Barlow Lens - Complete Overview and Function

hey guys this is Lyman today is going to be a complete overview on Barlow lenses so how Barlow lenses affect the telescope you know how they decrease the brightness and how they increase the magnification and what they do to your images and all the effects of it so it's gonna be a total overview of Barlow lenses so let's jump right into it alright well to show you how the Barlow lens functions and works I've decided to use the refractor design to show you how the light moves through the system and then eventually hits the Barlow but before I do that I have to stop for a second and get myself some props here I am NOT a graphic designer at all but I feel like I did a pretty good job on this refractor it took me a while and I know it's very simple but hey I think it looks good I even got a diagonal on here and I peace I have a focus or I feel like it did a good job so we're just gonna assume that this refractor has a 1000 millimeter focal length and that will you know make the calculations a little bit easier and I know refractors don't have you know a conical design here they usually come out stop get a little narrow and stop but hey I still feel like I did a good job so let's go ahead and take a look at what the light does through this system so to keep things simple we're just going to you know ignore the lenses that are found in the objective of the refractor so you know those could be a chromatic or a Poe chromatic you could have two lenses or three lenses in here to keep things simple though we're just gonna you know ignore those and just assume the light comes in and gets bent down to a focal point down here now I removed the diagonals but if you were using a diagonal obviously that light would be you know shot up this way spread out a little bit more than focus in an eyepiece or if you're directly using a camera go from that but to keep things simple we're just gonna just assume this is a very very simple refractor where the light comes in and it gets bent down and go from there so what happens when we put a Barlow lens in the system well a Barlow lens is a concave lens so you know concave on both sides here it's what we call a diverging lens so what happens to the light when it comes to this refractor and hits this this diverging lenss it will spread out the light more so instead of you know concentrating it down into one spot it spreads it out and makes it concentrate into one spot further down the the focal length so essentially your Barlow lens will apparently increase your focal length by a factor of two or three or depends on you know what kind of lens is used and if you put a 5x Barlow in your your system your apparent focal length will appear to be five thousand millimeters rather than you know the one thousand millimeters and that also increases your focal ratio let's say you know this was an f5 telescope well if you put a 5x Barlow in the system there you're going to increase that focal ratio to F twenty five so you're going to multiply that focal ratio by you know whatever the Barlow lens number is so if it's a 5x Barlow you just multiply your focal ratio by five so what does this do to your image well spreading out that light and not having it you know be so concentrated and spreading it over the sensor or the eyepiece or whatever is naturally going to make it less bright so it will dim the view now here is a closer view of you know what that Barlow lens actually does in terms of spreading out that light so this line here is where your you know light would be focused if you didn't have the Barlow lens in the system so if you just ignore the bar lens it just comes right through and focuses down right there but that Barlow lens will diverge that light and you know spread it out like I said it will make it less bright the nice thing about modern Barlow lens is most companies fully multi-coat their Barlow lenses with you know either proprietary coatings or you know fairly common coatings but that really helps the brightness issue and then some Barlow lens is you can't even tell that it decreases the brightness so you get that you know longer focal length or apparent longer focal length more magnified images and a lot of cases you get to keep that brightness which is nice the other thing that you kind of have to realize with Barlow ins is is as this you know light comes in and you know hits different parts on that Barlow lens and is diverged different amounts well the refractive index of each wavelength of light is a little bit different so if you didn't have a Barlow lens that had those those multiple elements in it to fix that you would get some chromatic aberration just the way you know lenses work if light came straight down through the middle it would basically go right through this lens unaffected whereas these ones that come down from the side would have to travel a further distance to get to that same point and so you know they really wouldn't focus at the same point so you know one color may be in focus where one color is out of focus and that's why we use multiple lenses to try and correct that so that you know all the light focuses at the same point and in modern Barlow's that are a Poe chromatic or four element they do a really good job of you know fixing those color issues so that they're not even there the best way to show how optics works though or Barlow lenses for that matter is to use real lenses and real lasers which is really fun so I have a converging ones here which is what you would find you know on the front of a refractor and a diverging lens I'm gonna show you how these work in real life so I have a beam of five lasers here and we have our diverging lens or our Barlow lens and the converging lens which is you know normally what you would find on the front end of a refractor so what's really neat though is right away when you put in this Barlow lens you