Blue Newt
Home Grown Newtonian Telescope
+110 dbi Parabolic Antenna for 1 Million GHz
Blue NewtHome Grown Newtonian Telescope |
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DX From Great Orion Nebula, 1600 Light Years |
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Electromagnetic waves, aka photons, from a distant source may be captured by a sensor, the amount of energy captured is proportional to the area (or aperture) of the sensor. Animals who see well in the dark tend to have large eyes and irises that open very wide. Likewise, we use large apertures to receive faint signals in all parts of the spectrum. Large parabolic reflectors are used to look deep into space, focusing waves collected over a large area onto a concentrated spot. Like a parabolic dish antenna focuses radio waves, so a parabolic mirror focuses light. Sir Isaac Newton invented the telescope shown below. In the top part of the picture, light (yellow arrows) is focused by the large primary (parabolic) mirror. The secondary (flat) mirror sits at a 45o angle and diverts the light out of optical axis where it can be observed without the observers head blocking the view. Outside the optical tube, at the focal point, an image is formed of the distant source. The image is visible if we place a white screen at the focal point, and we could use a microscope to inspect the screen at high magnification. It turns out the screen is not necessary, we can "eliminate the middleman" and use an eyepiece to directly examine the "virtual image" at the focal point of the primary mirror. The magnifying power is the ratio of the focal lengths of the primary mirror and of the eyepiece. However for night sky viewing, large aperture, or light gathering power, is more important than magnification. Image brightness is proportional to the square of the aperture diameter. Aperture Rules !
The lower half of the picture shows some typical construction details of the Newtonian telescope. An optical tube, usually round, houses the components on the optical axis. The primary mirror is mounted in a mirror cell, having some arrangement of screws and opposing springs which allows the mirror to be aligned with the axis. The optical tube can be as simple as a long rectangular wooden box with the primary mirror supported by adjustment screws on the wall of the far end. I used an 8 inch PVC pipe, and an aluminum mirror cell mounted in the rear of the pipe. The secondary mirror, or diagonal mirror, should be only as large as necesary to divert the rays returning from the primary mirror. Likewise its mounting, or spider, should hold it securely on axis while blocking as little of the aperture as possible. The legs of the spider may be small diameter threaded rod, for adjustment. My spider has legs of wide straps of thin sheet turned edgeways. An adjustable focuser holds the eyepiece, allowing some travel. Standard diameters for eyepieces are 0.965 inch, 1.25 inch, and 2 inch. Have a small assortment of eyepieces with different focal lengths for different magnifications. When calculating the dimensions, your primary mirror governs everything else. The arriving light reflects from the primary mirror back up the tube as it converges toward the focal point. Before the focal point is reached, the secondary mirror diverts the light out the side of the tube. THe focal point should coincide with the eyepiece. The sec dia and position all depends on where the focal point falls. I had to change mine in fact, it was easiest to move the rear mirror up a bit since I had already moutned the focuser, etc. Lets say for example, you have an 8" dimater f6 primary mirror. Your fp will be 8in * f6 = 48 in up the path. That should fall outside the tube far enough your focuser can get the eyepiece fp to meet it. Now remember the eyepiece fp will be as much as 25mm or an inch for low mag, so that's 49 inches total. At high mag the eyepiece fp is short. So your focuser needs to carry the eyepiece from about 48 in to about 49 in from the main mirror surface. Eyepiece fp I think is measured from the seating plane, that is the end of the focuser tube. Your tube diameter and thickness, as well as your focuser depth and travel will tell you how much of the 48to49 in has to be after the sec mirror. That leads to how big the sec mirror must be. Say your tube is 9 in outside diamter and the focuser minimum height is 2 in with 2 in travel. So your tube outer wall is 4.5 in away from the center axis, plus 2 in for the focuser and you allow another .5 in for error, so youre not right on the hard stop with a short fp eyepiece. So your focal point needs to be about 7 in past the sec mirror, leaving 41 in of optical path from pri to sec. You have left 1.5 in of focuser travel to allow for longer fp eyepieces, so you can get 25mm eyepieces and still have .5 in room for error on the outer end of travel. Pretty simple huh? Your sec is about 7/48 the dia of the pri. Now the tradeoff is you want the sec as small as possible, so that means keeping the focuser short and the main tube narrow. Ideally you would have a very thin wall tube exactly the pri dia and a focuser with zero height collapsed. Then your sec would only be 4/48 the pri dia. |
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The picture shows some typical construction details of the Newtonian telescope. For an optical tube, I used an 8 inch PVC pipe, and an aluminum mirror cell mounted in the rear of the pipe. I trimmed my pipe square to a length about as long as the primary mirror focal length. The secondary mirror, or diagonal mirror, should be only as large as necesary to divert the rays returning from the primary mirror. Likewise its mounting, or spider, should hold it securely on axis while blocking as little of the aperture as possible. The legs of the spider may be small diameter rod, threaded for adjustment. My spider has legs of wide straps of thin sheet turned edgeways. An adjustable focuser holds the eyepiece, allowing some travel. Standard mounting diameters for eyepieces are 0.965 inch, 1.25 inch, and 2 inch. The 1.25 inch is most common and is interchangeable with 1-1/4 inch chrome sink drain pipe in case you want to make your own eyepieces or adapters for cameras, etc. |
