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  1. #1
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    Default Setup According to Backfocus on a Celestron C8 SCT?



    Hello!

    So I wanted to explore the possibility of prime focus AP with the Celestron C8 SCT and since we're importing the stuff, we gotta do it in one go with the parts we need figured out. Basically I need to buy parts I'm very confused about. I'm sorry if the questions will be basic, but I wasn't able to find satisfactory answers during my time googling.

    1)I think we're going to be using the F/6.3 reducer for some images. Now a lot of setups I saw online didn't feature a focuser in the imaging train and kinda went SCT-reducer-T adapter-T ring-DSLR, relying on this length being the required 105mm. Couldn't the SCT focuser be used to get the focus accurately right on the sensor so that the imaging train could vary a little? Because that would mean adding an OAG, or a filter would require me using a whole different setup.

    2)For planetary and lunar snaps, people suggest using a 2X barlow, but again that would increase the backfocus. A lot of setups online seemed quite liberal in the kind of imaging train used, with significant variations so I couldn't get a good idea of what adapters to buy. Was is because the focuser could be used to adjust the focus after all?

    Thank you!

  2. #2
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    Default

    If you're using prime focus, or a Barlow, then you've got some leeway with the spacing. (Especially for planets, where you're only interested in a tiny part of the field of view where the planet is), so you can just use the focuser as normal.

    If you're using a reducer/flattener, then they're designed to give a flat result at a fixed distance from the flattener to the sensor - if you change the spacing, then you probably won't get a flat result, so the right spacing is important for best results.

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    Default Re: Setup According to Backfocus on a Celestron C8 SCT?

    The 105mm backfocus requirement for the FR is independent of focusing. Yes, you use the focuser to position the focal 'plane' at the camera sensor, but it still needs to be 105mm behind the FR. If you don't have the right distance, you won't have a flat field at the sensor, and parts of the image will be out of focus.

    You would never use a barlow and a focal reducer at the same time. They are opposites. If you want more image scale, you first remove the reducer, then, if it is still not enough, you add the barlow.

    The 105mm includes the 55mm from the front of the T-ring to the sensor. So you need another 50mm of adapter between the FR and the T-ring. Get a T-adapter that threads onto the back of the FR and that is 50mm long, and you're good to go.

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    Default Re: Setup According to Backfocus on a Celestron C8 SCT?

    I believe if you use the Celstron prime focus adapter it is 105mm in length already so no fussing around with spacers but I've never had one in my hand to measure. Maybe someone here can do that for us.

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    Default Re: Setup According to Backfocus on a Celestron C8 SCT?

    Hi Shamsi, and welcome here to the Forum!

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    Default Re: Setup According to Backfocus on a Celestron C8 SCT?

    This clears up so much!

    Thank you, jerryTheC and KeithBC for the curvature explanation. I wasn't going to use the reducer and the barlow together (my bad, should have clarified), but I am now at this setup:

    1)Reducer, The 50mm Celestron T adaper for SCT (Thanks Leveye!), T ring, and DSLR for DSOs and;

    2)Original 1.25" diagonal, barlow, T mount nose adapter, T ring, and DSLR for planetary.

    Could you please confirm if this is fine?

    Also, thank you spereira. Glad to be here

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    Default Re: Setup According to Backfocus on a Celestron C8 SCT?

    You don't need to use a diagonal for planetary, you should be able to just use the visual back straight into the barlow and then into the nose adapter. Usually for planetary DSLRs aren't optimal and you'd use a video camera instead, but it can be done.
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    Default Re: Setup According to Backfocus on a Celestron C8 SCT?

    Oh that simplifies things further. Just out of curiosity, what is the backfocus when using a barlow lens? In one forum someone expressed concern that attaching a barlow directly to the VB could mean imaging would be hard.

    And I'm also trying to get a webcam for planets. Just have to see what the budget allows.

    Thanks!

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    Default

    A Barlow is a diverging (negative) lens - so it effectively replaces the end of the original light cone with a longer, narrower angle version.
    This has two effects - it moves the focal plane further out (which is why it may let you reach focus in scopes that don't normally have enough back focus), and it makes it look as though the scope has a longer focal length, giving you a bigger image scale (and higher magnification with an eyepiece).
    Because it's spreading the same quantity of light out over a larger area, it increases the exposure time needed (just like a camera teleconverter).

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    Default Re: Setup According to Backfocus on a Celestron C8 SCT?

    Everything seems clearer now Thanks

 

 

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