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Thread: How to tell if a reflector will have enough backfocus for use with dslr??

  1. #11
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    Default Re: How to tell if a reflector will have enough backfocus for use with dslr??



    Last time I checked, several of the skywatcher newtonian OTAs that they usually sell with EQ mounts were designed to work with T Rings and a DSLR... some better than others (larger secondary, better focuser, etc).

    From a 2016 catalog, the following list of UK models quote "Direct SLR Camera Connection" as a feature:
    In the Explorer black diamond series: (number is aperture in mm).
    Explorer 150P (f5)
    Explorer 150PL (f8)
    Explorer 200P (f5)
    Explorer 150PDS (f5, Dual speed focuser, shorter tube for better AP performance)
    Explorer 200PDS (f5, Dual speed focuser, shorter tube for better AP performance)
    Explorer 250PDS (f4.7, Dual speed focuser, shorter tube for better AP performance)
    And their Quattro f4 imaging newtonians:
    Quattro-8CF, Quattro-8S, Quattro10CF, Quattro-10S, Quattro-12S. (Low expansion glass, with carbon fibre (CF) or steel (S) tubes (number is aperture in inches)
    <i><span style="font-family: Times New Roman"><font size="2"><b>SCT:</b> N11GPS <b>Refractors:</b> FS-60C/CSV, ZD-66, A80SS, FSQ106ED <b>Binoculars:</b> 8x42, 15x70<br /><b>Eyepieces:</b> 40mm Meade SWA, 28mm UWAN, 17mmT4, 12mmT4, 2.5mmT6 Nagler, 17mm, 8mm, 5mm LVW, HI-LE2.8<br /> <a href="
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    Default Re: How to tell if a reflector will have enough backfocus for use with dslr??

    Quote Originally Posted by CamelHat View Post

    So it can be done but it is difficult considering everything else (collimation screws, how much focus travel, coma corrector or not, etcetera). Unless you are handy, willing to try crazy methods and risk complete failure it is not recommended.

    Ah one thing, if you have a collapsible Dob you should be able to fiddle with the tube length, that way it is a heck of a lot easier to accomplish.
    Great points!

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    Default Re: How to tell if a reflector will have enough backfocus for use with dslr??

    Hi,

    Let me throw out a novel approach here. If you are buying from a dealer, call them and ask if the OTA will come to focus with a DSLR attached. If you're buying it from something like Kijiji then you may be stuck. Of course, you could always ask the seller the same question. Who knows, they just might know the answer.

    Cheers,
    JT
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    Default Re: How to tell if a reflector will have enough backfocus for use with dslr??

    The question has been answered. Buy an Astrograph.
    Else, you will be blaming everything else for poor images and wonder why no one sympathises
    If its for AP, why not buy the correct tools?

    https://www.365astronomy.com/ts-opti...iABEgJDv_D_BwE
    Enjoy
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    Default Re: How to tell if a reflector will have enough backfocus for use with dslr??

    Quote Originally Posted by UlteriorModem View Post
    Just a quick note, coma correctors are required for Newtonian 'astorgraphs' regardless of focal length. So yea that cost should be considered.
    This is true.

    And since a coma corrector is required anyway, it is worth noting that some coma correctors (notably the Paracorr 2, perhaps others) are slightly magnifying, and will likely add enough back focus to bring a DSLR to focus.

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    Default Re: How to tell if a reflector will have enough backfocus for use with dslr??

    So....I guess the actual answer to the question (buy an astrograph is not an answer lol, it's a solution) is: The information needed to plug in to a formula is either not available readily or only sporadically. So no data= impossible to calculate most of the time. Thanks for everyone's input. On a related note, will an extension tube for the camera to focuser connection rectify that issue of coming to focus the majority of the time? (without consideration to other issues like coma corrector at this time)

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    Default Re: How to tell if a reflector will have enough backfocus for use with dslr??

    No a spacer will not work, the problem is exactly the opposite. You cannot rack the focuser in far enough.

    A 'common' mod is to somehow move the primary mirror forward in the ota to 'push' the focal point out further.
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    Default Re: How to tell if a reflector will have enough backfocus for use with dslr??

    Quote Originally Posted by UlteriorModem View Post
    No a spacer will not work, the problem is exactly the opposite. You cannot rack the focuser in far enough.

    A 'common' mod is to somehow move the primary mirror forward in the ota to 'push' the focal point out further.
    See, that's what I had heard. About moving the mirror, but for some reason, I could have sworn I read people talking about extension tube or a barlow helping with this regard. Hence, the question to the communal "Astro moshpit"

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    Default Re: How to tell if a reflector will have enough backfocus for use with dslr??

    A barlow 'might' change the focal point to some degree.

    Some cameras have the imaging chip way up front and might require an extension.

    As an example I can come to focus with the ASI120mc in my dob with no alterations. A dslr would not.

    But typically what I stated is the case.
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    Default Re: How to tell if a reflector will have enough backfocus for use with dslr??

    Quote Originally Posted by Starlord101 View Post
    for some reason, I could have sworn I read people talking about extension tube or a barlow helping with this regard.
    Extension tube: no. Barlow: probably. They do opposite things.

    An extension tube consumes excess back focus. A bad thing to do when you already don't have enough. A barlow gives you more back focus. A good thing that is probably sufficuent, possibly not.

    SCT owners use extension tubes to consume excess back focus when they remove their diagonals. Newt owners use barlows when they don't have enough back focus.
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