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Blue Newt was designed around a mirror donated by Dr John Weaver. The Parabolic mirror is 6 inches dia (150 mm) with a 1250mm focal length. The f ratio is about 8.2, and the practial limit for magnification is about 60 times the aperture diameter in inches, or about 360 power. A 4mm eyepiece would give a magnification 1250/4 = The magnification is the ratio of the focal lengths of the primary mirror and of the eyepiece. . However for night sky viewing, large aperture, or light gathering power, is more important than magnification. Image brightness is proportional to the square of the aperture diameter. The primary mirror is mounted in a mirror cell, having some arrangement of screws and opposing springs which allows the mirror to be aligned with the axis. I used an 8 inch PVC pipe, and an aluminum mirror cell mounted in the rear of the pipe. The mirror cell that came with the mirror was too small for the inside of my pipe, so I used the trimmed off scrap of pipe end to make some curved blocks to fit between the mirror cell and the wall. The mirror cell holds the mirror against 3 alignment screws, that allow the mirror to be tilted slightly in any direction. |
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Light (yellow arrows) returning from the large primary (parabolic) mirror converges towards the secondary (flat) mirror which diverts the light out of optical axis, exiting the tube to be observed without the observers head blocking the view. Actually the secondary mirror itself blocks a bit of the view, but we minimize this as much as possible. Since the secondary is not in focus at infinity, it does not cause a "hole" in the view, just a slight loss of brightness all over. The secondary mirror, or diagonal mirror, should be only as large as necesary to divert the rays returning from the primary mirror. Likewise its mounting, or spider, should hold it securely on axis while blocking as little of the aperture as possible. The legs of the spider may be small diameter threaded rod, for adjustment. My spider has legs of wide straps of thin sheet turned edgeways. Like the primary mirror cell, the spider was made for a smaller optical tube, so some plastic pipe caps were unstalled as 'cups' to reduce the diameter until the spider's legs were long enough to stretch across. |
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Outside the optical tube, at the focal point, an image is formed of the distant source. The image is visible if we place a white screen at the focal point, and we could use a microscope to inspect the screen at high magnification. It turns out the screen is not necessary, we can use an eyepiece to directly examine the "virtual image" at the focal point of the primary mirror. The magnification is the ratio of the focal lengths of the primary mirror and of the eyepiece. The practial limit for magnification is about 60 times the aperture diameter in inches, or about 360 power in this telescope. A 4mm eyepiece would give a magnification 1250/4 = 312 power. Using a 3mm eyepiece increases the magnification to 1250/3 = 417 power, but the image will not be very clear, since the primary mirror can only support magnification up to about 360x. Department store telescopes often boast high magnification with small apertures, like 450x on a telescope with a 3 inch mirror. The package does include such an eyepiece, but the image at such high magnification will be very poor. An adjustable focuser holds the eyepiece, allowing some travel. Standard diameters for eyepieces are 0.965 inch, 1.25 inch, and 2 inch. The adjustable focuser included in my donated parts is a common rack and pinion type with a 1.25 inch diameter and about 2 inches of travel. The 1.25 inch is interchangeable with 1-1/4 inch chrome sink drain pipe in case you want to make your own eyepieces or adapters for cameras, etc. Have a small assortment of eyepieces with different focal lengths for different magnifications. For example, I bought three eyepieces with focal lengths of 25mm, 10mm, and 5mm. With my 1250 mm focal length primary mirror these eyepieces give me magnifications of 50x, 125x and 250x respectively. There are many types of eyepieces, such as Plossl, Erfle, etc. with prices ranging from $20 to $200 each depending on field of view, and color correction, etc. |
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Collimation is the process of aligning the components of the telescope with the axes of the optical tube. This becomes the optical axis. 1. Align the primary mirror with the axis of the optical tube. 2. Center the secondary (diagonal) mirror on the optical axis. 3. Align the diagonal mirror with the axis of the focuser and eyepiece. This may require a repeat of the previous step. Continue collimation until diagonal is aligned with both the main optical axis and with the branch axis exiting thru the focuser. Relax. Have a beer. Take your time. A long piece of chromed sink drain stuck into the 1.25" focuser will help keep your eye centered on the focuser axis. When everything is collimated and you look into either axis of the telescope you should see your eye looking straight at your eye, exactly on center. Good collimation gives good focus and eliminates 'coma' the tendency for each star to show a false tail or curved streak. |
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Stable mounting is vitally important ! My bargain $10 yard sale beat up Tasco junk telescope was difficult and disappointing to use. Later I repaired an equatorial mount for Dr Weaver and tried it out with my junky little telescope. It was like getting a whole new telescope ! ! Replacing the wobbly little tripod with a solid, stable, smooth mount made all the difference, literally, in the universe.
