How to install the Barlow lens on this telescope?

How to install the Barlow lens on this telescope?

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I've brought a 4.5inch astronomical telescope recently which has an aperture of 114mm and a focal length of 1000mm, and two eyepieces. The two eyepieces were of 25mm and 10mm. I enjoyed stargazing using the 25mm eyepiece. But the stuff that I could see with my 25mm eyepiece was not visible through the powerful eyepiece. I'll post some pics. Can someone tell me how to install the Barlow lens?

Can't say for sure what the problem is with the 10mm lens. More magnification will make the image dimmer, but you should still see something Practice during the daytime, focusing on distant landmarks. My guess would be that the 10mm lens was not sitting in the focusser properly, resulting in everything being so out of focus that nothing could be seen. So practice during daylight.

The Barlow fits to the telescope, and the the eyepiece fits onto the Barlow. Everything should just fit together, if it was all brought as part of one set.

Based on the length of the optical tube (which doesn't appear to be roughly 1 meter even though you mention it is a 1000mm scope) this is probably a "Bird Jones" telescope and not a typical Newtonian reflector.

A Jones-Bird reflector telescope (sometimes called a Bird-Jones) is a mirror-lens (catadioptric) variation on the traditional Newtonian design sold in the amateur telescope market. The design uses a spherical primary mirror in place of a parabolic one, with spherical aberrations corrected by sub-aperture corrector lens usually mounted inside the focusser tube or in front of the secondary mirror. This design reduces the size and cost of the telescope with a shorter overall telescope tube length (with the corrector extending the focal length in a "telephoto" type layout) combined with a less costly spherical mirror. Commercially produced versions of this design have been noted to be optically compromised due to the difficulty of producing a correctly shaped sub-aperture corrector in a telescope targeted at the inexpensive end of the telescope market.

Also see Astrowiki

The "Bird Jones" designs use a spherical primary mirror and then use (or attempt to use) a corrector installed in the focuser assembly. It also acts as a 2x Barlow. A 1000mm "Bird Jones" design scope would really be roughly 1/2 meter in length.

The optics are usually not great (not as good as a Newtonian) and not a good candidate for using a Barlow (because now you're basically putting a Barlow on a Barlow). The image will get faint and very fuzzy.

Have you tried using the scope with just a 10mm and no additional Barlow?

Remove the eyepiece (and Barlow if you've inserted one) from the focuser. When you look through at the secondary mirror… do you see any "glass" in the path (other than the secondary mirror?) If yes, then you have a "Bird Jones" design telescope and not a typical Newtonian reflector.

While you are certainly able to try using a Barlow, I suspect you wont be happy with the results no matter what you do. I suspect a Barlow with a 10mm eyepiece would be pushing the magnification well beyond the practical limits of the optical system of that telescope.

Barlow Lenses of a telescope

Barlow lenses were invented and developed by Peter Barlow and consequentially bear his name. The lenses are not only used in telescopes but also in microscopes to serve the purposes of medicine, science, etc. this type of optical lens is usually used in combination with other different types of optics as well as optical systems. The result of such use is a substantial increase of the focal length of the system where Barlow lenses are used.

Barlow lenses are widely used in telescopes. It is a very efficient way of increasing the magnification of your tool. The lens is simply placed between the eyepiece and the mirror or objective lens. The result of this action is pretty astonishing. The magnification can be doubled if the 2x lens is used and triples it if a 3x lens is placed in a telescope. Therefore this is a very effective and relatively cheap way of increasing telescope performance.

1 – Try different eyepieces

Eyepieces can make or break what you see through your lenses. This holds especially true for amateur telescopes where manufacturers sometimes include cheap eyepieces in order to meet a certain price point. This means the tube in your device can be more powerful than you think. It just needs some help to show you the best it can do.

Let’s start by analyzing magnification.

A telescope has a maximum magnification. This is how “big” can the image be when you look at it. But a bigger magnification isn’t always better. Lower magnifications will allow you to view smaller objects like a planet’s moons or to track moving objects with ease. A bigger magnification becomes useful when you want to get more detail out of bigger objects like planets.

