Posts Tagged ‘telescope’
- Milky Way, Credit: Ralph Clements
By Ralph O. Clements
When I was invited to write about this subject for Astronomysource.com, I must say I was flattered and a bit flabbergasted too, as I do not consider myself an expert on the subject, nor a writer by any means, but just a guy who likes to go out at night and take pictures of the sky. I stumbled into this hobby when my wife brought home an old 4” Meade reflector telescope with a manual equatorial mount from a yard sale that she paid $60 for.
I took that thing out in the country and set it up (completely wrong, I now understand) and as darkness approached, held my point-and-shoot camera up to the eyepiece and took a picture of Venus. Well, now that was very interesting…it was certainly not a very good photo and I have learned it is hard to get a good one of Venus, but I could tell it was not a star, it was not round but had a semi-circular shape. Wow! I took a picture of another planet! That got the gears turning in my head and I just had to do more….I mean who would think I could take a picture of another planet, with a point-n-shoot camera and an old yard sale telescope?
“Camera Only” Images
I do take images with newer telescopes and a decent equatorial mount which I have acquired since. Imaging galaxies and nebulae is an ongoing goal and interest, but I have learned that it is time consuming, tedious and has a fairly steep learning curve. My view of the sky at home is very limited. So for the time it takes to drive out to the country and get all that gear set up and working, I am limited to weekends and then only weekends when the sky is clear. Since clarity of skies does not always happen on Friday or Saturday night, I often image without the telescopes at all. All the tips and advice offered here is just what I have learned and I expect others may have better ways of doing things.
My research indicated that Canon cameras are preferred for astrophotography and the T1i is what I use for everything. I also have an older Nikon DLSR with two lenses, a 180mm fixed focal length and a 75-300mm zoom, for which I bought a Canon adaptor, but the 18-55mm “kit lens” that came with the Canon is what I use most often.
If you read up on astronomy and astrophotography equipment you will note it is often said that the mount is every bit as important as the telescope. My camera tripod is my mount and I fully agree that a sturdy tripod is a must. I am fortunate to use a tall Berlebach tripod with hardwood legs. The cheap aluminum department store tripods are not stable enough.
- Figure 1: Orion at Peaks of Otter, Credit: Ralph Clements
Widefield & Star Trails
Camera only astrophotos with a static tripod fall into these three general categories:
Widefield – Single Shot
These include what would be considered “scenic” or “landscapes” in daytime photography, that is, including some portion of the Earth, as well as constellations and shot of the Moon (See Figure 1). I try to shoot as long as possible without having oblong or streaked stars. A high ISO setting helps with this and I often use 3200 ISO unless it is twilight or too much man made light is around. Figure 1, Orion and the Peaks of Otter, is a 10 second exposure and the stars are a bit oblong but not too bad.
Widefield – Stacked Images
Images that are composed of multiple single exposures, stacked and aligned in the computer to reveal much more of the faint light features than what is visible to the naked eye. Sagittarius (Figure 2) was taken as series of short, 6 second shots and stacked in the computer using Deep Sky Stacker. The exposure time for shot like this can vary depending on the target, its location in the sky and ambient light conditions. I find that targets nearer the poles may allow a little longer exposure than those on or near the celestial equator, which appear to move more due to their location.
- Figure 2: Sagittarius, Credit: Ralph Clements
Star Trails Shots
Long Exposures or combined multiple exposures that show the apparent rotation of the stars above the Earth. Of course, the stars just appear to rotate because we are riding on the Earth which is really doing the rotating (Figure 3).
Taking star trails images is fun, easy and I like the look of them. Although a star trails image of say 40 minutes can be done on the “bulb” setting with a single exposure, this requires a remote timer and more importantly, a very, very dark site as the least ambient light will over expose the shot during that time. So I just take a series of 30 second shots and combine them using “Startrails” software, another useful and free program. This software is definitely easy to use and produces good results, although I do not notice much improvement when I use dark frames with it. For noise reduction I use “Noiseware Community Edition” in the final images instead. I recently became aware of another free software to do this, “Starstax”, and will be trying it soon as it offers more features.
On these star trails shots, sometimes it is good to have some moonlight on the subject and I will go out under a quarter to half moon and shoot them. I find a full moon makes it too much like daylight for my taste and if I lower the ISO under a full moon the stars don’t show up much. So depending on the amount of moonlight, artificial light and desire ground detail, I take these star trails shots at ISO setting of 800, 1600 or 3200. Generally, I try to get 40 to 60 minutes total exposure. Less than that and the trails are too short, more than that and chances are airplane will mess it up.
- Figure 3: Startrails, Credit: Ralph Clements
Foreground and Framing
I try to pick a good site with some interesting foreground , although “fore-ground” in this case doesn’t mean close to the camera, rather, it means the part of the Earth that is shown. I try to frame the shots so that the sky covers roughly ¾ of the frame, since the sky is the real subject and the foreground is really just a reference or point of interest.
To get crisp focus on the stars and the ground, anything in the image needs to be as far away as your camera’s “infinity” focus distance, which varies with the lens. So I try to take scenes that I would focus to infinity on if I were shooting them in daylight, such as the farm you see in Figure 3. For all my images I use the camera’s “live view “. This feature lets me zoom in on a bright star, or the moon and focus. If your target is too dim, aim at a brighter one or an artificial light a long way off and focus on it and re-aim at your target. Make sure your camera is not set to “auto-focus”, use “manual”. The Nikon I was using did not have “live view” but I used the same method, only I looked through the view finder at a bright star or light. Sometimes a few test shots were needed to get it right.
As for general advice for other beginners, I offer the following
- Read your camera’s instructions, particularly the section on manual control.
- Learn to work your camera’s controls in the dark, the corollary of which is….
- Don’t be afraid to experiment. I use the trial and error method, with lots of trial and plenty of errors. That’s okay though as I am having fun and try to learn from my mistakes, and I don’t have to buy film for a digital camera, so I don’t mind deleting the ones that didn’t come out.
….just do it! Have fun with it.
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
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.