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The Off-Axis Guider and Its Applications

The Off-Axis Guider (OAG) has a long history. It is the guiding method used from the beginning of astrophotography by all astronomers. Images of overheated or freezing astronomers, their eyes glued to the telescope guiding eyepiece fill biographies about these brave pioneers. There are reports of eyelashes frozen to the eyepiece and worse. Today, the OAG is still one of the most popular guiding methods. However,generally it is a CCD imager used as a guider that does the guiding job.

There are, unfortunately, a large number of OAGs and CCD guiders that are terribly difficult to use and do not work well. They have given, in many cases, the use of the OAG a bad name. With a high quality guider and a high quality CCD imager/guider it is a very reliable way to guide telescopes for film or CCD imaging. At the very start, I will say that the only guider that I know of that does the job well is the Lumicon giant OAG. The CCD guider of choice is the SBIG ST-4. There are other CCD guiders that do very well also. I think of the Meade 216XT or even the ST-7 imagers. I have used the Lumicon OAG and both of these CCD imagers with good results.

Idea of the OAG is to guide on a section of the star field that is created by the main imaging telescope, but just far enough out of the image field so that the "picked off" part of the field does not cut into the field being imaged. This is often a bit tricky to accomplish since it generally means that a deflecting mirror or more commonly a prism is placed at the very edge of the imaged field and directs a portion of the image off to one side. The tolerances are quite critical. It is not only off the main axis of the image but off far enough to avoid cutting into the desired image. Unfortunately, telescopes often have such a small image field of high optical quality that the picked off star images are somewhat distorted. This is generally coma distortion. Refractors are much better in this respect than SCT type telescopes. Nevertheless, it is not too hard to pick off a section of the star field that is totally adequate for guiding. This is where the quality of the CCD guider comes into play. Some CCD guiders are much better at capturing and locking onto the slightly distorted star images than others.

Also the quality and versatility of the OAG comes into play. It must be easy to rotate the OAG so that its prism can be set to intercept a good guide star. When the OAG is rotated of course, the star field rotates and the CCD guider has to be oriented so that it still guide the telescope properly. Thus, it is not trivial to adjust the OAG and guider combination to make it guide well. These factors are what have made the OAG difficult to use and have discouraged many amateur astronomers from using them.

I must admit that I have had all too much experience with crummy OAG/CCD combinations and thus grew to hate them thoroughly. But because the CCD guider is guiding on a part of the actual star field image being captured,it gives excellent guiding. It is probably closely equal to that obtainable with the SBIG ST type self guided imagers. The features to look for in the OAG are a large pickoff prism which can be easily adjusted to find a good guide star. There is none better than the Lumicon unit. Another feature is to have the takeoff tube for the guider that is very solidly built. And finally, the entire OAG should be very solidly built and have a convenient way to hold a focal reducer or expander is that might be used. The Lumicon OAG has all of these features.

The Lumicon unit has adapters on both ends that can be changed or modified to fit any telescope and imaging device. This includes small and large format film cameras as well as CCD imagers of any type. I can attest that it is extremely well built and that it operates exactly as expected. Considerable technique is required to operate even this fine OAG. Descriptions about these techniques are given by Philip Perkins at <>. He is without doubt a premiere astrophotographer and is willing to share his excellent techniques with everyone. I defer to this site for the operational details.

Below is shown the Lumicon giant Off Axis Guider. Notice its giant size. It is a full 4 inches in diameter. It is very solidly constructed. The adapter ring, lower right, screws directly onto the back plate of atypical SCT. It is very firmly held by three set screws with the grooved ring. An adapter for any telescope is either available or can be easily made. The supplied 2" adapter tube is shown to the left of the main body of the OAG. Internally there are stepped retainers and set screws to hold the large diameter focal reducer also available from Lumicon. The focal reducer is at the top right. This focal reducer can be adjusted to several positions and is a full 80 mm in size. This provides several levels of field reduction or none if the lens is not used. The pickoff prism is large in size and can be easily positioned by rotating the entire OAG about the mounting ring. A side tube is provided for the mounting of the CCD guider, in this case a 216XT. An eyepiece for setting up the guide star can be substituted for the CCD guider. This tube is very rigidly mounted but still has a good range of adjustment to aid in finding a suitable guide star. The back side of the giant OAG has other adapters available for almost any camera or imager. (including large format cameras)

I have made a few additional adapters for the unit to suite my own needs. One is an insert to hold a smaller and I think optically superior focal reducer Generally I use the Meade 0.63 reducer or it will take any other with a Schmidt thread. I also have made a similar adapter that will hold a projection lens for high magnification or a focal expander(Barlow lens). I also have made a 6 by 9 cm film and plate adapter with a fold away focussing screen and magnifier attachment. This is shown at the far left bottom. The latter substitutes for a medium format camera but is very much lighter. For exposing the film, the hat trick is used instead of a shutter since film exposures are usually a half hour or longer. A final attachment, a sort of coup de gras I think, is the flip mirror mounting I have made to use in place of the side tube. This allows finding a guide star and flipping the mirror to the guider quickly. It is shown at the upper left. All in all, the Lumicon is a fine OAG with or without the accessories. If you want to do guiding with an off axis guider, this is the one to use. I have found it a most desirable and useful accessory which works very well.


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