Low Scattered Light - Faint Surface Photometry Problems

S. M. Simkin ( 11 September, 1997)

There are many observational problems which require the accurate measurement of low surface brightness features. These include:
  1. Searching for faint dust comae in distant comets:
  2. Measuring q and lambda from observes scale changes
  3. Imaging the ISM surounding stars in formation:
  4. Identifying wakes in the ISM associated with ``run away'' stars:
  5. Low surface brightness measurements of galaxies:
  6. Discovery and measurement of difuse stellar light in clusters of galaxies:
  7. Discovery and measurement tidal tails for interacting galaxies:
  8. Low surface brightness measurements of optical synchrotron emission from supernova and radio galaxies:
Much of the discussion in the science working group centered around the structure of an ``ideal psf'' (either instrumental or in the presence of seeing.) However, contributions to the scattered light and spurious background noise from reflections, forward scattering, and direct incidence of starlight (or worse) on the imaging ccd are the effects which determine the irregular background which limits low surface brightness measurements in most of the examples above.

The type of background contamination which is important for these problems is, by its nature, not as amenable to simple analytic discussion as the instrumental psf. Nevertheless, it is at least as important if not more important than having a clean psf.

To estimate the level of baffling needed from scattered light, I have picked three examples from the literature which seem to have achieved the limits of present low surface brightness measurements. The most stringent is from a recent preprint by Trentham and Mobasher measuring stellar debris in the Coma Cluster of galaxies ( http://xxx.lanl.gov/form/astro-ph/9708248). The others are from earlier work (by SMS) on optical synchrotron emission in radio lobes (Ap.J., 309, 100, 1986) and faint steller emission from the disks of Seyfert galaxies (Sci, 235, 1289, 1987).

The first reference shows convincing evidence for faint, continuum emission at the level of 27 mag/sq arc-sec (in B). The other two references discuss features at the level of 25 and 26.5 mag/sq arc-sec which correspond to similar morphology in the radio region, thus implying that these are ''real''. If we take a value of 27.5 mag/sq arc-sec as our goal (ie we wish to have reliable surface photometry at this level) and limit the allowable rms from background scattering to roughly 10% of this value, we find an upper limit for scattered light of 30 mag/sq arc-sec. (An alternative way to approach this is to aim for a limit which is roughly 3% of the intensity at the ``Holmberg radius'' (26 mag/sq arc-sec in B).

Translating this limit into telescope parameters is messy. We can achieve some idea of what to look for by the following calculation:

Given a star of magnitude, m,

If a is the area of a ccd pixel

and n**2 the number of pixels covered by one square arc second on the ccd.

The flux from the (non-focused) star directly illuminating one sq-arcsec will be n**2(a*10**{-m/2.5})

If A is the area of the telescope aperture

Then a sky brightness of 30 mag/sq-arcsec on this same (n**2) area will have a flux of:

A*(10**{-30/2.5}),

Equating these and solving for m For a ccd with 15 millimicron pixels of 0.1" each gives a rough value of m=10.5 magnitude.


Thus any stray illumination of the entrance aperture to the camera by a 10.5 magnitude star or brighter will compromise this type of work. Since stars of 10.5 mag and brighter are numerous, this is why present measurements of this type are so difficult and time consuming.

Another way of puting this is that the scattered light analysis which determines the necessary baffling must prevent direct illumination of the ccd by stars of 10.5 mag and brighter and atenuate secondary illumination (scattering) by at least a factor 5x10**(-5).

Along the same lines, For indirect illumination (eg. scattering from parts of the telescope structure), any surfaces which scatters even 1 % of the incident light, may introduce unacceptable contamination from stars of 5.5 magnitude and brighter. Since the cone of acceptance for this type of scattering is much larger than for direct illumination, the configurations needed to properly baffle the telescope will be complex (and probably can only be solved by lengthy model analysis and experimentation.)

Finally, an entirely separate problem when doing faint surface photometry is the random fluctuations introduced by ``ghosting'' from instrument filters and corrective optics. Any camara design which requires a field corrector must be examined very carefully for this type of effect (as must any filters introduced into the light path).
Susan Simkin (Simkin@grus.pa.msu.edu) last updated: 11 Sep '97