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Scientific asteroid images for astrometry and photometry need to record the body as a sharp dot without trailing. For slow-moving asteroids this may not be an issue, but fast movers require short exposures or setups that track the asteroid itself. This is especially useful for the high-cadence photometry necessary to determine the light curve, and hence spin-rate, of an asteroid.
For more general appeal, in outreach material for example, a fast-moving asteroid provides a convenient way to produce a trail that would otherwise take many extended exposures to capture. In this instance, a correctly polar aligned telescope tracking at the sidereal rate or, better still, autoguided on the stars, will produce a sharp star field with the asteroid as a light trail.
A similar effect can be created by aligning shorter exposures on the stars, and stacking them with the brighter elements set to show through. Many asteroids are within range of a basic telescope and DSLR setup. For scientifically calibrated work, CCD cameras, preferably with specialist filters are recommended. By using planetary imaging techniques, larger telescopes may even be able to capture larger asteroids as extended discs during favorable oppositions, rather than the usual star-like dot.
Use software to help you plot the exact position of small space rocks. Measuring the position of an asteroid is an important step in determining and refining its orbit. This is especially true for asteroids on eccentric orbits, which have the capacity to pass close to Earth.
Smaller bodies returning to the inner Solar System may have been gravitationally perturbed, leading to changes in the previously established orbit, and these need to be monitored. The astrometry of asteroids is similar to comet astrometry, with the exception that asteroids are somewhat easier to measure, appearing as singular dots of light without the complexity that accompanies the expansive head of a comet.
It is recommended that serious astrometric measurements follow the guidelines set out by the International Astronomical Union's Minor Planet Center MPC , available online at www. The basic workflow for the astrometric measurement of an asteroid is quite straightforward. First you need to obtain a set of images that include the object you intend to measure. Then you'll need some software assistance to measure the position accurately; the shareware Astrometrica is highly recommended.
Astrometrica allows you to 'blink' your images, which should reveal the asteroid moving against the static star field.
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The software will need to identify the star field in the images in order to determine the asteroid's position. You can help here by manually identifying the star field and supplying Astrometrica with the correct RA and dec. Once entered, the program attempts to match the star field. If it doesn't quite get things right, you can adjust the alignment manually. Astrometrica's star template can be adjusted for scale with a focal length used setting, for rotation with a position angle setting and positionally with an onscreen arrow key pad.
Once the alignment has been set, clicking on the object will generate an MPC compatible log file of positional data which can then be submitted according to the submission guidelines. Accurately plotting of the brightness and shape of distant asteroids is a team effort. Occasionally an asteroid will pass in front of a star, dimming the star's light as it goes. There are numerous programs available to predict such events as well as websites, such as Euraster, which presents results without you having to having to calculate them yourself.
A typical asteroid occultation path will be a narrow track and may require you to travel to a specific location in order to view and record the event. This adds additional complexity in that it requires the use of a portable observing and recording setup and a means to accurately calculate your location and altitude.
The modern way to do this is with some form of GPS recorder. One of the hardest parts of observing asteroid occultations is to locate the star that is going to be occulted. This can be done using a Go-To system, but you often need to use very accurate star charts to augment the process, especially when the star to be occulted is very faint.
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A common way to record asteroid occultations is with a low light video camera. The resulting video, normally recorded in the AVI format, can then be analyzed by specialist programs such as LiMovie or Tangra, which are both available for free. A successful occultation should produce a light graph for the star that shows it dim as the asteroid passes in front of it and brighten as the asteroid moves out of the way.
Accurate timing of the star's dimming will produce a line profile across the asteroid. Interesting though this is, such profiles really become useful when multiple observers record and communicate these events. With multiple profiles recorded, it's then possible to produce a more complete profile of the asteroid.
Obviously for this to be of any worth, a highly accurate time signal needs to be used.
Astro-Imaging Projects for Amateur Astronomers
Astro imager Will Gater explores the photo opportunities presented by the myriad spacecraft that can be seen speeding overhead through the night. Nightscape images that contain glinting Iridium flares or space stations have been a staple of astro-imaging for decades. For beginners, they're great targets to practice your skills on, and it's possible to get really striking images with a basic setup consisting of nothing more than a DSLR and tripod.
If you have a bit more experience, don't dismiss shooting a satellite or space station; even advanced photographers can find fresh challenges in experimenting with the framing and foreground of such photos, and in finessing the quality of the final shot.
