Back focus

Discussion in 'Legacy Models (ST, STL, etc.)' started by larrys3255, Nov 25, 2015.

  1. larrys3255

    larrys3255 Standard User

    Joined:
    Oct 12, 2015
    Messages:
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    Can someone explain the concept of back focus. I have a Planwave CDK14 scope and it says it has 11 inches back focus. Where is this measured from? I am having trouble just focusing at all and perhaps I could determine if my optical train (SBIG SR2000 CCD) is too short or too long.

    Thanks and I apologize for such as dumb question.

    Larry
     
  2. Tim Puckett

    Tim Puckett Guest



    Larry,

    The back focus can be specified different ways.
    It can be measured from the front surface of the primary mirror to where the light cone comes to focus.
    It depends on the brand. Its actually a good question, not s stupid one.
    Some companies use a "metal back focal distance" usually from some sort of a reducer.

    NOTE: "Planewave states"
    Back Focus from Mounting Surface 11.09 inch (282 mm)

    http://planewave.com/wp-content/uploads/downloads/2015/06/CDK14_SBIG-adapters1.pdf
    Use the chart above to connect SBIG items.

    If you run into trouble you can call my number below.

    Best regards
    Tim
     
  3. larrys3255

    larrys3255 Standard User

    Joined:
    Oct 12, 2015
    Messages:
    7
    Tim

    Thanks. Where exactly is the "Mounting Surface for the CDK14 scope?

    My optical train has the ST2000 XCM, the AO8 and a 2 inch Pyxis Rotator.

    Larry

    cdk14 diagram.JPG Capture.JPG
     
  4. Tim Puckett

    Tim Puckett Guest

    It shows it in the drawing you provided, backfocus from focuser mounting plate.
     
  5. larrys3255

    larrys3255 Standard User

    Joined:
    Oct 12, 2015
    Messages:
    7
    Tim

    Thanks. It was looking me in the face. I also read in the scope notes a value for the back focus from the fully retracted Hendrick Focuser - around 8 inches. It looks like my imaging problem is that I am way far from focus.

    Since you work for SBIG, can you recommend an SBIG CCD that would take full advantage of the 70 mm field of view of this scope. I think it is time to upgrade from my old ST2000 XCM (providing I can ever get it to focus!).

    Larry
     
  6. Tim Puckett

    Tim Puckett Guest

    Larry,

    You are looking at tens of thousands of dollars to cover 70mm.
    You can cover a 52mm diagonal with our STX camera.
    It's a 4kx4k 9u pixel camera. The STX uses the KAF-16803 and we offer a good price on this camera $9995
    http://www.sbig.com/products/cameras/stx/stx-16803/

    [​IMG]

    Call me anytime next week and we can go over the focus issues and see about a new system.
    Cheers
    Tim
     

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