Off-Axis Guider
22 July 2010

The ASA DDM60 mount will give unguided exposures up to 5 minutes, but there is too much flexure in the 10" reflector to go much beyond this. For longer exposures I have bought a Starlight Xpress Off-Axis Guider (OAG) with a Lodestar CCD.
The OAG uses a small prism to direct light into the Lodestar camera. Pulseguide commands are sent to the mount to ensure that the selected guide star does not deviate from its original position.

Because the Lodestar is rigidly connected to the main imaging camera, flexure is totally removed.
This shows the prism and the main camera CCD.

The thumbscrew allows the prism position to be adjusted. If it is too close to the main CCD it will obstruct the light forming the image - if too far away not enough light will enter the guide camera to form a useable image.

I suspect trial and error will determine the optimum position.

The main camera is focused on the star field and then two small grub screws allow the guide camera to be adjusted to achieve focus also. Once this position is found it should not need to be altered.

This is a stack of 6x 6minute exposures of part of the Dumbbell Nebula. Stars are nice and round showing that the guiding is working well.

 Full size image