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Collimate your Ritchey-Chrétien

Your RC, collimated frame by frame: you upload a frame with defocused stars and we tell you which primary or secondary screw to turn, and how far. Everything runs in your browser: your frames never leave your computer.

Plan on an hour or two of clear sky and one short frame per step. It’s based on the Deep Sky Instruments method, simplified: first the primary removes the coma in the center, then the secondary balances the corners, and you repeat; the assistant measures every frame and tells you which screw to turn. Do step 1 before you head out. First time? The guide Collimating a Ritchey-Chrétien explains it all, with diagrams.

For the night, next to the scope: the screen in red and kept awake, and each step read aloud.

What’s wrong with your stars? Before you touch anything, check this

Take a focused frame and look at the center and the four corners.

  • One or two corners worse than the others: the sensor is tilted. The tilt analyzer fixes it.
  • All four corners equally bad, stretched toward the center or outward: it’s backfocus, the distance between the reducer or flattener and the sensor. The backfocus calculator tells you the spacing.
  • The center isn’t right either: defocus a star in the center; if the donut is brighter on one side or the hole isn’t in the middle, the scope is out of collimation. If it’s an RC, you’re in the right place. With a Newtonian or an SCT, the guide Colimar tu telescopio (in Spanish for now).
  • The whole field equally bad, and it changes from frame to frame: it isn’t the optics. Check focus (with a Bahtinov mask), guiding, or wait for the seeing to settle.
  1. Prepare
  2. Measure
  3. Primary
  4. Secondary
  5. Check

1. Prepare

Before you head out, tick what you already have:

ABC Primary XYZ Secondary
Looking from the back, starting at the top and going clockwise, as in the Deep Sky Instruments guide. B and Z are never touched: that way the mirrors don’t creep over time.

Step 2 · Measure

The first frame

    Drag your frame here

    FITS from NINA, SharpCap, ASIAIR, MaxIm and the like.

    See an example: the Deep Sky Instruments frames
    Which example
    The edge stars and the center one, as the assistant sees them (shape exaggerated). Coral: “pointed”, long axis toward the center. Blue: “flat”, short axis toward the center. The arrow in the center points to the bright side of the donut (the coma).

      Own an RC? Help us test it

      To collimate yours you don’t need to send us anything: drag your frames in above and they’re analyzed in your browser. This is separate: if you’d also like to help us improve it, email us those same frames.

      This assistant is in beta. We’ve tested it with the Deep Sky Instruments example frames (a 14" RC, collimated and out of collimation) and with a 1 m professional RC, but we’re short of real amateur RCs: 6", 8", 10", 12" or 14", well or badly collimated, with their cameras. If you send us yours, the assistant will get better for your scope and for everyone’s.

      1. A rich star field, high in the sky, with no reducer or flattener.
      2. Three FITS frames, uncalibrated, 10 to 30 s: one in focus, one defocused inward and one outward, with donuts about 30 to 50 pixels across.
      3. If you can, the same set after turning one primary or secondary screw 1/8: tell us which one and which way.
      4. Send them to astrolariusapp@gmail.com with a download link (WeTransfer, Google Drive…) and tell us the scope, the camera, the focal ratio and what you see in the corners.

      We only use them to test and improve the assistant. We never publish them without your permission.

      How it works

      In a frame of defocused stars, the assistant measures two things. In the center, whether the donut is brighter on one side: that’s coma, and the primary removes it. At the edge, the shape of each donut: in a collimated RC, every edge star looks the same as seen from the center (round, all “flat” or all “pointed”). If they’re “pointed” on one side and “flat” right across from it, the mirrors are misaligned, and the secondary balances it. To separate what’s the design from what’s misalignment, it fits the star shapes across the whole field with one term that grows with distance from the center (misalignment) and another with its square (design).

      You don’t need to know which screw sits on which side of the image: when you tighten a screw 1/8, it measures how much and in which direction the thing you want to remove changes, and with two screws per mirror it works out how far to turn each one. The donut size from one side to the other also gives away a tilted sensor, which this method would compensate with the mirrors: that’s why it warns you first. It’s Rich Simons’s procedure at Deep Sky Instruments (2011), measured with numbers instead of by eye.