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Old 12-09-2018, 03:25 PM
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Jason D (Jason)
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Jason D is offline
Join Date: Aug 2009
Location: California USA
Posts: 116
Originally Posted by bojan View Post

It may also be a bit of wrong secondary offset...
Because the light beam reflected from primary is conical, for it to pass the secondary unaffected by vignetting, it needs to be positioned a bit away from focuser (2~4mm, depending on f/ ratio and other factors).
I intend to use my Newt for AP, so I will replace existing secondary with larger one - and adjusting the correct offset will be even more important part of colimation process.
In most cases you do not need to offset your secondary mirror away from the focuser to achieve perfect collimation. There are three main reasons why the away-from-focuser offset might be necessary:
1- To reduce tracking error -- assuming your scope is equipped with a tracking system and that you are doing long exposure astrophotography. For visual, the error reduction is insignificant.
2- To accommodate a front corrective plate -- assuming your scope has one.
3- To avoid front aperture vignetting -- assuming the OTA opening it too tight.
Above three reasons do not apply to most Newtonan reflectors used for vsual observation; therefore, away-from-focuser offset is not needed in most cases.

Adjusting the spider vanes to move the secondary mirror assembly away from the focuser to offset the mirror can be more harmful since it will worsen the spider vanes diffraction.

If the secondary mirror requires an offset, it would have been done by the manufacturer. If it was not then it is not needed and you should not attempt to do it.

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