Reflectors (Catoptric) Use mirrors Problems: spherical aberration: For mirrors made with spherical surfaces, rays which are parallel to the optic axis but at different distances from the optic axis fail to converge to the same point. collimation: alignment of the optics Special types: Newtonian: Herschelian: Cassegrain: Dobsonian: basically just a newtonian but on an altaz base
Catadioptric Use mirrors and lenses Problems: spherical aberration: For mirrors made with spherical surfaces, rays which are parallel to the optic axis but at different distances from the optic axis fail to converge to the same point. collimation: alignment of the optics Special types: Schmidt-cassegrain Maksutov-cassegrain
Basic definitions f a f = focal length – the distance it takes for light to come to a focus after refracting through a lens or reflecting off a mirror a = aperature – the diameter of the main (objective) lens or mirror (primary) magnification = f scope /f eyepiece F/ratio = f / a f a f a primary secondary f a
Properties: Resolving Power The minimum angular separation two stars (or other features) can have and still appear as two stars. R = 252,000 × ( ) / (D) where is wavelength of light and D is objective diameter R will be in arcseconds If you substitute 550nm for, and D is in cm, then you have Dawes Equation. R = 12/D Very subjective Depends on seeing (atmospheric condition) Properties: Magnification Measure of how big something appears… M = angular size with aid/angular size without aid In optics, it is also expressed as M = F/f M max = 20 x D where D is in cm Unfortunately, most ‘department’ store telescopes will advertise that the telescope is capable of 900x! With the addition of some other optics (like a barlow lens) that may be true. But what is the quality of the view through that 2.5” scope? Well, a scope of 2.5” aperture has a maximum magnification of about 125x. That’s a far cry from 900x! Think of it this way… just because your car’s speedometer has 200mph on it doesn’t mean that the car can or should go that fast!!
Properties: Light Gathering Power Measure of how much light can enter a telescope to be brought to focus LGP = area of objective/area of pupil (eye) If the human eye opening with faint light is about 7mm, then LGP = D 2 /49 where D is the objective diameter in mm So a bigger light bucket is better! Seeing and Transparency Most people understand that to get the best views, most astronomers like clear and dark skies. What does that mean exactly? Well, dark skies mean skies that are free from light pollution – street lights, house lights, business lights… any light that is aimed at the sky rather than at the appropriate target (street, your own house or business). Basically, the contrast improves if you have darker skies. But more specifically, astronomers are concerned with “Seeing” and “Transparency” which can be loosely translated as how steady the air is and how clear it is respectively. Weather wise, it might be clear (no clouds) but it might be too hazy to view some objects. And then sometimes, a slightly hazy sky is an indicator of steady skies which is very important for observing planets.
Properties: Field of View The region of sky that can be seen through the instrument. (While there are formal methods to calculate the field of view, a much simpler method is to watch a star drift across your view.) Select a star near the celestial equator (why?) turn off any tracking motors so that the star drifts across the view adjust the telescope so that the star drifts directly across Place the star just outside the view and when it first drifts into view start a stopwatch or other timer Stop the timer when the star leaves the view The time may be several seconds to several minutes depending on the size of the telescope and eyepiece used Knowing that the earth spins on its axis once every 24 hours or sees 360degrees of sky/24hours 360deg = 15deg = 1deg = 60arcmin = 15arcmin 24 h1 h 4min 4min 1min So if it takes 150 seconds (that’s 2.5minutes) then the FOV is… 2.5min x 15arcmin = 37.5arcmin 1min
Mounts Telescopes must be supported by some type of stand, or mount -- otherwise you would have to hold it all of the time. The telescope mount allows you to: keep the telescope steady point the telescope at the stars or other object (birds) adjust the telescope for the movement of the stars caused by the Earth's rotation free your hands for other activities (focusing, changing eyepieces, note-taking, drawing) Alt-azimuth –basic camera tripod –dobsonian Equatorial –German equatorial –fork First, read the instructions that came with your telescope, but in general…. If you have a telescope on an altaz mount (ie, small refractor mounted on a camera tripod), then there’s not much to do except to plop the tripod down and aim your telescope. You will need to learn how to read starcharts and to starhop. If you have a telescope on an (german) equatorial mount, then things get a little more complicated! 1. Level the tripod. 2. Adjust the angle of the mount head to your latitude 3. Point the RA axis (stem of T) to the north (and the dec axis or top of T runs east- west) If you have a telescope on a fork equatorial 1. Level the tripod and attach the wedge. 2. Angle the wedge for your latitude. 3. Turn the tripod so that if you stuck your arm through the wedge from underneath, you’d be pointing North. 4. Attach the telescope.
