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Thread: How do I use these eye pieces?

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    tommyowen's Avatar
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    Unhappy How do I use these eye pieces?



    Hi.

    Very amateur question here. Pardon my ignorance.

    I just ordered two new lenses:

    - 2x Barlow
    - Super Plossl 32mm

    I'm not exactly sure how or when to use these. I assume that the 2x Barlow simply doubles the size of elements but in what situations would I use the 32mm?

    Any suggestions appreciated!

    T

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    Default Re: How do I use these eye pieces?

    you can use the 32mm without the barlow for less zoom to see larger objects (like nebulae and galaxies). If you want to zoom in to something (like planets) you put the barlow in between the eyepiece and the telescope focuser.
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    Default Re: How do I use these eye pieces?

    With the 2 x barlow your 32mm would be similiar in magnification to a 16mm so greater magnification.
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    Default Re: How do I use these eye pieces?

    You place the 2x Barlow into the focuser first and make sure that it's secure. You then place the eyepiece into the top of the Barlow and make sure that it's secure. Some Barlows come with one set-screw, some come with two set-screws.

    The purpose of the Barlow is to change the effective length of the telescope. If you have a 2x Barlow, then it will double the specified focal length of the scope. If you have a 3x Barlow, then it will triple the specified focal length of the scope.

    Assume your scope has a focal length of 1200mm. The 2x Barlow will double that to 2 x 1200mm = 2400mm.

    Magnification = focal length telescope / focal length eyepiece

    Using your 32mm Ploss eyepiece by itself:

    Magnification = 1200mm / 32mm = 37.5x

    Using your 2x Barlow plus your 32mm Plossl eyepiece:

    Magnification = 2400mm / 32mm = 75x

    As you can see, the 2x Barlow will double the magnification of the 32mm Plossl.

    The downside of doubling the magnification of the eyepiece with a 2x Barlow is that, as a result, you will also reduce the field of view (FOV) that you see in the eyepiece by half. In other words the circular portion of the sky that you see in the scope/2x Barlow/ eyepiece combination will now only be half as big.

    The objects seen in the FOV will appear twice as large when using the 2x Barlow but the diameter of the circular disc of Space that you can see will only be half as much as before.

    Some will consider that the 2x Barlow has the effect of halving the focal length of each of their eyepieces. This rather than considering the 2x Barlow as doubling the focal length of the scope. In the one case , the Barlow is working on the telescope. In the other case, the Barlow is working on the eyepiece. The net effect in either case is the same. You are affectively doubling the number of eyepieces that you have in your eyepiece case.

    Using a 2x Barlow can also increase the eye relief of those Plossl eyepieces that have very short focal lengths, say 12mm focal length and below. If you can not use 6mm Plossl eyepiece because its eye relief is too short for you, then you can add a 2x Barlow to a 12mm Plossl eyepiece which will yield an effective 6mm focal length and an eye relief that is marginally longer than that of a 12mm Plossl by itself. This increase in eye relief is best seen/experienced in longer focal length eyepieces such as your 32mm Plossl.

    Warning: Due to the last statement above, when using a 2x Barlow you might experience some initial difficulty placing your eye in the correct position behind/above the 32mm Plossl. This initial problem of eye placement when using the 2x Barlow and a 32mm Plossl, due to the increased eye relief, goes away quite quickly with practice and experience. You might find some advantage in using your one hand to steady your eye socket above the eye lens of the 32mm Plossl. Think of the fingers of your hand as an extendable eye cup.

    Hint: If you don't yet know what eye relief is, use Google Image search and the hits will show you graphically.

    Lastly, some Barlows come with a removable Barlow lens-set which can be screwed directly onto the bottom end of your eyepiece much in the same way that you can add a filter to the eyepiece. If yours can do this, then the Barlow factor for the lens-set, by itself on the eyepiece, is about 1.5x rather than 2x. All else is as above.

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    Default Re: How do I use these eye pieces?

    So it's not a good idea to use the Barlow when viewing galaxies and nebulae's?

    T

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    Default Re: How do I use these eye pieces?

    mostly correct. you usually want a wide field of view because deep sky objects are typically pretty large.

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    Default Re: How do I use these eye pieces?

    Quote Originally Posted by tommyowen View Post
    So it's not a good idea to use the Barlow when viewing galaxies and nebulae's?

