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Thread: dumb question

  1. #1
    Jan Owen's Avatar
    Jan Owen Guest

    Default dumb question



    "clf" <allspamautodeleted@trashcan.com> wrote in message
    news:3GvOe.30775$Tf5.8328@newsread1.mlpsca01.us.to .verio.net...
    now.
    scopes
    say -
    but
    viewing?
    I'm

    The diameter of the tube is dependent on the diameter of the objective,
    plus enough extra diameter from mirror edge to tube wall to minimize tube
    (thermal) currents.

    The length of the tube is dependent on the focal length of the objective
    (the mirror in your case). The longer the focal length of the objective,
    the longer the tube must be. And the shorter the focal length, the
    shorter the tube... Each objective will require a given length of tube to
    match it's focal length... You can't pick and choose your tube length
    independently of the focal length, though you CAN pick and chose a
    telescope with longer or shorter focal length...

    Shorter tubes can be good from a handling standpoint, but a short focal
    length Newtonian reflector will show stronger and stronger coma, and it
    will get closer to the optical axis, the shorter the focal ratio... Coma
    is an aberration that makes the stars in the outer portion of the field
    look more like seagulls than pinpoints of light...

    --
    Jan Owen

    To reach me directly, remove the Z, if one appears in my e-mail address...
    Latitude: 33.662
    Longitude: -112.3272



  2. #2
    Starlord's Avatar
    Starlord Guest

    Default dumb question

    It depends on the F stop of the mirror. I've seen 8inch scopes that have
    shorter tubes because they are F4 / f5, while my Babylon 8 has a tube that's
    as long as I am tall, it's a F8 8 inch scope.


    --

    The Lone Sidewalk Astronomer of Rosamond
    Telescope Buyers FAQ <<<<------
    http://home.inreach.com/starlord
    Astronomy Net Online Gift Shop
    http://www.cafepress.com/astronomy_net



    "clf" <allspamautodeleted@trashcan.com> wrote in message
    news:3GvOe.30775$Tf5.8328@newsread1.mlpsca01.us.to .verio.net...



  3. #3
    Chi-hung Yeung's Avatar
    Chi-hung Yeung Guest

    Default dumb question

    The length of the tube also depends on the design of the scope. From
    what you mentioned (a tube of about 2' and an 8" or so mirror), it is
    most probably a Schmidt Cassegrain (SCT). In this case, the focal
    length of the scope will be much longer than the physical length of the
    tube.

    For two scopes of same diameter and using the same eyepiece, the longer
    focal length one will give a higher magnification but narrower field of
    view.

    C. H. Yeung

    clf wrote:

  4. #4
    Starlord's Avatar
    Starlord Guest

    Default dumb question

    The "Slower" scopes like my 8inch F8 are easyer to aline too.


    --

    The Lone Sidewalk Astronomer of Rosamond
    Telescope Buyers FAQ
    http://home.inreach.com/starlord
    Astronomy Net Online Gift Shop
    http://www.cafepress.com/astronomy_net



    "Chi-hung Yeung" <bcchye@polyu.edu.hk> wrote in message
    news:1124779116.932583@nsserver1.polyu.edu.hk...


  5. #5
    clf's Avatar
    clf Guest

    Default dumb question

    I am wondering - not having seen any of my astronomy books in a while -

    My telescope isn't a really expensive one - but it does ok for me for now.
    It is one of the 4.5" reflectors commonly sold.
    With that said, you know about how long the tube is. Then, there are scopes
    with a wider tube yet seem somewhat shorter.

    My question is - since I"ve been lost in this field for some time - what
    determines the "length" of a telescope tube? To that end, could one say -
    use a tube maybe only a foot to two feet long tops and any size mirror - but
    the bigger the mirror - the more light gathering and thus better viewing?
    I"m thinking of a tube of about 2' and an 8" or so mirror.

    I said the question was dumb, but I've been away from this for so long, I'm
    trying to get my brain in gear with it - again.

    Thanks in advance for replies



  6. #6
    jonisaacs@aol.com's Avatar
    jonisaacs@aol.com Guest

    Default dumb question

    Shorter tubes can be good from a handling standpoint, but a short focal
    length Newtonian reflector will show stronger and stronger coma, and it
    will get closer to the optical axis, the shorter the focal ratio...
    Coma is an aberration that makes the stars in the outer portion of the
    field look more like seagulls than pinpoints of light...
    ------

    There are other issues involved with "fast scopes", ie F4-F6 and even
    higher.

    One is that the angle of the "light cone" entering the eyepiece is very
    steep. To handle this steep light cone and still provide reasonably
    sharp images across the field of view requires fancy and expensive
    eyepieces.

    With Newtonians, collimation becomes difficult, I own an F4 Newt but I
    don't recommend it except for portability reasons. F5, or F6 is much
    nicer IMHO.

    As others have said, there a variety of scope types, some like Schmitt
    Cassegrain, Schmitt-Newtonian and MAKs can provide a longer focal
    length in a shorter package, they have their own set of problems but in
    general, these are sound designs that are worth considering.

