Telescope Accessories

Newtonian Focuser Calculator

It is important to consider focuser parameters when replacing your existing focuser with a low profile, Crayford model. Before selecting the correct focuser configuration, you must check the existing focus range of your current focuser. This requires two measurements:

Measure the minimum "racked down" height and the fully "racked up" height. This should be measured from the OTA surface to the top of your focuser drawtube with no EP in place. This will be your min and max focus range that your original focuser provided.

With those two measurements in mind, pick the correct focuser drawtube option and spacer/curve adapters to allow the replacement focuser to meet the Min and Max height requirements.

Example: If your min focuser height is 3" and your max height is 4.5" then you could pick the focuser as follows.

Note: base thickness on all CR model focusers is 1.45" but let's round up to 1.5" to make it easy.

With a base thickness of 1.5", the min height becomes 1.5" (Draw tube is even with the top of the base). In the above example, 3" is needed so an install kit under the focuser would add an additional 1.5" giving a min height of 3" total. Or a 1" spacer plus a 1/2" spacer would produce the same 3" racked down profile. It comes out to (1.5" base thickness + 1.5" of spacers = 3")

Now lets look at the racked up requirements. The example original focuser above measured 4.5" when fully racked up. That would indicate that the new focuser should at least rack up to that height. With a 2" travel drawtube the focuser would rack up to 5". That is (3.0" min height plus 2" of travel= 5.0" total). A shorter travel drawtube could also be selected for this example. The 1.5" and also the 1.75" travel drawtube would also work.

Remember to consider how much draw tube may stick down into the OTA and possibly bump the light path. With that said, try not to pick too long of a drawtube, pick just what is needed and no more.

MoonLite now offers 5 different draw tubes for our Newtonain CR focuser models. Travel options are now 1.5" 1.75" 2.0" 2.375" 2.75" Note: The Brass compression ring option adds 3/8" to the total racked down height

There are three different types of curve adapters/install kits. We make curve adapters that fit from 7" up to 19" tube diameters. They are the same size as the focusers foot print (2.9" X3.5") Along with curve adapters we also make spacers (1/4" ,1/2" and 1"). THey can be stacked on a curve adapter to place the focuser at the required height.

Beside the curve adapters we make Install kits, these kits match up with the existing bolt holes and make installing a focuser much easier due to not having to drill new mounting holes.

There are 2 different styles, Large format install kits for bolt patterns around the 3.5" and narrow install kits for bolt patterns around 2.75".

The narrow install kits bolt on to the OTA from the inside and the Large format install kits bolt from the outside.

Please see Install kits on the newtonian focuser page for more info on bolt patterns and different curve diameters.