    T
    It is going to depend on the size of the object you are looking at. For example you would not use it when viewing the Andromeda galaxy. On the other hand observing the Crab nebula it could be used to reveal more detail.

    Don't stress too much. Not sure - stick it in and see what happens. You wont break anything, and if the view is rubbish, take the barlow out and enjoy inspecting the target. All up it takes 8 seconds to put a barlow in and take it out. Easy peasey
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    Default Re: How do I use these eye pieces?

    If you're concerned about the reduced FOV then, as far as galaxies are concerned, do not worry. For the most part galaxies are small and will fit into the FOV of high magnification eyepieces.

    This might be an issue with the larger wide spread out nebula, and if so, you can then scroll around the wide expanse of the nebula like you would on a very large image in your favourite graphic viewer.

    The point here is that if you can see the galaxy in a 12mm eyepiece, you will see that galaxy in a 24mm eyepiece having the same apparent field of view (AFOV).

    Although you appear to be beginner, you would be well served to read up on the following terms:

    Apparent field of view (AFOV)
    Magnification
    True field of view (TFOV)
    Focal ratio (f/ratio)
    Exit pupil

    Some will opine that the optimal eyepiece for viewing Deep Space Objects (DSOs) like galaxies will be one that yields an exit pupil of 2mm. In my 8" 1200mm f/6 scope that means a 12mm eyepiece.

    If the TFOV of this eyepiece, say a 12mm Plossl, is too narrow for me to comfortably fit the entire object into its FOV, then I must choose a 12mm eyepiece having a wider apparent field of view (AFOV), say a 68°, 82° or perhaps even one of those pineapple type monsters that come in with an AFOV of 100°.

    Your new 32mm Plossl will have a AFOV of about 50°, some claim 52°.

    The Orion Nebula (M42) in the constellation of Orion (well, duh?) is a useful example of a easily visible nebula using even the smallest AFOV/TFOV eyepieces.

    If you're not yet using the free planetarium program called Stellarium, give it a go.

    Stellarium has an ocular plugin/tool that allows you view an object with a specific telescope/eyepiece combination. This will show you the TFOV of that combination and whether or not the object will fit into it. Galaxies mostly do, well certainly the brighter parts of those galaxies that we can see. The Andromeda Galaxy M31 (some call it a Nebula) is a good example of this. It's a huge wide galaxy but for the most part, the typical backyard astronomer will only be able to see the smaller brighter core.

    Lower power, wider AFOV eyepieces are sometimes used as scanners and finders when searching for required DSO targets. Then when found, the observer will start increasing the magnification, and reducing the FOV, until the optimal view and/or resolution is reached.

    You might experience more difficulties when viewing larger open clusters or pairs of open clusters. Consider the Pleiades (Seven Sisters) or the Double Cluster for instance. These will not fit easily into the FOV of your new 32mm Plossl even without the 2x Barlow. (Of course, this is based on my 1200mm focal length scope as I don't know the focal length of your scope.)
    Last edited by PhilipLangley; 02-27-2017 at 07:20 PM.

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    Default Re: How do I use these eye pieces?

    What scope are you using?

    Whether or not it's appropriate to use the barlow will depend on the size of the object and the field of view your scope presents with a given eyepiece. There is no general rule that says don't use 16mm for galaxies because a 16mm in an f/12 scope is going to give a completely different view than it would in an f/5 scope. Aperture makes a difference as well. So an f/5 130mm (5") scope is going to present a less magnified view than an f/5 305mm (12"). A 16mm might be the perfect eyepiece for a particular object in an f/5 5" scope (1.28° view) but not in an f/5 12" scope (0.55° view).

    For my f/5 newtonians, 16mm gives a good midrange view for that scope. I've viewed galaxies with 9mm eyepieces in a 12" scope. But in an f/10 SCT, a 9mm would be too much magnification on just about everything but planets or the moon unless it was a perfect night in very dark skies. Most galaxies are small in the eyepiece, often looking like faint smudges. However, ones like the Andromeda Galaxy are so large, you want to use the lowest magnification possible to get as much in the field of view as possible.

    Bottom line: it depends.
    The best way to see if it helps is to try it. Center it with your 32mm and then add the barlow.
    Rob

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    Default Re: How do I use these eye pieces?

    Barlows also dim the view so for faint fuzzies like Galaxies I personally don't use them for that. Mostly On brighter objects.
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