I worked out a very simple modification to the very simple tangent barndoor tracker to double its exposure-time for same tracking error.
Eg, a standard tangent tracker has a similar amount of error as a 60min two-arm after only 10-12 mins of exposure.
With my mod, 20-25mins for same error as 60min two-arm exposure.
And much easier to build.
Got a photo here somewhere but can't find it.
Edit: Still no photo, but an edited version of the graph Ian posted might give an idea of what I did.
Using the graph for figures, the double arm has ~8 arc-sec(+- combined) error at 60min exposure, and ~3 arc sec error for exposures <50min.
The modded tangent with ~8 arcsec error will give approx 25min exposure and at around 3arc sec error(not shown on graph) will give approx 20min exposure.
Edit2: Found the photo. Can you spot the mod? Ignore the rough Alt-Az mount.
This barndoor was made about 5 or 6 years ago(Before I had a decent set of tools!), the mod seems so obvious and so simple(one piece of wood!) so I'm surprised I haven't seen someone else do similar.