Ive got no need and therefore no recipe for making a homegrown mount. But I know its gotta be stable. Here are some types of mounts, you can buy 'em or make 'em. If ya end up home growing a simple, solid mount, please send in your design to this web site. 1. Altitude-Azimuth Mount. Rotates around the horizon (in Azimuth) and also swings up and down (in Altitude). 2. Equatorial Axle Mount. Axle is mounted so its axis rotates (in Right Ascension) parallel to the axis of the earth, carrying a second axle mounted so it can rotate N and S (in Declination). 3. Equatorial Fork Mount. A big 'fork' mounted so its axis rotates (in Right Ascension) parallel to the axis of the earth, and the telescope is mounted between the tines of the fork so it can swing N and S (in Declination). The Altitude-Azimuth is simpler, but the Equatorial mounts match the relative motion of the earth and sky. A simple clock motor added to a properly aligned equatorial mount will counteract the motion of the sky as the earth rotates W to E at 15 deg per hour. |
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Knowing where to find unusual objects makes observing interesting. A planisphere is a simple star chart made like a circular slide rule that can easily be set to show the positions of stars in the sky at a certain day and time. They are cheap. Buy one. Variable locations of planets, moon phases, etc can be found from magazines or PC software. Appearances of comets and other irregular phenomena can be monitored via magazines or the www. |
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Outside the optical tube, at the focal point, an image is formed of the distant source. The image is visible if we place a white screen at the focal point which is called "Prime Focus". The sun is too bright to view directly but it can be safely viewed by a screen mounted outside the focuser. Dark filters reduce the intense light of the moon for easier viewing. Colored filters sometimes help reduce light pollution. A Barlow lens added between eyepiece and focuser is a cheap way to double the magnification of any eyepiece. |
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Just as the virtual image can be viewed with a screen, a camera may also be put in the same position. This is Prime Focus Astrophotography. On the other hand, a camera may be used to photograph the image through an eyepiece. This is Eyepiece Projection astrophotography.
The photo above was taken at Prime Focus using an old 35mm camera body mounted to the focuser with a homemade adapter fabricated with 1-1/4 inch chrome sink drain pipe.
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APERTURE (DIAMETER OF THE LENS OR MIRROR) APERTURE is the SINGLE MOST IMPORTANT FACTOR in choosing a telescope. At any given magnification, the larger the aperture, the brighter and sharper the image will be. The clear aperture of a telescope is the diameter of the objective lens or primary mirror specified in either inches or millimeters (mm). The larger the aperture, the more light it collects and the brighter and clearer the image will be.
