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Thread: Advice needed on 8" SCT vs 10# DOB

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    Default Advice needed on 8" SCT vs 10# DOB



    Hello,
    I have the opportunity to obtain a used scope in good condition.
    Currently I have the Orion 10XT DOB.
    The new scope is a used Celestron C8 SCT
    I had both out last night trying to compare views of the same between the two scopes.
    And I found the Orion with a 25mm lens resolved about 5 times more stars than the SCT with both a 40mm and a 25mm.
    As far as I know the SCT is not problematic but giving normal expected results.

    I was really surprised by the lack of stars viewed in the SCT!
    But someone told me it might be because I need some kind of lens adapter/reducer of some kind.
    Does that sound right?

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    Default Re: Advice needed on 8" SCT vs 10# DOB

    Are you talking about the difference in the field of view or that so many more dim stars were seen by the larger scope in the same field of view?

    The 10" scope will show you dimmer stars, but the difference is very marginal, certainly not 5 times as many. However, the difference in the field of view using the same EP is dramatic unless you have a focal reducer for the SCT and then the field of view is very similar.

    The Dob does have a small (but noticeable) advantage because of the aperture, the SCT has the advantage of portability, both are good scopes.

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    Default Re: Advice needed on 8" SCT vs 10# DOB

    Thank you!
    I think I need to look at the calculations for FOV, eye pupil, and how what to expect with the SCT and the focal reducer.
    I'll see if I can't find a web-site that has that info laid out.
    If you know of one also I would appreciate the link.

    Sincere thanks

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    Default Re: Advice needed on 8" SCT vs 10# DOB

    This may help, just enter the telescope and eyepiece information and it will do the calculations for you.
    N.A.A. Telescope Math Calculator
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    Default Re: Advice needed on 8" SCT vs 10# DOB

    Quote Originally Posted by Gabby76 View Post
    This may help, just enter the telescope and eyepiece information and it will do the calculations for you.
    N.A.A. Telescope Math Calculator
    Thanks!
    I remember that site now - visited it last year but forgot about it.
    I was doing the calcs by hand but that saves a lot of time. :-]

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    Default Re: Advice needed on 8" SCT vs 10# DOB

    I tend to do them by hand as well but it is nice to occasionally have a shortcut handy
    Refractors: Antares 105 f/15, Celestron 150 f/8, Stellarvue NHNG 80 f/6.9, TAL 100RS f/10, TS 102 f/11, UR 70 f/10, Vixen SD115s f/7.7
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    Default Re: Advice needed on 8" SCT vs 10# DOB

    It is not often you see someone considering downsizing in aperture unless there is a good reason like physical limitations or simply wishing to have a more portable scope.

    You didn't specify the eyepieces in use as to their AFOV, but based on the generic "25mm" and "40mm" description I am running under the assumption they are plossls. If that is indeed the case, then the 25mm plossl will have an AFOV of 50° and the 40mm one of 42° (based again on assumption of a 1.25" barrrel). In that case they would yield the following for comparison:

    XT10 using 25mm plossl gives 48x with an EP of 5.3mm and TFOV of 1.0
    °
    C8 using a 40mm plossl gives 50x with an EP of 4.1mm and TFOV of 0.8°

    That is the better comparison between the two as a 25mm in the C8 yields 81x, EP 2.5mm and 0.6
    ° TFOV. Agree with Oz that there is no way if the C8 is properly collimated and its condition is good that it should be out done by the XT10 to that extent. As said there is a definite difference in two inches of aperture, but how much is seen visually by the user depends on one's observing practices and skills. The short answer is that a two inch bump in aperture would yield a little more resolution and slightly deeper view. But if one observes mostly brighter objects then the visual differences can be difficult to discern. But when one tends to observe more at the threshold limits of their aperture, as I can often do since I predominantly chase galaxies, then two inches can be quite noticeable, but still in somewhat of a subtle sense.

    I look at observing as layers of an onion. An 8 inch will take you only so deep, and those objects that are just at the threshold of perception in the 8 inch will be more easily seen in the 10 inch. While the next layer, those objects that are just out of reach in the 8 inch can now possibly become threshold detection objects in the 10 inch, thus peeling back another layer.

    I will take the pragmatic approach and say that in my opinion, unless you have a specific reason for getting the 8 inch when you already have a 10 inch, then I would be looking at going larger rather than smaller. But we each have our own needs, wants and desires, and this may fill one of those for you.
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