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Going to the Limit.

Results from the Deep-Field Challenge are in: amateurs stand at the shores of a brave new world.

Last spring Canadian amateur Paul Boltwood obtained the CCD image reproduced here, which records objects as faint as magnitude 24.1. This is amazing when you consider that before the age of CCD cameras the deepest photographs with the venerable 200-inch telescope on California's Palomar Mountain reached stars only down to about visual magnitude 23. What makes Boltwood's accomplishment even more phenomenal is that it was done with an unguided 16-inch reflector in a suburban Ottawa backyard.

Boltwood made his image in response to the Deep-Field Challenge I announced in this magazine's May 1998 issue (page 119). I asked amateurs to make the deepest possible image they could of a small field in the constellation Serpens. Of the seven amateurs who submitted images, Boltwood recorded the faintest objects and is thus crowned reigning King of the Deep Sky.

The faintest stars in Boltwood's image are recorded with a signal-to- noise ratio (S/N) of 3:1. This is considered a standard threshold for detection even though objects at this level don't look very impressive to the eye. If I tried to measure brightnesses in this image to an accuracy of better than 10 percent, I'd have to restrict my measurements to objects that are magnitude 22.8 or brighter. These limits are based on two independent calibrations made using the visual- magnitude (V) system. Because Boltwood's CCD exposures were made without filters, the transformation from unfiltered magnitudes to visual magnitudes depends somewhat on the color of the star in question.

I compared Boltwood's limiting magnitude to the value predicted by my program published in last May's issue (and also available on the Sky & Telescope Web site at Calculations made using Boltwood's circumstances suggest that a 24.1- magnitude star near the zenith should have an S/N of 2.5, which is satisfactorily close to the observed 3.0, with the difference likely due to uncertainties in the CCD's quantum efficiency.

Among the fainter sources in Boltwood's image, few are ordinary stars. This is because even the feeble K and M dwarf stars in the tenuous outer regions of our Milky Way's halo still appear much brighter than the image's limit. Almost all the faintest objects in the image are normal galaxies with typical redshifts of 0.5 to 1.0. Depending on the cosmological parameters you use, this corresponds to distances greater than 8 billion light-years. Thus Boltwood's time machine is looking back across a significant part of the universe's age. Each galaxy (long ago and far, far away) contains tens of billions of stars, perhaps tens of billions of planets, and an unknown number of civilizations, some of which might be staring back at our own Milky Way.

As I stated in the challenge, the target field was selected because it is the location of a gamma-ray burst that occurred on January 11, 1996. The burst position was identified with data collected by spacecraft, including the BeppoSAX Wide Field Camera and the BATSE and Ulysses detectors. No optical or radio counterpart of the burst was recorded, but it is very likely that the event originated inside this field, and that a deep image would show the host galaxy.

The ongoing study of gamma-ray bursts is fueling a debate as to whether the distance to a typical event is defined by redshifts of 0.8 to 4 or beyond. Boltwood's image provides meaningful limits, since the brightest galaxy in the region is magnitude 22.3. If the 1996 burst luminosity was near the lower end of the debated range, then the distance must be only 211/42 billion light-years. If this is so, then the galaxy must have an unusually low optical output (within the bottom 3 percent). If, on the other hand, the burst luminosity was near the upper end of the debated range, then the derived distance would be greater, and a host galaxy of average brightness would appear consistent with those recorded by Boltwood. This implies that either the burst had a relatively high luminosity or the host galaxy is anomalously subluminous.

Why can today's backyard observers venture into territory that was once outside the limits of even the 200-inch telescope? The answer, of course, is CCDs. These detectors are typically tens of times more efficient at collecting light than photographic emulsions. They also have a linear response, which allows "stacking" many images to produce one with a long effective exposure. These are big advantages and they allow today's backyard telescopes to surpass the best that the previous generation of observatory instruments could muster.

Professional astronomers have also taken advantage of CCDs. My program predicts that very long exposures with one of the Keck 10-meter telescopes in Hawaii can penetrate to a depth of about 28th magnitude. The well-known Hubble Deep Field image gets to around 30th magnitude with a roughly 10-day exposure, since its space-based vantage offers diffraction-limited seeing and a low sky background.

Boltwood's breathtaking achievement was in less-than-optimum conditions - suburban light pollution, eastern skies, a moderate-size telescope, and no guiding. It serves as encouragement to other amateurs, since it proves that deep images are possible by those who possess reasonable dedication. No longer is the deepest sky the exclusive realm of professional telescopes.

In science, repeatability is a key point for recognizing trustworthy claims. In modern times, with much frontline science requiring expensive equipment, the cost of double-checking results can be prohibitive for professionals. The power of the CCD camera, however, has opened us to a world where astronomical claims can be tested by anyone with moderate equipment and reasonable skill. Today's amateurs can create color-magnitude diagrams and estimate the age of open star clusters by determining the turnoff point of the main-sequence stars. They can reproduce Henrietta Leavitt's period-magnitude relation for Cepheids in the Large Magellanic Cloud - an important yardstick for measuring the scale of the local universe. Amateurs can even derive the Hubble constant from measurements of distant Type Ia supernovae. Long- term projects include measuring the expansion age of the Crab Nebula, the proper motion of Barnard's Star, the rate of precession of the equinoxes, and changes in the amplitude of Polaris's brightness oscillations.

There's also cutting-edge science in the offing. Amateurs can use CCDs to monitor the brightness of field stars in a search for rare transits by orbiting planets. Long exposures allow amateurs to follow the shape of a supernova's light curve long after professionals have run out of telescope time. They can look for periodicities of young stars in scattered clumps throughout Orion to find the age at which their rotations begin to slow down. And backyard observers might discover whether the moon Nereid changes its rotation state chaotically after close passes to Neptune. With the soon-to-be-launched HETE2 satellite and potentially rapid notification of positions of gamma-ray bursts, amateurs will have an opportunity to capture the fading optical counterpart of a burst. They can look for light echoes around novae, Earth-crossing asteroids, or super flares on normal solar-type stars.

Don't think that any of these projects will be easy just because CCDs make them possible. There will be much trial and error, tedious exposures, complex calibrations, subtle systematic effects, and complicated mathematical analyses. But amateurs have already proved they are up to these tasks.

Boltwood's picture is a celebration of the power of the CCD in the hands of a backyard observer. It is a reminder that amateurs can help discover the deepest truths in our sky.

Bradley E. Schaefer is an astronomer at Connecticut's Yale University. His frequent contributions to this magazine cover topics as diverse as sundials and gamma rays.
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Title Annotation:astronomical photography
Author:Schaefer, Bradley E.
Publication:Sky & Telescope
Date:May 1, 1999
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