AAVSO CONFERENCE, OCTOBER 2005 - PART 3/4
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John Pazmino
NYSkies Astronomy Inc
nyskies@nyskies.org
www.nyskies.org
2005 November 27
Introduction
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The American Association of Variable Star Observers held it fall
convention in Newton, Massachusetts, on 13-15 October 2005. The
meeting was far too complex and lengthy to summarize in a single
article. I'm issuing a series of four to adequately treat the material
generated by the meeting. This is the third in the series, covering
talks and discussions about visual observing methods.
Visual observing - discussion
---------------------------
There were several presentations and open dialogs about visual
observing on Friday morning, October 14th. On other days, some papers
covered observing topics, which I moved to here. They are part of a
workshop on the art of assessing the brightness of stars by eye-&-
chart methods.
Visual observing was, and still is, the traditional means of
capturing the brightness of a variable star. In this method the
observer is provided with a starchart from AAVSO with the variable
star marked on it. Other stars in the vicinity are labeled with their
stellar magnitudes as comparisons.
The observer by eye at the telescope or binocular assesses the
variable against these comparison stars. He makes a judgement for its
magnitude. He then records on a form the magnitude, along with the
date and other indexing information.
The forms are either in paper form, like a ledger sheet. Until the
1990s virtually all reports collected by AAVSO were on paper forms
snailmailed or faxed to headquarters. The forms were typicly
handwritten, altho a few were generated in replica layout by word
processing.
There was a small, but energetic, section in AAVSO for capturing
observations by electrophotometry, or photoelectric photometry. The
light from the star is focused by the telescope onto a photomultiplier
cell and converted to an electric current. This current is compared to
that received from the comparison stars. The variable is gaged among
the current readings and converted into a magnitude value.
This method, as objective as it was, never caught on. It remained
a niche area of AAVSO mostly for the expense of the apparatus and the
squander of effort and time to operate and maintain it.
Never the less, important work was realized in the fields of
eclipsing binaries and Cepheid stars. This was due to the
electrophotometry's ability to operate automaticly over many hours. It
could record the moment of minimum or maximum more precisely than the
human observer. It was the moment of least or greatest current
generated by the star.
In the 1990s the CCDgraph entered home astronomy. Intended for
digital photography and astrometry, it was soon put into service for
photometry. While still an expensive accessory, it was far simpler to
operate and maintain than the electrophotometer. Also, by the 1990s,
pretty much every home astronomer had a home computer, unlike in the
1980s when they were still an uncommon feature. Also by the 1990s, the
diversity of computers, that made it just about impossible for easy
comms and transfer of data among them, was washed out. Home computers
are oow dominated by the IBM and MS operating systems.
CCD-based photometry is a steadily growing practice among variable
star observers, the more so as prices of the equipment continue to
drop and telescopes with solid mounts and precise tracking are more
common.
The rise of Internet as a public utility also pushed CCDs forward
thru its easy and spontaneous ability to send data between computers
by email, websites, or webcasting. Internet also made it far quicker
and simpler to send in the traditional visual reports by the same
means. With the data in digital form at AAVSO, it was a natural next
step to automate the initial processing and validation of the incoming
reports and then distribute them to astronomers who need them.
With CCDgraphy diffusing into home astronomy, is eye-&-chart
observing still viable? Definitely so. Despite the precision of the
CCD apparatus, it is still fiddly to work with and riddled with
errors. These are both patent and latent. Unless the observer
exercises a heightened care, a CCD-based report can be of worse value
than eye-&-chart one.
Aging eyesight - discussion
-------------------------
AAVSO found an amazing phaenomenon among its observers who have
decades of records. Their observations can monitor the deterioration
of their eyesight! This decline comes from yellowing of the ocular
lens, filtering out the blue end of the spectrum.
By the nature of variable stars, the easier ones to follow are
red, being the long-period and semi-regular red giants. The comparison
stars, which have constant brightness, tend to be main sequence stars
from the blue to white range of color.
In the ideal situation, you should select those comparison stars
that are of the similar spectral type as the variable. It is hardly
ever possible to do so, specially for the red variables. So the
measure is made of a red star against white, blue-white, or blue
stars, these being the only ones to hand on the charts.
As the eye lens yellows, it filters out the blue end of the
spectrum, making the comparison stars dimmer against the red variable.
The result is that the variable is gaged brighter than it would be for
a clear-eye, that of a young observer!