There is a limit to how much magnification you can get from a telescope. Beyond that point, no matter how big of an eyepiece you use, you will not get a better image. if you don’t know what the maximum magnification of your device is, it is easy to figure it out. You will simply need to know your telescope’s aperture as this will be the limiting factor. Then multiply it by 50.

So, for example, if you have a telescope with a 3″ aperture, your max magnification would be:

Your maximum magnification would be 150x.

Once you know that, you can calculate how much magnification you are getting with your current telescope/eyepiece combination by using this calculator. If you are not using eyepieces that match that number, you can buy an eyepiece that does.

The second consideration when it comes to eyepieces is design.

There are many types of eyepieces. You don’t need to know them all, but your telescope will most likely come with one of these:

If your device came with a Kellner or Edmund RKE eyepiece, you should definitely consider upgrading. Both of those designs are older and only use 2 pieces to project the images instead of the more sophisticated eyepieces that use 4 or more. They were very common in older telescopes but now they have been relegated to budget ones. The difference you will see switching from a Kellner to a Plossl will be significant even if they have similar specifications.

Finally, in order to finish upgrading your eyepieces, try getting a Barlow lens.

Think of a Barlow lens as an accessory to your eyepieces. It is basically an extensor that connects to them and gives you an additional 2x, 3x or 4x magnification depending on which one you got.

What does that achieve? well, a Barlow lens gives you access to a wider variety of magnifications to try. If you have eyepieces with a magnification of 20x and 60x, you would have four available magnifications with a Barlow lens of 2x, these would be the following: 20x, 40x, 60x and 120x.

Pretty much every eyepiece comes in the standard 1.25″ size so you don’t need to worry much about it fitting your telescope.

There are some great sets of eyepieces that include a few plus a Barlow lens for a very reasonable price. Check out for example this one by svbony that covers most of the eyepiece sizes a hobbyist astronomer could need.

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Hi, i have made a video where i show a coople of eyepieces that i use and like, and also show how a barlow works, compared to the "Powermate" from Televue. And how is the FOV affected when changing the magnification with different eyepieces?

Feel free to comment and give me feedback - I hope you like the video!

I am excited to join this big star gazing community! I realize I am abusing of your kindness, so that you in advance for any help you can give.
I have an old telescope, what appears to be a Meade ETX 90 from the early 2000s. I cleaned it pretty well after many Youtube videos and online research. I found full blown spider webs and dead spiders!
I have encountered a few of issues as I'm putting it back together and I wonder if anyone might be able to help.
1. I took off the screws of the back of my OTA (I know I shouldn't have!) and now I'm not sure how to put them back on (see attachments). When I try it seems to push the primary mirror instead of fastening to anything.
2. My secondary mirror flip is broken, the plastic piece part fell off. Is it possible to fix? The mirror itself is also very dirty (see attachments) and I don't think it can be cleaned further. Any tips on how to replace it and where to buy the parts? Or is it possible to not use the 90 eye piece viewport and instead use the front one so I don't need to fix the flip or the mirror?
4. My secondary baffle is sliding off. I've read that this is a problem w/older models. I tried pushing it to the right place and it requires some force, but after 1 minute it slides back to its original place. What should I do? Does this matter?
5. The plastic attachment to hold the smaller lens (finderscope?) wobbles no matter how tightly I screw it on. I don't know if it has anything to do with the front plastic part being a little broken.
Thank you for your help in advance, and I am still learning terminology so be easy on me

