - 3. 5791f 3 2.8389f4)/244. 14
B= (-8.7554f1 + 4 .0296f2 + 6.8381f3 - 4 .5711f4 )/39.063
C= -39.063A - 6.25B
From my Boat recent Foucault test
(reading)1= .014
(reading)2= .077
(reading)3= .157
(r e a d i n g )4=.209
These give
f 1 = + .2152
f 2 = + .3786
f 3= + .5629
f4= + 1.008
and then
A= .003265
B= -.02860
.05120The resulting 4th-order equation is now plotted in
the accompanying graph. One can see that the maximum
to minimum distance is about 0.29 wave and so this is
the surface accuracy. The wavefront accuracy is twice
this value or 0.58 wave. So the mirror is not finished
yet.
The great strength of this method is the ease with
which the result is interpreted. The equation gives the
deviation y in wavelengths. It is also quite general as
presented here, so it will work with any mirror for any
mask. The method will probably only interest the makers
of larger mirrors and the beginner will probably do well
to start with a small mirror for which the usual literature is adequate.
I have a 70-line BASIC program which carries out the
above calculations and plots the results and I will make
this available to anyone who is interested. If you have
any questions, feel free to write me at:
Doug Welch
89 Charles St. W.
Toronto, Ont. M5S 1K6
Have fun!
References
Sinnot, R.W., Sky and Telescope, Pp 54, 144 (1977)
Texereau, J., How to Build a Telescope, Doubleday, (1963)
#########THE ASTRONOMER Fred Werthaan
High upon a mountain top
Above the atmospheric dust
The vast unending universe
Awaits in silent beckoning.
And moving with the speed of light
So can be seen this wonderous sight
Of time onto eternity
When all that is was born.
From Orion's stellar nursery
Her vast and glowing majesty
Her unborn stars will shine their light
When all that's known has turned to dust.
Beyond our own galactic isle
Andromeda shines with the light
Whose journey to these distant shores
Began in man's own infancy.
And quasi-stellar entities
Whose power spans all else conceived
By puzzled minds in ecstacy
To ponder on such mysteries.
And so unto the mountain top
Above the atmospheric dust
Go those who gaze out on a sea
As timeless as infinity.
#########
CRYSTAL CONTROLED R.A. DRIVE (REVISED) Frank Roy
In the April issue of Astronotes I published a right
ascension drive circuit. There are a number of problems
with it and a second circuit appears in this Astronotes.
The upper and lower frequencies have been found to be
too far from 60 Hz and as a result the R.A. motor may
not have enough torque to drive the telescope. The new
circuit has a lower frequency of 48 Hz and an upper frequency of 84 Hz. Also, the fuse for the dc supply is toosmall, and a 2A or 3A fuse would be preferred. The status
circuit employing the 4049 should be omitted and the
4018 will drive the LED directly through a 470-ohm
resistor. Some of the pin numbers on the 4018 werewrong and have been corrected.
The truth table for the frequency selector was inverted, and this has also been corrected. The output
from the regulator is about 11V. A 2 k-ohm pot may be
put in place of the 1 k-ohm resistor and the desired
- 1 1 -voltage adjusted to.
This circuit has been built by the author and found
to work extremely well.
# # # # # # ###ASTRO NOTES
TO MS. ROSEMARY FREEMAN CAST
NATIONAL SECRETARY
THE ROYAL ASTRON. SOC. OF CAN.
124 MERTON STREET
TORONTO, ONTARIO M4S 2Z