The Hubble Program
By Mac Gardiner
Why Hubble?
For many years, astronomers have wanted to study celestial objects that are further away and dimmer. The finite speed of light means that all we see coming from further objects are emitted at an earlier date, and it turns out that cosmic events in those earlier times are more interesting and beautiful than recent ones. On Earth, the limitation has been the background brightness of the sky, and the shimmer of the image caused by differential refraction through the Earth’s atmosphere. A location in space eliminates both, and permits an order of magnitude improvement in resolution and sensitivity. The exciting new development of Charge Coupled Devices permits astrophotography to be coupled with telemetry downloading, to permit unlimited use without film replenishment. It was felt that the system could be made to last as long as ten years in orbit.
History
At last NASA had a program that would be a worthy successor to the Apollo Moon program. The cost appeared to be about right, $2 billion (the cost of the Manhattan project). Other countries would contribute equipment, instruments, advice and cash. In return, they would receive (reluctantly) recognition, and, more valuable than gold, guaranteed time on the Hubble, with priority on celestial objects, including the right to delay distribution of raw data until after publication.
What was built was actually an observatory with five telescopes, including a Wide field/planetary camera, a faint object camera, a Faint object spectrograph, a high resolution spectrograph and a high speed photometer. All shared the same primary and secondary mirrors, but occupy separate areas in the image plane.
The Challenger catastrophe in 1986 added four years to the schedule delay of three years (of 20 years), and this was used to clean up the Hubble. However, the 94.5” mirror system was never “system tested” end to end.
The big mirror was ground so accurately that a ground test to verify that accuracy would have cost tens of millions of dollars and subjected the system to possible damage from manipulation. Senator Gore had suggested that it be given a quick “Sea Trial” end to end test but his advice was ignored. In retrospect, the simplest of Foucault tests (flashlight and razor blade!) would have shown that the system had spherical aberration to a devastating extent.
The launch and deployment of the Hubble was technical and political hype at its utmost. Never had so many millions of people waited so breathlessly and confidently to see the “First Light” images. They waited and waited. After hiding the fact for about a year, NASA finally admitted the problem. The technicians simply couldn’t focus the system. There were other problems, bad ones, but correctable or capable of being worked around. All of the positive feelings of the press and the public turned into ugly, negative ones, and Congress nearly killed the program. The scientists didn’t want to use their valuable “priority time” on a weakened system. There was concern about suicides by key personnel.
Finally, the scientists got good pictures and data and made discoveries. The resolution was fantastic, as it was possible to “deconvolute” and remove the defocused “halos,” but the process meant throwing away 85% of the precious photons and thus losing “magnitudes” of sensitivity. One could discern lovely details, but couldn’t probe into the distant dim past. The glass was then half full, or half empty.
The first repair mission, undertaken a year ahead of schedule, installed a corrective eyeglass for the myopia, at the cost of one of the camera systems, and restored most of that vital sensitivity. Also, it and a subsequent mission replaced failing gyros, and attempted (unsuccessfully) to correct vibration problems caused by the fluttering solar power panels.
The result was a flood of discoveries and solid data that vindicated the investment, and restored the Hubble to the preeminent position it currently holds. It still is a cranky system, calling for the coordinated action of hundreds of people, at a cost of $250 Million/year. Like the venerable DC3, it has its faults, but the personnel know what to do to compensate.
This last mission is different. Finally, instead of fixes, we are getting upgrades! The “Advanced Camera for Surveys” has both a wider field of view and higher sensitivity, particularly in the blue end of the spectrum.
The “window shade” solar panels have been replaced by smaller solid panels of higher efficiency, meaning more power, less drag, and the fact that ten minutes of vibration at each crossing of day to night (45 minutes) is eliminated.
A satisfactory non-replenishing cooling system for the Near Infrared Camera and Multi-Object spectrometer means the availability of those essential tools for the rest of the mission
What is unique and important about Hubble?
