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Questions & Answers

  • Why could I see through the ONAG®?
  • Although the dichroic beam splitter reflects 95% or more of the visible light, the 5% left allows to see through when looking at a bright source, like day light. This is because the human eye is a non-linear very sensitive receiver. Also the dichroic beam splitter is designed to work at 45 degrees with a small divergent incoming light beam (few degrees). Which is much less than in a daylight scene situation where rays can come with very large off-axis angles. At those angles the dichroic cut-off wavelength may be quite different than in the context of your scope, adding to this effect.
  • From the ONAG® imager port the reflected light seems colorized, is this normal?
  • The dichroic beam splitter (DBS) uses a multi-coating interference technology. Like anti-reflection (AR) for lenses and optical surfaces it exhibits changing colors, function of the light incidence angle. The DBS is optimized for a 45 degree reflection angle, which is its working nominal condition. In astronomical applications light beams have a very little divergence, 1 to 2 degrees at most. Under those conditions the DBS acts like any normal mirror would. However when you look at the ONAG®® alone, "naked eye", on the open you are very far from this nominal condition. Diffused illuminations, such as day light scenes, and absence of any optical element, will result in a wide reflection angles leading to noticeable color variations.
  • What is the best procedure to focus a guide star?
  • The ONAG® guider port has a built-in focuser, with an effective travel of 9mm (about 1/3"). Its allows you to fine focus the guide star without disturbing your imager focus point, or involving your scope focuser mechanism. First be sure you have selected the right extension tube combination for your set-up, by considering both imager and guider back-focus (including any other element, such as a filter wheel). Please refer to the ONAG® user manual. When you move the ONAG® guider focuser drawtube all the way, the guide star should change form from a vertical ellipsoidal shape to a horizontal ellipsoidal shape, or the opposite in function of your CCD reference frame position. The optimal focus point is achieved when both ellipsoid collapse becoming a spot, or a little cross. This is normal and not a source of concern. This feature becomes handy when manually seeking for best focus. Since most scopes and optical components are not optimized for the near infrared (NIR) there is maybe small distortion involved. Autoguider algorithms are mainly based on centroid algorithms and are not sensitive to this. They average pixels from the all guide star area, so the maximum pixel value or FWHM are not much relevant in this case, unlike for imaging. If you use computer assisted focusing software, such as Maxim DL, the right figure of merit should be the half flux diameter (HFD), or ½ FD. The half-flux diameter is the diameter in pixels that contains half the energy in a star image. In other words, if you add up the pixel values (less the background) inside the diameter, and outside the diameter, you will get the same number. This measurement gives a very similar answer to FWHM, but it is much more robust in the presence of seeing, noise, and can handle non circular distorted images, even out-of-focus like "donuts". The HFD varies linearly with focus position making it reliable to locate the best focus regardless the star shape. If you use the PHD guiding software watch the SNR value, you should seek for its maximum. If you do not use any software, the best focus will be achieved when the guide star cross like shape is minimized and symmetric.
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