My lovely wife bought me a SW Explorer 130 with the basic RA drive for my 40th birthday 8 years ago, and I've enjoyed using it on and off ever since. I've started getting more into it - it's a great beginner scope, but I'm - like lots of others on this forum - looking at upgrading. One of the things I found tricky with the scope was getting the focus just right at high magnification (I'm sure that's kind of obvious!). It does wobble quite a bit on the EQ2 mount when focusing. I get pretty good - if small - views of Jupiter, Saturn, (the Moon, of course) and have had hints of seeing some DSOs.
I bought a 8mm BST Starguider, and it's a great improvement on the standard 10mm that comes with the scope, but if I push the scope by adding the 2x Barlow and the 8mm it's a blur and the focus seems to keep missing the sweet spot. I saw a post elsewhere on this forum by MakTheNight, (looks like they've left now - last post was 2 years ago) who added a Baader Helical Focuser that attached directly to the T-thread on the focusing apparatus on the SW. Link to that post is below. I got one and it is indeed a great improvement on fine tuning - the problem is I can't use it with the Barlow as the helical focuser is about an inch long and the focuser can't move far enough into the scope to focus with the Barlow and any of my eyepieces (the 8mm BST, or the 10 or 25mm standard eyepieces)
Clearly MakTheNight managed to get their setup to work with some specific eyepieces (a Luminos 10mm and 32mm Plossl) and a specific Barlow, so I tried looking up how I could work out what combination of eyepieces and Barlow would work, rather than randomly buying kit and having to return it because it won't work with my scope, but I haven't found anything useful yet. So I thought I'd take the plunge and ask here

Can anyone help me figure out how I can calculate what combinations of eyepiece and Barlow would work with this setup? I'm also looking at upgrading the OTA and mount to a 200P and EQ5 at some point, so ideally would have a combination that could work with that too - the focal length and mirror is quite different - 1000mm on the 200P parabolic vs 900mm spherical on the 130.

What a Barlow Lens Means and Does for a Telescope

For those of us that do astronomy as a hobby, we dream of having the best telescope available in the market to get the most out of our hobby. However, the cost can thwart our plans quickly. Fortunately, there are certain things that we can do to drastically improve our viewing experience without having to break the bank. One way to do this is by using a Barlow Lens.

A Barlow Lens invented by Peter Barlow (1776-1862), is made up of multiple lenses that have special lens coatings and shape to increase their light transition capability. More simply, it is a lens that is designed to increase the focal length of your telescope while maintaining a wider field of view. A Barlow lens is a cost-effective way of improving your viewing experience without buying new, more powerful and expensive scopes.

How Can a Barlow Lens Effectively Increase the Number of Your Eyepiece Collection?

We already know that it is essential always to have a set of good eyepieces with different magnifications. We need a low powered eyepiece to view larger objects for a wider field of view. On the other hand, we need a high powered eyepiece to view smaller objects like planets for higher magnification. A Barlow Lens can also be handy in this area as it can effectively increase the number of your eyepiece collection. For example, if you only have a 32mm and a 20mm eyepiece, having a 2X Barlow Lens allows you to double the number of your eyepiece collection by also having the magnification capabilities of a 16mm and a 10mm eyepiece. However, if you have a 10mm, 20mm, and 40mm lenses, a 2X won’t help much. In this case, the magnifications would end up the same as your eyepieces.

What Size Barlow Should I get?

The idea here is to go with a Barlow magnification that increases your lens abilities. Like the example above of having a 10, 20, and 40mm lens set. If you added a 2X Barlow, your 40mm would be the same as your 20mm eyepiece. To expand your viewing capabilities, you should look for a Barlow that is 1.5X or 2.5X. For example let us use a 1.5X Barlow. Because, this lens would make your 40mm into a 30mm, your 20mm into a 15mm, and the 10mm into a 7.5mm. Thus, by adding 1 Barlow lens, you expanded your eyepiece set to the equivalent of 6 eyepieces. A 40, 30, 20, 15, 10, and 7.5mm effective lens set with the one Barlow. Pretty cool, right?

How Can a Barlow Lens Help with Your Eyepiece’s Eye Relief

Eye relief is defined to be the distance required from your eye to your telescope’s eyepiece to be able to see its full field of view. Using high powered eyepieces requires a smaller eye relief. For example, using a 10mm eyepiece does require you to place your eye very close to your eyepiece to see through it.

If you are wearing glasses, using a high powered eyepiece can be a huge problem. Also, eye relief is essential so you can avoid having your eyelashes brush against your eyepiece because this can leave streaks of eyelash oils that could need to cleaning during your viewing.

Having a Barlow Lens allows you to use an eyepiece with lower power for its more considerable eye relief and still get the magnification of an eyepiece with a higher power. Pretty cool again, right?

How Do Barlow Lenses Work?

