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Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

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Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory
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Page 1: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Telescope BasicsTelescopes_v20140124.ppt

Elizabeth Warner

UMD Observatory

Page 2: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Refractors (Dioptric)

• Use lenses• ‘first’ telescopes

Problems:• chromatic aberration: A lens will not focus different colors in exactly the same place because

the focal length depends on refraction and the index of refraction for blue light (short wavelengths) is larger than that of red light (long wavelengths). The amount of chromatic aberration depends on the dispersion of the glass.

• spherical aberration: For lenses 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.

Special types:Achromatic: telescope has been color-corrected with the use of multiple lenses and/or coated lensesApochromatic: corrected for both chromatic and spherical aberration

©Nick Strobel www.astronomynotes.com/telescop/s2.htm

Page 3: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

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

Page 4: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

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

Page 5: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

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 = fscope/feyepiece

F/ratio = f / a

f

a

f

a primarysecondary

f

a

Page 6: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Properties: Resolving PowerThe minimum angular separation two stars (or other features) can have and

still appear as two stars.R = 252,000 × () / (D)

where is wavelength of lightand D is objective diameterR 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: MagnificationMeasure of how big something appears…M = angular size with aid/angular size without aidIn optics, it is also expressed as M = F/fMmax = 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!!

Page 7: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Properties: Light Gathering PowerMeasure 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, thenLGP = D2/49 where D is the objective diameter in mm

So a bigger light bucket is better!

Seeing and TransparencyMost 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.

Page 8: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Properties: Field of ViewThe 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 h 1 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

Page 9: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

MountsTelescopes 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 latitude3.   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 equatorial1.  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.

Page 10: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

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%).

•www.company7.com/library/clean.html•www.televue.com/engine/TV3b_page.asp?return=Advice&id=103 •www.astro-tom.com/tips_and_advice/cleaning_optics.htm•www.sctscopes.net/SCT_Tips/Maintenance/Cleaning_Your_Optics/

cleaning_your_optics.html

Page 11: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Collimating your scopeOver 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•www.fpi-protostar.com/bgreer/collim.htm•www.amateurastronomy.com/collimate.html•ngc1514.com/Celestron/collimate.htm•web.telia.com/~u41105032/kolli/kolli.html

•www.google.com/search?q=Collimating+telescopes

Thierry Legault

Page 12: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Polar AligningAs 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.

•www.astro-tom.com/tips_and_advice/precision_polar_alignment.htm•www.minorplanet.info/ObsGuides/Misc/DriftPolarAlignment.htm•www.minorplanet.info/ObsGuides/Misc/ccdpolaralignment.htm

•www.astrocruise.com/articles/articles.htm

Page 13: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Balancing your scopeNot 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

•www.telescopes-astronomy.com.au/telescopes019.htm

Page 14: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Star Testing Your OpticsOne way to find out the quality of your optics is to do a star test. The resultant image might

show that you need to collimate your scope. Or it could reveal more fundamental problems such as a bad figure to your mirror. Interpreting the results takes some practice and experience. Don’t be afraid to ask others for help!

•astunit.com/astunit_tutorial.php?topic=startest

•aberrator.astronomy.net/html/star-testing.html

•starizona.com/acb/basics/using_startesting.aspx

•www.willbell.com/tm/tm5.htm

•www.telescope-optics.net/star_testing_telescope.htm

© Pacific Telescope Corp.

Page 15: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

FocusingWhen 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!

Page 16: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Actually Finding ObjectsThe 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.

•www.skymaps.com•www.skyandtelescope.com/observing/ataglanceOther online sites might be more interactive but still generally just give a view of the

constellations.•calsky.com/•www.astroviewer.com/index.php•www.space.com/skywatching/•www.heavens-above.com/

Page 17: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Actually Finding Objects, pt 2Starhopping 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.•www.skyandtelescope.com/observing/ataglance

Page 18: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Actually Finding Objects, pt 3So 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).

•www.astroleague.org/

•www.aavso.org/

•www.lunar-occultations.com/iota/iotandx.htm

•cfa-www.harvard.edu/icq/icq.html

•www.minorplanet.info/

•www.minorplanetobserver.com/

Page 19: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Actually Finding Objects, pt 4Astronomical League Observing Clubs

www.astroleague.org/observing.html

(These are just a few of many programs!)Introductory

Constellation Hunter Club Lunar Club Sky Puppy Club Universe Sampler Club

BinocularBinocular 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 ClubUrban Observing Club

Topical Asteroid Observing Club Comet Observers Club Double Star Club Earth Orbiting Satellite Observing Club   Lunar II Club Master Observer Club   Meteor ClubOutreach ClubPlanetary Observers Club Sunspotters Club

The Messier Marathon

•www.richardbell.net/marathon.html

•seds.org/messier/xtra/marathon/marathon.html

•10minuteastronomy.wordpress.com/messier-marathon-tools/

Other challenging lists…•www.astronomy-mall.com/Adventures.In.Deep.Space/

Page 20: Telescope Basics Telescopes_v20140124.ppt Elizabeth Warner UMD Observatory.

Actually Finding Objects, pt 5Once 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).

•www.aavso.org/ •www.lunar-occultations.com/iota/iotandx.htm•cfa-www.harvard.edu/icq/icq.html•alpo-astronomy.org/•www.cfa.harvard.edu/icq/cometobs.html•www.minorplanetobserver.com/•www.minorplanet.info/


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