{"id":15138,"date":"2026-05-23T17:27:29","date_gmt":"2026-05-23T15:27:29","guid":{"rendered":"https:\/\/deep-space-astronomy.ch\/?p=15138"},"modified":"2026-05-26T07:41:49","modified_gmt":"2026-05-26T05:41:49","slug":"sky-watcher-stargate-400p-probleme-de-courroie-daltitude-qui-sort-au-zenith","status":"publish","type":"post","link":"https:\/\/deep-space-astronomy.ch\/en\/sky-watcher-stargate-400p-probleme-de-courroie-daltitude-qui-sort-au-zenith\/","title":{"rendered":"Sky-Watcher Stargate 400P: Solve altitude belt problem at zenith"},"content":{"rendered":"<p>On some Sky-Watcher Stargate motorized Dobsons, in particular the <strong>Sky-Watcher Stargate 400P GoTo<\/strong>, it may happen that the <strong>altitude drive belt<\/strong> gradually shifts as the telescope rises towards the zenith.<\/p>\n<p>The symptom is fairly typical:<\/p>\n<ul>\n<li>at low altitude, everything seems to function normally; ;<\/li>\n<li>around 45\u00b0, the belt begins to shift; ;<\/li>\n<li>close to 90\u00b0, it becomes clearly off-center; ;<\/li>\n<li>In some cases, it may even partially emerge from the throat.<\/li>\n<\/ul>\n<p>The problem can be worrying, especially on a 400 mm Dobson, but in many cases it's not a serious breakdown. Rather, it's a problem of alignment, belt tension or slight mechanical bending.<\/p>\n<h2>Observed symptoms<\/h2>\n<p>The observed behavior was as follows:<\/p>\n<ul>\n<li>At 0\u00b0 or close to horizontal, the belt seemed well centered.<\/li>\n<li>At around 45\u00b0, it began to migrate laterally.<\/li>\n<li>Around 80\u00b0 to 90\u00b0, i.e. close to the zenith, the shift became very visible.<\/li>\n<li>The belt came very close to the edge of a roller.<\/li>\n<\/ul>\n<p>The fact that the problem increases progressively with altitude is extremely important for diagnosis.<\/p>\n<p>This usually indicates a <strong>dynamic misalignment<\/strong> the system appears aligned at rest, but under load the structure works slightly as the tube rises towards the zenith.<\/p>\n<h2>Photo of belt system<\/h2>\n<p><a href=\"https:\/\/deep-space-astronomy.ch\/wp-content\/uploads\/2026\/05\/mecanisme-1.pdf\">mechanism<\/a> <img fetchpriority=\"high\" decoding=\"async\" class=\"aligncenter size-full wp-image-15142\" src=\"https:\/\/deep-space-astronomy.ch\/wp-content\/uploads\/2026\/05\/mecanisme.png\" alt=\"\" width=\"1536\" height=\"1152\" srcset=\"https:\/\/deep-space-astronomy.ch\/wp-content\/uploads\/2026\/05\/mecanisme.png 1536w, https:\/\/deep-space-astronomy.ch\/wp-content\/uploads\/2026\/05\/mecanisme-300x225.png 300w, https:\/\/deep-space-astronomy.ch\/wp-content\/uploads\/2026\/05\/mecanisme-1024x768.png 1024w, https:\/\/deep-space-astronomy.ch\/wp-content\/uploads\/2026\/05\/mecanisme-768x576.png 768w, https:\/\/deep-space-astronomy.ch\/wp-content\/uploads\/2026\/05\/mecanisme-16x12.png 16w, https:\/\/deep-space-astronomy.ch\/wp-content\/uploads\/2026\/05\/mecanisme-600x450.png 600w\" sizes=\"(max-width: 1536px) 100vw, 1536px\" \/><\/p>\n<p>At first glance, everything may seem perfectly normal:<\/p>\n<ul>\n<li>the belt looks clean; ;<\/li>\n<li>the pebbles seem to be in good condition; ;<\/li>\n<li>nothing seems broken; ;<\/li>\n<li>the transmission works.<\/li>\n<\/ul>\n<p>However, on a belt drive, even the smallest defect can cause lateral migration.<\/p>\n<h2>Why does the belt shift at the zenith?<\/h2>\n<p>On a large 400 mm Dobson, mechanical stresses change dramatically with tube altitude.<\/p>\n<p>When the telescope is pointed at the zenith :<\/p>\n<ul>\n<li>weight acts differently on the trunnions; ;<\/li>\n<li>the motor support may bend slightly; ;<\/li>\n<li>the tension applied to the belt changes; ;<\/li>\n<li>the rollers can be set at a very slight angle.<\/li>\n<\/ul>\n<p>And that's the point: a shift of less than a millimetre can be enough to cause a timing belt to gradually migrate.<\/p>\n<p>There are many possible causes:<\/p>\n<ul>\n<li>a slightly misaligned pebble; ;<\/li>\n<li>a belt that is too tight; ;<\/li>\n<li>a bearing with slight play; ;<\/li>\n<li>an incorrectly positioned spacer ;<\/li>\n<li>slight bending of the motor mount ;<\/li>\n<li>a pulley that is no longer perfectly parallel.<\/li>\n<\/ul>\n<h2>The very important role of belt tension<\/h2>\n<p>In this case, belt tension probably played a major role.<\/p>\n<p>Intuitively, you'd think you'd have to tighten a belt to avoid backlash. However, on the Stargate 400P, this is generally not a good idea.<\/p>\n<p>Why?<\/p>\n<p>Because :<\/p>\n<ul>\n<li>the belt is wide; ;<\/li>\n<li>many teeth are in contact; ;<\/li>\n<li>the transmission surface is large ;<\/li>\n<li>the torque transmitted remains relatively low.<\/li>\n<\/ul>\n<p>Excessive tension, on the other hand, can :<\/p>\n<ul>\n<li>increase lateral stress ;<\/li>\n<li>accentuate geometric defects; ;<\/li>\n<li>make the rollers work crooked; ;<\/li>\n<li>promote derailment.