Longest focal length eyepiece

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  • #574133
    James Dawson
    Participant

    What is the longest focal length eyepiece commercially available, any why is anything longer than 40mm so uncommon?

    James

    #580025
    Peter Anderson
    Participant

    Personally I have 50mm, 60mm and 70mm eyepieces. All are 2 inch size because otherwise the field of view would be very restricted. Very simply for standard SCT’s at F10, the 50mm, 60mm and 70mm translate to 5,6,and 7mm exit pupil, the last mentioned being as large as the (young) human eye can accommodate.  Now, for the simple reason that most eyepieces tubes are 1.25″, a 40mm is still capable of producing a field width in the low 40’s (degrees) and any longer focal lengths need the 2″ tube.

    Other factors are the cost and weight of these larger, longer focal length units, though some might be cheaper (say) if they are simple 3 element Kellners.

    The old visual professional refractors ( say think of the 24″ at Lowell Observatory and its ilk), had some quite long focal length eyepieces in regular use – longer than I have quoted above.  But they were special.

    For myself, and perhaps I am lazy, I usually just use the standard 1.25″ eyepieces and a 32mm focal length is about as far as you can go with a 50 degree field. On the C14 when I observe my lunar occultations, I use a 25mm with a very pleasant 60 degree wide field.

    #580026
    Peter Anderson
    Participant

    From a quick search on the internet, probably the cheapest reasonable unit (around USD$80), is a Meade 56mm Plossl with 52deg field in a 2″barrel unit. So if you crave a long focal length eyepiece for your F10 SCT, then this should do it with an exit pupil of 5.6mm.

    Alternatively you could purchase a screw on focal reducer and field flattener. (I have one that lives on the C11 for photography.)  This reduces the focal ratio from F10 to F6.3. and means you would get the same magnification and exit pupil with a 35mm eyepiece and so the standard 32mm Plossl in a 1.25″ tube would still provide a 50 degree field of view and give a large 5.08mm exit pupil. It is swings and roundabouts.

    #580028
    Andrew Smith
    Participant

    James, in addition to the points Peter has made on the size of the exit pupil another constraint is the size of the field stop that can fit into a 2″ focuser. You need a larger field stop at longer focal length.

    By the way if you do get the exit pupil larger that that of your eye you just lose some light and have to shift your head to look around the field.

    Regards Andrew

    #580032
    Callum Potter
    Keymaster

    There are some eyepieces with a 3 inch barrel diameter.

    There is a ‘short’ 30mm from Explore – which I saw at a show last year – a monster of an eyepiece.

    But there is also a Japanese 80mm eyepiece available from APM – but not cheap – though if you have €2k spare…

    Of course you need a scope with a 3″ focuser too…

    Callum

    #580033
    Denis Buczynski
    Participant

    Hello all,

    Talk of large eyepieces made me think of the thread on Antique Telescope Forum about the Great Melbourne Telescope and the low power eyepiece used on it. Here is a picture of the eyepiece with a field lens of 8 inch diameter! That is a large eyepiece.

    #580044
    David Arditti
    Participant

    This reminds me of the visit I recently made to Berlin, for the European Planetary Science congress. On the last day there was a visit to the longest refractor in the world, the Archenhold Telescope, which has a focal length of 21m and aperture 68cm (f/31). The longest eyepiece they have is 100mm, giving x210. This is the power they nearly always use. The telescope is in regular use for public observing, particularly by school groups. The eyepiece is the green bit in the photo.

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