The magnification of telescopes is the focal length of the telescope divided by the focal length of the eyepiece. One way to increase the magnification of your telescope is to use an eyepiece with a smaller focal length, and this is what we do when we use a high powered eyepiece. A Barlow Lens, however, increases the magnification by effectively increasing the focal length of the telescope instead. Thus, giving us the ability to use a wider field of view for lenses to achieve the same magnification.

A 2X Barlow lens doubles your telescope’s focal length while a 3X Barlow lens triples it. For example, a 20mm eyepiece on a telescope with a 1000mm focal length gives a magnification of 50. Using a 2X Barlow Lens with this eyepiece creates a focal length of 2000mm thus doubling its magnification to 100x.

What Sizes of Barlow Lenses Are Available

Barlow Lenses come in different magnifications: 1.5X, 2X, 2.5, 3X, and 5X for some standard sizes. Some even have screw on lenses that allow for even more diversification and modification. Further, some are made to adapt right onto a DSLR camera. The 4X and 5X Barlow’s are best intended for direct insertion of small chip digital cameras. Like these CMOS cameras from Amazon that fit right where your eyepiece typically goes.

Which Barlow Magnification Is Best?

It is very tempting to immediately say that a Barlow Lens with a higher magnification is better in the same way that most beginners in astronomy usually think that a high powered eyepiece is always better than a low powered one.

In astronomy, higher magnification is not always better because of two reasons:

The same is the case for Barlow Lenses. A 2X Barlow Lens is the most common sized available because it usually works well with most telescopes. It usually gives a better combination of magnification and image quality and brightness compared to a 3X Barlow Lens.

The Diameter, Or Barrel Size Explained

Before you think and decide about which Barlow lens to use, you must first determine the barrel size of your eyepieces that you intend to use with it. The most widely used eyepiece barrel size is 1.25”. Some larger telescopes use 2” while cheaper telescopes use the smaller 0.965” size. Whatever the barrel size of your eyepiece, you select a Barlow lens with the same diameter.

What are the Different Types of Barlow Lenses?

Generally, there are 2 types of Barlow Lenses:

  • Standard or Long Barlow Lens – are usually 5″ to 6″ in length. They are used primarily in Newtonian reflectors. They have the disadvantage that they cannot be used in all types of telescopes because of their length. They can still be used, however, on telescopes that use a diagonal but it has a risk of protruding too far into the diagonal and damaging the mirror.
  • Short Barlow Lens – are about half the length of standard Barlow Lens. Because they are shorter “shorty’s,” they are compatible to be used in any telescope. It uses a stronger negative lens to be able to achieve the same magnification as that with the longer Barlow Lens. Short Barlows tend to introduce additional imperfections to the image quality concerns, mainly lateral color fringing and vignetting.

If you have a telescope compatible with a Long Barlow Lens, then this is the best choice. However, if your telescope is not compatible with a Long Barlow Lens, you have no choice but to use a “shorty” the short type. A good quality Achromatic “Shorty” will work wonders though.

What Makes a Good Quality Barlow Lens?

Different Barlow Lens from different manufacturers can give the same amount of magnification, but what separates a good quality Barlow Lens is the resulting image quality. The optical coatings depend much on the type of and quantity of coatings used on the lenses. They are ensuring maximum light transition and reducing internal reflections. If you have a chance to look in through the end of a Barlow, it should not be reduced down, or smaller internally. By gyrating it slowly toward a light source, you should not see glares.

Enjoying Your Barlow Lens

Using a Barlow Lens can be as satisfying as having a brand new, more powerful telescope without the huge added cost. However, this is not a magic tool that solves all of your telescope viewing problems and instantly makes your experience perfect. Astronomy does require knowledge and skill, so practice with your Barlow Lens. Practice makes perfect, so never stop learning about other aspects of astronomy. There are a lot more resources available on this website that can help guide you on the learning curve of this hobby.

Choosing & using a Barlow Lens

Considering all of the above — how do you actually choose the best possible Barlow lens for your telescope? Well, there are a couple of things that you need to keep in mind. But primarily — you need to make sure that it will be a good fit with your eyepiece’s tube. Generally, there aren’t many different diameter sizes of eyepiece tubes — it’s either going to be two inches or 1.25. If you remember that, you won’t have any issues with finding a lens that fits your telescope.

