We've already talked about the need to guide your long exposures to spot and correct tracking errors using guidescopes and off-axis guiders before the errors show up on your film. Now it's time to top learn HOW to use them! Regardless if you use an off-axis guider or a separate guidescope, the principle is the same:

• Accurately align, balance and level your mount to reduce tracking errors.
• Focus your camera and frame your subject.
• Aim your guidescope or guider (not the camera scope) at a suitable guidestar near your subject.
• Start your exposure.
• Watch the guidestar for errors in tracking.
• Correct the mount's tracking before those errors show up on your final image.
• Keep guiding as above, until you stop the exposure.

Other then the Guidescope or Off-Axis Guider and your Illuminated Reticle Eyepiece, you will need a way to make changes to your mount's position in very tiny, super accurate movements. This is often accomplished with your mount's own tracking system as an added feature. Check your instructional manual for GUIDING to see if you already have this capability. If not, you may be able to get a separate add-on drive corrector.

You will need to use a hand controller to make guiding corrections. All mounts that offer "GOTO" computer service or built-in guiding will have a hand controller that will allow you to adjust the tracking in both Right Ascension (RA), and many will allow you to change the DEC too. If you have the DUAL control option, you will have a much easier time, and will be able to guide for longer periods.

Polar Alignment is absolutely critical!
Visit the Polar Alignment page to learn how to do this!

SELECT A GUIDESTAR

The first thing you should do is align your subject in your camera, and then look for a suitable guidestar in the GUIDESCOPE or OAG. DO NOT MOVE THE CAMERA SCOPE! You can certainly shift the guidescope around to help you locate a suitable star. The guidestar should be bright enough to see easily in the reticle eyepiece, but not so bright as to be painful to stair at for extended periods. Some people will find it easier to slightly blur the star to make it larger, and less intense.

IMPORTANT: If you can, always choose a star who's declination position is the same as your subject or as close as possible. Do not move too far from your subject or you will introduce field rotation error no matter how well you guide. If you end up with a photo that shows all the stars slightly trailed AROUND your chosen guidestar, then you know you have selected a star too far from the subject. If you must choose a star with a different declination position, try to pick one that is located between your subject and the celestial equator. This will help reduce the effect of field rotation. The bottom line.. stay as close to your subject as you can!

ALIGN THE ILLUMINATED RETICLE
Now rotate your guiding eyepiece so that the reticle lines are orientated with the Right Ascension (RA) axis of your mount. When you move the scope in RA you should see the guidestar move along a straight reticle line. This is CRITICAL to accurate guiding. You can do this quickly by slewing the scope in RA back and forth by pressing the right and left buttons with the hand controller. Be sure the slew speed setting is not too fast, start with a speed of 2 or 3 and work from there. Next, orientate your hand controller so that the LEFT button moves the guidestar to the left on the guiding eyepiece, and when you press the UP button the guidestar moves up. It seems obvious, but it isn't. The goal here is to match what you SEE with appropriate buttons in your hand. It's instinctive to have buttons in your hand that are orientated LEFT/RIGHT and UP/DOWN that match a left/right/up/down movement in the eyepiece. This may mean that you place the controller in your hand sideways or upsidedown! Even experienced astrophotographers often make the mistake of pressing the up button when they meant to press the right button. If you drop the hand controller in the middle of an exposure, be sure you place it back in your hand appropriately. Also, if your system will allow, you can often switch the N/S and E/W directions on your tracking system to match what you see in the eyepiece. Meade's Autostar does this quite nicely.
SELECT GUIDING MODE Next, be sure the correct tracking rates are selected for your hand controller. You don't want to press the RA button while guiding and have the scope slew half way across the sky! Some mounts call this GUIDE SPEED. It is usually set up so that the RA speeds are simply 2X normal for forward corrections, and STOP for reverse corrections. The logic is that by stopping the scope's RA tracking, the Earth will rotate under your guidestar and catch up to any noticed tracking error in RA. Now you re-center the guidestar either in the guiding box, or along a box reticle line. The choice is yours, just be sure you stick with your decision once the guiding has begun. I like using the line, as you can spot error much faster. RECHECK THE CAMERA! Go back and recheck your camera and be certain everything is ready. Check that the cable release is functioning, film is advanced and that the camera's shutter is cocked and ready. Then give the scope a minute or two to settle down. Take this opportunity to get some coffee, or other caffeine beverage, pee, grab warm clothes, kick the cat out of the observatory, and get ready to take a great astrophoto! This is also a good time to RECORD the date, time, subject, seeing conditions, film type, and frame number. You will never remember all this when you get your film developed.
MAKE GUIDING CORRECTIONS Now you simply sit there and watch the guidestar and keep everything tracking well with simple guide corrections. As you notice the guidestar moving in ANY direction, you should use the appropriate buttons on your hand controller to put the star back where it was. If you have placed the guidestar behind one of the reticle lines, instead of between them, you will be able to spot tracking errors much faster. When you see the star appear from behind a line, you can quickly cover it back up. This is far more accurate then trying to spot tracking error when the star is free floating between the lines. In the example to the right, you can see some error being corrected back to center. Note that this will almost certainly be too much of an error to allow. This was done to show that the guidestar is moving left and right, and must be corrected, but in real life you should make the corrections way before the star leaves the guiding box. As soon as you spot the slightest error, you should correct it. See Guiding Tolerances (below) for more info about this. If you spot DECLINATION (up/down) errors, those should be corrected too. However, you should remember that those are almost always POLAR ALIGNMENT errors and will simply add field rotation to your photograph. If you have to make more then one DEC correction every 10 minutes, you should recheck the mount's polar alignment. When you have finished your exposure, write down the duration of the shot, and any problems you noted during the exposure. These can include number of DEC corrections made, errors in tracking you may have let slip, wind, poor seeing, aircraft and clouds... anything that you noted during the exposure that will help you when the pictures come back and you are scratching your head trying to figure out what happened to your perfect exposure!

