Elongated Stars in One Corner of Images

Discussion in 'STX and STXL' started by Mike Hambrick, May 2, 2020.

  1. Mike Hambrick

    Mike Hambrick Cyanogen Customer

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    I had to switch from the original dovetail nose piece that came with my STXL16200 camera to a custom nose piece in order to be able to achieve focus on my various imaging trains. Since making this switch I have been noticing that the stars in the upper left corner of my images are elongated. I need some advice on how to try to solve this problem. I have attached a copy of one of my recent images.
    I have checked the flatness of the custom adaptor on a surface plate and it seems to be OK. I don't see any burrs or nicks on any of the surfaces that might be causing uneven contact. Note that there were no shims included with the original dovetail nose piece that came with the camera (Serial # X18040006).
     

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  2. Doug

    Doug Staff Member

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    Did your camera come with shims under the plate? These are thin little "washers" that slightly adjust the tilt of the adapter plate. There would have been a shim sheet included with the camera, noting what shims go where. If you remove the dovetail then you need to replace the shims when you install the new adapter.
     
  3. Mike Hambrick

    Mike Hambrick Cyanogen Customer

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    Hi Doug. There were no shims that came with the original dovetail faceplate (see comment in original post). If you add shims, what do they look like, and how thick do you generally have to use. Based on what you see in my image how much would you have to shim ?
     
  4. Doug

    Doug Staff Member

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    I looked up your serial number and no shims were required at manufacture.

    First thing to check is the instrument collimation. If that has shifted for some reason, that could easily account for the entire issue. It could be coincidence that it happened when you installed the adapter.

    Be aware that the flatness requirement is on the order of thousandths of an inch. If your custom adapter wasn't built perfectly, you can't measure the problem without a good micrometer.

    I can't tell from your picture how much adjustment might be needed. It really depends on your equipment - f/ratio, etc. In the shop we measure the tilt of the sensor directly using a specially designed instrument, and calculate the shims required based on those measurements. We use ring shims from McMaster Carr.
     
  5. Mike Hambrick

    Mike Hambrick Cyanogen Customer

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    Thanks Doug. I don't think it is a collimation issue because I see the exact same image behavior regardless of which telescope I am imaging with.

    I will check McMaster for ring shims. One thing I like about the SBIG dovetail nose piece is that it has the o-ring in the face of it. I presume that this is there to prevent stray light from entering the camera in case you have to add any shims. That is a very clever design. My custom adapter does not have any o-ring, so I am not sure how I will keep stray light out.

    In the meantime I have reinstalled the original dovetail face plate on the camera because I have an adapter that will allow me to use it with my Astro-Physics scope. Next time I am out (not until moon and weather allow) I will see if the images are better, and go from there.
     
  6. Doug

    Doug Staff Member

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    Yes but fortunately most people haven't needed to use an O-ring in the groove. If it's just shimmed a few thousandths it's a pretty small gap for light to leak through, and it's a good distance from the camera's aperture. It was more of an issue for the old STL cameras because the overall diameter of the adapter was a lot smaller. I'd try it first, and then figure out how to mask it if you have a problem.
     
  7. Mike Hambrick

    Mike Hambrick Cyanogen Customer

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    I was able to get out last night and take a few images after some difficuty (see separate thread on my STi). With the original OEM dovetail faceplate the elongated stars have disappeared.
     

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