As we’ve mentioned above, there are a couple of different levels of magnifications you can achieve — depending on which Barlow lens you purchase. You can encounter the 2x lens most often — but there are 5x and 3x out there as well. However, at least for starters, we recommend you pick the 2x one.

Once you choose the Barlow lens that you want, you’ll proceed to install it luckily, it’s really a no-fuss addition to your telescope. So, instead of putting your eyepiece inside the focuser — you insert it into your Barlow lens. And you insert the eyepiece into that combo. Sooner than later, you’ll be able to enjoy all of the advantages of seeing the night sky closer than ever before!

Pros and Cons of Barlow Lenses

A Barlow lens is not an eyepiece, but it is a special lens that when placed in between the optics and the eyepiece, it increases the magnification of that eyepiece. Depending on the lens, it can double, triple, and even quadruple or quintuple the magnification!

How it works is they are inserted into the eyepiece barrel first, and then the eyepiece itself is placed on top of it into a special barrel.

Eyepieces and Barlow lenses are a very personal thing for every telescope user, as each individual will have their own tastes, viewing objects, eye relief needs, etc… It’s up to you to decide if you should invest in one. Here are the pros and cons of using them!

Pro – They Essentially Double Your Eyepiece Collection

Telescopes that accept standard 1.25″ eyepieces can accept any 1.25″ Barlow! Any eyepiece in your collection can be paired with the Barlow, which means your eyepiece collection has doubled.

Barlows virtually change the focal length of a telescope, thus a 1000 mm telescope is now virtually a 2000 mm focal length scope. That means you can say you have “two telescopes in one.”

Con – They Won’t Be Needed As Often

It just depends on what you’re observing.

Barlows can be a treat to use for high magnification views of the solar system objects, plus resolving double stars and tight star clusters, but they will more than likely not be used when viewing most deep sky objects as they get noticeably dimmer at higher magnification.

Another factor is of course the atmosphere, which when the seeing conditions are bad, then using them for higher magnification becomes pointless due to the image appearing too blurry and non resolvable.

Pro – They Can Give High Magnification Views in a Lower Power Eyepiece

Remember that the wider the mm of the eyepiece, the lower the magnification.

A 2x Barlow lens will double the magnification of any eyepiece you use. If your 20 mm eyepiece gives you 50x magnification, then using the Barlow will give it 100x.

I have found that when showcasing the solar system at high magnification, casual viewers have an easier time looking through wider eyepieces than through narrower. The eye relief is simply more comfortable.

This comes in handy especially for people who need glasses, as it’s much easier to look into a wider eyepiece than it is into a narrow, and the wearer still gets the views they desire.

Con – The Same Views Can Simply Be Achieved with the Correct Eyepiece

A lot of users don’t like messing around with changing magnification too much. It is easier to insert a single eyepiece into the barrel than to insert a Barlow into the barrel and then the eyepiece into that.

While a Barlow can increase the magnification, so can simply switching to a higher powered eyepiece. On a 1000 mm focal length telescope, a 10 mm gets you 100x, an 8 mm gets you 125x, and a 6 mm gets you 150x. But when you’re using a refractor or an SCT with longer focal lengths well over 1000 mm, then you may not need a Barlow lens if the eyepieces are already doing the job.

Pro – Barlow Lenses Can be Stacked

Let’s say you happen to have more than one Barlow, whether it being by separate purchase or from buying kits that included them. You might have a couple 2x Barlow lenses on your hands, but you see that attractive looking expensive 4x lens on the market. Good news, you can stack two 2x Barlow lenses and get the same 4x power!

Stacking Barlows doesn’t add the factors, it multiples them – thus stacking a 2x with a 3x gets you 6x. Now, on your 1000 mm focal length scope, with a 25 mm eyepiece (40x) paired with that 6x stack, you’re getting a 240x magnification view!

Con – They Add Too Much Bulk and Give Narrow Views

Even when just using a 1.25″ shorty 2x Barlow, you’ll notice that it makes the eyepiece you’re using appear to stick out of the tube like a cancerous tumor. 3x’s and higher usually have longer barrels, and thus stick out more. When you’re stacking them, or simply using larger 2″ eyepieces, then they are even longer!

