avandonk
10-08-2008, 12:28 PM
Some time ago I installed a dial indicator on my 150mm Achro so I can focus very accurately after changing filters without losing the framing of the object no matter how dim. They come in two flavors analogue or digital. Either will do but the analogue are only about $40 from Bunnings.
It is simply a matter of taking test exposures say 0.2mm apart and then checking which position is closest by inspecting the images on your computer screen. The animated gif is three images (100% crop)using a Halpha filter on the achro and they are 0.2mm apart. It is then just simply a matter of resetting to the best focus position with the dial indicator.
If you note the temperature of your optic on different time/nights and focus position, you can also get an accurate idea how focus changes with temperature.
Bert
It is simply a matter of taking test exposures say 0.2mm apart and then checking which position is closest by inspecting the images on your computer screen. The animated gif is three images (100% crop)using a Halpha filter on the achro and they are 0.2mm apart. It is then just simply a matter of resetting to the best focus position with the dial indicator.
If you note the temperature of your optic on different time/nights and focus position, you can also get an accurate idea how focus changes with temperature.
Bert