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  1. #1
    bcoley's Avatar
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    Default Eyepieces for SCTs



    In several threads on these forums, I have read the suggestion that when it comes to eyepieces, Schmidt-Cassegrain telescopes tend not to be as demanding as other kinds of scopes. I have three questions about this:

    1) Is that true?

    2) If it's true, why is it true?

    3) If it's true, does it also mean that SCT owners who purchase higher quality EPs are in effect wasting their money because the optics of their scopes will not benefit from the extra quality of those EPs?

    Thanks for your time.
    MY SETUP: Celestron Nexstar 8SE / SkySync GPS accessory / GSO quartz dielectric 2" diagonal / EYEPIECES: Celestron eyepiece/filter kit; GSO 2" 30mm; Russell Optics 2" 19mm SuperWide Konig XL; Russell Optics 2" 13mm SuperWide Konig; Zhumell 2" Barlow / Telrad finder / POWER: Celestron Powertank 7 / Peak Portable Power System 450 Plus (x2) / Celestron vibration suppression pads / Dew shield / Astro Zap dew strap w/power controller / Connection to laptop via Tripp Lite USA-19HS Hi-Speed USB serial adapter

  2. #2
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    Default

    SCTs suffer by field curvature from design. This has the worst effect at F10. Better, more complex eyepiece designs give a wider, better corrected field. If you use a .63 reducer/corrector, you will noticeably flatten the field and widen the true field for a given eyepiece. This allows you to use 1.25" eyepieces such as plossls, to give you reasonably wide and corrected fields that compare to some more expensive eyepieces at F10. However, more expensive, well corrected eyepieces used at F6.3 (with the R/C) will give better corrected fields than the less expensive eyepieces. All of this is important at low to medium powers. At higher powers, on axis (the center of the field of view) is generally most important and simpler eyepiece designs such as plossls and orthos are preferred by many.

    SCTs are a special case since using a F/R to get to a lower F ratio also flattens the field. In general, for most scope types, the longer the focal length the better the field looks for both inexpensive and expensive eyepieces, however a premium, well corrected eyepiece will give a better looking low to medium field than a cheaper design at all focal lengths. Keep in mind, this is my opinion and others may reasonably disagree.

    Randy

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  4. #3
    kencrowder's Avatar
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    Default

    The typical sct is f/10. The focus is a surface with the radius determined by the f/no. So an f/10 will have larger radius than an f/5 and thus be easier for the ep to accomodate. The market, however, is everything in between. So the manufacturers tend to compromise.

    A typical plossl will have less coma around the edges on an f/10 than it will on an f/5. If an ep has been designed for an f/5, however, it may over correct on an f/10.

    A flat field is especially important to AP folks. That is where the field flateners are a must.

    An inexpensive focal reducer doesn't change the flatness. It only reduces the apparent focal length. Thus on an f/10 it gives an apparent f/5. So the effect of focal surface curvature is apparent through an ep.

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    bcoley's Avatar
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    Default

    Ken,

    Ads for available focal reducers claim to correct views, which I have assumed meant to flatten the field of view. When you talk about inexpensive reducers, to what models/price ranges are you referring?
    MY SETUP: Celestron Nexstar 8SE / SkySync GPS accessory / GSO quartz dielectric 2" diagonal / EYEPIECES: Celestron eyepiece/filter kit; GSO 2" 30mm; Russell Optics 2" 19mm SuperWide Konig XL; Russell Optics 2" 13mm SuperWide Konig; Zhumell 2" Barlow / Telrad finder / POWER: Celestron Powertank 7 / Peak Portable Power System 450 Plus (x2) / Celestron vibration suppression pads / Dew shield / Astro Zap dew strap w/power controller / Connection to laptop via Tripp Lite USA-19HS Hi-Speed USB serial adapter

  7. #5
    MitchAlsup's Avatar
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    Default

    1) Is that true?

    Yes, the slow F/ratio is not very demanding of the EP optics.

    2) If it's true, why is it true?

    The slow F/ratio (F/10)

    3) If it's true, does it also mean that SCT owners who purchase higher quality EPs are in effect wasting their money because the optics of their scopes will not benefit from the extra quality of those EPs?

    No, not at all. In fact, due to the slowness of the scope it necessarily has a long focal length. This means you need a long focal length EP to see a FoV as big as the scope can deliver.

    Given the choice between a 55mm Pl÷ssl, a 41mm PanOptic, of a 31mm NT5 (all giving a similar FoV) the NT5 is the most emersive of the three.

    But there is a question you did not ask. And that is of field curvature. SCTs have field curvature, younger eyes hardly see this curvature as younger eyes have greater accomodation than older eyes (Presbiopia is the tecnical term). So young people tend to to see the field curvature in SCTs, while older people generally do. The new EdgeHD series has gone to great lengths to eliminate this field curvature (and coma).

    EPs with greater field width are a bit more prone to display the inherent field curvature but only to the extent that they are delivering a wider field at the same power as a lesser EP. Do not blame the EP for this phenomenom, the problem is from the scope.

  8. #6
    kencrowder's Avatar
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    Default

    Quote Originally Posted by bcoley View Post
    Ken,

    Ads for available focal reducers claim to correct views, which I have assumed meant to flatten the field of view. When you talk about inexpensive reducers, to what models/price ranges are you referring?
    I have one which I use with my video camera. It was bought from OPT. It does the job nicely, screws in like a filter and the coma doesn't bother me.

    I also have one from Meade which includes the field flattener. It does flatten the field, but it is a 2 inch and attaches to the scope. It requires adapters to use with an ep. My scope is an LX90.

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    Default

    Just for the record the common 6.3 focal reducers from both Meade and Celestron do flatten the field of view ...

    The point I want to make is that although I own and use SCTS for the most part buying high end well corrected eyepiece is NEVER a waste of money... a good well corrected eyepiece will work in any darn scope while a inexpensive eyepiece may not work all that well in a "good" fast telescope.

    Someone on this forum was selling a GTO Proxima 2 inch 70 degree eyepiece which was the very first 2 inch eyepiece I ever purchased years ago..It works darn well in both my 8 and 11 inch SCTS.. BUT it has a "flock of seagulls" along the edges of the FOV when used in my 100 mm f/6 refractor...

    Not a thing wrong with that eyepiece when used in a slow scope like a Sct but with a flock of seagulls present in a f/6 scope I would hate to see that eyepiece in a even faster Dob...

    Spend your money wisely but you never loose by buying quality...

    Bob G.
    CPC1100 housed in a slotted domed observatory (Exploradome) 4 and 5 inch refractors for use from the lawn, a 8" Sct (NS 8i) for star parties...
    I Hate the winter so I use heated Motorcycle clothing to stay warm while observing in winter
    Retired, also have 2 other hobbies
    1. tinker with older Corvettes (6 in garage)
    2. make a heck of a lot of sawdust in my wood shop.

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