An example of stacked Barlow lenses. The eyepiece is inside the 2X, the 2X is in the 3X, and the 3X is in the focus barrel. It definitely sticks out of the tube!

This can not only throw the telescope off balance and put more strain on your focuser, but the views through the longer barrels are narrow, and your eye needs to be near perfectly aligned to see the object, otherwise all you’re seeing is blank.

When viewing the Moon, the longer barrels can reflect the bright moonlight, giving a tough glare to view around.

114GT mod to remove in-focuser lens?

  • topic starter

#2 StarWars

The cheap lens/barlow in the focuser in my 114GT has to go. I heard tell of directions around suggesting if you remove the lens, you need to shorten the telescope tube by about 1/2". Anyone heard of such a thing? Or perhaps know of an URL describing the proceedure?

The NexStar 114GT Project to improve the scope.

#3 Syzygy

Why go through all those gyrations on an inexpensive tube when you could get a better one for just a few more bucks?

#4 Tom L

Exactly John! It isn't worth it. The mount by itself, now. that is something else entirely! I have a 114GT tube sitting on my workbench but an StellarVue 80/9d firmly attached to the mount.

That is the beauty of this scope. You can outgrow it and still put it to good use. The Baader Nexstar bracket can help you convert your mount (or a block of wood like I used and save $80).

To turn your mount into a GoTo AltAz mount for a small refractor

and Voila! A small scope with GoTo for a minimal amount of money! Here's mine.

How to Pick a Barlow

A Barlow is an auxiliary lens system in a tube that mounts in front of the focal
point of a telescope, between the telescope and eyepiece. It increases the power
of any eyepiece used with the telescope by increasing the telescope's effective
focal length. (Since magnification equals telescope focal length divided by eyepiece
focal length, a longer telescope focal length means higher power with any given
eyepiece.) A Barlow effectively doubles the number of eyepieces you own - at less
cost than buying that many new eyepieces individually.

While a Barlow increases eyepiece power, it does not change the longer eye relief
typical of lower power eyepieces (in fact, actually increasing it somewhat in some
cases). This allows eyeglass wearers to see a full field of view at high power without
having to remove their glasses to get close to the eyepiece, as they would have
to with a shorter focal length eyepiece alone.

Barlows have an undeservedly poor reputation among those who start astronomy with
inexpensive toy store telescopes that use toy plastic one-lens Barlows. A good Barlow
is not a toy, and does not deserve a toy's reputation. It is a precision multi-lens
all-glass optical system, just as an eyepiece is, and can actually improve the performance
of many eyepieces. While most Barlows are achromatic doublet (two-lens) systems,
some are triplet (three-lens) apochromatic systems or even four-lens systems consisting
of two complementary doublets.

By increasing the effective focal length of a fast to medium focal ratio telescope,
the diverging lens optics of a Barlow give inexpensive or moderately priced eyepieces
a slower-converging and easier-to-handle light cone to deal with. This results in
lower astigmatism and better color correction at the edge of the eyepiece field.

A Barlow can similarly improve the edge sharpness of good quality wide angle eyepieces
used with fast focal ratio telescopes. It should be considered in lieu of higher
power eyepieces alone for star cluster and planetary nebula viewing with such a

Barlows that are designed for long focal ratio telescopes may vignette (cut off)
the edge of the image when used with low power wide angle eyepieces on a fast focal
ratio scope.

An eyepiece and Barlow combination will have a slightly (2%-4%) dimmer image and
slightly lower contrast than the short eye relief/short focal length eyepiece alone
that is needed to get the same power as the eyepiece/Barlow combination.

The lower-priced Barlows we carry may introduce minor astigmatism and spurious color
at the edges of the field with fast focal ratio scopes. However, since a Barlow
is normally used to look at objects at high power in the center of the field (where
the image is unaffected), this is a scarcely insurmountable drawback. Many observers,
particularly eyeglass wearers, find that the potential of long eye relief at high
powers with a Barlow, and its ability to multiply their eyepiece collection at modest
cost, more than outweigh this minor shortcoming.