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GUIDING SIMULATOR

	To show you what guiding is like, you may wish to use the free Guiding Simulator. The simulator will let you practice manual guiding and polar alignment with the drift method. Click on the image of the simulator after you finish with this tutorial to try it.

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The better your polar alignment is and the better trained your PEC the less time must be spent continuously observing your guidestar. Also, the shorter your focal length, the less accurate you must be.

HOW ACCURATE MUST YOU GUIDE? This is called your guiding tolerance.

Guiding Tolerances

Now that you are guiding with your new guidescope, you still may want to know how well you are doing. How do you tell if you've allowed too much tracking error? How much error is too much?

The goal of guiding is to quickly spot any errors in your mount's tracking and correct it before it shows up on your photograph.

First REMEMBER that you should only correcting errors in RA. Most errors in DEC will accumulate over time - no matter how well you correct them. DEC errors are almost always POLAR ALIGNMENT ERRORS and the only way to fix them is to readjust your mount and start again. Use the DRIFT method to gain as much accuracy you can. Ideally, you want to shoot your entire exposure with very few (if any) DEC corrections - although in practice you can often get by with 1 DEC correction every 10 minutes or so. We'll get back to this later.

RA corrections can be corrected as often as you need to.. in fact, you can consider your mount's drive as continuously correcting for RA - so adding a few of your own won't make any difference.

The KEY is to:

• Detect the error BEFORE it can show up on film (or CCD chip) and blur your image (star trails)
• Correct it with fine movements using the hand controller.
• Do not do this any more than necessary, so you don't go blind or destroy your neck and back.

How much can a guidestar drift in the guidescope before the error shows up? While I say, none, the correct answer is that some small amount of error can be allowed that would never show up on your final image anyway. There is a mathematical way to express just how much error you can tolerate.

A nice formula by noted astrophotographer Michael Covington is:

- Where F is the CAMERA focal length in millimeters.

Also, this assumes that an acceptable error on film is 1/40 mm. CAMERA can be a telephoto lens, prime focus or eyepiece projection. Note that in eyepiece projection you must know the new effective focal length of the camera system.

		GUIDING TOLERANCE TABLE
Here is a table of focal lengths versus guiding tolerances (in arc sec), and how many seconds in TIME that tolerance looks like with an undriven telescope at the equator. (HINT: Don't turn your scope off to measure this.. simply press the EAST button on your hand controller which will STOP the mount if it's in the GUIDE mode.) In this example, with a 50 mm lens, you could let your guidestar drift for 105 arc seconds, and still have good star images on your final print. Looking at the table, this is how far a star would drift in SEVEN seconds of real time if the telescope drive were turned off. That's a lot of drift, far more then most modern mounts will give you with NO guiding at all as long as your polar alignment was good. However, when shooting at higher focal length things are not quite so easy. If shooting through a 2500 mm scope--typical 10 inch SCT, your guiding can only be 2.1 arc seconds off--less than 1/10 of a second of untracked drift before the effect would be visible on your image! Now all you have to do is figure out what that amount of drift looks like in your guidescope. The table above shows this… Here's an example:		Lens Tolerance (arc sec) Drift Time (seconds) 18 mm 290 " 20 28 mm 185 " 12 50 mm 105 " 7 135 mm 40 " 2.5 200 mm 25 " 1.7 800 mm 6.5 " 0.4 2500 mm 2.1 " 0.10 !!!!

Find your own setup. Let's say you use a Meade LXD75 8" f/4 SN. This scope about 800mm in focal length (812mm to be exact).

When you do the calculation above you find that you can be off by no more than 6.5 seconds of arc before the error would be noticeable. 6.5 seconds of arc is .4 seconds of real TIME.

Watch through your guidescope for about a 1/2 second with the drive OFF. That tiny amount of movement is slightly MORE than you should tolerate (because the actual calculation is .4 not .5 seconds).

Note what that error looks like in your guiding setup. Is it 1/2 a guide-box? 1/4? 1/8? If you find it's 1/5 or less, you will have a hard time catching the error before you could do anything about it! So in this case you would want to INCREASE the magnification of the guidescope (Barlow or smaller F/L guiding eyepiece) or get a longer F/L guidescope.

IMPORTANT: Correctable guiding error will usually not show up on your final image, IF you correct the error immediately. If you are shooting something very faint, you may get away with lots of small errors that you quickly catch and put back on track. If you have bright stars in the field, however, that may not be the case though! To be safe, always correct errors as soon as you spot them.

Keep on guiding and making small corrections as necessary to keep the guidestar within your tolerance.

If your polar alignment is not perfect, keep a mental note of any error in declination. As you correct this error, keep track of how much TOTAL error you have “corrected” with the understanding that this is how much DRIFT you have allowed, and not really corrected. Remember, you cannot guide-out poor polar alignment declination errors.

For example. You see 1/4 of guidebox worth of error. You correct it. Then 5 minutes later you make another 1/4 of a guidebox correction. That's now 1/2 a guidebox worth of FIELD ROTATION you have just allowed. This means your entire photo will show signs of rotated stars around the guidestar in the photo - to the amount of whatever 1/2 a guidebox worth of drift is. If you can tolerate that much, keep going. If not, stop the exposure, readjust the polar alignment, and begin again.