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Thread: Meade 6.4 mm Plossl (52°) for Astromaster 90 Az ?

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    Default Meade 6.4 mm Plossl (52°) for Astromaster 90 Az ?



    Hello all!

    I have a Celestron Astromaster 90 Az (1000mm) that comes with 20mm and 10mm eyepieces

    This is a very basic refractor telescope and does its job good for planets and the moon. Im happy with it and Im not looking for high end gear.

    But of course, it has its limitations. Starting with 20mm eyepiece it takes me some time to find a star or planet. Then moving into 10mm eyepiece and I struggle to get it. The Az mount doesn't help

    I was thinking about getting a 6.4 mm inexpensive eyepiece, just to see how the telescope performs. Maybe that would be its limit.

    What do you think? Is it worth? Im happy with 10mm basic eyepiece, just curious in going farther.,...

    Thanks!!
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    Default Re: Meade 6.4 mm Plossl (52°) for Astromaster 90 Az ?

    That's really pushing the abilities of the scope. I believe you would get a lot more use from an 8mm EP and still there would be a lot of times when that would be too much. I would opt for a 15mm EP to fit between the 20 and 10 along with a 2x barlow. The barlow wouldn't be useful with the 20 as you already have a 10 but you could use it with the 15 for 7.5 and on the very rare occasion with the 10 for 5. then you would have 20, 15, 10, 7.5, and 5.
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    Default Re: Meade 6.4 mm Plossl (52°) for Astromaster 90 Az ?

    I think you should be able to push your scope to 156x on moon and planets with 6.4mm Plossl. However, it will require good seeing conditions, so you may not be able to use it every night.

    Another thing to take into account is that 6.4mm Plossl has only 5mm eye relief and at higher magnification objects will move faster out of the view. Thus, it will take some practice to use it, but it is definitely doable.
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    Default Re: Meade 6.4 mm Plossl (52°) for Astromaster 90 Az ?

    Quote Originally Posted by Ozman13 View Post
    That's really pushing the abilities of the scope. I believe you would get a lot more use from an 8mm EP and still there would be a lot of times when that would be too much. I would opt for a 15mm EP to fit between the 20 and 10 along with a 2x barlow. The barlow wouldn't be useful with the 20 as you already have a 10 but you could use it with the 15 for 7.5 and on the very rare occasion with the 10 for 5. then you would have 20, 15, 10, 7.5, and 5.
    I had 90mm/660mm (F7.3) achro and was getting very nice views of Jupiter, Saturn and Moon last summer using 5mm EP (132x). Also used it to split doubles quite often. Joe has F11 scope, so I think he should be able to push it a bit further, assuming good seeing
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    Default Re: Meade 6.4 mm Plossl (52°) for Astromaster 90 Az ?

    Hello again, thanks for your well thought answers.

    Actually, I saw this Zoom EP Meade 8 mm(40°AFOV) -24 mm (60° AFOV)

    Meade Series 4000 8mm-24mm Zoom Eyepiece (1.25")

    Are Zoom EP worth in terms of quality? Aren't fixed focal length better?

    Thanks

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    Default Re: Meade 6.4 mm Plossl (52°) for Astromaster 90 Az ?

    Quote Originally Posted by Joe GM View Post
    I have a Celestron Astromaster 90 Az (1000mm) that comes with 20mm and 10mm eyepieces

    This is a very basic refractor telescope and does its job good for planets and the moon. Im happy with it and Im not looking for high end gear.

    But of course, it has its limitations. Starting with 20mm eyepiece it takes me some time to find a star or planet. Then moving into 10mm eyepiece and I struggle to get it. The Az mount doesn't help

    I was thinking about getting a 6.4 mm inexpensive eyepiece, just to see how the telescope performs. Maybe that would be its limit.
    Take a quick look at these web pages and be sure to scroll all the way through.

    Telescope Equations

    How to Size and Set Up a Scope

    Joe, I'm sure that you don't necessarily want to get bogged down in the equations when you're just wanting to acquire one additional eyepiece. It might however be a good idea for you to have a quick look at the range of magnifications that your scope could realistically handle.

    Your scope's aperture = 90mm and focal length = 1000mm

    From those specs alone, you can calculate the focal ratio as follows:

    f/ratio = focal length ÷ aperture = 1000mm ÷ 90mm = 11.1 say 11

    Some rules of thumb then:

    Maximum useful magnification on an average night = aperture in mm = 90x

    You should not expect that scope to yield much more than 90x unless you have pristine atmospheric conditions. Even after four years of doing this, I don't know what a pristine evening looks like yet.

    The maximum useful magnification is given to you when you use an eyepiece whose focal length is the same as your f/ratio in mm.