Be certain when you buy a Barlow that you do not duplicate eyepiece focal lengths
you already have - or plan to buy. If you have a 26mm eyepiece now, a two power
(2x) Barlow would effectively give you a new 13mm eyepiece, so it wouldn't make
sense to buy a 12mm or 13mm eyepiece later, since you'd merely be duplicating magnifications.
A 15mm to 17mm or 8mm to 10mm eyepiece would make more sense, as any of them would
give you unduplicated powers when combined with the 2x Barlow and 26mm eyepiece.

Magnifications specified for Barlows only hold true if the eyepiece is mounted directly
in the Barlow and the Barlow is used with a reflector, with a refractor used without
a star diagonal, or mounted in the star diagonal of a catadioptric or refractor
scope. Generally, if the Barlow is installed between the visual back (or
focuser drawtube) and the star diagonal of a refractor or catadioptric (in other
words in front of the diagonal), the projection distance from the Barlow's lens
to the eyepiece will be increased and the resulting magnification can be as much
as 50% higher than that specified on the Barlow. However, some Barlows are inconvenient
to use in a star diagonal because of their length, or are physically incompatible,
and are so noted in the individual Barlow descriptions.

Barlows are optimized for one specific lens to focal point spacing. Using them at
other spacings, such as in front of a diagonal as mentioned above, can reduce the
performance slightly at these other spacings. Variable Barlows, that change magnifications
by varying the lens/focal point spacing, tend to be optical underperformers for
this reason. Also, variable Barlows require a layer of lubrication between their
lens cell and the tube it rides in to allow free movement of the cell. At the least,
this can introduce internal reflections that may degrade the image, and the possible
creep of lubrication onto the optics due to summer heat at the worst. It is for
these reasons that we don't carry variable power Barlows.

The Barlow lens was invented in 1834 by English physicist and mathematician Peter
Barlow (1776-1862).


In its astronomical use, a Barlow lens may be placed immediately before an eyepiece to effectively decrease the eyepiece's focal length by the amount of the Barlow's divergence. [1] Since the magnification provided by a telescope and eyepiece is equal to the telescope's focal length divided by the eyepiece's focal length, this has the effect of increasing the magnification of the image.

Astronomical Barlow lenses are rated for the amount of magnification they induce. Most commonly, Barlow lenses are 2x or 3x, but adjustable Barlows are also available. The power of an adjustable Barlow lens is changed by adding an extension tube between the Barlow and the eyepiece to increase the magnification.

The amount of magnification is one more than the distance between the Barlow lens and the eyepiece lens, when the distance is measured in units of the focal length of the Barlow lens. A standard Barlow lens is housed in a tube that is one Barlow focal-length long, so that a focusing lens inserted into the end of the tube will be separated from the Barlow lens at the other end by one Barlow focal-length, and hence produce a 2x magnification over and above what the eyepiece would have produced alone. If the length of a standard 2x Barlow lens' tube is doubled, then the lenses are separated by 2 Barlow focal lengths and it becomes a 3x Barlow. Similarly, if the tube length is tripled, then the lenses are separated by 3 Barlow focal lengths and it becomes a 4x Barlow, and so on.

A common misconception is that higher magnification equates to a higher-quality image. However, in practice, the quality of the image generally depends on the quality of the optics (lenses) and viewing conditions, not on magnification.

Teleconverters are variations on Barlow lenses that have been adapted for photographic use. [2] A teleconverter increases the effective focal length of the photographic lens it is attached to, making it a telephoto lens. A true telephoto lens uses a configuration similar to a Barlow lens to obtain a shorter tube length for a given focal length.

In microscopy the Barlow lens is used to increase working distance and decrease magnification. The lenses are "objective lenses" that are mounted in front of the microscope's last objective element. Barlow lenses for microscopes can be found with magnifications ranging from 0.3× to 2×. Some standard lenses are 2×, which decreases the working distance by half and doubles the magnification, 0.75× (3/4×), which increases the working distance by 4/3× (1.33×) and decreases the magnification by 0.75×, and a 0.5× Barlow doubles the working distance and halves the magnification.