|Guidescopes and Off-Axis Guiders (OAG)|
Guiding is the act of ensuring that your telescope is tracking accurately during a long astrophotographic exposure. This section describes what manual guiding is, the differences between a guidescope and an off-axis guider, and how to mount them. Details on using them, and how to determine guiding tolerance will be covered in the Techniques section.
First, the difference between TRACKING and GUIDING.
TRACKING is how your telescope's mount compensates for the Earth's rotation. It is the accurate movement of the Right Ascension (RA) axis on your mount and is what keeps your subject in view. Most scopes have basic tracking built in, while others can add it as an option. These are quite adequate for visual use, but may not be enough for astrophotography. There are many reasons for this inadequacy; the mount may not be polar aligned well enough, the mount gears are not perfect, the sky refracts light, wind pushes your scope, etc... There is always a way that TRACKING ERROR will cause your telescope to drift from where it should be. This error can ruin your photograph.
The longer the focal length, the more accurate the tracking must be. For example, with a very short focal length, such as an ordinary 50mm camera lens, you can usually get by with just good polar alignment and basic tracking. But as you increase the focal length, tracking errors will become visible on your image as trails and other blurred effects.
The photo on the right is a photograph of M31 by amateur astrophotographer Jeff Bullard in which the camera was left to the scope's tracking system, but NOT guided. While it shows a huge amount of detail compared to an UNTRACKED photo, it suffers from not being GUIDED.
You can see two problems in this shot, one is that the stars trail off at an angle. The other is that the star trails have a zigzag pattern to them.
Accurate GUIDING can reduce and even eliminate these effects, as seen in the shot of the same galaxy. Stars are pinpoint, and fine detail is not blurred. For any shots taken with a 150mm lens or longer, you really must guide them.
GUIDING is the process of making SURE your scope is tracking well. Somehow you have to monitor what your mount is doing and correct problems before they get to the point that the error is visible on your photos.
Guiding is done either with your eyes (manual or hand guiding) or a small CCD camera and computer (autoguiding). The new rave in astrophotography is to use an autoguider, and let a computer make small corrections in your telescope's tracking. Some astrophotographers though (like me) enjoy the hands-on approach to guiding. It gives us complete control of the tracking, and makes you feel more deeply involved in the resulting photograph.
The principle is simple. You watch a star (guidestar) in the same area of the sky you are photographing, and monitor it for any tracking error. Once you spot your "guidestar" moving away from it's correct position, you make corrections to the scope to bring it back on track.
Before you begin guiding, you really must have your scope mounted on an equatorial platform and aligned with the celestial pole. If you want your equipment to track the heavens, it must be able to move in the same direction, rotation, and speed, as the sky is moving. Remember, your equipment is accomplishing the actual TRACKING; only the guiding CORRECTIONS are being done by you. Therefore, set up your mount with as perfect polar alignment as you can. Once this is done, you will still come across some errors in tracking, but now you'll be in a better position to correct it!
To learn HOW TO GUIDE with a guidescope or OAG, please read the tutorial on GUIDING after you complete this section.
You can GUIDE your scope one of two ways..
OFF-AXIS GUIDER: You attach a small device called an off-axis guider between your camera and your scope. It has a tiny prism to grab a small piece of light that would never reach your camera anyway. You look through it with an eyepiece and focus on a star NEAR the object you are photographing. This way you can watch that star and detect any error in time. To the left is a photo of Meade's Off-Axis guider.
An off-axis guider is a T-shaped camera mount. One short arm of the T threads onto the rear cell of Schmidt-Cassegrains or slips into refractor or reflector focusers. Your 35mm camera is attached to the other short arm by means of a T-ring (most CCD cameras have built-in T-threads and dont need a T-ring). An illuminated reticle eyepiece (which also have their own separate category in Photographic Accessories) is inserted into the eyepiece holder that forms the long third arm of the guider.
GUIDESCOPE: You attach a separate telescope to your main scope and use one for the camera and one for guiding. This is called a GUIDESCOPE and it allows you to find many more stars to guide on, as well as giving you brighter stars. The downside is that IF this guidescope and your camera slip in respect to one another you may think there is error when in fact there isn't. This is called FLEXURE. With care, you can reduce the chance of this. I, and many other astrophotographers prefer the GUIIDESCOPE method over the off-axis guider for most astrophopto requirements.
The longer your guidescope's focal length is, in respect to the main imaging scope the better. 2 to 1 is ideal (the guidescope being twice as long as the imaging scope). To the right is a photo of a Meade LXD75 SN with an Orion 912mm focal length, 80mm diameter guidescope attached.
In both cases you need to see the star you are guiding on (GUIDESTAR) and detect any movement. This is made easier with illuminated crosshairs in your eyepiece, as you can then see very tiny movements of the guidestar against the black background of space. The Illuminated Reticle Eyepiece or Guiding Eyepiece does this.
ILLUMINATED RETICLE EYEPIECE: To monitor the guidestar we use an Illuminated Reticle Eyepiece. This is an eyepiece that has dual crosshairs that makes a small BOX in the center of the view. This is called a GUIDEBOX.
The lines are illuminated with faint light from a battery or other power source so you can see them against the dark sky. A better system actually blinks the reticule lines to reduce eyestrain. You then put a star in the box and watch for movement. If the scope is TRACKING well, you will see no movement. If there is error, you will note the star moving within the box. This is TRACKING ERROR and you must correct it before the error is large enough to show up on your final image. While you should strive to catch any detectable error, you may be able to get away with a small amount of drift. The amount of allowable error is called your GUIDING TOLERANCE and that amount depends on the main scope's focal length, the guidescope's focal length, and the eyepiece focal length. I will describe a way to determine this later in this article.