The effect can be dramatic, a full magnitude of excess brightness
or more. It shows up as an annoying scatter on the lightcurve when the
data from the older observers are included. It proved impractical to
adjust the estimates of these elders because the decline of eyesight
is progressive and irregular. It was best to note the error and give
greater weight to the younger clear-eyed observers.
Visual observing advantages - Mike Simmons
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Visual observation of variable stars has many advantages, which
seem unlikely to be overthrown any time soon. It is available to any
observer, without the barrier of cost for the newer CCDgraphs and
hefty telescope. There are plenty of important variable stars in the
brighter range for small telescopes and binoculars. Even the charts
are free for downloading from the AAVSO website.
There is the crucial need to keep the continuity of record with
the same general method since the 19th and 20th century. By comparing
records of the same star using visual and electronic methods, AAVSO
finds subtile, but important, deviations that prevent naively pooling
the two together.
Visual observing is quick and spontaneous, allowing for fast
response to novae and sudden changes in other variable stars. The
sheer number of visual observers across all longitude zones gives
good coverage of these special events. CCDgraphs tend to cluster in
the more affluent parts of the world with large longitude gaps between
them.
All that's required for visual observing is the same instrument
used for regular stargazing, plus starcharts for the variables. These
charts are free for downloading from the AAVSO website in PS or GIF
form for any computer to display or print. The observer can fit
himself for variable star work on a stargazing trip merely by packing
a folder of these charts.
The learning curve is shallow with competence coming after a few
observing sessions. Visual observing makes a fun and worthy project
for camps, schools, clubs, even for children. It is quickly regained
if the observer is dormant for a long period, like for career or
family burden.
Human eye for photometry - Mike Linnolt
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Linnolt gave a detailed physiological description of the human
eye, with comparison to several other animals. Humans are one of the
few species that specificly distinguish color. More accurately, humans
register different responses in the brain for each wavelength of
incoming light. It is unknown what other animals 'see' in their own
wavelength-dependent responses, but it surely is NOT a straight
mapping of human colors.
Colors are registered by one of the two kinds of cell in the
retina, the cones. The cones decrease in density from the center of
the retina to the periphery. At the center, in the fovea centralis,
there is an extra dense packing of cones. When you look directly at a
target, you place its image on the fovea to get the maximum resolution
and color,
Altho each person can describe and match the colors he sees, he
can not visualize what colors others see. That is, the 'orange'
observed by person A, altho in lab tests can be matched to that of
person B, can not be objectively compared. As yet there is no way to
tap into the brain's rendering of a scene and compare it with the
rendering of an other person's brain.
Color is, therefore, NOT a physical property of the target but
only the human interpretation of the target's mix of wavelengths. The
spectral distribution of wavelengths is the objective reality; color
resides entirely within each human observer.
At night, below a certain illumination level, the cones fail to
record light and the far more sensitive rod cells take over. These do
not have color discrimination, so colors are absent in pure night
vision. The rods are densest around the periphery of the retina, at
the edge of the visual field, and thin out in the center. This gives
rise to the 'averted vision' trick to see dim celestial objects.
Apparently contradicting the behavior of night vision is the fact
that humans DO see color at night. The brighter stars and planets show
real tints. Brighter lights in the landscape show colors, like for
advertising and traffic control. This color perception is a localized
effect on just the part of the retina the brighter light hits. The
illumination at that spot is high enough to trigger the cones back
into action, while the surrounding cones stay dormant.
The electric signal produced by the inpinging photons contains only
a percent or so of the photon's energy, making the eye a very weak
detector compared to ordinary selenium photocells and photographic
film. Part of the cause for the low conversion efficacy is that the
human eye can not accumulate the incoming energy like photographic
film. The image is 'erased' every 1/15th second or so.
For human survival this is a good feature, else the scene would be
dangerously smeared by ordinary movements of the eye and body. On the
other hand, motion pictures and fluorescent lamps depend on the
latency of human imaging.
The human eye is remarkably good at matching the brightness of two
adjacent sources, notably points or small discs. Ideally they should
be of the same spectrum, but the eye is amazingly tolerant about
mismatched color.
The eye can not give quantitative measure of different
brightness. It can not, for example, claim that source A is equal to x
number of source B. On the other hand, if a source can be boxed in
brightness between two others, B and C, the eye is pretty good at
proportioning the target A brightness relative to B and C.