    On the other hand, there is a class of scope that uses a fast
    (F4)spherical mirror and a built-in Barlow/corrector to achieve a long
    focal length in a short package. The most common of these are the 4.5
    inch "Short Tube Newtonians." With a tube length of under 20 inches
    but a focal ratio of F8 or so, these scopes offer a compact design but
    marginal performance. The standard 4.5 inch F8 Newtonian has a OTA
    that is about 3 feet long but will give nice sharp stars across the
    field of view. THere are larger versions, some are 6 inch scopes and
    often sold on Ebay under a variety of labels, Baytronix is a common
    one. These scopes and low priced Ebay scopes are wise to avoid.

    Jon Isaacs


  7. #7
    jonisaacs@aol.com's Avatar
    jonisaacs@aol.com Guest

    Default dumb question

    Shorter tubes can be good from a handling standpoint, but a short focal
    length Newtonian reflector will show stronger and stronger coma, and it
    will get closer to the optical axis, the shorter the focal ratio...
    Coma is an aberration that makes the stars in the outer portion of the
    field look more like seagulls than pinpoints of light...
    ------

    There are other issues involved with "fast scopes", ie F4-F6 and even
    higher.

    One is that the angle of the "light cone" entering the eyepiece is very
    steep. To handle this steep light cone and still provide reasonably
    sharp images across the field of view requires fancy and expensive
    eyepieces.

    With Newtonians, collimation becomes difficult, I own an F4 Newt but I
    don't recommend it except for portability reasons. F5, or F6 is much
    nicer IMHO.

    As others have said, there a variety of scope types, some like Schmitt
    Cassegrain, Schmitt-Newtonian and MAKs can provide a longer focal
    length in a shorter package, they have their own set of problems but in
    general, these are sound designs that are worth considering.

    On the other hand, there is a class of scope that uses a fast
    (F4)spherical mirror and a built-in Barlow/corrector to achieve a long
    focal length in a short package. The most common of these are the 4.5
    inch "Short Tube Newtonians." With a tube length of under 20 inches
    but a focal ratio of F8 or so, these scopes offer a compact design but
    marginal performance. The standard 4.5 inch F8 Newtonian has a OTA
    that is about 3 feet long but will give nice sharp stars across the
    field of view. THere are larger versions, some are 6 inch scopes and
    often sold on Ebay under a variety of labels, Baytronix is a common
    one. These scopes and low priced Ebay scopes are wise to avoid.

    Jon Isaacs


  8. #8
    Paul Winalski's Avatar
    Paul Winalski Guest

    Default dumb question

    What determines the tube length in a Newtonian reflector or in a
    refractor is the focal length of the mirror or the lens. The
    longer the focal length, the longer the distance must be between
    the eyepiece and the mirror (or lens), and hence the longer the
    tube that you need.

    The diameter of the lens or mirror, as you say, affects light
    gathering--the wider it is, the more light you take in. Light
    gathering affects how dim an object you can see, and how well the
    scope can resolve objects.

    The focal length affects magnification. The magnification that you
    get is the ratio of the focal lengths of the primary mirror (or lens)
    and the eyepiece. Thus, if you have a mirror with a 600mm focal
    length, and you use a 20mm eyepiece, the magnification is 600/20 or
    30x.

    You'll also see the term focal ratio or "f ratio". This is the focal
    length of the mirror/lens divided by its diameter. Thus, a mirror
    with a 100mm diameter and a 600mm focal length has a focal ratio of
    f/6. In general, faults in the optics (such as the coma inherent in a
    Newtonian's primary mirror or the chromatic aberration inherent in the
    lens of a refractor) are more of a problem the shorter the focal ratio
    is. But a scope designer has to balance that against the longer tube
    needed for higher f ratios.

    Clear skies,

    -Paul W.

    On Mon, 22 Aug 2005 22:17:19 -1200, "clf"
    <allspamautodeleted@trashcan.com> wrote:

    ----------
    Remove 'Z' to reply by email.

  9. #9
    clf's Avatar
    clf Guest

    Default dumb question

    "Jan Owen" <janowen1z@cox.net> wrote in message
    news:C7wOe.70245$DW1.39416@fed1read06...

    Thank you very much for that reply. To further my past - I got into
    Astronomy at like 12 and was into it up into my 20s, but then life started
    happening. I'm now in my 40s, so though I have the scope and stashed away (I
    hope) my books - as you can see it has been a long while and after that many
    years, one can tend to forget. Now, I have some time to break back into the
    hobby and have to a minor degree - even having taken pics with my somewhat
    cheapo digital camera through the scope. I'm hoping to get into it more as I
    go, and look to maybe get a better scope down the road. I believe one of my
    books "was" or well - with my memory - maybe it was just a book I had read
    from a library - was on the subject of building telescopes. Perhaps it did
    explain that principle, but again - it has been so long.
    Thanks again! I appreciate it.
    clf



  10. #10
    clf's Avatar
    clf Guest

    Default dumb question

    Thanks to all for your replies, I appreciate them. I don't post in here
    much, but I've been reading many posts with interest. This is one of the
    best groups I could have found.

    Thanks again. Hopefully one day I can post to help someone.

    clf



 

 

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