There are already some happy users of premium telescope mounts that do not need to use any guiding at all. But still most people who deal with astroimaging are obligated to guide their setups for long exposure photography. Guiding itself is pretty old technique that origins in the negative film astrophotography. Those days amateurs used long focal auxiliary refractors with cross hair high magnification eyepiece and they spent whole session observing the star and trying to keep it centered. Guide corrections have been introduced manually. Human eye and brain performed then all guiding logic. Then electronic detectors and computer era has came and whole process has been automated. Small CCD camera is doing short exposures, computer analyses the image and determines star or stars position, calculates error and sends correction commands to the mount. It is now much easier and also more precise. However there is one important thing worth to remember: guiding does not prevent mount from tracking errors. Guide only reacts to the error that already happened and tries to compensate it. Guiding algorithm is being controlled with several parameters that define how it should behave to achieve best outcome – that is astropicture without tracking error. But this is a topic for another entry, that will come in near future 🙂
In this entry I would like to compare off axis guiding (OAG) with guiding using guidescope. These two are probably the most popular guide techniques. Both have advantages and drawbacks and it is quite useful to know them, so we can choose better one. I used for quite a long time each of these two and all following text is basing on my experience.
In off axis guiding in the main optical train there is a small prism (usually a few mm large) that redirects small path of light to the outside of the tube, and there some small CCD camera waits for this light to do short exposures, detect stars and calculate guide errors. When you have filter wheel or drawer, probably you need to put OAG device before filter, so the light that comes to guide camera will be unfiltered. Otherwise guide star signal will be limited, and it will be even harder to find proper guide star. The prism itself must be placed in such way it does not interfere with the light cone that comes to main sensor. Otherwise the prism would obscure main detector and that is usually not acceptable. Off axis guiding has several drawbacks. First of all it is less suitable for small aperture and/or slow telescopes (with slow focal ratio). Field of view of such device is quite small, so it can be quite hard to find any guide star. Basing on my experience, with 8″ f/4 newton I almost always was able to find a guide star in OAG camera. With 6″ f/5 newton and 130 f/5.5 refractor I sometimes needed to adjust telescope position to find proper guide star – especially in relatively empty areas like in Lynx for example or Cetus.
And basically with OAG you should only use high sensitive mono cameras. Color OSC cameras see even less stars. With OAG I have successfuly used QHY5 and ASI120, but more sensitive ASI290MM gives more stars. And 16 bit cooled QHY6 also worked for me pretty well. Another issue is field curvature / coma. As its name indicates, off axis guider works with light that is running off the telescope axis (so it will not interfere with main sensor). If we have large main detector, we need to put prism quite far away from optical axis, and the image quality in this area may suffer from different distortions, so the star images will no longer be pinpoint. In my 8″ f/4 telescope stars in the guider camera were already little comas (even when using coma corrector). However depending on guiding software algorithm it may handle well with calculating of the centre of such stars. At least MaxIm DL guiding works very well even when stars are not perfectly round. Another thing to consider is mechanics. OAG device plus guide camera are additional load to the focuser, so it needs to handle with more weight. Plus there is usually not much space around main camera, so guide camera for OAG cannot be large. So, as you can see there are pretty many drawbacks of using off axis guiding. But there is one essential benefit: off axis guiding works within the same image train as main detector. It handles very well with all bending and tilting that can be present in amateur setup. If you have not premium grade newtonian with not quite stiff steel tube, or not perfect focuser, or wobbly main mirror cell, then all these components can bend or tilt during session. OAG will catch all these misperfections and will correct them, because it uses the same light and the same image as main detector. For me it was a real cure for my first steel tube 6″ imaging newtonian. With separate guidescope I noticed that each subsequent 5 minutes subframe was a little bit misplaced, so I was not able to take longer pictures, because of the tube bending. And the OAG was the solution in this case.
And the second common guiding device is separate guidescope. It is usually small telescope (like 50-80mm aperture refractor, or sometimes telephoto lens) piggybacked to the main optical tube. And guiding camera is attached to this small instrument. Some time ago only long focal length refractor were used for guidescopes, so the guider scale was similar to the main detector pixel scale. But it quickly turned out that is not neccessary at all. Guiding software is able to determine guide star center with 1/10 px precision, so the guider scale can be actually much less. Hovewer there are limitations. First of all guidescope optics quality is quite important. For example I found out that some noname 50mm nice CNC guidescope optics has pretty poor quality. Star diameter (FWHM) in this scope can be as large as 8-10 arc seconds! Cheap GSO or Skywatcher 8×50 finderscopes give better picture. I needed to use barlow plus Fringe Killer filter to obtain acceptable results with this noname 50mm guider. This is even more important when you guide with small pixel modern CMOS camera, like ASI290MM. Then all optics flaws are clearly visible.
Also in theory it is possible to have guider with 10 times larger pixel scale than main detector. I would be very conservative in this however. It may work, when you have decent quality guidescope with 16 bit guiding camera. In all other cases I would not recommend guider pixel scale more than five times larger than main detector pixel scale (it also depends of course on the mount tracking capability). I was able to achieve good guiding results with 50/165mm focal length guidescope and ASI290MM camera (2.9um pixel) on 130/910 mm refractor, but I think it worked close to its limits. Guiding this setup worked better with 60/240 guider and also popular 80/400 Skywatcher scope used for guiding (this is my current stationary setup). So, what are the drawbacks of using guidescope? I think most important is that it uses different optical train than main detector. So all mechanical faults and tilts will not be corrected. And that can be easily noticed when we use poor mechanics telescopes and/or long exposures. Another guidescope drawback can be te optical quality – if that is poor, then star image diameter will be pretty large and software will not be able to calculcate star centroid precisely. You can fight chromatic abberations with Fringe Killer filter, but it will not help in all cases and will not remove all optic flaws. Another thing to consider is mounting guidescope to main tube (this connection must be solid) and additional weight of guidescope, that the mount must handle. But still guiding with additional scope is pretty common – it is easy to implement, you do not need to correlate main and guide cameras focal planes (as in OAG case), you should not have problems with finding guide stars even with color cameras. Plus there is no additional load for focuser, and also you do not need to figure out how to put any more devices between camera, filter wheel and corrector to keep perfect backfocus distance.
So there is no easy answer, when you need to decide on guiding setup. I try to sum it all up in a table, where basing on my experience I will present my recommendations for either using OAG or separate guidescope.
|Off axis guiding||Separate guidescope|
|When you have doubts about your telescope stiffnes and mechanics (steel tube newtonian, focuser tilt, etc)||When you have good confidence in your instrument mechanics (good quality focuser, rigid tube refractor, etc)|
|When you have medium or large aperture instrument with fast focal ratio, so you will always find proper guide star.||When your instrument has modest aperture and relatively slow focal ratio, so finding guide star in small OAG field of view may be problematic.|
|When you have sensitive monochromatic guide camera.||When you use color OSC camera camera, with relatively low sensivity.|
|OAG device must be packed into the focuser, between main camera and corrector. Usually not much space, plus more load for focuser.||Guidescope needs to be piggybacked to main tube. This connection must be stiff. More load for the mount.|
|Guide camera cannot be large, because there is usually not much space around.||Guiding camera can be relatively larger - for example Atik 16 with cooling.|
|Stars visible through OAG prism can be distorted (coma, field curvature), because they are taken from the edge of the field of view.||Poor guidescope optics (usually achromatic doublet) can give stars with large FWHM diameter, so centroid calculation may not be precise.|