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.
So you think you are interested in amateur astronomy. Well, you are not alone. Every one of us who has become “hooked” on our hobby started out right where you are now. Hopefully we can get you started in such a way that you will find our hobby interesting and rewarding.
Most beginners get involved in amateur astronomy because they have looked up at the majestic night sky and found it fascinating. We have all marveled at the beautiful images from the Hubble Space Telescope. Just remember, if you expect to see images like those from Hubble, you are going to be deeply disappointed. After all, if we amateurs could see “stuff” just like Hubble, NASA would never have spent billions of dollars putting the observatory into orbit.
First of all, and this is extremely important:
DO NOT GO OUT AND BUY A TELESCOPE UNTIL YOU ARE ABSOLUTELY SURE YOU KNOW WHAT YOU WANT/NEED.
There are lots of telescopes out there and some are total junk. So do not go out and make a major purchase that could very well disappoint you once you gain additional experience.
If you have not already done so, stretch out in a lawn chair and look up at the night sky. You will be amazed what you can see on a clear dark night just by “looking up”. Most people have binoculars so try using them from the lawn chair, you will see even more. Just scan the sky and look for areas of interest. The moon is visible almost every clear night of the year. Look along the “terminator”, the region between the light and dark parts of the moon. This is where you will find the most visible detail. Light pollution (along with clouds) is the biggest adversary for observers. If you live in an area where there is a lot of light pollution, try going out into the country where the sky is much darker. State Parks are a great place to view the night sky and you will see a lot more.
As you gain observing experience you will want to learn more about “what’s up” in tonight’s sky. Most people know about constellations, but which ones are currently visible? Will I even recognize them if I see them? What are the names of the bright stars that I can see? Are any of the planets visible tonight? Are there any Deep Sky Objects (DSO’s) that I can see? To answer these questions, and many more, you will need a roadmap of the sky. These roadmaps are called sky charts and planetariums. A simple free version of a sky chart is available online at http://skymaps.com/.
Have you ever wondered how a telescope works? Is it just plain old magic? Nope, there is a real reason that we can see all of those objects in space that are so very far away. If you would like to know more, take a look at http://science.howstuffworks.com/telescope.htm. This site does a great job of introducing the beginner to the kinds of telescopes that exist and how they work.
Find an astronomy club in your neighborhood and visit them at their public nights and star parties. You will meet great people who are glad to talk with you about astronomy and equipment, and will be glad to show you celestial objects with their telescopes.
Once you have attended some astronomy events you will discover that there is a lot more to amateur astronomy than you ever thought possible. You may have even found out that your real area of interest is not what you thought it was. Hopefully you have had the opportunity to view through various types of telescopes (and other observing tools) that are available. You may have even decided what type of telescope best fits your interests. You have been introduced to a whole new vocabulary of names and terms. You have found additional sources of information including books, the Internet, monthly publications and computer software (just to name a few) that will allow you to continue to learn about your new hobby.
Yet you have just scratched the surface. Amateur astronomy is a lifelong pursuit and the only regret that many of us “old timers” have is that we did not start it early enough in our lives. For many, amateur astronomy is a family happening. Regardless, now is the time for you to really get started.
Posted with friendly permission of : Indiana Astronomical Society
- Sky & Telescope Guide: Getting Started in Astronomy for Northern Hemisphere or Southern Hemisphere. Great free pdf guide from Sky & Telescope. It offers valuable tips and advise for astronomy beginners, and it comes with bi-monthly star charts as well as a map of the moon.
- German Equatorial Mount – Shown Celestron CG4
This article explains how German Equatorial Mounts work. It consists of 2 parts: The first part provides a general overview about these mounts. GEMs require polar alignment; it will be discussed what this is and how it is done. Part 2 talks about more specific subjects like Declination, Right Ascension, setting circles, mount balance and load capacity.
Most people are familiar with camera tripods that offer vertical movement and head rotation. Telescope mounts based on this principle are called Altitude-Azimuth mounts, or Alt-Az mounts for short. German Equatorial Mounts (GEMs) are more complex, but they are preferred by astrophotographers and observers who like to view particular objects for long periods. Furthermore, GEMs are ideal for finding faint celestial objects just by coordinates.
German Equatorial Mounts are designed to compensate for the earth’s rotation. They can keep a telescope steady at any object in the sky. For this, GEMs have special rotating axes and additionally a gearbox. Astronomers can either turn a slow motion gear knob manually to keep the object in view, or they can attach a small motor to move the telescope for them. If the mount is properly aligned and the motor is running (or the slow motion knob is turned), an object will stay in the field of view (FOV) as long as the observer desires.
The earth rotates, and so do observers who are standing on it. This gives the appearance as if stars circle in the sky. If observed with bare eyes, stars move very slowly (one revolution in 24h), but the telescope magnification increases this speed proportionally. A telescope with a power of 100, let stars “move” 100 times faster. If an observer uses for example an eyepiece with apparent FOV of 50 deg, and a telescope with magnification of 100, it takes only about 2 minutes for a star to wander from one side of the eyepiece to the opposite site. What might be slightly annoying for stargazing is completely unacceptable for astrophotography. GEMs offer a solution
- German Equatorial Mounts need to be polar aligned. Startrail photo credit: RClements
Exact compensation for the earth’s rotation is only possible if both, the polar axis of the mount and the polar axis of the earth are in perfect alignment. Other than Alt-Azimuth mounts, which offer only 2 axes of movement, German Equatorial Mounts have no less than 4 axes. These mounts use Altitude and Azimuth settings solely for aligning the polar axis of the mount. Azimuth adjustment turns the mount so that its polar axis points north. Altitude variations set the angle to point exactly at the north or south celestial pole (NCP or SCP). Astronomers in the northern hemisphere use the Pole Star as reference. Polaris is conveniently located within 0.8 degree of the actual NCP. Observers in the southern hemisphere refer to the constellation Octans for aligning the mount to the SCP. All GEMs are equipped with a tight tube like opening for aiming at the NCP or SCP. At most GEMs this opening is actually a bore in its polar axis.
Astrophotography demands very precise polar alignment because imaging faint galaxies or nebulae requires very long exposure times. Polar scopes improve alignment accuracy dramatically. These are small telescopes to be inserted in the aiming tube of the mount. These scopes have a small magnification but more importantly, they come with etchings on a lens. Etchings show constellations and a marker for Polaris. Using these markers provides a much more accurate polar alignment. More information about precision polar alignment methods will be provided in future AstronomySource articles.
Once the German Equatorial Mount has been properly aligned, latitude and azimuth screws are secured; they should not be touched during the remaining observing session. At first glance it seems rather strange that these axes are secured and left alone; are these not exactly the degrees of movements that are used with Alt-Az mounts to point at the objects we want to see? Yes – however GEMs offers two more degrees of movement to set Declination and Right Ascension, or DEC and R.A. for short.
Continue reading Part 2 of this German Equatorial Mount article on Declination, Right Ascension, Setting Circles, Mount Balancing, and Load Capacity.