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  1. #1
    Hawker22's Avatar
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    Default Question about EP's



    I have a Helios(SkyWatcher) 200P Newtonian Reflector with a 1000mm FL. I reckon that makes it an f5 scope. Two EP's and a 2X Barlow came with the scope--a Plossl 10mm and a Plossl 20mm. From what I know, that gives the scope a 50X--100X--200X mag. power.
    So why would there be that particular combo? The barlowed 20mm EP gives me the same power as the 10mm EP without the barlow.
    Is this not duplication, or is there any advantage to using a barlow other than boosting power?

    Also, there is a new Zhumell PL9mm fully coated 1.25" EP advertised locally for $35. Is that a decent price and would it be of any value to me in this case--or should I be looking for a 25mm EP?

  2. #2
    DaveW's Avatar
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    Default

    Does the scope come with a 2" or a 1.25" focuser?

    I wouldn't bother with the 9mm Plossl, it's not different enough from your 10mm to be worthwhile. If you have a 2" focuser, I'd look for something like a cheap GSO 30mm 2" 70 degree eyepiece or the Meade one on sale at Agena.
    Meade Series 4000 2" QX Wide Angle Eyepiece - 30mm
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  3. #3
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    Default

    Good question! answer "WHO KNOWS!" Ha!

    You probably should get one 2 inch wide field eyepiece say something like a 30-40mm

    and also one 6 mm for the few times when you can apply that much magnification. Then stop buying and use what you have. On second thought FORGET about the 6mm.

    Clear skies to you.
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    Hawker22's Avatar
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    Default

    Yes, it has a 2" focuser with a 1.25" adapter. So----look for a 2" EP that is 30-40mm. I'm guessing that this will give me a wider FOV.
    Thanks

    And what about the other part of my question--my 10mm and 20mm EP's

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  6. #5
    Joe Lalumia's Avatar
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    Default

    With the barlow you already have a 5mm.

    Yes-- get a wide field low power 2 inch-- as DaveW mentioned above.

    I predict it will become one of your most used eyepieces.
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    Default

    There is no good rhyme or reason about it (20mm, a 10mm, and a 2X barlow).

    There is one thing the barlow brings to the party, and that is eye relief. The 20mm EP will have greater eye relief than the 10mm EP. Barlowing the 20mm into a 10mm equivalent will preserve the eye relief of the 20mm while giving the magnification of the 10mm. In fact the way a barlow works increases the eye relief of the EP due to a lot of semi-complicated optical jargon.

    Given that you have a 20mm and 10mm Plössl EPs already and a 2X barlow already:: You should be looking at something in the 13-15mm range or something in the 26-30mm range.

    The 13-15mm one would barlow into a 7mm EP--probably about as high as you will need to go most of the time.

    The 26-30mm one would barlow into the 13-15mm range and fill in the missing range between 20mm and 10mm. The 26-30mm use will be a wide field of view as low a power as you can muster in 1.25" that will make finding things easier.

  8. #7
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    Default

    You guys sure do have a way of getting things into perspective. That is exactly the reasoning that I should have used in the first place.
    Question though. An f5 scope is medium fast, I would think, and many reviewers of a 2" 30mm wide fov EP claim that it gets beaten up pretty bad by a fast scope-but barlowing it will help. I guess there is no way to pair up a 2" EP with a 1.25 barlow, so that would mean a possible 2" barlow purchase.
    Or will I be happy just accepting the fuzzy outer edges. And what about the talked about limited eye relief?

    I am thinking about jumping on that Meade one at Agena.
    Helios 200P f5 Reflector on EQ5 mount and aluminum tripod--1.25 Plossl 10 & 20 EP's with 2X barlow

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  9. #8
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    Default

    Back again. I just spent about two hours surfing and reading. I almost pulled the trigger on the Meade 2" 30mm wide angle EP from Agena.
    But before I do it, I need to know this. Is there a comparable 1.25" EP ou there? And will I need to end up buying a 2" barlow if I get the 2" EP. Will it perform OK with my f5 scope?
    Answers to these questions should help me decide.
    Thanks
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  10. #9
    DaveW's Avatar
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    Default

    Quote Originally Posted by Hawker22 View Post
    Back again. I just spent about two hours surfing and reading. I almost pulled the trigger on the Meade 2" 30mm wide angle EP from Agena.
    But before I do it, I need to know this. Is there a comparable 1.25" EP ou there? And will I need to end up buying a 2" barlow if I get the 2" EP. Will it perform OK with my f5 scope?
    Answers to these questions should help me decide.
    Thanks
    There isn't a comparable 1.25" EP out there. With a 1.25" OD barrel there are limitations on the maximum size of the field stop. The formula for apparent FOV is AFOV=57.3*d/fe, where d is the field stop diameter and f is the eyepiece focal length in mm. For a given focal length (say 30mm) if the limit of your field stop is say 25.4mm (1") then the maximum apparent field of view would be 48.5 degree. My 1.25" 32mm Plossl has an AFOV of 44 degree compared to the 52 of all my other Plossls for just this reason.

    If you want a long focal length wide field eyepiece, you just have to move up to a 2" barrel EP.

    As for the barlow, I have a couple 2" EPs but no 2" barlow. It would be a nice thing to have, but not a requirement. If you want a higher magnification, you can use one of your existing 1.25" EPs until you either add a new 1.25" EP to your collection or buy a 2" barlow.

    As for how good this EP would be in your f/5 scope, I'm not sure. I know that Meade outsources their EPs to Taiwan, and the 5 elements in 3 groups sounds very similar to my GSO Superview. With my Superview at f/5, the stars are soft at the edge of the FOV, which I do find a little annoying at times. I don't find it annoying enough to spend the $310 on a 27mm Panoptic to get sharp stars to the edge of the FOV though, at least not yet.
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  11. #10
    Hawker22's Avatar
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    Default

    Thanks, DaveW. I just re-read some of Star Drops stickies, and it is amazing how much more I got out of it the second time around. And now, I think I understand the limits of the 1.25" EP's when it comes to fl and fov. So, to get a more relaxed fov EP, I think I need to take your initial advice and shop for that 2" 30mm 70* fov.
    Can't seem to find a better deal than at Agena--even their EBay stuff.
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