There are several ways to address this question. The simplest is shown in the figure below
What this says is that man could see detail down to about
1.5 arc minutes of arc all the years up to Galileo in the 17th century. He jumped man’s effective light resolution up to around 5 arc seconds and got himself house arrest for the remainder of his life. Herschel gave resolution a ten percent kick 200 years later with his big refractors, and Palomar gave it another nudge of a few percent toward 1 arc second in the 1930’s. Then Hubble came along and raised it 10/1 to 0.1 arc sec! Cold numbers follow. The NASA spec said:
Given the following:Nominal Parameters of Hubble Space Telescope
Primary D = 2400 mm, R' = -11040 mm, f/2.3, K~ = -1.0022985Secondary R2=-1358mm, K2=-1.496Overall m = 10.43 5, '3 = 0.2717, k = 0.1 11 2, scale = 3.58 arc-sec/ mm = 279 ~m/ arc-secThe image quality must meet the following requirements:The optical image, including effects of optical wave-front error, pointing stability, and alignment of the scientific instruments to the Optical Telescope Assembly, should satisfy the following on-axis requirements at 6,328 angstroms and be a design goal at ultraviolet wavelengths: Image resolution using the Rayleigh criterion for contrast of 0.10 second of arc. A full-width half-intensity diameter of 0.10 second of arc, 70 percent of the total energy of a stellar image must be contained within a radius of 0.10 second of arc. After correction for astigmatism, these specifications shall apply to the image quality over the entire usable field of Space Telescope.
This has been met, though it took more than a little time and effort to achieve it.
How sensitive is it? Again, given the following:
Detector and Telescope Parameters
Detector ? = 15 µm, R= 10 counts/pixel, C = 0.01 count/(pixel sec), ? = 0.8, Q = 0.5
Telescope t = (0.9)2 = 0.81 Filter t = 0.8, ?? = 100 nm (V-band) Relay optics t = 0.5 Other m´ [mag/arc-sec)2] ø (arc-sec) F
WFPC 23 0.2 2.9 Ground 22 1.0 2.5 It should be this good:
Sensitivity
Left side: (SNR) Signal Noise Ratio Bottom: Celestial Magnitude (m)
What this says is that the current system can get adequate signal/noise ratios of celestial imagery down to 28th magnitude stars, given a single exposure maintained for the duration of time that the earth could not occlude the image. That is significantly better than what is obtained with a 15 meter (49.2 feet) diameter terrestrial reflector under optimum conditions! Then the Hubble guys took successive multiple images in those delicious dark skies, stacked them and went way out further!
So what is new?
The key factor of gain is in the Quantum Efficiency of the CCD detector element, rising from 0.5 to 0.85, with a major gain in efficiency in the blue region of the spectrum. This change, in an element 2 inches on a side, is the equivalent of increasing the big mirror area by 70%. This results in much shorter exposures, and a corresponding increase in the throughput of the telescope system. The other change is (Cont. on p. 8) the increase in the total pixel count on the detectors. This gives more serendipity in the collateral images obtained, and also reduces the incredible precision required to acquire and track the dimmest images. Right now, the faint object camera has 0.02” resolution within a 11” field, implying that acquisition cannot be assured unless the system is pointed to within 5” of dead center.
So all looks good, but still, the proof is in the pictures taken, and here the new system is showing itself off in grand style.
The first breathtaking pictures have been released. We have every right to expect many more beautiful pictures. We also expect exploration into much earlier eras of cosmic development. We can now access the NASA Web site and browse through the beautiful, incredible images that are now pouring out. Just use the following: <http://hubblesite.org/news> and <http://hubblesite.views/pr.cgi.2002+11>. As Diane Colvin emailed: “Wow”.
And we wonder if they mean it when they say that the Hubble’s life is coming to an end. Will we have the “Next Generation” telescope ready by that time? Will the ISS Amateur Telescope be the only picture taker in space?
Keep up your membership in BPAA, attend Paul Middents’ lectures, read the BPAA Newsletter to find out. And, hold your breath for the next 6 years.