Manual Guiding

MANUAL GUIDING
By Peter Kennett and Suk Lee

Guiding is the act of ensuring that your telescope is tracking accurately during a long astrophotographic exposure. This article describes what manual guiding is, the differences between a guidescope and an off-axis guider, how to mount and use them, and how to determine guiding tolerance.

First the basics: The difference between TRACKING and GUIDING. This is point of confusion for many beginners.

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 it's what keeps your objects in view. Most scopes have basic tracking built in, while others can add it as an option. These are quite accurate for visual use, but may not be enough for astrophotography. The scope may not be polar aligned well, gears are not perfect, the sky refracts light, wind pushes your scope, etc... There is always a way that TRACKING ERROR causes your scope to drift from where it should be.

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.
Here's a shot of M31 by Jeff Bullard in which the camera was left to the scope's tracking system, but NOT guided:

You can see two problems in this shot, one is that the stars trail off at an angle and the other is the star trails have a zigzag pattern. The mount not being perfectly aligned with the Celestial pole usually causes the continuous trailing. It may also be that the mount's drive system is not moving the scope at the correct speed. The zigzag pattern is classic for a problem called Periodic Error (PE). This is due to the RA gear and/or the worm gear not being perfectly round, which causes the scope to lag behind the Earth's rotation and then speed up with each revolution of the gears. It is found in every tracking system, regardless of cost.



Accurate GUIDING can reduce and even eliminate these effects.

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.

This article will deal with the demanding art of MANUAL (HAND) GUIDING only. I'll let someone else describe Autoguiding.

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 ÒguidestarÓ moving away from it's correct position, you make corrections to the scope to bring it back on track.

There are several causes of tracking error. Each of these can be either fixed or compensated for. It is important to understand what can NOT be compensated by guiding, and MUST be fixed.
Any error in RIGHT ASSENSION can be CORRECTED with your telescope's hand controller by speeding up the scope a tiny amount, or stopping the scope's tracking depending on if you need to correct forward or back. RA error is a simple tracking issue. You can always bring the scope back into sync by speeding up the mount or slowing it down. Your telescope mount is always moving, thus it is making a continuous correction to the scope's position anyway! You are just keeping it on track.

The #1 reason for RA tracking error is inaccurate gears, causing a periodic error (PE). This causes your scope to move slightly ahead and then slightly behind where it should be. Since gears are round and rotate throughout the exposure, this error shows itself as a regularly repeating error with a specific PERIOD related to the gear size and speeds of movement. The good news is that many scopes have a system to help reduce this error by making these corrections for you. This is called Period Error Correction (PEC). While not 100% effective, it helps a great deal by reducing how much work you must do.

Other sources of RA tracking error are poor mount design, low battery power, an over weighted mount, poor mount balance, wind and even falling asleep and bumping the scope with your head! All of these need to be checked to ensure good tracking.

Declination errors are more troublesome.

The #1 reason for declination tracking error is inaccurate Polar alignment of the mount. Tracking error in declination (when caused by poor polar alignment) MUST be corrected by re-aligning the mount. You CAN NOT use guiding to fix this error. As you make guiding corrections in declination (when the cause was polar misalignment) you are in effect causing image rotation / blurring.

You will ruin the photo with each declination ÒcorrectionÓ. You must stop the exposure and re-align the mount. Since this is the #1 cause, you should always suspect poor polar alignment when you see declination errors.

Other causes of DEC errors are mechanical imperfections such as non-orthogonal axes, and slight play in the gears. These will manifest themselves as DEC tracking errors. Also, atmospheric refraction effect can be surprisingly strong even at 30 degrees elevation. As the scope tracks the sky at a specific rate, the sky bends light as water does in a pool. Thus, you will need to make adjustments to the tracking to compensate for this.

My advice. ALWAYS suspect poor polar alignment first before simply correcting declination errors. If you can drift align for 10 minutes with seeing any DEC drift, then you should check other sources of declination error.

GUIDING TOOLS

You can GUIDE your scope one of two ways..

OFF-AXIS: 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.



Or ..

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 most astrophotographers prefer the GUIIDESCOPE method over the off-axis guider.

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 guidescope attached.

In both cases you need to see the star you are guiding on (GUIDESTAR) and detect any movement.