Bottom line summary: For beginning imagers, which means WAY less than 2000mm of focal length, use a guidescope. It is a whole LOT easier, and if you're starting out in imaging with an Schmidt Cassegrain Telescope you're asking for a lot of frustration so I don't recommend it. Why?
What are the fundamental decision criteria?
It comes down to two things, ease-of-use versus freedom from differential flexure.
Below a certain focal length, say around 2000mm to 3000mm, the decision can be based on ease of use, which leads to guidescopes. Above that range you're forced into off-axis guiders because OAGs essentially eliminate differential flexure.
What is differential flexure?
Differential flexure is where your guidescope and imaging scope physically move/flex with respect to each other. The effect can be severe where there's so much jiggling that the corrections being made for tracking errors are wrong (because the guidescope is moving with respect to the imaging scope. Or worse, it can be extremely subtle, showing up as a very slow differential movement caused by the two scopes slowly changing alignment as the scopes track across the sky. In this case you can have perfect guiding, but still get trailed stars. This latter case is particularly important for film imaging where typically you take very long (30 min to multiple hour) exposures.
Differential flexure can happen ALL OVER the place. In the mounting of the guidescope, in the guidescope rings themselves, in the focuser itself. Above 3000mm it can take heroic efforts to debug and cure differential flexure.
By contrast, an OAG is a physically very solid unit, with the off-axis pick-off mirror mounted very close, and solidly to the imaging camera. Even if the OAG moves, the guide-port (where you look through or stick in your autoguider) and camera port will move together, so no more differential flexure. But there are some real downsides, which I'll get to.
Advantages of using guidescopes below around 2000/3000 mm
Guide stars are ROUND. With an OAG, you're picking up rays off-axis. With some telescope designs (SCTS, Mewlons, Mak Cass) the off-axis rays suffer from a lot of coma. Which means your guidestar can be a nasty little seagull which makes guiding or autoguiding a judgment call - "Hmm... where did I decide the center is, the wing tip or the head?".
Disadvantages of using a guidescope below 2000/3000mm
A guidescope is added weight. So if you're getting close to the imaging load limit of your mount, you may need to go to a heavier mount or image very carefully under light windy conditions.
If you're using an Schmidt Cassegrain Telescope without a mirror-lock, the shifting of the mirror can force you into using an OAG. A guidescope with a shifting mirror would introduce guiding errors due to differential flexure.
How to mount a guidescope - side by side or over/under? You can mount a guidescope over/under as shown by my scope before, or you can use a side-by-side cradle mount:
Side-by-side is typically heavy because of the need to have a very solid mounting plate. But over/under is probably as much load on the mount because the guidescope is farther away from the rotational axis of the mount. In terms of mount loading, side-by-side vs over/under is probably a wash.
There's a very subtle disadvantage to side-by-side. Since the two scope are offset laterally from the DEC axis, the scopes will actually track very slightly different arcs through the sky as the guide scope makes corrections in DEC.
BUT, you say, my polar alignment is perfect, so I'll have no DEC corrections!
Not necessarily true. Long exposure tracking virtually ALWAYS needs slight and continuous DEC corrections as the star traverses the sky because of refraction in the atmosphere. As the star gets lower in the sky, it "bends" off of the theoretically perfect arc due to atmospheric refraction. So you have to make DEC corrections. But the axes of the two scopes are offset from the DEC axis so you'll get a very slightly different arc for the imaging scope. And for very long exposures that will show up as trailing that can look like differential flexure, but is actually due to atmospheric refraction.
The effect gets more pronounced as the spacing of the scopes gets larger, the focal length of the imaging scope gets larger, and the greater the distance between the guidestar declination and the image declination.
Over/under completely eliminates this effect, and the effect is virtually invisible for short exposures (< 30 minutes) and short focal lengths (< 1500mm).
So which? If you do a lot of camera with short lens work, side-by-side is pretty flexible because of the ability to mount all sorts of weird things (like large format cameras!), especially with a flexible system like the Losmandy DSBS.
If you're doing prime focal imaging with refractors < 2000mm, over/under is extremely convenient.
Guidescope Mounting Rings
No matter whether side-by-side or over/under, you need guidescope rings with alignment pins that allow you to align the guidescope with the imaging scope. More grief can come in here.
You want the pins to make a solid connection to the guidescope so the guidescope can't move around. You don't want pins with squishy soft plastic tips (meant to keep from scratching up your guidescope) because they'll let the guidescope move around. The very pretty Megrez80 guidescope rings are an example of a poor design.
A better solution is the Losmandy guiderings with hard derlin plastic tips. You can really crank down (enough that I've slightly dimpled my Megrez80) on the tips without them deforming. Some people have reported problems with these tips but I've never experienced any.
The best, ideal, solution, is the guidescope offered by Astro-Physics. The tips are METAL, and the guidescope has strong metal reinforcing rings on the OTA that the tips fit into, so that you can REALLY crank down and yet not deform the guidescope.
Finally, make sure your guidescope has a really solid focuser that doesn't have any play, and can lock. After all this work you don't want the focuser introducing flexure.
Into the wild blue - beyond 3000mm
Above 3000mm things can get pretty hairy. Everything flexes/moves.
It can be virtually impossible to get rid of differential flexure (I gave up once I got to 2400mm). The way out is the OAG, but it's a challenge.
Challenges of using an OAG
To learn HOW TO GUIDE with a guidescope or OAG, please read the tutorial on GUIDING!