<\/li>\n<\/ul>\n<p>A slightly less taut belt is better able to absorb minor mechanical faults.<\/p>\n<h2>Step 1: Observe belt altitude<\/h2>\n<p>The first test simply consists of observing the behaviour of the belt at different angles:<\/p>\n<ol>\n<li>Near-horizontal tube.<\/li>\n<li>Tube at approximately 45\u00b0.<\/li>\n<li>Tube close to zenith.<\/li>\n<\/ol>\n<p>If migration increases progressively with altitude, the problem is probably related to the structure or alignment under load.<\/p>\n<h2>Step 2: Check the rollers<\/h2>\n<p>After disassembly and inspection, the lower roller appeared to be slightly offset.<\/p>\n<p>The offset was small, almost hard to see with the naked eye, but on a belt drive it can be enough to cause continuous migration.<\/p>\n<p>Check that :<\/p>\n<ul>\n<li>parallelism of rollers ;<\/li>\n<li>the presence of washers ;<\/li>\n<li>bearing condition ;<\/li>\n<li>no axial play ;<\/li>\n<li>condition of spacers; ;<\/li>\n<li>tightening screws.<\/li>\n<\/ul>\n<h2>Step 3: Slightly slacken the belt<\/h2>\n<p>The most effective test was to reduce the belt tension slightly.<\/p>\n<p>To do this:<\/p>\n<ol>\n<li>Slightly loosen the idler.<\/li>\n<li>Gradually reduce tension.<\/li>\n<li>Do not make the belt soft.<\/li>\n<li>Re-test tube movement.<\/li>\n<\/ol>\n<p>The result was an immediate improvement in symptoms.<\/p>\n<p>The belt remained slightly offset, but its tendency to run out was greatly reduced.<\/p>\n<h2>Should you aim for perfect alignment?<\/h2>\n<p>Not necessarily.<\/p>\n<p>On a large Dobson GoTo locksmith, there is always:<\/p>\n<ul>\n<li>microflexions ;<\/li>\n<li>mechanical tolerances ;<\/li>\n<li>a little elasticity; ;<\/li>\n<li>load-dependent variations.<\/li>\n<\/ul>\n<p>The most important thing is :<\/p>\n<ul>\n<li>that the belt remains engaged; ;<\/li>\n<li>that she doesn't jump out of her teeth; ;<\/li>\n<li>that it doesn't come out of his throat; ;<\/li>\n<li>that the follow-up remains good; ;<\/li>\n<li>that GoTo movements remain fluid.<\/li>\n<\/ul>\n<p>Slight lateral migration may be acceptable as long as it remains stable.<\/p>\n<h2>When should you really worry?<\/h2>\n<p>The problem becomes serious if :<\/p>\n<ul>\n<li>the belt jumps off the teeth; ;<\/li>\n<li>it rubs strongly; ;<\/li>\n<li>she's out regularly; ;<\/li>\n<li>monitoring becomes irregular; ;<\/li>\n<li>GoTo loses precision; ;<\/li>\n<li>a mechanical noise appears.<\/li>\n<\/ul>\n<p>In this case, consider :<\/p>\n<ul>\n<li>a complete realignment ;<\/li>\n<li>replacing a roller ;<\/li>\n<li>bearing replacement ;<\/li>\n<li>or improved lateral guidance.<\/li>\n<\/ul>\n<h2>Conclusion<\/h2>\n<p>On the Sky-Watcher Stargate 400P, an altitude belt that shifts towards the zenith does not necessarily indicate a serious malfunction.<\/p>\n<p>In this particular case, the problem seemed to stem from a combination :<\/p>\n<ul>\n<li>a very slight misalignment; ;<\/li>\n<li>and belt tension probably too high.<\/li>\n<\/ul>\n<p>Simply reducing the voltage has already significantly improved the system's behavior.<\/p>\n<p>Before replacing any parts, it is therefore advisable to :<\/p>\n<ol>\n<li>observe behavior according to altitude; ;<\/li>\n<li>check the rollers carefully; ;<\/li>\n<li>check tension ;<\/li>\n<li>and gradually test the settings.<\/li>\n<\/ol>\n<p>On this type of drive, extremely small differences can have a major impact on belt behavior.<\/p>","protected":false},"excerpt":{"rendered":"<p>Sur certains Dobson motoris\u00e9s Sky-Watcher Stargate, notamment le Sky-Watcher Stargate 400P GoTo, il peut arriver que la courroie d\u2019entra\u00eenement en altitude se d\u00e9cale progressivement lorsque le t\u00e9lescope monte vers le z\u00e9nith. Le sympt\u00f4me est assez typique : \u00e0 basse altitude, tout semble fonctionner normalement ; vers 45\u00b0, la courroie commence \u00e0 se d\u00e9caler ; proche [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":8886,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[34],"tags":[],"class_list":["post-15138","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-telescope"],"_links":{"self":[{"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/posts\/15138","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/comments?post=15138"}],"version-history":[{"count":3,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/posts\/15138\/revisions"}],"predecessor-version":[{"id":15152,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/posts\/15138\/revisions\/15152"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/media\/8886"}],"wp:attachment":[{"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/media?parent=15138"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/categories?post=15138"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/deep-space-astronomy.ch\/en\/wp-json\/wp\/v2\/tags?post=15138"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}