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  1. #1
    Holographic's Avatar
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    Default Plossl, barlows, etc.. what are they good for?



    I see alot of people around here have a huge amount of atachemnts, some or most of it i dont quite understand what are they for or what do they do, or if i should have any of them.

    Maybe there are more people like me, that are new here, and havent quite figured it out too, maybe this would be a good place to put some questions


    So, my questions are.

    What exactly is a Plossl and what does it do? I know it is an eyepiece (i think), but is it diferent from "regular" eyepieces? In what type of telescopes should you use a plossl? Is it better to see planets than my regular 20mm eyepiece that came with my telescope?

    And barlows. Are barlows used together with your eyepiece to increase magnification ? Do you recomend geting one to observe planets?

    My telescope is a celestron astro master 114eq, and i was wondering if i need, or should get any of those accessories in the future.

    thanks in advance to anyone who can give a little help!

  2. #2
    OleCuss's Avatar
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    Quote Originally Posted by Holographic View Post
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    What exactly is a Plossl and what does it do? I know it is an eyepiece (i think), but is it diferent from "regular" eyepieces?
    You are correct about the Plossl being a particular kind of eyepiece. It has 4 elements in it and for many purposes is a pretty decent EP (EyePiece). Because of its popularity I actually consider the Plossl-type eyepieces to be the "regular" eyepieces.

    In what type of telescopes should you use a plossl?
    There really isn't a great answer to these questions - and there is some disagreement. IMHO, a Plossl is a good EP for use with telescopes with a focal ratio of 6 or more. But there are people who happily use them with much faster (focal ratio less than 6) telescopes.

    Is it better to see planets than my regular 20mm eyepiece that came with my telescope?
    It depends on what you mean. In order to be able to give you a really good answer we'd need to know what telescope you have and what kind of eyepiece you currently have (it actually might be a Plossl).

    But part of the problem is that your question may be skirting a misconception.

    Most of us prefer rather low power (magnification) for most of the DSO's (Deep Sky Objects) and rather higher power for use on the planets. Last night I had my LX200 out and when I was looking for nebulae I was mostly using around 83x but when I was looking at Saturn I was kicking it up to about 120x (the seeing was not good enough for higher power). Usually for planets most of us really want to get up to 150x or higher (sometimes a lot higher if we've got a big telescope and great seeing conditions).

    But power is determined by the ratio of your telescopes focal length divided by the focal length of your telescope.

    Your telescope has a focal length of 1,000mm. To achieve a magnification of 150x you would need an eyepiece with a focal length of about 6.7mm.

    The thing is, however, that your telescope's objective (primary mirror) measures about 114mm. That means that it will be very seldom that you will have good enough seeing to be able to use a magnification of greater than 114x - doing so under typical conditions would mean that you have a larger but blurrier image.

    Now, changing the focus just a little. Your telescope seems to come from Celestron with 20mm and 10mm eyepieces. Celestron does not tell use what kind of eyepiece they are so it is likely that at best they are of the Kellner design. A Plossl eyepiece or two would be highly likely to be an improvement over what you have.

    [quote]And barlows. Are barlows used together with your eyepiece to increase magnification?[quote]

    To put it simply, yes.

    But a little longer answer might be that a Barlow effectively increases the focal length of your telescope (at least so far as your eyepiece is concerned). That means that your magnification will be greater with any given eyepiece.

    Do you recomend geting one to observe planets?
    I would not necessarily recommend a Barlow for absolutely everyone to use for looking at planets. But many people will benefit from using a good Barlow for looking at the planets.

    There are several issues I can think of that come into the considerations, but let's look at yours:

    1. Your 10mm eyepiece will give you a magnification of about 100x. It won't be very often that you will be able to use more than 100x so if that 10mm eyepiece is of much better quality than I suspect, a Barlow won't often be all that beneficial.

    2. But I would guess that your 10mm eyepiece is not of great quality and will likely have poor eye relief (meaning that you have to get your eyeball right up against the EP in order to see anything and if you move your head a tiny bit that you'll not be able to see anything). Your 20mm EP likely has better eye relief so using a 2x Barlow with your 20mm EP may be a much more pleasant experience than is using your 10mm EP.

    3. If you had extremely excellent seeing conditions you might be able to push your magnification as high as around 200x. That means that on very rare occasions you might benefit from having a 3x or even 4x Barlow with which to use your 20mm eyepiece.

    4. Depending on the quality of your eyepieces a 2x Barlow may or may not be of any benefit to you. If your 10mm eyepiece is of very high quality (and has good eye relief), then using a 2X Barlow with your 10mm EP would give you 200X for those very rare occasions on which you have the seeing conditions which will allow that.

    My telescope is a celestron astro master 114eq, and i was wondering if i need, or should get any of those accessories in the future.
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    That will be a personal choice for you depending on what you want or need and your budget.

    Your telescope is f/8.77 (has a focal ratio of 8.77 - calculated by dividing the 1,000mm focal length of your telescope by the diameter of its 114mm objective mirror). I consider that to be a medium speed telescope and I really like Plossl eyepieces for that type of telescope.

    Since I doubt your eyepieces are as good as Plossl eyepieces would be, I would at least think about getting several. But that's what I think I would do - which is not necessarily what you should do.

    If you did get new eyepieces for your telescope, then getting a Barlow may or may not be a good idea depending on what you purchased. Just to make a guess, a 3x Barlow would probably be occasionally useful with your current 20mm EP and if you purchase new eyepieces there is a good chance that buying a 2x Barlow will be a good idea. There are too many unknowns to be certain.

    FWIW

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  4. #3
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    It looks like your questions have already been answered, but I was wondering about the different types of eyepieces too yesterday, and found this page with diagrams and descriptions of them all: Eyepieces Telescope Eyepiece Types Design Variations oculars

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    Well, OleCuss. Thats one hell of a detailed anwser! Thanks alot! It really answered alot of my questions, including some i had not yet posed in here, but were definitely around my head.

    VincentMcKenzie:
    I had seen something like that in wikipedia too ( Eyepiece - Wikipedia, the free encyclopedia ), but my doubts were more towards how and in wich conditions should the diferent eyepieces be used, and not how they actually work.
    Although knowing how it works its pretty cool too.

    As we say in here "Knowledge doesnt take physical space". roughly translated.

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    You CAN USE Plossl eyepieces in SLOW telescopes , let's say with a F number greater than about F6 and they will give good views for the low cost. In faster scopes like an F6 or below you need newer designs and higher cost eyepieces. AND don't forget about eye relief if you wear glasses, and a wider field of view (over 50-55degrees), which are both available in higher cost eyepieces.

    Great answer by OleCuss!
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