Quote:
Originally Posted by lineout
What are the advantages and dis-advantages of a manual focuser over a motor driven one? Can they be hooked up to a laptop for automation?
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There are three levels: manual, motorised (no computer interface) and motorised automated (with computer interface - there are actually two additional levels of automated focusers – relative or absolute positioning but lets not confuse the situation for the time being).
To specifically answer your question (well partly); If we take a simplistic look, when focusing a telescope manually you need to wait for vibration/other movement to stop after you’ve moved the focuser. A motorised focuser allows you to rack the focuser in/out without touching the telescope. This is generally not a major issue, but something to note.
Of more importance is to be able to measure the movement of the focuser drawtube or sct knob turn. Without measurements, it is difficult to return to precise focus. This is not a problem for visual work, but I would consider very important for astrophotography. You need to be able to repeat the focus consistently through different filters etc. If you are out of focus by 50 microns or more the image can appear soft and lack detail. You can purchase both manual and motorised focusers with digital or analogue readers to assist you with the measurements.
Automated focusing obviously also performs measurements, but this is noted on a computer not a digital or analogue reader. These measurements are analysed by specific focusing software, along with star values such as brightness and FWHM (Full Width Half Maximum). The computer then understands what position the focuser drawtube or sct knob turns to reach the most optimal focus. The computer can also be told of the differences in temperature and how to correct for this to maintain precise focus as the ambient temperature drops. In addition you can also tell the computer that if a specific non-parfocal filter is used, shift the focus by x microns to reach focus. If you are serious about astrophotography, an automated focuser set up is a time saver and ensures you reach precise focuser ~95% of the time. Its never 100% because the computer is analysing a star in real time, thus seeing conditions and star saturation can significantly affect focusing precision. I usually focus on the mag 4 star nearly the object I’m going to image, then slew back to the object. I refocus every 30 minutes as I haven’t finalised the temperature compensation as yet – on the todo list.
An example of all three-types can be see at Van Slykes web site;
1
http://www.observatory.org/master12.htm - manual
2a
http://www.observatory.org/master4.htm - analogue reader
2b
http://www.observatory.org/master5.htm - digital reader
3
http://www.observatory.org/mastauto.htm - autofocus (automated)
There should be nothing stopping you getting a RoboFocus motor on your WO to make it automated. If you intend doing this, you may also want to lock down you SCT primary mirror to reduce image shifts.