Is there a simple sure fire way to test the squareness/collimation of a refractor focuser? thank you, Bill
How to test focuser collimation?
Started by Bill in Ballard, 08/16/2005 12:34PM
Posted 08/16/2005 12:34PM
Opening Post
Posted 08/16/2005 01:24PM
#1
The easiest way that I have found is to use an alignment tool such as a Cheshire eyepiece or a laser collimator.
Remove the diagonal and insert the aligment tool. It should be aimed at the exact centre of the objective. The centre can be identified by using crossed threads or tape on the outside of the objective cell (not touching the lens) or if the lens cap is perforated in the centre it may be installed to indicate the centre. It is not necessary to have the lens installed to check the focuser alignment.
Rotate the alignment tool in the eyepiece holder to check the fit and alignment of the tool. If the centre wanders as the tool is rotated then there will be errors in the final alignment.
Rack the focuser all the way out and check the alignment, then rack it all the way in and see if the aligment has changed. If the focuser is well aligned, there should be no change from one position to the other.
clear skies,
dan
Remove the diagonal and insert the aligment tool. It should be aimed at the exact centre of the objective. The centre can be identified by using crossed threads or tape on the outside of the objective cell (not touching the lens) or if the lens cap is perforated in the centre it may be installed to indicate the centre. It is not necessary to have the lens installed to check the focuser alignment.
Rotate the alignment tool in the eyepiece holder to check the fit and alignment of the tool. If the centre wanders as the tool is rotated then there will be errors in the final alignment.
Rack the focuser all the way out and check the alignment, then rack it all the way in and see if the aligment has changed. If the focuser is well aligned, there should be no change from one position to the other.
clear skies,
dan
Posted 08/16/2005 06:08PM
| Edited 08/24/2005 07:19AM
#2
If you don't have access to a laser or Cheshire tool...
If the "squareness" of either end of the scope tube is untrue to a significant degree, a T-square or combination square may show it (remove the focuser from the tube). However, if the degree is too subtle for either tool, then another quick & sure-fire way to test the focuser/tube interface is to simply twist/rotate the focuser in its place while sighting a fixed target. For many scopes, this involves the removal of three screws that attach the focuser housing to the tube. Obviously, a very tight flange/tube interface can make for some stiff & stubborn rotation. Make sure the focuser/tube interface is maintained (focuser butted up against the tube end when rotating). If focuser rotation induces similar circular motion of the target within the field of view, then the focuser's optical axis is not parallel to the tube's/objective's optical axis.
Focuser/tube alignment is especially critical for faster refractors utilizing non-adjustable objective cells. If one or both ends of the scope tube are not perpendicular to the tube's ocular axis, realignment can be tedious. Both ends of my 60mm f/5 refractor tube are non-perpendicular, so realignment required shimming the objective within the non-adjustable cell and a focuser tweak. Luckily, the focuser tweak involved a simple 120-degree rotation of the focuser (screw holes in an alternate "clock" position).
Cheers & luck.
-Dan
If the "squareness" of either end of the scope tube is untrue to a significant degree, a T-square or combination square may show it (remove the focuser from the tube). However, if the degree is too subtle for either tool, then another quick & sure-fire way to test the focuser/tube interface is to simply twist/rotate the focuser in its place while sighting a fixed target. For many scopes, this involves the removal of three screws that attach the focuser housing to the tube. Obviously, a very tight flange/tube interface can make for some stiff & stubborn rotation. Make sure the focuser/tube interface is maintained (focuser butted up against the tube end when rotating). If focuser rotation induces similar circular motion of the target within the field of view, then the focuser's optical axis is not parallel to the tube's/objective's optical axis.
Focuser/tube alignment is especially critical for faster refractors utilizing non-adjustable objective cells. If one or both ends of the scope tube are not perpendicular to the tube's ocular axis, realignment can be tedious. Both ends of my 60mm f/5 refractor tube are non-perpendicular, so realignment required shimming the objective within the non-adjustable cell and a focuser tweak. Luckily, the focuser tweak involved a simple 120-degree rotation of the focuser (screw holes in an alternate "clock" position).
Cheers & luck.
-Dan
Funding Member