Cleaning the Optics DON’T!! You should only clean your telescope optics when needed but not much more than twice a year, remember less is more. To help keep optics clean always replace the telescope cover when not in use and put your eyepieces back in their containers. [You can be creative here – eyepieces can be stored in plastic sandwich bags and the telescope can be covered with a shower cap or those new food covers.] Never cover your telescope optics or eyepieces if they have dew(or frost) or condensation on them, instead use a hairdryer on low heat until they are dry then cover them. If you must clean your optics, research what is the best method for your scope. Some telescopes require that you remove the main lens or mirror to clean them. Others can be cleaned in place. Never scrub optics as the abrasions will scratch your lens and mirror and can remove any special coatings. Only the outer surface of the optics usually need to be cleaned so don’t take apart eyepieces! Eyeglass lens cleaners and general glass cleaners are NOT designed for the delicate optics of your scope. Find out what the manufacturer recommends but sometimes simpler is better. A very diluted mix of water and a mild dish detergent followed with a rinse with distilled water and isopropyl alcohol (99%-97%).
Collimating your scope Over time the alignment of the optical components will no longer be perfect and the image will appear fuzzy. Making minor tweaks (and sometimes major tweaks) to the alignment of the optics in relation to each other can drastically improve the performance of the telescope. Collimating your telescope is not hard but it does require some practice. Collimating is important for newtonians, dobsonians and cassegrains. Refractors rarely need to be collimated because of the way the lenses are mounted in the tube. Here are several sites that have details for different telescopes. Collimation procedures can vary slightly between the different types of reflectors. Be sure to find the right instructions for your telescope. legault.club.fr/collim.html ngc1514.com/Celestron/collimate.htm web.telia.com/~u /kolli/kolli.html Thierry Legault
Polar Aligning As the world turns… the stars will drift through your field of view. If you have your telescope polar aligned, you might just need to push your telescope in the RA axis to catch up (assuming you are on an equatorial mount). If your telescope is powered and polar aligned, then objects should stay centered in the field of view for quite a while. For general observing, rough alignment is sufficient since most people only observe an object for a few minutes and it doesn’t matter that the object drifts out after 10 minutes. But if you plan on doing any astrophotography then this is a critical procedure to learn. [If you are on an alt-az mount and unpowered – no GOTO – then polar aligning is pretty much irrelevant for your situation.] There are several sites that give good descriptions of the rough alignment procedure as well as the more accurate star-drift method of aligning telescopes.
Balancing your scope Not often mentioned or covered, is how to balance your telescope. A small refractor on an alt- az tripod does not need to be balanced; however, a refractor or reflector (newtonian or SCT) on an equatorial mount will have a counterweight shaft and need to be balanced for you to be able to use the telescope and for the tracking to operate optimally. Some fork-mounted scopes might also need to be balanced if you piggyback other equipment or attach heavy cameras. starizona.com/acb/basics/using_balancing.aspx
Focusing When my students complain that they can’t see anything in the telescope, I check 1.the dust cover, make sure it’s off 2.the focus 3.where they are pointing 4.the eyepiece Usually, they are out of focus and not pointing at anything. Or, they are using the wrong eyepiece! Learning how to focus is simpler than learning where to look in the sky! –During the day, point the telescope at a very distant tree or lightpost. –As you watch through the low-power eyepiece, turn the focus knob first in one direction, and if nothing happens and it stops turning, turn it in the other direction. (You should end up halfway between the two extremes so if you count the number of turns…) Eventually, you should see your target appear blurry then sharper as you improve the focus. Changing eyepieces will require that you change the focus but it should only be a few turns at most. –That night, point the telescope at a bright star. Hopefully, you’ll see a big blob (out of focus star), but you may either have to adjust the pointing or really turn the knob. If you have a newtonian or cassegrain the blob will actually look like a donut when it is way out of focus. Very dim stars pretty much disappear when they are out of focus so be sure you are looking at a bright star!
