INSA y Herschel Science Centre,

1
Catalogues for Asteroid Photometry Fact and
Myths

• INSA y Herschel Science Centre,
• European Space Astronomy Centre,
• European Space Agency Villafranca del Castillo
  Satellite Tracking Station, Madrid

2
An Astronomers Task

• Basically, all observational astronomy is simply
  a matter of counting photons, be they visible,
  infrared, gamma rays, or radio.
• We are trying to do one of two things
• See the two-dimensional distribution in space of
  the photons (take an image), or...
• Count how many photons arrive per unit of time
  (take photometry).

3

• The concept of astronomical photometry dates back
  to Ptolemy in the 2nd Century AD.
• Ptolemy divided the stars into 6 degrees of
  magnitude from the most brilliant magnitude 1
  to the faintest visible to the naked eye
  magnitude 6.
• It is Ptolemy who we should thank/curse for
  introducing photometry and photometric
  calibration.

4

• All modern photometric systems are based on
  Ptolemys original one.
• With a few exceptions, the magnitudes given by
  Ptolemy agree with modern photometry to the limit
  of accuracy of his estimates.
• Today the magnitude scale is defined
  matematically such that a difference of 5
  magnitudes is a factor of 100 in brightness.
• In other words, a difference of 1 magnitude
  represents a factor of 100.4 in the irradiance of
  a detector.

5

• Originally, the magnitude scale only had to cope
  with a range of 3 orders of magnitude in flux
  the difference between the brightest and faintest
  stars visible to the naked eye.
• Now, it has been extended over a range of 55
  magnitudes (11 orders of magnitude in flux) from
  the Sun magnitude 26.7 to the faintest
  objects detectable with the HST.
• And it has been extended to other bands than just
  the visible.

6

• The modern CCD astronomer can reach magnitudes
  close to (and sometimes beyond) what was the
  limit of the largest optical telecopes 30 years
  ago.
• He or she needs to be able to calibrate
  photometry down to at least magnitude 18 and
  preferably fainter.

7
The trick is to know how to calibrate the
photometry correctly Badly calibrated data is
useless If you do not know that it is bad, it is
misleading
8
What is the aim of calibration? It is to give a
magnitude In a known system or one that can be
converted to one. It should give a realistic idea
of the error in the zeropoint.
9
The Photometrists Commandments

• Be systematic
• Whatever you do. However you calibrate, being
  careful and systematic is better than being
  fancy.
• Understand your limitations
• If you have s/n10 you cannot EVER calibrate to
  better than 10 and will be lucky to manage 20.
• Dont claim that you are accurate to 1 when your
  systematic errors are at least 10 times larger.
• A large but honest error is far more believable
  than a tiny and unrealistic one.
• For a single night in good conditions, good
  equipment and with careful calibration the limit
  to accuracy is about 3.

10
The Photometrists Commandments

• Be realistic size does matter!
• Use the smallest aperture consistent with your
  conditions.
• For absolute photometry, about 4 times your
  seeing.
• For relative photometry 2-3 times your seeing.
• And remember that aperture kills
• The sky background in a 20 aperture in average
  amateur conditions is equivalent to a star of
  magnitude 11.2.
• For a 30 aperture the sky is equivalent to a
  magnitude 10.3 star not ideal if your asteroid
  is magnitude 19 and just 0.01 of the integrated
  brightness of the background.

11
Tread Carefully With the Sky Lest it Tread on You
1000 observations of 29P/S-W1 by 30 observers
over 6 years
12
The Photometrists Commandments

• Thou shalt spend a minimum of 30 of your time
  calibrating
• The average amateur astronomer rarely has
  genuinely photometric conditions
• Even in clear sky, aircraft contrails are
  non-photometric
• You should either take very frequent calibrations
  with nearby stars and calculate the dispersion in
  the measures, or
• Take relative photometry against field stars
• It does not matter how accurate your calibration
  stars are if the atmosphere is unstable and the
  calibrations infrequent.

13
What do we know about this?

• Since 1990 the group in Tenerife has dedicated
  itself to high-precision photometric calibration
  of stars and fields in the visible and infrared.
• The aim is to produce a definitive calibration
  for the Spanish 10-m telescope (Fabiola Martins
  PhD thesis).
• More than 200 nights of observation
• Almost 80 000 measurements of stars from
  magnitude 7 to 19.
• Median uncertainties on magnitudes around
• 0.003 magnitudes (near-IR)
• 0.008 magnitudes (visible)

14
A sample of our data
s 0.012 mags 50 within ?0.007 mags Median
zero point difference 0.002 mags
15
Like it or not size matters!
Limited by flat fielding, calibration standards,
atmosphere, etc.
Limited by photons (telescope size)
82-cm IAC-80 Telescope (Teide Observatory)
16
Available resources for amateurs

• Tycho
• A scanning instrument on the Hipparcos satellite
• 2 539 913 stars, almost complete to V11
• Good sky coverage
• Average 130 measures per star
• Typical error 0.05 magnitudes per measure (bright
  stars)
• Magnitudes in B and V
• The Tycho-1 catalogue had significant issues with
  it and should not be used

17
The Myth of Tychos Infallibility (I)
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The Myth of Tychos Infallibility (II)
19
Use and Misuse of Tycho

• For best results
• Use only V
• Stick to stars with V lt 9
• Select stars with small colour index
• Use only nearby stars (within 1 degree)
• Take several measures before and after the
  observation to be calibrated to measure the
  dispersion in the calibration.
• Unless conditions are exceptional, calibrate at
  least every 10 minutes.
• Stay well above 30º altitude.
• For really accurate results, transform the Tycho
  magnitudes to Johnson V.