can see that light diverges immediately so you got the five straight lines they all diverge exactly how it works in the telescope which is pretty darn cool again you know that laser that comes through the middle isn't diverged at all and then the further away you get from the middle the more the light diverges so that's kind of you know just how Barlow lens works if you want to see you converging lens so just put that in so when the light comes into the refractor it comes in straight and it gets bent right down to here where all that light converges now what's really cool if I keep my finger right here and then I put this Barlow lens in here so right here notice the new point is right here so that light does get diverged it gets lengthened so it's an apparent increase in focal length so it's kind of fun to to play with these but you can see it actually does happen and it's pretty cool optics or neat and you know actually I'm gonna show you what an image would look like without a Barlow lens first to show you kind of a difference so again I'm not the best graphic designer in the world but hey I'm pretty proud of my little Jupiter I even put a little shadow on it here but with you know a normal telescope could be a refractor schmidt-cassegrain whatever if you're looking at the planet you know you get a nice bright image you know moons typically visible depending on the telescope great red spot some banding so this is kind of your your typical view without a Barlow lens now when you switch over to a Barlow lens let's just say that you're using the exact same equipment all you did was put in a Barlow lens then this is what you would expect to see now it's kind of fun about this as I took the previous image and assumed you were using a 2x Barlow lens and so I magnified it exactly how a 2x Barlow lens would so obviously you get this a lot bigger of an image exactly you know two times longer of a focal length but we really notice is the brightness went down a little bit and the focus is just not quite there like the other image and this is one aspect of Barlow lens I haven't mentioned yet is when you you know increase that magnification it's a lot more difficult to get a really crisp focus and a lot of that kind of has to do with the telescope a lot of people will you know use Barlow lenses in their telescope for visual use that they really shouldn't be you know the really small almost department store like newtonian z-- will often come with like a six millimeter eyepiece and a 3x Barlow and you put those in and you literally cannot see like anything and so the bigger you know the aperture of the telescope or the diameter of the telescope the more resolution it has so bigger telescopes can handle you know that increase of magnification a lot better than the really small ones another factor that goes into how well a Barlow we'll perform is the sky conditions so if you know the sky isn't very transparent for the evening and you just don't have great conditions it will be even more difficult to get a nice crisp contrasted view of you know a planet or whatever you're looking at so moving away from my graphic design skills this is a real image of Jupiter that I took with a one shot color camera quite a while ago it's not you know the greatest image or anything like that in fact this is one that I took when I was first starting out but it illustrates what bar lenses can do a little bit so this is without a Barlow lens and you got four four moons in the image and Jupiter itself again not the greatest image but at least it's a base for what we're doing here so this is just a shot of Jupiter without a Barlow lens now moving over to the Barlow lens so this image of Jupiter was shot using the same telescope and the same camera but using a 3x Barlow lens and you can immediately tell that the night was better just because well for using a bar lens the focus isn't terribly bad I mean it's nothing special you can get the banding here you still see the Great Red Spot so it's a decent image it's you know nothing great though but it's really nice is you still notice the brightness is there why well like we said earlier modern Barlow lenses with their multi coatings really help on the brightness issue so they really compensate for that but the other nice thing is in your camera hardware or the software that you use you really can pull out some brightness and some details after you've acquired the images so that's what's really nice about using Barlow lenses for Astro photography now specifically planetary imaging is you don't really have to worry so much about the brightness as you did before because you can kind of you know touch it up how you want after you're done acquiring your images which makes things really convenient another really cool thing that you can do with Barlow lenses is you can use them to slow down really fast telescopes so if you have eyepieces that you know can't handle fast telescopes very well and you get a lot of koma at the edges can always use a Barlow lens to slow them down so this is a celestron c6n it's an f5 Newtonian telescope so if you use you know just fair quality eyepieces and this you're going to get some koma towards the edges of your field of view but if you throw in a 2 X Barlow you will make it an f10 focal ratio telescope and a lot of those aberrations that you see through the eyepiece won't be as bad so you can actually use Barlow lenses you know to slow down your telescope so that you get a more crisp and better view out of it even though we do have all those you know modern coatings that make images a lot more bright I do think it's important to show you the difference between an image that was taken with a Barlow lens and without so this image here was taken without of our lens this was shot using a Canon t3i on a celestron 8 inch Schmidt Cassegrain telescope and it was shot at ISO 801 16 hundredths of a second so