Get a telescope with as large an aperture as you can afford. This is the distance (usually in mm.) from the lens (or primary mirror) to the point where the telescope is in focus (focal point). The longer the focal length of the telescope, generally the more power it has, the larger the image and the smaller the field of view. For example, a telescope with a focal length of 2000mm has twice the power and half the field of view of a 1000mm telescope. If you know the focal ratio but not the focal length, you can calculate it: focal length is the aperture (in mm) times the focal ratio. For example, the focal length of an 8" (203.2mm) aperture with a focal ratio of f/10 would be 203.2 x 10 = 2032mm. This is the ability of a telescope to render detail. The higher the resolution, the finer the detail. The larger the aperture of a telescope, the more resolution the instrument is capable of, assuming the telescope optics are of high quality. For telescopes this is referred to as "Dawes limit." It is the ability to separate two closely-spaced stars into two distinct images measured in seconds of arc. Resolving power is a direct function of aperture such that the larger the aperture, the better the resolving power. The theoretical resolving power of a telescope is 4.56 divided by the aperture of the telescope (in inches). For example, the resolving power of an 8" aperture telescope is 0.6 seconds of arc (4.56 divided by 8 = 0.6). However, resolving power is often compromised by atmospheric conditions and the visual acuity of the observer. Maximum image contrast is desired for viewing low-contrast objects such as the moon and planets. Newtonian and catadioptric telescopes have secondary (or diagonal) mirrors that obstruct a small percentage of light from the primary mirror. It is NOT true that image contrast is severely reduced with Newtonians or catadioptrics because of this obstruction, unless more than 25% of the primary mirror surface area is obstructed. To calculate the secondary obstruction, use the formula (pi)r² for the primary and secondary mirrors which gives you the surface area of each. Then calculate the percentage of obstruction. For example, an 8" telescope with a 2¾" secondary obstruction has an 11.8% secondary obstruction:
primary 8" = (pi)r² = (pi)4² = 50.27 Seeing conditions (or air turbulence) is the single most important factor that degrades image contrast and planetary detail. LIGHT GATHERING POWER (LIGHT GRASP) This is the telescope's theoretical ability to collect light compared to your fully dilated eye. It is directly proportional to the square of the aperture. You can calculate this by first dividing the aperture of the telescope (in mm) by 7mm (dilated eye for a young person) and then squaring this result. For example, an 8" telescope has a light gathering power of 843. ((203.2/7)² = 843). When you view a star in a properly focused telescope you are not going to see an enlarged image since stars, even at high power, should look like points of light rather than disks or balls. This is simply because stars are very, very far away. But, if you magnify a star’s image by about 60x per inch of aperture and look carefully you may be able to see rings around the star. This is not the star’s disk you are seeing but the effect of having a circular aperture in your telescope and due to the nature of light. Under close inspection, when the star is at the center of the telescope’s field of view, this highly magnified star image will show two things; a central bright area called the airy disk, and a surrounding ring or series of faint rings called the diffraction rings. The airy disk becomes smaller as you increase the aperture. Airy disk brilliance (the brightness of a point-source stellar image) is proportional to the fourth power of aperture. In theory, when you double the aperture of a telescope, you increase its resolving power by a factor of two and boost its light gathering ability by a factor of four. But more importantly, you also reduce the area of the airy disk by a factor of four resulting in a sixteen-fold gain in stellar image brilliance. The exit pupil of a telescope is the circular beam of light that leaves the eyepiece being used and is measured in mm. To calculate exit pupil, divide the aperture (in mm) by the power of the eyepiece being used. For example, an 8" aperture telescope (203mm) used with a 20mm eyepiece is working at 102 power and has an exit pupil of 2mm (203/102 = 2mm). Or, you can calculate the exit pupil by dividing the focal length of the eyepiece (in mm) by the focal ratio of the telescope. POWER (MAGNIFICATION) One of the least important factors for a telescope is the power. Power, or magnification, of a telescope is actually the ratio between two independent optical systems – (1) the telescope itself, and (2) the eyepiece (ocular) you are using. To determine power, divide the focal length of the telescope (in mm) by the focal length of the eyepiece (in mm). For example, a 30mm eyepiece used on the C8 ( focl length = 2032mm) telescope would yield a power of 68x (2032/30 = 68) and a 10mm eyepiece used on the same instrument would yield a power of 203x (2032/10 = 203). Since eyepieces are interchangeable, a telescope can be used at a variety of powers for different applications. There are practical upper and lower limits of power for telescopes. These are determined by the laws of optics and the nature of the human eye. The maximum usable power is about 60 times the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually give you a dim, lower contrast image. For example, the maximum power on a 60mm telescope (2.4" aperture) is 142x. As power increases, the sharpness and detail seen will be diminished. The higher powers are mainly used for lunar, planetary, and binary star observations. Manufacturers who advertise a 375 or 750 