The eye can tolerate considerable amount of optical noise, stray
light, glare, gray sky. As long as the variable and bracketing
comparison stars are clearly seen, the estimate ends up being about as
good as one taken under ideal dark conditions. Never the less, for
comfort, do try to exclude as much extraneous light from the eye as
possible.
The magnitude scale in astronomy is a logarithmic one, like the
decibel scale in acoustics. A 1/10th magnitude leeway in assessing the
brightness of a star is almost a 10% swing in illumination from the
star. At first this sounds like a terrible precision. For a lone
observer with no collaboration, it is. For a multitude of observers
within AAVSO, the individual precisions combine rapidly and tighten
with increasing numbers of assessment.
Custom starcharts - Erne Henden
-----------------------------
The special charts required for variable star work are compiled by
AAVSO and distributed thru its website. In early years they were
printed on blueprint paper and sold for about 10c each. Blueprint was
used for being the only handy way to make copies in the era before dry
photocopy machines. It had the benefit of looking very sexy, with
their deep blue 'night sky' background.
AAVSO has several scale of chart for each star to suit various
instruments and the amplitude of the star's variations. To satisfy the
needs of observers, a supply of each chart was on hand. If one was
depleted, AAVSO ran out to a printing depot and cut more blueprints
from a tracing paper original. The charts were snailmailed to the
observer in a flat envelope.
With plain paper photocopiers in the mid 1960s, duplicating the
charts as needed was done in house, saving the cost and hassle of
outside blueprint services. The major change for the observer was that
the chart had black stars on white background. One of the earliest
examples of this new chart was for nova Delphini in 1967, which had to
be issued in a hurry to catch the star while still active.
Until the website was established, AAVSO had cabinets of these
charts, idly waiting for a request. With the website the charts were
converted to GIF and PS format and placed in directories for each
star. The observer picks the chart specs from a webpage and within
seconds the chart is ready to download or display.
But there still was the inventory problem of all those graphic
files on disc. a project of AAVSO is to eliminate the inventory of
charts and create each chart on the fly as requested. The idea is to
draw the chart from suitable catalogs, fill in the lines, header
block, labels, and then hand it to the user for download or display.
The advantage to this scheme, besides saving immense amount of
disc space, is that the chart base data is always up to date with the
current catalogs. The paper and computer graphic chart was frozen as
at its drawing date. To update it, you had to revise the original and
upload it to disc. Or pull out all the old paper copies from the
cabinet and replace them with new ones.
The new chart-on-the-fly will replicate the appearance of the
standard AAVSO chart and is restricted to just the area around a
variable star. You can not pick an arbitrary celestial spot for a
chart. or, yikes!, tile the whole heavens to build a master star
atlas.
Tests so far are going well. The response time is hardly longer
than the fetch time for the current chart-from-inventory. However,
there are a few glitches to work out. It is anticipated that the new
system will be in operation by the 2006 fall meeting.
Johnson-Cousins filters - Erne Henden
-----------------------------------
The magnitude now used is a redimension of the stellar magnitude
scale by Johnson in the 1950s. To realize the photometry, Johnson
selected a set of Shott filters to isolate the near ultraviolet, blue,
and visual (yellow) part of the spectrum. These when placed in the
optical path in front of the photograph plate, enabled a standard
method of UBV colorimetry.
It quickly replaced the Harvard Photometry, which caused
artificial changes in the magnitude ratings of many stars. SOme
astronomers were fooled to think that the stars either altered their
brilliance or were mismeasured in the past.
In the 1960s Cousins added a filter for the red, completing the
present set of UBVR filters for colorimetry. In the 1970s, an other
set was selected to match the spectral sensitivity of photomultiplier
tubes, so they would yield magnitudes calibrated to the photographic
standard.
By the 1990s these filters were no longer made by Shott.
Observatories relied on legacy sets, which gradually dwindled by
breakage or loss. In some cases, replacement filters were eyeballed
from prevailing offerings. They are not suitable for calibrated
colorimetry or photometry. In fact from off-the-shelf items there is
no Johnson-Cousins equivalent.
Photometric filters for home astronomy are close but not
calibrated well enough for rigorous work. It is not at all proper to
use RGB color separation filters, a common accessory for home
astronomy.