20
Landolt Stars

• 756 stars with very accurate UBVRI photometry.
• Magnitudes 6 lt V lt 17.
• Published between 1973 and 1990
• Landolt, 1973, AJ, 78, 959
• Landolt, 1983, AJ, 88, 439
• Landolt, 1983, AJ, 88, 853
• Landolt, 1992, AJ, 104, 340
• Landolt, 1992, AJ, 104, 372
• Generally acknowledged to be the very best
  astronomical photometry for calibration
• To say calibrated with Landolt stars is a mark
  of quality.
• Landolt is the de facto standard photometric
  system in the visible.

21
Limits on Landolt

• Almost all the stars are in the celestial
  equator.
• Usually there will not be a star close by.
• This limits Landolt stars to use in genuinely
  photometric conditions.
• We generally observe some 60-70 Landolt stars
  each night to define
• Transparency and instrumental sensitivity changes
  with time.
• Colour transformations
• Unless we can observe for at least 3 hours and
  take multiple calibrations we do not bother to
  try to calibrate.

22
Care with Landolt Stars

• Some Landolt fields have numerous calibrated
  stars
• Multiple stars with different colours and
  magnitudes can be observed simultaneously.
• However, a number of these fields have only a
  single night of data
• Use with care and check the number of
  observations used to give the calibration
• Some fainter stars have relatively large errors.
• A single Landolt star/field is never sufficient
  for calibration.
• Always use several fields to check the zero point
  of your calibration.

23
The Photometrists Burden

• The sad reality for most astronomers who do not
  have mountaintop observatories in good sites, is
  to have to cope with
• Variable atmospheric transparency
• Aircraft contrails are definitely cloud.
• Variable seeing.
• Both on short time scales.
• In other words far from photometric conditions
  on the vast majority of nights.

24
One Catalogue Does Not Fit All

• The most professional solution is to do a full
  calibration with Landolt stars
• The most accurate and reliable
• But only if you have completely trustworthy
  conditions.
• The more paranoid you are, the better you will do
  it!
• Do it well or dont do it at all.
• If the night looks good, but you are not
  completely sure (perhaps a little cirrus, a
  little haze, or aircraft), a local calibration
  with Tycho stars
• Its not as reliable, not as accurate, but it is
  good enough, if done properly
• Calibrate every few measures and check for
  consistency or you will never know how reliable
  your data is.
• Select your Tycho star with care.
• Otherwise, swallow your pride and

25
Go Relative its often better in the long run!

• A relative calibration is the simplest and is
  often almost as good.
• Calibrate against field stars that are already
  there.
• You forget the atmosphere
• You can do it even if it is cloudy
• Variable seeing does not matter
• Quasar and variable star observers have done it
  for years.

26
Just a Small Problem

• We need a high-density, accurate photometric
  catalogue that reaches at least magnitude 16.
• No such beast exists.
• The nearest available catalogue to this, at
  present, are the USNO products
• USNO A2.0
• USNO B1.0
• UCAC

Millions of stars to magnitude 16-21 Very high
density Sadly, not high photometric quality
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What is USNO?

• USNO is a digitised, recalibrated, updated
  version of the Palomar Sky Survey.
• Astronomers spent nearly 50 years breaking their
  hearts trying to calibrate the POSS plates
  photometrically.
• Tycho has changed all that
• Each POSS plate has an average of about 60 Tycho
  stars.
• The bright end of USNO is tightly tied down by
  Tycho.
• The faint end is fitted by a multi-parameter fit.

28
USNO R magnitudes are actually not so bad
To magnitude 18 USNO R is within 0.2 magnitudes
of Landolt
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What is USNO?

• Of the 4 USNO products (USNO A2.0, USNO B1.0
  1st epoch, USNO B1.0 2nd epoch UCAC), by far
  the best is USNO A2.0.
• 38 of USNO A2.0 stars have an R magnitude
  accurate to lt0.1 mags.
• 62 of USNO A2.0 stars have an R magnitude
  accurate to lt0.2 mags.
• 78 of USNO A2.0 stars have an R magnitude
  accurate to lt0.3 mags.
• After transformation, the typical error on the
  USNO A2.0 magnitude is 0.20 mags in B, 0.17 in
  V, 0.15 in R.
• If you have 50 stars in your field, who cares?
  Use the best ones and throw the worst ones away!
• But, getting a good s/n is important for
  reliability.

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Does it work?
9P/Tempel 1
Dispersion 0.15 magnitudes (23 observers) Clearly
shows the 4.5/9 day period in outbursts,
published in The Astronomer and on an IAUC and
later discovered by the HST/Deep Impact.
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Does it work?
32
Relative or absolute?

• Absolute calibration
• Do it well or not at all.
• Not suitable for most sites or nights
• Tycho stars
• It can work very well, if done properly
• Short cuts make it unreliable and lose accuracy.
• Relative calibration with USNO
• The only realistic solution for the urban
  astronomer.
• Uses all the stars in the field to beat down
  errors.
• Can end up being of similar quality to Tycho.