p POOR COLLIMATION Ensuring your collimation is spot-on is an important step, particularly when shooting with fast Newtonian astrographs with coma correctors. This image of van den Bergh 15 displays comatic stars along the right side. These are particularly visible in the top right. COLLIM ATION: SE A N WA LK ER / S&T PERFECT GUIDING Perhaps the most critical step in deepsky astrophotography is ensuring well-tracked images. This detailed image of NGC 7000 (left) and IC 5067 (right) includes more than 16 hours of perfectly tracked, unguided exposures using a Takahashi FSQ-106ED astrograph and a Moravin G3-16200 CCD camera. Unless otherwise noted, all photos are courtesy of the author. the sensor. Fast optics (those with a low f/ratio) are particularly sensitive to imperfect collimation. In astrophotography, not only must the optics be aligned, but the camera's sensor must be properly positioned. Heavy cameras and filter wheels can cause some focuser drawtubes to sag, introducing misalignment. One clue that a focuser on a refractor or Cassegrain telescope is sagging is when stars appear round across a photo only when the telescope is pointed straight up towards the zenith and gravity is pulling the focuser holding a heavy camera square to the optical axis. You can solve this sagging issue by tightening up any loose rails or p UNWANTED TRAILS While some star elongation doesn't detract from wide-field nightscape images, the same can't be said for deepsky astrophotography. If your results look like the image above, plan on spending some time identifying the cause of the tracking problem. p HALF IN FOCUS Diagnosing the source of sensor tilt - seen in this image as the right half of the frame is out of focus, while the other side is not - can be tricky. Take an exposure pointed at the zenith then another pointed about halfway towards the horizon to make sure the problem isn't due to a sagging focuser drawtube. sk yandtele scope.com * J U N E 2 019 65http://www.skyandtelescope.com