Posts Tagged ‘imaging’
- Star trails, image credit: Ralph Clements
Star trail images are beautiful to look at and they are captivating because they make time visible. These images can be made either by exposing one single image over a very long period or by taking many shorter exposures and combine them afterwards. Digital cameras deliver images as electronic files, making combining very easy – particularly with software that does all the combining work automatically. One of these software tools is the program Startrails. This software has been developed by Achim Schaller, and he did an outstanding job. Not only is his software really easy to handle, but it comes with powerful features – and it is free. Startrails can be downloaded at Achim’s website.
Startrails is designed to automate star trail image processing by combining a series of input files into one final image. It is Microsoft Windows based and runs on Windows XP and later versions. Microsoft .NET framework has to be installed on the computer. Simplicity is a key attribute that sticks out immediately. Only 6 buttons maneuver through all functions of this software. It processes input files in JPEG, TIFF and BMP format and stores results also in these formats.
Processing Images with Startrails
Processing star trail images consist of the following steps:
Select all star trail images at your HD or memory card. It is recommended to go through every single image and unselect the ones that have flaws. Air traffic or blinding lights of a passing car can sometimes spoil a good series. Taking out one or two sequential images is virtually not noticeable in the final star trail image but improves the overall appearance considerably. Sometimes however, “disturbances” are wanted; the firework of a meteor or the bright reflection trail by the International Space Station (ISS) are usually considered an upgrade to most images.
Startrails software handles dark frames, or Darks for short. Darks are taken with the lid on the camera lens. This way only noise that is caused by the CCD chip is recorded. Noise in the actual images is random, but each CCD has a particular noise pattern which, can be subtracted from the final image pixels. This step improves the image quality significantly. If more than one Dark is available, Startrails averages them. Using Darks is highly recommended when images are taken with high ISO speeds.
Averaging images improves the signal to noise ratio. As a result it reduces (random) noise of the images and increases brightness (luminosity) of the stars. The averaged image becomes brighter and is blended into the resulting image.
No interaction is required during the run. After it finished, results can be saved in JPG, TIFF or BMP file format.
Note: Images can often be improved by post processing. Stars can be made brighter, background sky can be darkened. Good image processing software is available from many companies or organizations. Photoshop is probably the most common software for astrophotography, but freeware like GIMP performs also quite well.
Time Lapse Movies
Startrails has another powerful feature: it can create time lapse movies. Of course, it can create any kind of time lapse movies, not just astronomy movies. The output format is AVI with the following possible compressions:
Full Frames (uncompressed), Microsoft RLE, Microsoft Video1, Microsoft H.263 Video Codec, Microsoft H.261 Video Codec, Indeo® 5.10, Cinepack codec by Radius, Logitech Video (I420), Intel Indeo Video R3.2, Intel Indeo Video 4.5, Intel IYUV Codec.
Startrails converts the image input resolution during compilation into the wanted AVI screen size. The same is the case with the desired frame rate (fps) and some compression settings to set the streaming speed (kbps).
I’m often asked the question: What do I need to take star trail photos? Well, the answer is: not much. A camera with remote capability and a solid tripod is basically all that is needed. The camera should make JPEG, TIFF or BMP images. The tripod has to hold the camera steady, even when light wind gusts occur.
DLSR cameras are perfect, but simple point-and shoot cameras may work as well. Even smart phone cameras may be suited. There are no real strong demands on camera optics; it simply depends on what image quality level you desire. The better the optics the better the resulting images will be. The faster the lens (1/f) the less ISO speed is needed and with that, lesser noise is introduced. Other things to consider:
The memory card must offer sufficient space to hold the truly huge amount of data that will be stored during the shooting session.
Remote Control Capability
Some cameras offer image-series capability, these cameras can work on their own. Most other cameras need to be remote controlled. Whether continuous shooting mode is used or a remote control with multiple timer capability (intervalometer) is connected to the camera, depends on the preferences of the photographer. I prefer remote controls with timers because they give me most flexibility and I can choose any delay between two images. This allows setting longer intervals for time laps movies.
Wide Angle Lens
Star trail photos look better with many stars on the image. Foreground objects are improving visual image balance. Most photographers use their widest angle lens (or zoom setting) for these photos to capture as many stars as possible for best effects.
Most digital cameras have great difficulties focusing on stars when the rest of the image is extremely dark. Manual focus capability is required for star trail photography; any auto focus feature on camera or lens has to be off.
Startrails is a powerful, yet easy to use all-in-one program for star trail imaging. It allows to create time laps movies in AVI format. Startrails is great freeware and well suited for beginners and advanced amateur photographers.
