Last updated on
4/11/
2006

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Guiding an SCT
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[Guidescope vs.Off-Axis Guider] [Autoguiders]

35mm film astrophotography requires long exposures, which usually requires correction of the scope’s position during the exposure since few mounts can track completely accurately for a long time. Even CCD photography may require guiding depending on the length of exposures you are doing.  There are two ways to manually guide a scope during astrophotography: with an off-axis guider or with a guide scope. There is a lot of vigorous discussion (to use a term from international diplomacy) among astrophotographers as to which of these two methods works better.  Click on the Guidescope vs. Off-Axis Guider button above for some references on the internet espousing each position. At the risk of oversimplification I would summarize the respective positions by saying that the long exposures needed for film photography generally require an Off-Axis Guider, while a Guide Scope works acceptably for the much shorter exposures needed by CCD cameras.  I also conclude from the discussions on the Internet that an Off-Axis Guider tends to be more difficult to learn to use, but once you gain experience with one you will get better results even with CCD cameras. Conversely, smaller-aperture SCTs have a harder time producing a sufficiently bright guide star for an Off-Axis Guider, and thus these owners often use a Guide Scope.

Note that the advent of inexpensive CCD cameras specifically designed to automatically guide a GoTo scope during photographic exposures (but not to take pictures) can really reduce the tedium of staring through an eyepiece for a long time and correcting the scope’s position during an exposure.  Click on the Autoguiders button above for a discussion of these CCD cameras, which are mounted on either an Off-Axis Guider or a Guide Scope - they just replace your eye in either case. 

Off-Axis Guiders
An Off-Axis Guider, often called a Radial Guider (Meade’s is shown here on the left) mounts between the scope’s rear cell and your camera (film or CCD) and contains a small diagonal mirror at the edge of the field of view, that reflects a little light up a tube (as shown in the diagram here on the right), on which you can mount a guiding eyepiece or CCD autoguider.  When you have centered the object you wish to photograph in the camera, you then adjust the off-axis guider’s small diagonal mirror to center on an off-axis star that you use to guide on. The small mirror doesn’t interrupt much light so it doesn’t interfere with the camera’s exposure.  Unfortunately, you may not be able to find a star that you can guide on, that is both properly positioned near your intended photographic subject and bright enough to see (because the small diagonal mirror doesn’t interrupt much light so the guide star needs to be relatively bright). Frustration with this process has caused some astrophotographers to abandon off-axis guiders in favor of a Guide Scope. Nevertheless, experienced astrophotographers do claim that off-axis guiders allow better guiding and thus better photographs. My assumption is that those who like off-axis guiders have larger-aperture scopes so they can find more dim stars, one of which can be positioned in the guider’s diagonal mirror.  If you have a smaller-aperture scope you are likely to have more difficulty seeing a star you can guide on , because the scope doesn’t reveal stars that are very faint

Off-axis guiders are offered by Meade or Celestron (at around $100), and by Lumicon whose “Easy Guider” includes an internal f/6 focal reducer (which is probably useful for you), is regarded as the best off-axis guider, and not surprisingly is more expensive at around $250, focal reducer included..

Guide Scopes
A guide scope is a secondary scope mounted on your SCT.  You could use the finder scope provided with your SCT to guide it during exposures, except that finder doesn’t include, or even accept, a right-angle diagonal and it doesn’t offer enough magnification to allow detailed guiding. (I can’t imagine that you would be willing to stare in a guide scope for any length of time without a diagonal in the guide scope, any more than you would be willing to do much visual observing without a diagonal mounted on the SCT.) 

Celestron previously sold a 60mm Guidescope (part #22212, shown on the right) that included a diagonal; this is a long (twice as long as the SCT itself) refractor scope with a 60mm objective lens (a little over 2” in diameter) and a 700mm focal length (making it an f/12 scope). It mounted on an SCT with quick-release dovetail brackets so it is readily removable when you don’t need to use it. (It included its own small finder scope which is easily removed since it isn’t really needed - the standard Celestron finder scope is much better.) The Celestron Guidescope appears to be out of production, but if you find one on the used market it had a street price of $115 new without an eyepiece (which is good since you want to get a specific illuminated reticle eyepiece as described below). 

Another guide scope previously sold was the Orion 60mm achromatic guide scope (part #9419, shown here on the left) that appeared to be of substantially higher quality than the Celestron.  But you got what you paid for - the Orion guide scope sold for $330, without a diagonal, and since the mount wouldn’t easily mate to an SCT (it was made to mount on Orion refractors) you would have needed to order the Orion Guide Scope Rings (part #5770) for $50 (and unfortunately they weren’t quick -release dovetail rings), or if you use a dovetail plate you would have needed to get corresponding scope rings for it.  So your cost would have likely been well north of $400 for this guide scope. 

I mention the two guide scopes above in case you want to find one on the used market.  But many astrophotographers have used an 80mm “ShortTube” refractor (an “ST-80”) as a guide scope - they sell for around $200 (and their value for the price was probably responsible for the demise of the two guide scopes discussed above). Although a wide-field scope like an ST-80 is not really what you want for guiding your main scope during astrophotography, an ST-80 serves double duty as a rich-field scope for visual observing. The ST-80 and other rich-field scopes are described on the Rich-Field Scope page on this site.

I have the Celestron 60mm Guidescope and although it is OK optically I feel that its mechanical construction is a little flimsy. Even though a guidescope doesn’t need to have the same quality as your main scope, this one certainly doesn’t match the quality of Celestron SCTs themselves although in fairness it did cost about 20 times less.

Illuminating Reticle Eyepiece
Regardless of whether you choose an off-axis guider or a guide scope, you will need to put a specific eyepiece in it - an eyepiece that has an illuminated reticle (set of crosshairs) that you can center on a star and use while watching that star to correct any drift caused by inaccuracies in the main scope’s drive system.  An excellent guiding eyepiece for astrophotography is the Meade “9mm Illuminating Reticle Cordless Eyepiece” which includes an illuminator and sells for $150. (The eyepiece reticle pattern and the eyepiece itself are shown on the left.) It has a better pattern of reticles than the simple cross-hair in other guiding eyepieces, but the feature that most distinguishes this eyepiece from others is that the reticle is movable - you can readily position it on a star that doesn’t happen to be in the center of the view in the off-axis guider or guidescope, which a bright star very well may not be when you are photographing a deep-sky object or a planet. (The reticle positioning screws are visible in the photo.) This eyepiece is commonly regarded as one of the most practical reticle eyepieces available. Also, it is parfocal with the Meade Pictor 201XT autoguider.  This means that if you use the 201XT as an autoguider you can focus with this eyepiece, and then replace it with the 201XT autoguider and have the 201XT be very close to properly focused, a real time-saver with CCD autoguiders.

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