    In your case, since f/ratio = 11, you will need an 11mm focal length eyepiece minimum.

    The shortest focal length eyepiece that you should consider would probably be an 11mm.

    If you try to use an eyepiece having a focal length shorter than 11mm, you might find that the delivered magnification starts to increase beyond that which the scope can reasonably deliver.

    Lastly, the lowest magnification that you should want out of your scope would be that calculated from

    Exit pupil = eyepiece focal length ÷ f/ratio which, when rearranged mathematically gives:

    Eyepiece focal length - exit pupil x f/ratio

    Assuming that your maximum dilated pupil is 7mm, then the longest focal length eyepiece that you should be wanting to use would be

    Longest focal length (lowest power) eyepiece = 7mm x 11 = 77mm

    Since it's a tough call finding those, you may have to settle for your longest focal length eyepiece as something in the order of 30mm - 40mm.

    You can use these shortest and longest focal lengths, along with the recommendations of your other readers, to answer your own question in post#5 as it pertains to an 8mm - 24mm zoom eyepiece.

    And as always, these are just 'rules of thumb' for which almost everyone has an anecdotal alternative. This includes even yourself since you're already using a 10mm eyepiece as your your minimum.

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    Default Re: Meade 6.4 mm Plossl (52°) for Astromaster 90 Az ?

    Quote Originally Posted by Joe GM View Post
    Hello again, thanks for your well thought answers.

    Actually, I saw this Zoom EP Meade 8 mm(40°AFOV) -24 mm (60° AFOV)

    Meade Series 4000 8mm-24mm Zoom Eyepiece (1.25")

    Are Zoom EP worth in terms of quality? Aren't fixed focal length better?

    Thanks
    Generally speaking fixed FL EPs deliver better quality than zoom. Most of the time it is also cheaper to get two fixed length EP than a zoom EP.

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    Default Re: Meade 6.4 mm Plossl (52°) for Astromaster 90 Az ?

    Concerning the zoom, many people like them for the variety of focal lengths in one EP but it has a comparatively narrow field of view so many do not. A dedicated EP is generally considered to be superior (I also prefer dedicated EPs) but they have a place in many people's grab and go set up just for the convenience.

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    Default Re: Meade 6.4 mm Plossl (52°) for Astromaster 90 Az ?

    The eye relief will be considerably tight on a 6.4mm plossl. For a little more perhaps look into the 7mm X-Cel LX, which can be found for around $64 if you browse around on Amazon. The eye relief will be much better and for the price the X-Cels are excellent performers. I actually did a comparison between a $64 12mm X-Cel LX and a $340 12mm TeleVue EP, and while the TV was definitely better, the results were surprisingly close!
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    Default Re: Meade 6.4 mm Plossl (52°) for Astromaster 90 Az ?

    Hello all again!
    Thanks for your well thought answers and your time!

    Phillip, I appreciate your post, well written and you already did the job for me, thanks a lot!
    Great websites those you pointed, Im still reading. I downloaded the Power Point file, these are clear explanations any novice should go thoroughly through. Those pages are worth spending time on them...

    As a result of all of your answers, and based on my own limited experience, with the 6.4mm EP will have a hard time. Exit pupil would be 0.5mm thats truly tiny. I already struggle with my current 10mm EP that has an exit pupil of 1mm.
    This is in addition to the EP delivering past the point of 90x.

    Another point is that I have an Az mount which is not precise and hard to handle. With such a mount I sometimes have a hard time finding objects even with the 20mm. And then moving into 10mm EP then often have to adjust the position of scope (no fine tuning screws). Now going down to 6.4mm will be difficult. It remains the issue of tracking the object with the Az mount as the Earth moves.

    As wingsfan mentioned, the eye relief is another important issue. I think the 6.4mm EP gives a 2mm eye relief which is too tight. I don't wear glasses but its still too tight.

    Ok, I guess I probably better look for a longer focal length EP, it is, going on the opposite direction, maybe a 40mm EP that allows me to find objects easily...

    Just another question, out of curiosity, how about those "advanced" (at least they are "advanced" for me) EP. Ive seen this one:

    Meade Series 5000 HD-60 6.5mm 6-Element Eyepiece (1.25")

    Meade claims it has a 17mm eye relief which I think sounds pretty comfortable...


    Ok, thanks again for your time and answers, I guess after all you have said, I rather will look for a low magnification, brightest, wider FOV eyepiece, with a longer eye relief and exit pupil.....

    I also enjoy low magnification! A 40mm Ep will give me 25x which is close to a binocular and I love watching with my 15x binos!!

    (by the way, I refer to Meade products but it is not that I love Meade products, they are good and I certainly like them, but it is more that my Local Astronomy Shop sells mainly Meade stuff)

 

 
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