Send us your star trail photos. Tell us where and how you made them.
Star Trails and Night Photography
Great website by Steven Christenson – highly recommended. Steven provides a wealth of information on star trail photography. His gallery shows breathtakingly beautiful star trail images.
Star Trail Photography
Harald Eden’s website offers also good information on star trail imaging. Additional information is provided on Lighting and Clouds photography.
The first part of this article gave an introduction of German Equatorial Mounts (GEMs) and discussed polar aligment and how it is been done. This second part of the article talks about Declination, Right Ascension, setting circles, balancing the mount and load capacity.
The celestial equator is a projection of the earth’s equator in the sky. Declination of an object describes the angle to the celestial equator. Similar to the latitude scale of the earth, Declination is measured in degrees. Values are positive (0 to 90 deg) for objects north of the celestial equator, and negative south of it (0 to -90 deg). The advantages of an equatorial type mount are clear: because we have aligned the mount exactly with the polar axis, we have calibrated the mount’s celestial equator as well. The celestial equator is always orthogonal (right angle) to the polar axis. With that, the Declination of an object is completely independent of location or observing time.
It may be a bit confusing for beginners to imagine the celestial equator and the related Declination. A really simple way to picture this is to replace the sphere of the earth with a flat, circular platform on which the telescope stands. The platform has the size of the earth’s equator and the axis is the polar axis. Even when moving the telescope to any place on this disc, the
Declination of the object will be always the same. The famous Andromeda Galaxy (M31) can for example always be found at DEC 41° 16’, this is independent if we observe it from New York, Los Angeles or Munich.
The last degree of movement at a GEM is called Right Ascension (R.A.). After proper alignment, the R.A. axis points exactly at the NCP / SCP and with that, any R.A. rotation describes a circle as do the stars in the sky. Motors (or hand controls) can follow the apparent movement of a celestial object perfectly. Right Ascension is commonly used in units of time (hours, minutes and seconds).
- The star Omega Pisces in the constellation Pisces is very close to the zero point of Right Ascension and serves astronomers as easy to find reference.
Greenwich has been arbitrarily selected as start point for the earth’s longitude scale (0°), similarly the celestial zero point for Right Ascension (00h 00min 00sec) has been chosen arbitrarily to be the vernal equinox. A useful reference for astronomers is located in the constellation Pisces. With R.A. of 23h 59m 19s, the star Omega Pisces is currently very close to the zero point. Right Ascension of all other objects are designated by how long they lag behind this coordinate after it passes overhead moving toward the west.
German Equatorial Mounts are equipped with setting circles for R.A. and DEC and slow motion controls (or motors) that move the telescope in these directions. Setting circles are scaled discs attached to the R.A. and DEC axes.
- Even faint celestial objects, like nebulae and galaxies, can be found with setting circles.
They are marked with a 90 – 0 – 90 degree scale for DEC and a 0h to 24h scale for R.A. Setting circles are ideal to point the telescope at a particular object just by setting its coordinates. Most GEMs have movable setting circles. Their use is quite easy; astronomers point the telescope to a star with known R.A. and DEC coordinates close to the wanted object. They adjust the setting circles to the exact coordinates of the known star and move the telescope until the scales show exactly the wanted coordinates. It is surprising how easy it is finding faint objects with this method.
Balancing the Mount
All German Equatorial Mounts need one or more counterweights to balance the telescope. The position of counterweights is variable for balancing any load. Balancing is of utmost importance because it minimizes the burden on the gear and bearings. Clutches that hold the telescope in place are also much less stressed with a balanced setup. Conversely, a highly unbalanced set-up will most certainly damage the gearbox (and probably the motor). Furthermore, it is much more convenient to work with a balanced telescope that stays put when released instead of shifting and slipping at its own.
It is often forgotten that the focuser is extracted when the telescope is in use. Due to the weight of diagonal, eyepiece and sometimes even a camera, this extraction shifts the center of gravity significantly. This has to be considered during the balancing procedure.
Most manufacturers are quite “generous” when describing load capacity of their GEMs. Specifications should be taken with care, particularly for entry level mounts. Load capacity includes, with the exception of the balance weights, everything that is put on the mount: telescope, finder scope, diagonal, eyepiece, accessories, additional mounting hardware, and when shooting astrophotography, camera and addition accessories.
Do not overload the mount, it will make it unstable and results in unwanted mount wiggling and mount shake. This effect is multiplied by the magnification power of the telescope. Weak, wiggly mounts can make observing indeed quite unpleasant. Stable solid mounts are absolutely paramount for any kind of astrophotography. Astrophotographers tend to load their mounts only with 50% of the actual load capacity for best stability. There is much truth in the astrophotographers proverb:
“A good mount with a mediocre telescope will provide much better Images than a good telescope on a weak mount.”
