Posts Tagged ‘magnification’
Amateur astronomers have to face some challenges with equipment, weather and environment during their stargazing sessions. The tips listed below help to overcome some initial hurdles and make the celestial night show even more exciting. If you have further tips, please contact us. We will be glad to share them with our fellow amateur astronomers.
Some telescopes come with finderscope others with red dot finder. Either way, the finders need to be perfectly aligned with the telescope axis. Alignment can be carried out during the day. Point your telescope at a stationary object far away. A chimney, an antenna mast or a mountain peak will serve well. Move the telescope so that the object is exactly in the middle of the eyepiece view. Lock the telescope in that position: Re-check if the object is still in the middle of the view. A adjust the finderscope / red-dot-finder by turning the adjustment screws so that the cross hair / red dot is exactly on the object. Do not compromise. Time spent for propper aligning means less time searching objects and more time observing fascinating objects.
Finding Objects with Finderscope / Red Dot Finder
I look for objects with both eyes open, one directly aimed at the object, the other through the finder. One has to get used to this way but once acquainted with it, objects are found much easier. Using the finder view with two eyes provides not only a small cutout of the sky but you can see a large part of it.
Most telescopes come with one or two eyepieces. If you have more than one, start your search for objects always with your lowest magnification (the eyepiece with the largest focal length). The longer the EP focal length, the wider the field of view and the easier it is finding objects. Once the object is centered in the eyepiece, you can increase magnification with shorter focal length eyepieces.
Besides setting the equipment up, stargazing is really not a very physical activity. Due to the excitement about celestial objects it is ofter forgotten that our body can loose temperature – quite quickly that is. Make sure to wear sufficient layers of clothes to keep you comfortably warm. The winter, nights in the desert or at high altitude remind us permanently to stay warm, but it can become very cold during a night after a nice, warm day in Spring or Fall. Check the weather forecast for the night. Take particularly windchill factors in account and dress accordingly.
How does dew form? The dew point is the point at which the air is saturated with water vapor. Warmer air is able to absorb more water vapor than colder air. If the air it cools down, excess water vapor has to go somewhere. On a greater scale, it will rain, on a smaller scale, exposed objects get wet. Whenever air cools down our equipment cools down as well and the exposed surfaces radiate excess heat. As this happen, moisture condenses at a greater rate than that at which it can evaporate, resulting in the formation of water droplets.
Moist lenses disturb the view significantly. Because they have their mirror deep down in the tube, Newtonian telescopes are relatively save, but all other types of telescopes, including binoculars are unfortunately susceptible to dew. A simple but a little bit cumbersome trick is to point the telescope downwards between observing sessions. When pointing it down, it is more difficult for the radiated heat to escape, and lenses will keep their temperature longer; and with that, they stay longer moist free.
Another defense against dew are large(r) dew shields. Most telescopes come with some dew protection but these shields are often far to small. Larger dew caps are available for every telescope but they can be be made easily with readily available material. Dew shields delay dewing, unfortunately, they cannot prevent it completely.
When the telescope gets moist, take utmost care when de-fogging the lenses. Best is not to touch the lenses at all, especially when they are coated. A fan is often the preferred option. If you have to use a cloth, take a very soft one. Do not rub, only dab the lens to make sure the lens coating is not damaged during cleaning.
Heating telescopes with special telescope heating bands is most convenient. These dew heaters keep the telescope just a bit warmer than the air temperature and with that, preventing dewing and moist surfaces. Heating bands need however a 12 V power source, and they can draw quite some current.
If you bring telescopes from the cold into the warm, lenses will moist almost immediately. Do not put the protection caps on right away, wait until the lenses are completely moist free.
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.
- Refractor 70mm on alt-azimuth mount
This article is for people who are in the market for their first telescope – but have only a very limited budget (say $100) at their disposal. Some amateur astronomers will say spending $100 on a telescope is wasted money, simply because there is no great telescope for that amount. Good optics is expensive, but not everyone has hundreds of dollars to spend and there are indeed some decent telescopes that are fun and do not break the bank. A tight budget should never prevent anyone to start a fascinating hobby.
Telescopes are available in three basic types: refractors, reflectors and catadioptrics and they come with different mounts. An Astronomy Source article “Introduction to Telescopes” in coming soon. The link from souledout.org provides also a nice overview on telescope types.
What can you see with a beginners telescope?
Quite a lot. I heard people complaining that the view through their basic telescope did not reveal bright and colorful images. Well, do not expect Hubble quality images with a $100 telescope. Suitable objects for these telescopes are the moon, the bright planets Jupiter and Saturn, some of the brighter deep space objects, and stars. Observe Andromeda Galaxy (M31) or Orion Nebula (M42), you can also see the famous Hercules Cluster (M13). Stars shine in different colors due to their different temperatures and consistence, and there are many exciting double or ever multiple star systems to watch. Did you know that Polaris, the North Star, is actually a 3-star system?
When looking for a budget telescope, keep in mind:
- Do not consider department store telescopes that come with a ton of accessories, parts and pieces, and promise magnifications of whopping 500x or even more. These items are usually toys. There is nothing wrong with toys, but really, don’t expect clear images or lasting fun and excitement with those. What you can expect is inferior optics a lot of plastic parts that will break soon.
- Glass lenses. Plastic lenses simply do not deliver the view quality of well polished and adjusted glass lenses.
New versus used
Telescopes can last very long if they are treated well. Used scopes are often available for half the price of a new one and many astronomers choose to buy used, simply because they want more: for the same money they get a much better (used) telescope, offering better images.
- Be very careful buying a telescope at a yard sale. You may get lucky and find the deal of the century. More likely is however that the item is in a bad shape, because it has been neglected over years.
- However, used equipment sold by serious amateur astronomers is usually in pretty good condition. True astronomers care about their equipment. A good source buying used telescopes and accessories is www.cloudynights.com. Check their classifieds and see what it offered here. Sellers describe their items in detail and provide usually very reliable information about the condition of their goods. Look for other dedicated astronomy sites that offer classifieds.
- If you decide to buy used, you should wait – and be quick. This sounds like a paradoxon, but you should watch the classifieds for a while to get a feeling about a fair market price of the the item you have in mind. When you see the right one, be quick to get it. Good items are often sold within a couple of hours.
Further items for amateur astronomers:
- Planisphere. Easy to operate map like device, that shows you where stars, constellations and planets are – all year long.
- Red Flashlight. Eyes adapt to the dark, this is called night vision. Night vision takes a while, about 15-30 minutes, but allows to see much more and much fainter objects in the dark. Exposure to bright light, with the exception of red, erases light vision immediately and it takes another 15-30 minutes.
- A good astronomy book about telescopes and star watching gives helpful hints for the beginner and advanced astronomer.
Beginning astronomy is the start of a great, very exciting and fun hobby. A simple 70mm or 80mm refractor on alt-azimuth mount is probably the best choice as starter telescope for around $100. These telescopes are very easy to set up and simple to operate – just point and see the universe.