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  1. #1
    Derek Overdahl's Avatar
    Derek Overdahl Guest

    Default f Ratio and planetary viewing

    I currently own a fast f/4 8'' eq Newt.

    I have gotten many years of heavy use out of it mostly on DSO's (the scopes
    intend use), unfortunately I have no basis for comparison as I have never
    used any other scope, but I find myself a little unsatisfied with its
    planetary performance.

    In addition I am considering my first 'Second Scope' purchase.... something
    smaller... more of the ever referenced 'grab and go' type.

    * What I want is a good (doesn't have to be spectacular) planetary/solar and
    doubles scope with good color correction (I have been a little spoiled by
    the Newt).
    * What I don't want is a refractor or Mak version of what my Newt already
    is. And looking at many of the small refractors they are often tagged as
    "Rich Field" and don't seem to support the magnification required.

    I believe this comes down to f ratio.

    I have seen the numbers on comparing the trade offs with f
    ratio/magnification/exit pupil as well as the effect of different
    ep's....all that being said how big a factor is f ratio in good viewing for
    planets/solar and doubles?

    Orion Starmax 127 Mak at f 12.1
    Celestron5 at f 10
    Orion 80ed at f 7.5
    Stellarview 80/9D at f 9.4
    Stellarview Nighthawk at f 6
    TV Ranger/Pronto at f6.8

    This is the class I am considering... is a higher f ratio better for my

    Any help in understanding this would be appreciated!

    Thanks in advance

  2. #2
    Jon Isaacs's Avatar
    Jon Isaacs Guest

    Default f Ratio and planetary viewing

    >I believe this comes down to f ratio.

    Fratio allows the use of long focal length eyepieces but with the refractors,
    its all in the color correction. Of the ones you mentioned, I believe the
    Orion ED-80 is the scope of choice, it is the only one that offers APO
    performance so it will be the best planetary/doubles scope and will still
    provide a nice wide field of view.

    The C-5 and large MAKs are nice for higher powered viewing but do have central
    obstructions and cool down issues. One can fit a focal reducer to a C-5 but in
    the end, it is limited to 1.25 inch eyepieces and IMHO just does not provide
    the sorts of views one gets with a small refractor.

    It is unlikely that any of these scopes will provide comparable planetary views
    to your Newt.

    My suggestion would be to consider this second scope as a companion to your
    Newt. And though I own a Pronto, which does a nice job, the scope that stands
    out here is the ED-80, its pretty much in a class alone at this price point,
    similar to the TV-85 but at a small fraction of the cost.


  3. #3
    Bill Ferris's Avatar
    Bill Ferris Guest

    Default f Ratio and planetary viewing

    Derek wrote:

    At f/4.8, your Newtonian isn't too fast to provide excellent planetary images.
    When the seeing is very good, my f/4.5, 10-inch reveals a wealth of detail on
    Mars, Jupiter and Saturn. ( )

    You mention owning your scope for many years, so the following may be old news
    to you. But it bears saying for the benefit of newcomers to the
    hobby...collimation is critical in a fast optical system. It doesn't matter how
    good the seeing is or how smooth the optics are, if the collimation is off in
    an f/6 or faster system, planetary images will not be sharp.

    Anybody interested in getting the absolute most from their fast Newtonian
    should invest in H. R. Suiter's "Star Testing Astronomical Telescopes." The
    cost of the book is small compared to the dividends the information will pay at
    the eyepiece.

    Have you visited the "Marsobservers" forum on Yahoo? If you go through the
    messages from last August-September, you'll find observation reports from
    people using a wide variety of scopes. One or more of the instruments you're
    considering may be represented. In any event, perusing the Mars observing
    reports from last year's opposition should give you some perspective on what
    can be seen with a variety of scopes.


    Bill Ferris
    "Cosmic Voyage: The Online Resource for Amateur Astronomers"
    Email: Remove "ic" from .comic above to respond

  4. #4
    Derek Overdahl's Avatar
    Derek Overdahl Guest

    Default f Ratio and planetary viewing

    Perhaps the quality of my Newt is what I should be look at investing in....

