How to Choose Eyepieces - Orion Telescopes and Binoculars

hi everyone I'm Kent with Orion telescopes and in this video we're going to be talking about eyepieces how to choose them what the different specifications are its how to get a good range for the telescope that you might have just purchased or maybe if you need to add on to an existing telescope and I think it's collection the first thing you need to know is that the telescope collects the light and it's the eyepiece in the back of the telescope that actually does the magnification different objects in the sky require different magnifications so you're not usually just using one eyepiece for everything deep sky objects for the majority at the time require a fairly low power and a wide field of view there's a few exceptions but in general low-power for the big deep sky objects and planetary detail you usually require some pretty high power because planets even though they're closer to us than a nebula they're very small in the sky so high power is good for for planetary detail when you get done with your collection usually you'll have a range you'll have a low power maybe a medium power for some certain things and then high power for planets you might also consider a Barlow lens which is a doubler or in some cases a Tripler it will take the eyepiece that you've got and double the magnification so when you're choosing your eyepieces you might want to keep up Barlow in mind so you don't overlap with some of the magnifications anyways let's talk about some of the specific details of the eyepieces and get a little further into it one of the most basic specs of an eyepiece is the focal length that will tell you what the magnification will be and if you just look at an eyepiece you can't tell what power it is because it depends on what telescope you put it in the reason for that is the focal length of the eyepiece interacts with the focal length of the telescope so in this case this is a 80 millimeter refractor and if you look on the label you'll find out what the focal length is this one is 400 millimeters if you look on an eyepiece it also has a focal length to its own 18 14 and a half 12 so just for example if you put a 10 millimeter eyepiece into this the formula to figure the power out is the focal length of the telescope 400 divided by the eyepiece focal length 10 millimeter so in this case 400 divided by 10 that's 40 power so if you think about that formula it's the exact opposite of what you usually think this is a 10 millimeter eyepiece which is moderate to high power a 25 millimeter in this case would be 16 power so the lower the number on the eyepiece the higher the magnification another important specification of the eyepiece is the field of view and that's usually listed on the eyepiece itself in this case this is a 20 millimeter focal length and right next to it it says 66 degree apparent field of view each eye piece has its own inherent apparent field of view that's not the true field that's the 66 degree apparent field is the circle that you see when you look through them it's a certain degree of of angle to figure out the true field of view which is the actual field of view in the sky that you'll see there's another little formula it's a fairly simple one it's the apparent field via the eyepiece divided by the magnification so in this case with that previous telescope I showed you 20 millimetres 400 millimeter focal length so that's 20 power and the apparent field of use 66 divided by the magnification 20 yields a 3.3 field of view 3.3 degree field of view with this eyepiece now another eyepiece might have a different field of view this is a Plusle there are generally 50 52 degree field of view so at the same magnification this shows you a little bit less field of view in the sky the apparent field of view 52 divided by 20 magnification so this is a 2.6 degree field and this is a 3.3 degree field of view wider field even though there are these act same magnification another important specification of an eye piece is the eye relief now I relief is the vertical distance from the top of the IPS upwards to where your eyeball is supposed to sit in order to see the entire field of view if you're too far away from the eye relief you'll just see the central area of the image and you're losing all the stuff at the edge in the case of this Plusle this is a six point three millimeter Plusle with a fairly small window because as plus let's go higher magnification the window gets a little smaller and therefore the eye relief to get smaller as well so in this case it's 4.1 millimeters that means your eye has to sit just four millimeters above the surface in order to see the entire field of view that's not a very long eye relief there are other eye pieces that have long eye relief as one of their features in this case this is a six millimeter eyepiece so about the same magnification as this pasal but this one boasts the 20 millimeter eye relief you can already see the window is bigger on this and your eye sits 20 millimeter is almost one inch above the surface well why is that important if you are glasses it's impossible to get your eyes four millimeters away from this IPS your glasses are in the way your rice it's a bit behind the glasses so this is probably not very usable for an eyeglass where where is this your eyeglasses will sit right on the surface and your eye sits comfortably above 20 millimeters above the lens you got to glue your eyeball to the top of one of these so in general long eye relief is a nice feature to have low magnification eyepieces don't matter so much because pretty much all low magnification eyepiece is have long eye relief but as the powers go up so my piece is it gets a little small so a long eye relief eyepiece it's a nice feature to have another thing you might have noticed behind me are the different sizes of the eyepieces in addition to the standard 1.25 inch barrel which pretty much any telescope can accept there are too much eyepieces now you can see the difference two inch barrel inch and a quarter barrel two inch eyepieces exists for very low power very wide field of view there's no such thing as a high