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Thread: How to scale image in prime focus photography.

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    Default How to scale image in prime focus photography.



    Hello Guys,

    Thanks for all your help with my last question. Continuing on the same train of thought:

    So as there's not much in the way of night, I thought I would get a look at the sun. I got the sun filter sorted out thanks to your help (Baader film) and finder scope sorted (shadow on the ground) but I'm struggling to get decent videos.

    I find when I look at the image through the 20mm eyepiece I get a full size image nearly filling the view (no sunspots, however, for the last few days). But, when I switch to the camera, I get only a quarter or so sized image. I also get dust particles on the objective in focus on the viewing screen.

    I've tried using a Barlow lens with x.5 instead of x2, but I can't get it into focus.

    What to do.

    Regards

    Steve.
    Celestron C8n, focal length = 1000 mm (ie f/5) + CG5 mount.
    Celestron 114EQ, focal length = 900 mm (ie f/7.9) + motorised EQ3-2 mount.
    Bosma TCE-200, CCD 2MP Camera.

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    Default Re: How to scale image in prime focus photography.

    Do you:
    a- Find yourself unable to focus INwards?
    b- Find yourself unable to focus OUTwards?
    ...
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    Default Re: How to scale image in prime focus photography.

    You need either a shorter focal length scope or a camera with a bigger sensor. Image scale is directly proportional to focal length and inversely proportional to sensor size.

    The dust spots you are seeing will be on the protective front glass of the camera sensor, not on the objective.
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    Default Re: How to scale image in prime focus photography.

    Quote Originally Posted by KeithBC View Post
    You need either a shorter focal length scope or a camera with a bigger sensor. Image scale is directly proportional to focal length and inversely proportional to sensor size.
    Hi Keith, sadly I'm coming to the same conclusion, but it implies that I need a separate camera for each scale of object that I wish to image. Is that how it is? I thought a multiplier or reducer near the focal plane would help me, but I can't get the distances right. I tried to calculate where the focal plane was by measuring 900 mm from the objective and it seems to be in the middle of the range of the focuser. So with a reducer before and the camera after I thought I'd get it right, but I don't seem to be able to get the right mixture of extender parts. The empty Barlow won't go in far enough and then camera is too far from the reducer. When I screw the reducer on the end of a lense (where the filter goes), it's fine, but obviously I'm not imaging.


    Quote Originally Posted by KeithBC View Post
    The dust spots you are seeing will be on the protective front glass of the camera sensor, not on the objective.
    And so I assumed, but it is still there when I remove the protective lens and when I rotate the camera it doesn't move. The secondary mirror seems clear and I also think I can see dust particles on the objective when I peer down the tube.

    I shall probably remove the whole objective attachment to remove the dust, but I wanted to avoid recollimating the telescope.
    Celestron C8n, focal length = 1000 mm (ie f/5) + CG5 mount.
    Celestron 114EQ, focal length = 900 mm (ie f/7.9) + motorised EQ3-2 mount.
    Bosma TCE-200, CCD 2MP Camera.

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    Default Re: How to scale image in prime focus photography.

    Quote Originally Posted by nicoyenny View Post
    Do you:
    a- Find yourself unable to focus INwards?
    b- Find yourself unable to focus OUTwards?
    ...
    I'm not sure, but I think I can't get inward enough for the reducer and then the camera is too far from it.

    What I'd like to do is draw the telescope and optical train on a piece of graph paper. The raw telescope and camera I can do, but I'm not sure how to do the calculations for the refracted angles for the reducer. I imagine this solution will also introduce some aberration in the image, although I hope not too much.

    Regards

    Steve.
    Celestron C8n, focal length = 1000 mm (ie f/5) + CG5 mount.
    Celestron 114EQ, focal length = 900 mm (ie f/7.9) + motorised EQ3-2 mount.
    Bosma TCE-200, CCD 2MP Camera.

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    Default Re: How to scale image in prime focus photography.

    Quote Originally Posted by LyulphTracks View Post
    Hi Keith, sadly I'm coming to the same conclusion, but it implies that I need a separate camera for each scale of object that I wish to image. Is that how it is?
    THis is why many astrophotographers have several scopes.

    I tried to calculate where the focal plane was by measuring 900 mm from the objective and it seems to be in the middle of the range of the focuser.
    Sounds about right. So that is where the sensor of the camera needs to be. Unless you are using a focal reducer, in which case it needs to be even closer in.

    So with a reducer before and the camera after I thought I'd get it right, but I don't seem to be able to get the right mixture of extender parts. The empty Barlow won't go in far enough and then camera is too far from the reducer.
    So you are using the tube from the barlow as a holder for the focal reducer? That is the problem. The reducer moves the focal plane forward and the tube moves the camera back. So there is no way to get the sensor to the focal plane.

    Your camera must have a nosepiece to fit into the barlow tube. Can the focal reducer attach to that? It would be worth a try even if you don't get the correect spacing for the FR.

    And so I assumed, but it is still there when I remove the protective lens and when I rotate the camera it doesn't move.
    If the dust doesn't move on the screen when you rotate the camera, it confirms that the dust is on the camera. There is no way that dust on the mirror will be visible in the image. It will be so out of focus that it is invisible.
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    Default Re: How to scale image in prime focus photography.

    Quote Originally Posted by KeithBC View Post
    So you are using the tube from the barlow as a holder for the focal reducer? That is the problem. The reducer moves the focal plane forward and the tube moves the camera back. So there is no way to get the sensor to the focal plane.
    Yes, That's it.
    Quote Originally Posted by KeithBC View Post
    Your camera must have a nosepiece to fit into the barlow tube. Can the focal reducer attach to that? It would be worth a try even if you don't get the correect spacing for the FR.
    Sadly, it's such a cheap camera, that it doesn't even have a thread on the nosepiece. There is a guy at my local astronomy club whole makes things with a lathe, maybe I could ask him nicely to help me with a replacement nosepiece that fits the camera at one end and the reducer at the other. Of course, I'd still need to know how long to make it!!

    Actually, the reducer works on the 12.5 mm eyepiece I have, maybe the tube could be the same dimensions. If it's not exactly right, I assume that's what the focuser is for, if it's, say 5 mm out, I just refocus by a few mm (it may not be 5, but I hope only a few).

    Regards

    Steve.
    Celestron C8n, focal length = 1000 mm (ie f/5) + CG5 mount.
    Celestron 114EQ, focal length = 900 mm (ie f/7.9) + motorised EQ3-2 mount.
    Bosma TCE-200, CCD 2MP Camera.

 

 

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