    I have a Discovery - and one thing that I know contributes to my contrast
    issue is this bulky secondary support. I have thought of replacing it with
    a secondary from Protostar.... they also make a "Arch" mounted secondary
    that looked interesting.

    I think I am comfortable with my ability to collimate the scope - both with
    a star test and my laser collimator (I don't own H. R. Suiter's book - I
    will look for it) I just read allot about people being able to pull
    planetary details out of their scopes that I don't feel I can. It has made
    me wonder if an f 4 was just not cut out for producing the contrast.

    Looking at your site(great sketches by the way!) my scope is just not
    producing images at that level.

    "Bill Ferris" <billferris@aol.comic> wrote in message
    ( )
    matter how
    pay at

  5. #5
    Frank Bov's Avatar
    Frank Bov Guest

    Default f Ratio and planetary viewing

    I don't think you will find a telescope on your list that is superior to a
    top-quality 8" Newtonian. The f-ratio issue you raise is one of achieving
    the quality an 8" aperture can provide. It's harder to figure a faster
    scope, and it's also harder to collimate, but both can be done.

    Suiter's book is an excellent recommendation, so you can evaluate your
    mirror. If it proves less than perfect, even when well collimated, you can
    have your current mirror refigured for less than the price range you imply.
    You will get a brighter image, and you will have the capability for far
    better image quality than 3-5" scopes can achieve.

    Plus, you'll start to see things in your DSO work you never saw before.

    HAve fun,

    "Derek Overdahl" <> wrote in message

  6. #6
    Stephen Paul's Avatar
    Stephen Paul Guest

    Default f Ratio and planetary viewing

    "Bill Ferris" <billferris@aol.comic> wrote in message

    I think you misread that Bill. Look again (8" F4).

    I agree with your post completely though. As you may recall, I had a very
    frustrating time with my XT10 back in February, and got the collimation all
    straightened out just in time to catch one of the very rare nights around
    here where 50x per inch is possible. I was viewing Saturn at over 400x (my
    available eyepiece limit), and the images were superb. That really set the
    standard for me, and completely changed my perspective on good collimation
    and good seeing (I want more!!).


  7. #7
    Jan Owen's Avatar
    Jan Owen Guest

    Default f Ratio and planetary viewing

    To reply, remove the "z" if one appears in my address
    "Bill Ferris" <billferris@aol.comic> wrote in message
    detail on
    ( )

    Correction; the OP said it was an 8" f/4. If it has excellent optics, the
    on-axis planetary views can still be quite good. But at f/4, the
    diffraction-limited field is going to be quite small...

    old news
    matter how
    off in
    pay at
    planetary/solar and

  8. #8
    Bill Ferris's Avatar
    Bill Ferris Guest

    Default f Ratio and planetary viewing

    Stephen Paul wrote:

    Right, you are, Stephen. Somehow, I managed to slip a non-existent decimal
    point in there. Good catch.


    Bill Ferris
    "Cosmic Voyage: The Online Resource for Amateur Astronomers"
    Email: Remove "ic" from .comic above to respond

  9. #9
    andrea tasselli's Avatar
    andrea tasselli Guest

    Default f Ratio and planetary viewing (Jon Isaacs) wrote in message news:<>...

    It's hard to believe that a 5" mak (or SCT), provided that is good
    optically, can be matched, let alone surpassed, by a 80mm (near) APO
    in planetary/doubles viewing.

    None of which can be though to be limiting performance.

    Andrea T.

  10. #10
    Chris L Peterson's Avatar
    Chris L Peterson Guest

    Default f Ratio and planetary viewing

    On 19 Jul 2004 23:58:09 -0700, (andrea tasselli) wrote:

    I agree. Certainly the Mak will be equal or better for planetary work, and
    superior for doubles and for DSOs. Unless cool down issues are some kind of
    serious problem, I'd have to say that would be the best choice for a planetary


    Chris L Peterson
    Cloudbait Observatory


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