power telescope which is only 60mm in aperture (maximum power is 142x), are misleading. Most of your observing will be done with lower powers (6 to 25 times the aperture of the telescope [in inches]). With these lower powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view. The lower limit of power is between 3 to 4 times the aperture of the telescope at night. During the day the lower limit is about 8 to 10 times the aperture. Powers lower than this are not useful with most telescopes and a dark spot may appear in the center of the eyepiece in a Catadioptric or Newtonian telescope due to the secondary or diagonal mirror's shadow. Astronomers use a system of magnitudes to indicate how bright an object is. The larger the magnitude number, the fainter the object. Each object with an increased number (next larger magnitude number) is approximately 2.5 times fainter. The faintest star you can see with your unaided eye is about sixth magnitude (from dark skies) whereas the brightest stars are magnitude zero (or even a negative number). The faintest star you can see with a telescope (under excellent seeing conditions) is referred to as the limiting magnitude. The limiting magnitude is directly related to aperture, where larger apertures allow you to see fainter stars. A rough formula for calculating visual limiting magnitude is: 7.5 + 5 LOG (aperture in cm). For example, the limiting magnitude of an 8" aperture telescope is 14.0. (7.5 + 5 LOG 20.32 = 7.5 + (5x1.3) = 14.0). Atmospheric conditions and the visual acuity of the observer will often reduce limiting magnitude. Photographic limiting magnitude is approximately two or more magnitudes fainter than visual limiting magnitude. DIFFRACTION LIMITED (RAYLEIGH CRITERION) A diffraction limited telescope has aberrations (optical errors) corrected to the point that residual wavefront errors are substantially less than 1/4 wavelength of light at the focal point. It is then acceptable to be used as an astronomical telescope. In compound optical systems, the individual components must be better than 1/4 wavelength for the wavefront error at the focal point to be at least 1/4 wavelength. As the wavefront number gets smaller (1/8th
or 1/10th wavelength), the optical quality is progressively better. FOCAL RATIO (PHOTOGRAPHIC SPEED OR F/STOP) This is the ratio of the focal length of the telescope to its aperture. To calculate, divide the focal length (in mm) by the aperture (in mm). For example, a telescope with a 2032mm focal length and an aperture of 8" (203.2mm) has a focal ratio of 10 (2032/203.2 = 10). This is normally specified as f/10. Many people equate focal ratios with image brightness, but strictly speaking this is only true when taking pictures of "extended" objects like the Moon and nebulae. The brightness of stars (point sources) is only a function of telescope aperture-the larger the aperture, the brighter the images. When viewing extended objects, the apparent brightness seen in the eyepiece is a function only of aperture and magnification, it is not related to focal ratio. Extended objects will always appear brighter at lower magnifications. Telescopes with small (sometimes called "fast") focal ratios do, however, produce brighter images of extended objects on film, and thus require shorter exposure times. The main advantage of fast focal ratios with a telescope used visually is a wider field of view. "Fast" focal ratios of telescopes are f/3.5 to f/6, "medium" are f/7 to f/11, and "slow" are f/12 and longer. An f/8 system requires four times the exposure of an f/4 as an example. This is the nearest distance you can focus the telescope visually or photographically for close terrestrial work. The amount of sky that you can view through a telescope is called the real (true) field of view and is measured in degrees of arc (angular field). The larger the field of view, the larger the area of the sky you can see. Angular field of view is calculated by dividing the power being used into the apparent field of view (in degrees) of the eyepiece being used. For example, if you were using an eyepiece with a 50 degree apparent field, and the power of the telescope with this eyepiece was 100x, then the field of view would be 0.5 degrees (50/100 = 0.5). An eyepiece with wide apparent field (in degrees) and lower powers used on a telescope allow wider fields of view. OPTICAL ABERRATIONS Chromatic Aberration -- usually associated with objective lenses of refractor telescopes, is the failure of a lens to bring light of different wavelengths (colors) to a common focus. This results in a colored halo around bright objects. It usually shows up more as speed and aperture increase. Spherical Aberration -- causes light rays passing through a lens (or reflected from a mirror) at different distances from the optical center to come to focus at different points on the axis. This causes a star to be seen as a blurred disk rather than a sharp point. Coma -- associated mainly with parabolic reflector telescopes which affect the off-axis images and are more pronounced near the edges of the field of view. The star images produce a V-shaped appearance. The faster the focal ratio, the more coma that will be seen near the edge although the center of the field (approximately a circle, which in mm is the square of the focal ratio) will still be coma-free in quality instruments. Astigmatism -- a lens aberration that elongates images which change from a horizontal to a vertical position on opposite sides of best focus. It is generally associated with poorly made optics or collimation errors. Field Curvature -- caused by the light rays not all coming to a sharp focus in the same plane. The center of the field may be sharp and in focus but the edges are out of focus and vice versa. COLLIMATION |
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