Motorized GEMs come with one or two motors to control either only R.A. or both, R.A. and DEC. Most manual mounts can be retrofitted with small motors. For stargazing it is often more convenient to have only a R.A. motor drive and correct possible DEC deviations manually.
Completely different types are computerized GoTo mounts. These mounts have not only coordinates of thousands of celestial objects stored in their memory; they have also electronic versions of setting circles built in; they are called rotary sensors. A computer calculates the position, counts the necessary sensor impulses and drives the motors to the right point. Before the computer can find any object, it needs calibration on reference points, which are usually three known stars (three-star-alignment). Some stargazers conclude falsely that this alignment substitutes for proper polar alignment of the mount – this is not the case. After a successful three-star-alignment, the computer can point the telescope automatically at any wanted celestial object.
German Equatorial Mounts offer many advantages that are desired for observing, but are definitely required for astrophotography. Portable GEMs are heavier and more difficult to set up than Alt-Az mounts and they always need to be propperly aligned before they can be used in the intended way. The biggest advantage of GEMs is that they are suited to eliminate apparent star movement and with that, they are clearly the mount of choice for astrophotographers.
German Equatorial Mount – Part 1 @ Astronomy Source
The March Equinox @ timeanddate.com
Eyepiece projection is a great way to take detailed shoots of moon and planets. Photographed objects in these images are considerably larger and show more detail than such taken with prime focus shots. Prime focus techniques replace the camera lens with a telescope OTA (no diagonal, no eyepiece), but eyepiece projection adds an eyepiece into the optical path, increasing focal length and magnification considerably. The image below shows the typical eyepiece projection setup.
Greater magnification and increased focal length come however at a price. Higher focal length (at the same aperture) results in a higher focal ratio number (1/f). The higher the focal ratio number the fainter the image becomes. This demands longer exposure times or higher ISO speeds to achieve a decent image brightness. Furthermore, constantly moving air layers diffract incoming light. That means, with stronger magnification distortion is magnified as well. The same is true for any mount and telescope shake or vibration.
Typical eyepiece projection setup with refractor telescope an DSLR camera.
How to do it?
The following paragraphs describe equipment that is needed and such which is additionally recommended to make photographer’s life easier. I will share some experiences that I had to learn the hard way; it will help you getting good results sooner.
- The mount needs to be strong and sturdy. It has to carry all the weight of telescope, camera and all accessories, furthermore it has to stand steady, even with light breezes.
- Many manufacturers are quite “generous” when listing weight capabilities of mounts and tripods in their data sheets. Unfortunately, this leads often to unsatisfactory imaging experiences.
- Never max out a mount load. The old astrophotographers’ rule still applies: actual equipment weight should not exceed half of the mounts specified load capability.
- Many astrophotographers do not extend the tripod legs for better stability and minimal vibration.
- Balance the mount very carefully with camera and all accessories attached.
- Polar align German Equatorial Mounts (GEM) with great care. It helps “keeping the object in the field of view”, even with highest magnification.
Telescope & Accessories
- Finder scope and main scope axis need to be perfectly aligned. This helps to “find” the object and framing it in the very narrow field of view (FOV).
- Screwed accessory connections,like tube extensions, are preferred over slide-in joints. Screwed connections offer better stability, less flex and are less receptive to shake and vibrations.
- Eyepiece projection requires usually significant focuser back travel, particularly with refractors. The required length can exceed the telescope’s focuser travel, which will render the projection out of focus. One or two 2” extension tubes provide the required additional focusing way. My telescope has sufficient travel way but I still use extension tubes because it keeps the, relatively heavy, focuser tube more inserted. This has the advantage that the telescope’s weight distribution is somewhat closer to the center of the mount (less vibrations).
Astrophotography: Typical Eyepiece Projection Assembly with DSLR T-Adaptor
Note: M42 and T-thread accessories have different threads. While the diameter is the same their thread pitches are different (M42: M42x1mm and T2: M42x0.75mm). Accessories with M42 and T-threads should never be mated.
- Remote control for the camera is strongly suggested. Pressing the shutter release manually will cause shake and vibrations. If your camera does not have remote capability use your longest shutter release delay, minimum is 10 seconds. Some cameras offer only 2 seconds shutter delay. This time is usually too short because many mounts are still shaking 2 seconds after the shutter button has been pressed.