power too much eyepiece but at low power you can get a wider field of view than you can at inch-and-a-quarter any well most refractors most high-end refractors above 80 millimetres will accept a two inch eyepiece they have a two inch focuser a reflector usually eight inches or above will also have a two inch focus or so if you've got that kind of a telescope that has a 2 inch focus or you can use a 2 inch eyepiece now just to compare there's a 40 millimeter Plusle internal quarter and 38 millimeter 2 inch so they're roughly the same magnification well applause all your limited 243 degree apparent field of view because the barrel inch-and-a-quarter that that low power the eyepiece can't see past its barrel very well so it has a 43 degree apparent field at 2 inches this eyepiece has a 70 degree field of view so same magnification not quite double the field but pretty unclose to double the field of view 70 versus 43 so a much wider expanse in the sky so you're looking at the Andromeda galaxy that's a very big object you can foot several you can fit several full moons across the andromeda galaxy it's many degrees wide well to get the entire thing in one field of view usually requires a very low-power 2 inch IPS so 2 inch eyepiece is a good extension to your eyepiece range for the very wide fields of view you might run across a feature of some eyepieces where they say the eyepiece is parfocal with other ones in the line parfocal means it's at the same focus point as the rest of the eyepieces meaning you don't have to change focus when you change eyepieces so if you look at a line like these guys the long eye relief there parfocal along the line which means when I stick this eyepiece in the into the telescope and I focus for it I can pull it out and pop the six millimeter in and expect the focus to remain the same I don't have to tweak the focus when I change eyepieces that won't be the case when you're going from this eyepiece to say this IP is an entirely different line of eyepieces but along the same line many eyepiece lines will be parfocal so those are the standard eyepieces different focal lengths different fields of view different high reliefs there are some specialty eyepieces that have different features that you may not find on the regular eyepieces one thing is a zoom eyepiece so far I've talked about a specific focal length IPS a 20 millimeter 40 millimeter well an IPS like this has a range this is a 7 to 21 millimeter so you adjust the barrel and it zooms from 21 millimeter at the low power up to 7 millimeter at the high power there's different zooms they have slightly different focal lengths but they all do about the same thing they they change the magnification without having to pop different eyepieces into the telescope that could be pretty handy when you don't have a lot of room in your case or you just you don't want to take a lot of stuff with you find something at low power and then zoom up the high magnification it's very nice there are some trade-offs the biggest trade-off is the field of view zoom eyepiece is never or it never really has wide field of view as a fixed focal length so something like this eyepiece if I compared it with the same focal length of a fixed magnification eyepiece I'd see a bit wider field of view out of the fixed IPs but the fixed IPS if I want to change power I got to pop it out put a different one in so a zoom eyepiece can be very handy to give you different magnifications all in one another specialty eyepiece design is a illuminated reticle eyepiece it can be any different design of eyepiece but the feature is a crosshair through the middle of the IPs and then usually a illuminated reticle on the side or an illuminator to illuminate the reticle so at night you can see the crosshair this one's a twenty millimeter this is great for when you're using a computerized telescope and you're doing the alignment and you want to make sure the star is dead center in the field of view pop the reticle eyepiece inside put the star right on the crosshair and you know you're dead center for the for the computer alignment another function of these eyepieces would be for guiding if you didn't want to use an auto guide if you wanted to manually guide usually use a fairly high power illuminated reticle put the star right on the crosshair and as you're taking your exposure you're keeping the star dead center using the hand controller and you know the picture will come out good with very good tracking at the end all right so there you have a basic overview of eyepieces the basic specs from the focal length to the field of view to the eye relief among among other things hopefully that will give you a good understanding of how to choose your next eyepiece in the line if you're looking for a set let's say you have a telescope it comes with a 25 and a 10 millimeter which is some standard eyepieces that may come with your telescope the next step might be higher magnification so say a five millimeter that will be good for moon and Planetary high magnification detail you can add a low power eyepiece on the other end of the spectrum of 32 or a 40 millimeter giving you a nice range 5 10 25 40 a nice range to cover the entire view of any object that you can see keep in mind a barlow as well number that doubles the power so if you have your 25 and your 10 well now you've got a 12 and a half and a 5 so do a little math and see where the focal lengths end up so you don't duplicate something using a barlow but a bottle is a good efficient way of getting more eyepieces without having to spend the money on more eyepieces if you've got two eyepieces in a Barlow you basically have four magnifications anyways I hope this helps you in your decision on how to buy different eyepieces thank you very much clear skies you

Channel: Orion Telescopes & Binoculars

Views: 191,167

Rating: 4.9433026 out of 5

Keywords: orion, telescope, telescopes, binocular, binoculars, eyepiece, eye, piece, optics, glass, fov, field, of, view, focal, length, parfocal, illuminated, reticle, barrel, apparent, magnification, mm, space, astronomy

Id: m7u9Q5hV7yc

Length: 12min 50sec (770 seconds)

Published: Fri Mar 30 2012