Some candidate objects for all-sky photometry projects, 1998 Autumn

• Star clusters - Open clusters
  Magnitudes and colors of open clusters can be analyzed for distance and age of the cluster using a ``color-magnitude diagram'', a type of ``H-R diagram'' for star clusters.

                      V of      approx.   contains
                      brightest limiting  red
  object        NGC   member    V needed  giants?  notes
  ------------  ----  --------- --------- -------- ------
  M45 Pleiades           2.9       8      no
           M34  1039     7.3      11      three
    chi Persei   884     6.4      13      five     half of Double Cluster
      h Persei   869     6.5      13      one      other half of Double Cluster
          M103   581     7.2      14      one
  
  

  The column ``approx. limiting V needed'' is a crude estimate of how faint the photometry needs to go in order get about 0.2 mag. of change in B-V along the cluster's main sequence below the turn-off.

• Unfortunately for fans of globular clusters, even the brightest well-placed globular cluster for us (M15 in Pegasus) is too faint (mag. 14-20) to yield useful results with our equipment.

• Asteroids (1998 Autumn)
  Calibrated V magnitudes of an asteroid obtained over a range of solar phase angles can be analyzed for the "magnitude-phase" dependence of the asteroid, and yield values for its phase function parameters H and G. Note however that the success of the analysis critically depends upon getting brightness measurements of the asteroid over as wide a range of solar phase angles as possible.

  For an asteroid the longer-term phase-induced brightness changes are entangled with short-term rotation-induced brightness changes. Understand that a result disregarding rotational phase is a perfectly good project goal, but that it's almost certain not to agree with published values from which the effects of rotation lightcurve have been removed.

                           predicted  predicted max.                        
                             mean     chg. in V mag.                          
      object       opp'n     V mag.     10/1-11/9     ephemerides
  -------------    ------  ---------  --------------  -----------
  (161)Athor       Sep 29  11.3-12.6  0.76            Geocentric
  (423)Diotima     Nov 07  11.7-12.4? 0.50?           Geocentric
   (69)Hesperia    Nov 09  10.7-11.8  0.48            Geocentric
  (804)Hispania    Sep 10  11.0-12.2  0.39            Geocentric
  

  "Full Disclosure" notes on these asteroids:
  • (423) Diotima - The G phase coefficient is unknown so the predicted brightnesses are uncertain. The rotation period is also uncertain, so one or more possible ``alias'' periods haven't yet been ruled out.

    Diotima is an intrinsically more risky project target object because the phase function could be too shallow for us to measure even with a successful observing plan. Writing up a negative result takes a bit more effort because you need to be very careful with error analysis at every step, so that you can precisely specify at what level you didn't detect the sought-for signal.

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Last modified 2005 Oct 16