- Most cameras allow shooting movie clips (avi). Even if the movie mode may provide less pixel resolution, shoot movie clips, particularly for planetary imaging. Movie clips consist of many single frames and software like RegiStax convert the movie clip into a string of single images, which can be stacked. With a frame per second rate (fps) of typically 10 fps to 30fps, a 10 second clip results in a large number of single frames. This is important because air movement and other distortions will blur many images. The probability of getting a few good ones increases with the number of available images.
- Stacking good images helps to pronounce object features and texture.
- If your camera has no movie (avi) feature take at least 30, better 50 (or even more) images to increase the probability hitting some really good ones with little of no air movement.
- DSLR cameras use mirrors that flip up during the exposure. If shooting images (not movie clips) use mirror lock if available. Even if the mirror is very light, the fast movement can create enough momentum to cause shake, which again blurs the image.
- Jupiter – Image taken with eyepiece projection technique (telescope: 900/120mm, eyepiece: 20mm)
- Take shots at planets when they are high in the sky rather than low at the horizon. Positions high in the sky minimize air refraction distortion. Light that travels through the atmosphere is scattered by aerosol droplets and absorbed by dust. These effects cause diffraction rings and reduce the image brightness. High in the sky, light’s atmospheric path is much shorter, reducing distortion effects significantly.
- There are also disadvantages of high object positions. Particularly when shooting with a large refractor, the camera position is very low. Also, a large refractor with extension tubes and camera mounted may hit the tripod legs in this position. Make sure enough space is left when moving the telescope to the desired object.
- Remote controlling the camera with a computer is strongly suggested, particularly with a large refractor. Looking in upright position at the computer screen is simply much (!) more convenient than crawling on the ground trying to peek in the – very low hanging – camera screen or finder.
- The image on a much larger computer screen allows more precise focusing.
- Take your time when focusing. High magnifications combined with moving air layers can make this quite a challenge.
- Re-check with some test shots that the focus is still optimal.
- Check the histogram and ensure that neither end (black or white) is clipped. If data is lost (clipped) it is lost for good, and can no longer be used to build the image. Even the best post processing effort can not bring lost information back.
- Shoot several movie clips. My recommendation is 10 by 10. Ten clips each ten seconds long. Depending on the fps rate this will provide you 1000 to 3000 single frames, a good base to work with.
- Some photographers prefer much longer clips to increase the probability of catching better results. With very long clips it is more likely that shake, vibrations and drift errors are introduced as well. CCD chips get hotter and start to introduce additional noise and hot pixels. Besides, long movie clips result in very large files, making processing somewhat cumbersome.
- Powerful software like RegiStax (freeware) converts the movie clip (avi) into single images. Furthermore, it aligns the images, selects the best ones and stacks them for best detail. It allows improving the resulting image even more with a great set of post processing features.
Question for Power
It is possible to calculate how much more magnification we get with eyepiece projection over a simple prime focus setup. To determine this, we need to know some dimensions: focal length of telescope and eyepiece, and the telescope aperture. Furthermore we have to measure the distance from the eyepiece lens to the camera’s CCD chip.
The dimensions used in the following example are from an actual eyepiece projection setup that was used when I shot the Jupiter image: Orion EON 120ED refractor with 20mm Eyepiece, 2 extension tubes each 2 inch ( about 50mm) and a Canon EOS T1i DSLR camera.
Focal length of telescope (FLtele): 900mm
Focal length of eyepiece (FLep): 20mm
Distance eyepiece to CCD (Depccd): 100mm
Telescope aperture (TA): 120mm
Magnification over prime focus set up (Mopf)
Mopf= (100mm-20mm)/20mm = 4
The image is 4 times larger than that of a prime focus setup.
Focal Length overall EP setup (FLoEPs)
FLoEPs = Mopf * FLtele
FLoEPs = 4 x 900mm = 3600mm
This setup has a focal length of whopping 3.6 meters (141 inches)! The number shows that eyepiece projection focusing can really be a challenge and has to be done carefully in minute steps.
Focal ratio overall EP setup (1/f oEPs)
1/f oEPs = FLoEPs / TA = 3600 / 120 = 30
The original telescope focal ratio of 7.5 has now become 30. The image will be much darker than that of a prime focus setup. Higher ISO speeds particularly for planetary images may be necessary.
Is it worth the challenge?
Most definitely: YES. Eyepiece projection astrophotography is for more advanced star shooters. It is easily among the most challenging processes in amateur astrophotography, not because of the setup but because of the effects that have to be considered and factored in. But with the right equipment and some practice it can be mastered – and the results speak for themselves: clearly visible features of the moon landscape, surface coloration and visible ice caps of Mars or detailed cloud bands of Jupiter make eyepiece projection imaging indeed quite rewarding.