Done well, these pictures can really spark the imagination in ways that other types of astro-images might not. The timing, brightness and location on the sky of any potential Iridium flares is dependant on your location, so — just as with ISS and other bright-satellite passes — in order to find out when and where one will be visible from your site you'll need to consult a website like Heavens Above www. Once you have this information you can set about planning your shot. The free planetarium software Stellarium www. By cross-referencing the Stellarium view with the information and star chart from Heavens Above, you can identify the path and position of whichever satellite you're aiming to catch and try out different compositions.
Shooting a series of consecutive to second exposures at a mid-range ISO with a DSLR, kit lens and static tripod will pick up most bright satellite passes. With Iridium flares, aim to start capturing images about 90 seconds prior to the predicted flare time and end the series about the same amount of time after the flare reaches its brightest; this way you'll capture a pleasing trail that slowly builds in brightness, peaks, then fades away.
You can then bring the series of images you've captured into processing or stacking software and combine them, so that the short trail in each photo joins the others to form a longer one. Since most satellites zip across the sky, capturing a series of photos from a static tripod will result in gaps in the final 'combined' satellite trail due to the short delay between exposures. To get around this you can mount your camera on a tracking mount and take one, much longer, single exposure. This requires balancing the exposure length — which will need to be several minutes — with the lens aperture, ISO setting and sky brightness, but can produce attractive unbroken satellite trails against rich, starry skies.
Remember, if you do this any foreground will be slightly blurred. One of the most exciting areas of satellite astrophotography to develop in recent years is imaging the International Space Station passing in front of the Sun or Moon. Imaging these 'transits' requires extensive planning, but the resulting pictures are extraordinary.
A typical transit might last seconds, — sometimes much less — and will only be visible from within a narrow strip of Earth's surface. If you intend to image a solar transit, where the space station is silhouetted against the disc of the Sun, you'll need to use a certified solar filter for your telescope and be sureto remove any finderscopes. Here are the key steps required to capture this thrilling phenomenon with a scope and DSLR camera. You may have to travel to be in a position to capture the event.
Use planetarium software to check where in the sky the Sun or Moon will be.
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Step 2: Setup Next set up your scope and have it track at the solar or lunar rate depending on your target. If you're imaging the ISS transiting the Sun, fit a specialist, certified solar filter and remove any finder scopes. Step 3: Focus and exposure Focus the view — use the terminator if viewing the Moon, or sunspot or the solar limb if viewing the Sun. Whether you capture stills or video, make sure that the exposure length is very short so that the ISS does not blur. Step 4: Capture video or a rapid burst of stills Start capturing video or a burst of stills as the moment of the transit approaches; that way if there is a slight error in your timing you'll still get the shot.
For a DSLR video use the highest frame rate that the camera allows. Step 5: Review, extract and process Review and process the frames from our video or still images that show the ISS. Then process and enhance the images.
If, like us, you remember fondly the days of NASA's Space Shuttle, you may well recall that on occasions the spacecraft and its — just-detached — external fuel tank would be visible passing over the UK shortly after launch. There was nothing quite like watching the rocket roar off the pad live on NASA TV then seeing the very same shuttle and orange fuel tank — both appearing as points of light; the orbiter appearing white, the fuel tank a subtle ochre tint — silently glide overhead. Though the Space Shuttle is no longer flying, there's still occasionally a chance to catch a similar spectacle thanks to one of the new generation of ISS-servicing spacecraft: SpaceX's Dragon capsule.
Whether you'll be able to see the capsule on its way to the ISS just after lift off depends on the conditions of its launch. For the capsule to be visible, it needs to be dark or deep twilight in the UK, but the Dragon itself has to be in sunlight as it flies over. Helpfully, the CalSky website www.
The pass you want to look out for — if it's listed — is the one that's about 20 minutes after the expected launch time, as that'll be the Dragon making its first flyover after departing the Florida coast. It's worth keeping an eye on either the NASA TV or SpaceX online video stream that usually accompanies the launch too, as it'll let you know if the lift off gets scrubbed. One of the things that's so exciting about catching the ISS-bound Dragon just after lift off is that, from here in the UK, it's not just the capsule you get to see.
Dragon is propelled into orbit by a SpaceX Falcon 9 rocket, and the separated upper stage of that rocket is visible next to the capsule as it passes over. Not only that, but Dragon itself jettisons two solar-panel covers after lift-off and these appear either side of the spacecraft as two points of light which repeatedly brighten and fade during the pass as they tumble away.
It's a truly electrifying sight and one that can be captured easily using a DSLR, static tripod and mm lens, and the same basic technique described in 'Nightscapes with a sparkle'. We've even been able to film Dragon firing one of its thrusters during a pass, using a DSLR and a telephoto lens. Ordinarily, high frame rate cameras are used to create detailed images of targets like the lunar surface and planets.