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Thread: Help choosing Eyepiece

  1. #1
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    Default Help choosing Eyepiece



    My old red Tasco (80mm, 900 FL) refractor has a single 23mm ep, of the old 0.965" type.
    If I hold a piece of paper 30cm behind the ep and open both eyes, I can see a projected circle of 16cm diameter on the paper.
    That gives an apparent FOV of 30deg. At 40x mag it means I am currently seeing a TFOV of 0.75deg.

    The ep has very little eye relief, so I cannot use it with my glasses. Since it will be a while before I can afford a decent, worthwhile upgrade on the scope, I would like to rather invest in a proper ep with nice eye relief and wide AFOV that will let me use the refractor to its full potential and that can also be used in subsequent scopes. I would like to push up the magnification to 100x or even a little more (I've read somewhere that these old red tascos can handle 150x if need be) for planetary viewing and splitting double stars, so I am looking at something with a FL of 6-9mm.

    One thing I am not sure about is the limit of the scopes usable true FOV. There is a baffle in the drawtube with an ID of 15mm. If I understand correctly, maximum TFOV will be:

    15/900 x 57.3 = 0.96deg

    Does that mean I will only start getting vignetting at 100x with an ep with AFOV with 100deg?

    I have only been able to find two options in that FL range that also satisfy the criteria of at least 20mm eye relief and wide AFOV:
    William Optics SPL 6mm (72deg AFOV) - about $130
    Orion Stratus 8mm (68deg AFOV) for around $195

    Skywatcher has a 6mm and 9mm in 66deg AFOV, but eye relief is not specified. Price around $60

    Any other suggestions?

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    Default Re: Help choosing Eyepiece

    Sorry I don't have any experience with the 0.965" type (I assume the diagonal is restricted to taking this size unless you buy an adapter to get up to 1.25" ? If the baffle tube is indeed only 15mm that is going to be a much more severe restriction.

    However, for what you've just quoted for the eyepieces (if you're really serious about spending nearly $200 on an EP for that scope?) I would seriously think to hold fire on that idea and look around for a second hand scope or even save up a bit more for a new one as the cost will not be much more (perhaps cheaper second hand) than what you're stating for just an eyepiece?
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    Default Re: Help choosing Eyepiece

    Being able to get 150x out of it might be a little optimistic, but it could work on exceptionally steady nights. Your math re field of view sounds correct.

    Your eyepiece options at 0.965" are severely limited. Are you planning on using a hybrid diagonal and 1.25" eyepieces? If so, your options are practically unlimited.
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    Default Re: Help choosing Eyepiece

    Thanks for the input.
    I will indeed need an adapter, since all the available ep's I have come across are 1.25". I have a friend who is a wizard on a lathe so that should not be a problem. They can also be bought for 25 bucks.
    The reason I do not want to get just an affordable 10mm Plossl is because any future scope I might buy will almost certainly include exactly just that, and will make the one I buy now instantly obsolete. A good quality wide angle ep will always find a place in a future arsenal of ep's, regardless of what scope I end up buying later.

    One of the reasons I want to get a wide angle ep in particular is the fact that I need to track with my eq mount slo-mo controls. At 40x the 30deg AFOV allows me just enough time to just observe before I need to twirl the controls again. At 100x, I will need a 75deg AFOV to be afforded the same time before the object whizzes out of view.

    I have considered having a round corrective lens made by my optometrist so that I can discard the long eye relief requirement, but even then there are no additional 70deg+ options that become available for less overall cost.

    My concern was using a wide angle ep with the restricted TFOV of the scope, but since TFOV = AFOV/mag, and rounding my TFOV to 1deg, I will not be wasting any AFOV as long as the magnification is equal to or greater than the AFOV, e.g. 72x mag for a 72deg AFOV ep.

    Apart from the quality of the optics and seeing conditions, what other factors limit the practical upper limit of usable magnification? Does exit pupil come into play at all?

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    Default Re: Help choosing Eyepiece

    Hello Eyescout,

    ther is not a problem with a 0.965"-to-1,25" adapter, Amazon.com : Solomark 0.965 to 1.25 Inch Telescope Eyepiece Adapter : Camera & Photo , the problem may arise with the vignetting on the ultrawide 1.25" eyepieces.
    The most recommendable way would be to get the adapter, and then to try out some eyepieces at the dealer's, or during the star parties.

    Otherwise, you can still get on the 2nd hand markets the discontinued 0.965" Carl Zeiss Jena PK-series (or A-series) eyepieces for the microscopes, like Carl Zeiss Jena Microscope Eyepiece PK 16 x 12 D 23 | eBay , which are basically derived from the Plössls, and the last production series with the T* multicoatings are excellent performers. I am still keeping them in my socks drawer, should I once find a nice refractor for these EPs. The field of view is like that of the other Plössls, but they seem to be better corrected. The A-series may be better suited for the achromatic optics, and it works fine on my 72mm F/4 fast Zeiss telelens converted to scope. With the old Zeiss eyepieces, the view is clear and crisp, also thanks to the Zeiss-proprietary glass materials.

    The Zeiss eyepieces would not adderss your looking for wider field of view, but they are worth of having.

    Best,

    JG
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    Default Re: Help choosing Eyepiece

    Since virtually no scope lists maximum TFOV angle in the specifications, how would you know if a certain wide-angle ep will cause vignetting?

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    Default Re: Help choosing Eyepiece

    Quote Originally Posted by eyescout View Post
    Since virtually no scope lists maximum TFOV angle in the specifications, how would you know if a certain wide-angle ep will cause vignetting?
    Hello Eyescout,

    not the scope or eyepice may cause vignetting of the field of view, but anything small sized in front of the eyepiece field lens.
    Now, I have in my hands the UWAN f=16mm 82°, and holding in front of its field lens (in this case it is the Smyth lens assy) a short 0.965" tube, which is smaller than the EP 1.25" thread. A vignetting towards the edge of the field of view is clearly visible.
    So, in some cases, a 0.965"-to-1.25" may cause a vignetting.

    Best,

    JG
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