Actually Finding Objects The telescope is aligned, balanced, collimated and focused, what should you look for?? The Sun (with proper filters!) Moon Planets Asteroids and comets Stars, binaries, variables, open and globular clusters Nebulae and Galaxies, oh my! Learn to read starcharts and to starhop. Even with an automated GOTO scope, you should have a rudimentary awareness of where certain constellations and stars are in the sky so that you can confirm the telescope is ‘heading’ in the right direction. Monthly starmaps can be found online but these will just show you the constellations and positions of the planets. Some will also list interesting events occurring that month. Other online sites might be more interactive but still generally just give a view of the constellations. calsky.com/
Actually Finding Objects, pt 2 Starhopping is a very common and easy way of finding objects. You don’t need to know anything about coordinates, but you will need to be able to read a starchart and match the chart to what you see in your telescope. First, make sure your finder scope is co-aligned with your main scope. During the day, point at a distant distinct object and center in the telescope field of view. Then adjust the screws holding the finder scope so that the object becomes centered in the finder. You should not be moving the scope to get the object in the center of the finder! Assuming that you don’t knock the alignment putting the scope back into its case or carrying it out to the yard, if you point towards a star and get it centered (by moving the scope) in the finder, then it should be centered in the main telescope. Second, study the starchart containing your target. Find a bright star nearby. This will be your start star. Make note of any strings, chains or other patterns of dim and/or bright stars between the starting star and your target. Go outside and find your starting star, then use the finderscope to hop from it to the chain of stars. You may have to refer to your starchart often to match up the patterns, but you should eventually get to your target. [Another trick if you don’t have a finder is to learn how to look along the length of the telescope tube. The near end frame should just line up and obscure the far end so that the side of the telescope tube “disappears.” Your starting star should sit on the edge of the far end frame.] Starhopping can be very rewarding since you are finding objects yourself. In addition, along the way to finding your target, you may see other objects that you’ll want to see later on! Knowing what’s up is also a key factor, so check some websites to find out what is happening.
Actually Finding Objects, pt 3 So what can you look for? If you have a GOTO telescope, you can let it give you a tour. I personally don’t find that very satisfying because for me, part of the fun is finding my target. Another common mistake of newbies, is that they look but they don’t observe. In other words, they’ll find an object, glance at it, then move on. Oftentimes, they miss the exciting but subtle details this way. To help slow yourself down when observing, try sketching the object. Once you’ve looked at the common objects like the Messiers and planets, check out the webpages for the Astronomical League. They currently have over 25 “Observing Clubs” for which you can earn a certificate (if you are an at-large member or member of a member club). Some of the clubs are easier and can be completed over a few weeks. But there are some that could take you years to finish! You might also want to think about observing projects like tracking comets and asteroids, occultations, variable and binary star observations… These are just a few of the areas where there are already professional-amateur collaborations. You the amateur can observe an object for a long time and the professional combines your data with data from big scopes (on which he might only have a few data points). cfa-www.harvard.edu/icq/icq.html
Actually Finding Objects, pt 4 Astronomical League Observing Clubs (These are just a few of many programs!) Introductory Constellation Hunter Club Lunar Club Sky Puppy Club Universe Sampler Club Binocular Binocular Messier Club Deep Sky Binocular Club Southern Skies Binocular Club Telescopic Arp Peculiar Galaxy Club Caldwell Club Herschel 400 Club Herschel II Club Galaxy Groups & Clusters Club Globular Cluster Club Messier Club Open Cluster Club Planetary Nebula Club Southern Sky Telescopic Club Urban Observing Club Topical Asteroid Observing Club Comet Observers Club Double Star Club Earth Orbiting Satellite Observing Club Lunar II Club Master Observer Club Meteor Club Outreach Club Planetary Observers Club Sunspotters Club The Messier Marathon seds.org/messier/xtra/marathon/marathon.html 10minuteastronomy.wordpress.com/messier-marathon-tools/ Other challenging lists…
Actually Finding Objects, pt 5 Once you’ve exhausted the common observing lists, you might want to try your hand at more technical or scientific observing, such as observing one specific object to see how it changes over time (lightcurves for variable and binary stars, comets and asteroids) or getting a better position for new objects (newly discovered asteroids and comets) or getting a shape of an object (asteroid occultations and grazing lunar occultations). cfa-www.harvard.edu/icq/icq.html alpo-astronomy.org/