Under study and experiment are currently available filters,
overlaid with coatings, to match the transmission profile of the
Johnson-Cousins set. In addition, the filters for U, B, and V will
have a red-infrared blocking coating so they can be used on CCDgraphs.
Nothing on the street yet, but results are so far encouraging. The
tests are about complete so that within a year a new set of Johnson-
Cousins equivalents can be announced.
Value of visual observing - Elizabeth Waggen
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Visual observing is sometimes criticized for not being precise,
pinning at 1/10 magnitude leeway. CCDgraphs can extract a magnitude
rating to the millimagnitude! However, this extreme precision is
filled with errors, often not recognized during the observing session.
They may sometimes be latent for years after the record is
incorporated into the AAVSO database. These errors degrade the
CCDgraphy to the same 1/10 magnitude precision as visual observing.
Yet this seemingly low precision is quite adequate to bring out
new and wonderful features of variable stars. They also are thoroly
adequate for supplementing observations in other parts of the
spectrum, whether from ground or space observatories.
The example presented was the outburst of U Geminorum in 1985.
This is an erupting star with irregular outbursts. I with other
AAVSOers witnessed one in a similar star, SS Cygni, at the 1997 fall
meeting of AAVSO in Chicopee, Massachusetts.
The key to digesting visual observations of 1/10 magnitude
precision is the large number of the points collected. By ordinary
statistical operations, simple to perform on the data in digital form,
U Geminorum was found to have irregularities in the up and down swing
of the outbursts. These were first observed by satellites, but theory
called for them to show up in the optical zone, too.
There is one little problem, one that can be fixed for special
occasions by instruction. The Julian day, the count of days used for
variable star work, is routinely given to the 1/10 day, or 2.4 hours.
For a rapidly changing star like U Geminorum in outburst, the time
resolution must be to the minute or so, or three places of Julian day
decimal. This time blurring smeared out the bumps somewhat, tho,
fortunately, not to mask them.
Observing from light polluted town - Gary Poyner
----------------------------------------------
This talk was a treat! Poyner could not attend the meeting, so he
emailed his digital slideshow to AAVSO and spoke to us by web
telephone! He lives in Birmingham, England, which he described as so
badly grayed or 'oranged' over that only 2nd or 3rd magnitude stars
are seen by eye. Yet he does good variable star observing with an
open-frame reflector in his garden. His 'hood is infested with
security lights.
The goal is to keep stray light out of the eyes and telescope and
to get the maximum number of optical path rays into the image. He sets
up cloth-on-wood-frame baffles around his scope to block the nearby
direct lights. These are taken down by day to leave the yard looking
as normal as the others around him. Covering the head with a black
hood also keeps local lights off of the eyes.
Use a cloth shroud around the open-frame tube. This is a common
accessory for American scopes, but his is homemade. He notes that the
cover has enough circulation to let the scope cool down before
observing, unlike a solid tube that may require an hour or so.
Be seated and comfortable. Any excess strain of the muscles
detracts from concentration and eye sensitivity. Be well nourished and
rested. A hungry or tired observer will not perform well even under
optimum sky conditions.
Keep the optics clean of dirt, soot (England houses still use
coal), industrial pollution. This reduces lost image quality from
absorption and light scattered into the eyepiece. Parallel to this
tactic is the strategy of getting the best optics you can afford, even
if it sacrifices aperture. This applies to both the primary elements
in the tube and eyepieces. Good optics put more of the rays into the
central disc of the image, less in the diffraction rings.
Turn off the clockdrive to catch very faint stars. The eye is far
more able to spot a dim moving point than one fixed in the field of
view. This is good for studying a faint variable while leaving the
field stars in sight. Tapping the eyepiece jiggles every thing in the
field and distracts the attention.
I asked about the neodynium filter touted for eliminating certain
light pollution. These are used in arts and crafts to filter out the
sodium flame in torches or kilns so the artist can inspect the
workpiece. Some British observers claim there is a dramatic
improvement where the illumination is from low-pressure sodium lamps.
Alas, in Birmingham the light pollution is so varied that this
filter doesn't work. It removes only the low-pressure sodium, the D
lines, the same emission as sodium in fire. It passes thru every other
part of the visual spectrum. There so much other radiation in
Birmingham's illumination to pass right thru the filter.
Continuation
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This is the third of four articles about the AAVSO 2005 October
convention. The articles are named 'aavso05a.htm', '...b.htm',
'...c.htm', '...d.htm'.