you see you know a pretty mountainside picture here you got a nice blue sky pine trees snow everywhere nice shadows being cast so just you know a fairly typical picture basically using the Schmidt Cassegrain as a spotting scope I guess you could say but pay particular attention to the center here where these trees are in this little small tree here so look what happens when I put in a celestron luminous 2.5 X Barlow lens and keep in mind the luma nose Barlow lens is fully multi-coated BAM look at that difference it's a major difference it's the same camera same settings taking you know 30 seconds apart but the brightness is much less and so you know we can talk about multi coatings and using hardware to increase our brightness all we want but when it comes to unedited images you simply cannot beat the laws of physics a Barlow lens will make your image less bright that's just how it is flipping back and forth between the two is actually kind of fun too because you can see no the difference in brightness but the difference in sharpness of the focus so this you know pretty mountainside view it's a little bit more sharp than the view that you get through the Barlow lens but not by much which goes show you know that the seeing was pretty good obviously I wasn't looking through too much atmosphere and the Barlow lenses of decent quality as well so it's kind of fun to just flip between those those images and just see the difference in brightness and feel the view it's it's quite a difference to really understand how Barlow lenses works it helps to look at it mathematically and it's actually kind of fun too so remember to increase the magnification of an image the Barlow lens has to spread the light out over the sensor right so this image was you know 116 hundredths of a second of an exposure which I mentioned so how long would I have to expose for to brighten it up well the answer is four times and think about it this was a 2.5 X Barlow lens so I took a 1/4 hundredths of a second exposure here for calculation ease let's just assume the Barlow lens was 2 X instead of 2.5 so we'll just round it down the area of a circle is PI R squared so the radius is squared so in this previous image when I put the Barlow lens in the system I doubled the magnification right but I also in doing so made the radius half of what it was well what is 1/2 squared it's 1/4 so this image is receiving 1/4 of the photons that it originally was without the Barlow lens so if it's only getting a quarter of the photons that it was getting to get it brighter how long do I need to explode for well I need to expose 4 times as long right 1/4 times 4 is 1 and that's exactly what I did I expose for 4 times longer and voila the image is bright again so to illustrate the math and make it a little bit more clear imagine you were taking an image of an object using one second exposures and no Barlowe ends and then you know let's say you wanted to use a 2 X Barlow lens well if you're gonna use a 2 X Barlow lens how much longer would you need to expose to get the same amount of light well remember that the radius is cut in half when you use a 2 X Barlow lens so the radius is 1/2 but the radius is squared C's square that and that's equal to 1/4 so a 2 X Barlow is going to only be picking up a quarter of the amount of light that the one second exposure is with no Barlow so to get the same amount of light you'd have to expose for 4 seconds 3 X Barlow same situation so you know 3 X Barlow your radius is gonna be a third of what it was but it's gonna be squared because that's how the area works so that's 1/9 so you would need to take 9 second exposure is using a 3 X Barlow to get the same amount of light that the one second exposure is 4 X Barlow well can just do the same thing fourth squared is equal to 1/16 so you need 16 second exposures and 5x Barlow the same thing one fit the radius square that equals 125th so you'd need 25 second exposures so that is primarily the reason why people don't use Barlow lenses for long exposure deep-sky astrophotography because it would take forever to get the same amount of light so for example if you took a 1 minute exposure with no Barlow lens and let's say you wanted to use a 4x Barlow lens well we get the same amount of light you'd have to take a 16 exposure that's not realistic so that's why you know most people are not going to use a Barlow lens at all for deep sky imaging because the exposure times would have to be so much longer so that's why people primarily use Barlow lenses to image or view the planets in the moon because those objects are so bright that when you use a Barlow lens you really can't even tell that the brightness drops at all which is really nice and convenient and so you know if you were imaging Jupiter and you wanted to use one mm of a second exposure and then you said hey I want to use a 2 X Barlow lens well on the to X bar no lens you just multiply that by 4 and to get the same amount of light you'd only take one 500th of a second exposure those two numbers aren't terribly different so hence Y bar lenses are really useful on the planets all right well that is the ins and outs of Barlow lenses and how they function so I hope you found this video helpful and that you learn something and also if you are you know interested in how focal reducers function and their total overview I've actually made a video on focal reducers you can find that by clicking here just a fair warning though I mispronounced the word vignette about 50 times so if you can handle that go for it but anyways I again I hope you found this helpful thanks so much for watching and clear skies

Channel: AstroBlender

Views: 78,088

Rating: 4.8972602 out of 5

Keywords: Barlow Lens, Televue, Celestron, Orion, Explore Scientific, Astrophotography, Planetary Photography, Astronomy, Telescope

Id: 34OGJKuNP0U

Length: 19min 58sec (1198 seconds)

Published: Wed Mar 13 2019