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. The 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.



WHICH SYSTEM IS BETTER ? GUIDESCOPE OR OAG?

Bottom line summary

For beginning imagers, which means WAY less than 2000mm of focal length, use a guidescope. It's a LOT easier. The rest of this is WHY.

The caveat is that if you're using an SCT without a mirror-lock, the shifting of the mirror can force you into using an OAG. And if you're starting imaging with an SCT you're asking for a lot of frustration so I don't recommend it. Why?

Vignetting, mirror flop, focus flop, long focal length, small FOV, slow (f10) optics, etc. etc. etc.

What's 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

The major advantage of using a guidescope is ease of use:

- You can guide directly on your imaging subject (with an OAG by definition you have to be guiding off of something near but slightly away from your subject)
- The image is bright (OAGs are dim because the pick-off mirror is typically very small)
- 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 - "where did I decide the center is, the wing tip or the head???Ó

Disadvantages of using a guidescope below 2000/3000mm

It's 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 wind conditions.

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.

How to mount a guidescope - 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.

* The guidescope mount.
* The guidescope focuser.
* The imaging scope focuser.
* The imaging scope OTA itself.
* The mirror for SCTs or Newts.

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

* Stars are dim. The off-axis pick-off mirror is small, which means that stars are very dim, limiting your choice of guidestars.

* Finding a guidestar can be hard. You typically can't see the guidestar in the imaging scope. You may have to rotate the OAG body to find a guidestar, compromising image composition.

* The guidestar can be very comatic (as discussed above), making deciding where the centroid is tricky.

* Just getting to focus the first time with an OAG is a challenge. First you focus your main scope on a nice bright star. You peer through the off-axis port with your reticle eyepiece. It's dim. It's out of focus. Typically you can't see anything. You have to find a bright star -- gradually move the scope over until the bright star you originally focused the image scope on is kind of visible in the off-axis port. You focus your reticle eyepiece in your off-axis port by fiddling with its placement in the port. Yay! You put in your camera, refocus on the bright star and... oh, you changed focus, which for a Cassegrain of any kind means that the focal point in the off-axis port also changed appreciably compared to the on-axis image because the focal plane is curved, which means your guide-port is out of focus again. ARG!!!!!

So that's it. A long winded answer with a short simple conclusion - if you can get away with it, use a guidescope!

TECHNIQUE

Now that you know how to monitor and spot tracking errors, you use the telescope's hand controller to make guiding corrections. Since you should have already aligned the polar axis with the celestial pole, you should only have to make Right Ascension (RA) corrections and only very few Declination (DEC) corrections.

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. 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.

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 chose 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!

Now lock everything down tightly, and then rotate your guiding eyepiece so that the reticle lines are orientated east/west and north/south. When you move the scope in RA or DEC you should see the guidestar move along a straight reticle line. This is CRITICLE to accurate guiding. Be sure you note which lines are east/west (RA) and which are north/south (DEC).

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. 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, of you need to, 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.

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 star and catch up to any noticed error.

Now either re-center the guidestar in the guiding box, or along an intersection of two reticle lines. The choice is yours, just be sure you stick with your decision once the guiding has begun.
Go back and recheck your camera and be certain everything is ready, and 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!

Now you simply sit there and watch the guidestar and keep everything tracking well!

GUIDING TOLERANCE

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:

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

How much can the star 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 image. There is a mathematical way to express just how much error you can tolerate.

A nice formula by noted astrophotographer Michael Covington is:

Guiding Tolerance= 2 arc tan * 1/40 F

- 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.

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.

Lens	Tolerance(arc sec	Drift Time (seconds)
18 mm	290."	20.0
28 mm	185."	12.0
50 mm	105."	7.0
135 mm	40."	2.5
200 mm	25."	1.7
800 mm	6.5"	0.4
2500 mm	2.1"	1/10th of a second!

In the above 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.

Find your typical setup. Let's say you use a Meade LXD75 8" f/4 SN. This scope about 800mm F/L (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 effect 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.

Note: 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, that may not be the case though!

Keep on guiding, 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 ÒcorrectedÓ 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 _ guidebox correction. That's now _ 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 _ 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.

Enjoy!
Clownfish & Suk Lee