Setting up the auto guider

Any imaging that involves sub frames of longer than 30 seconds or so will be better if auto guiding is used, this involves one of several methods and each has their own advantages and disadvantages. Firstly SBIG make cameras with two ccd chips, one for imaging and one for auto guiding. The advantages are that only one telescope is required and issues regarding differential flexure don't arise. The disadvantages arise when using narrowband filters as the guiding chip also sees the light through the filters that are being used and also if no suitable guide star is found in the field of view. Some of the Starlight Xpress cameras can guide and image at the same time but the exposure time has to be doubled. Another option is to use a separate guide scope and camera, the advantages are that a suitable guide star should always be available as the guide scope doesn't have to point at the same area of sky as the imaging scope, the disadvantage is that especially when using heavy equipment there could be a problem with differential flexure where there is some movement between the imaging scope and camera and the guide scope and camera. This movement could be miniscule but sufficient to cause star trailing over long exposures. The last option is off axis guiding where the guiding is done through the same telescope as the imaging but with a separate camera which is set in front of the filters collecting its light 'off axis' via a small mirror. This resolves the differential flexure issue and the use of narrow band filters as the light reaching the guiding camera is unfiltered, the only diadvantage is finding a suitable guide star which may result in long guiding exposures or binning.

When setting up the auto guider an exposure has to be taken for the guider and this will depend on a number of things such as the mounts inherent accuracy (the less accurate the mount the shorter the exposure times/more rapid corrections will be needed), the seeing conditions (too short a guiding exposure will result in the guider chasing the seeing, ie correcting for seeing conditions rather than periodic error). I usually run my guider at 10 seconds because the Paramount is accurate enough and also it avoids chasing the seeing.

Once an exposure is done I then select a suitably bright star and then click on calibrate, this causes exposures to be taken followed by the mount being moved a certain amount and then back, this is done automatically by Maxim DL. If everything has worked ok a right angled red line will appear on the screen showing where the mount has moved.

 

Now it is ok to click on track, the auto guider will now continually take exposures of 10 seconds and make any necessary corrections at which time the screen changes to show a small box with the guide star in, you have the option now to put cross hairs on the box which make any movements/corrections easier to see and you can also open up a guiding graph which show the degree of corrections.


There are other programs such as PHD, Guide Dog and others that do a similar job and it is a matter of choice as to which works best for you.