Telescope Reviews: Guidescopes and Manual Guiding Technique

Guiding is the act of making sure your telescope tracking is doing it's job during an astrophoto exposure. Here is a detailed lesson.

First the basics: The difference between TRACKING and GUIDING.

TRACKING is what your scope's mount does to compensate for Earths rotation. It's what keeps your objects in view. Most scopes track. It's accurate for visual use, but not photographically. The scope may not be aligned well, gears are not perfect, the sky refracts light, wind pushes your scope, etc... There is always a way that ERROR gets into your tracking and causes it to drift from where it should be. For visual use, you may never notice it.

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). I hand guide exclusively, but most people these days prefer to Autoguide.

While the new rave in astrophotography is to use an autoguider, and let a computer make small corrections in your telescope's tracking, I am old-fashioned and still make the corrections myself.

These days, you use the mount's hand controller to make guiding corrections. You really need to set your scope up with as perfect polar alignment as possible, this way you should only have to make Right Ascension (RA) corrections and no (or very few) Declination (DEC) corrections.

For me, I choose to hand guide for two reasons. One is so I can monitor the exposure and sky conditions, and fix other problems that may occur. Since I am out there, if a plane flies near, I can block the scope to prevent a trail etc.. But the MAIN reason I hand guide is to be 100% responsible for TAKING the photo. I feel distant to the craft if I let a computer guide the shot.

This posting will deal with HAND GUIDING only, as it is the most demanding and is what I am most familiar with. I'll let someone else describe Autoguiding.

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. Here 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. Here is a photo of my scope with my Orion guidescope attached:

In both cases you need to see the star you are guiding on (GUIDESTAR) and detect any movement. We use an ILLUMINATED RETICLE EYEPIECE to do this. This is an eyepiece that has dual crosshairs which make a small BOX in the center of the view. This is called a GUIDEBOX. The lines are illuminated with faint light from a battery so you can see them against the dark sky. 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 GUIDING ERROR and you must correct it before the error is large enough to show up on film. The amount of error you can let slide is called GUDING 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 the way to figure this all out later in this lesson.

The #1 reason for error when GUIDING is inaccurate Polar alignment of the scope. This alignment must be very precise, and is usually done with the DRIFT METHOD. If you detect any error in DECLINATION then the alignment is not good, and should 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.

To guide by hand requires three things..

1. A way to watch a guidestar. The guidestar is any star near your target that you can see easily and monitor. I use a separate GUIDESCOPE, while others use a radial OFF-AXIS GUIDER. The guidescope allows a larger selection of stars, and allows you to guide on fainter stars.

2. A way to MONITOR the guidestar. We all use an illuminated reticle eyepiece. This is eyepiece (I use a 5mm) that has duel cross hairs that are illuminated against the dark background of space by a red LED. You can center your guidestar inside the tiny "box" made by the intersection of the duel cross-hairs and quickly detect if the star "MOVES" from it's stable position. As you see the movement you can correct it, to prevent any noticeable effect on your photo.

The QUESTION is how much movement is too much? I'll address that in detail below.

3. A way to make corrections to the tracking of your scope. Most everyone uses a hand controller - the one that controls most modern scopes. If your mount allows, and you have the system, you can set your hand controller to make very tiny corrections, so instead of the right button slewing your scope forward in RA, it simple doubles the very slow RA speed so you get a very small correction. The LEFT button will simply STOP the RA motor and allow the Earth to rotate under the stars, thus giving you the ability to effectively back up.

So while guiding, just 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 you need to understand that you can only CORRECT errors in RA. Errors in DEC will accumulate over time - no matter how well you correct them. DEC errors are 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 without a single DEC correction - 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. And NOT do this any more than necessary so you don't go blind or destroy your neck and back.

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SIDE NOTE: Non-astrophotographers often ask me how difficult astrophotography is. I point to a sink tap and say "Go over and twist and bend until you can look UP into the tap. Now stay there for an hour without touching the tap or the sink!" God I love this hobby!
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OK, so now you have perfect polar alignment.
You have a camera piggybacked or attached to the main scope.
You have your guidescope or off-axis guider centered on a bright star near your target.
Everything is LOCKED down, camera focused and ready.

As you look in the guidescope you will see the guiding box. This box is only a guide. I never use it; instead I place my guidestar on the intersection of two cross-hairs and keep it there.

How much can the star drift in the guidescope before the error shows up?

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.

I have included 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
28 mm---- 185"------------------ 12
50 mm---- 105"------------------- 7
135 mm---- 40"----------------- 2.5
200 mm---- 25"----------------- 1.7
800 mm----- 6.5"---------------- .4
2500 mm-----2.1"---------------- 1/10th of a second!

In the above example, with a 50 mm lens, you could let your guidestar be 105 arc seconds off, 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.

If shooting through a 2500 mm scope--typical 10 inch SCT, it could only be 2.1 arc seconds off--less than 1/10 of a second of untracked drift!!

Now all you have to do is figure out what 2.1 arc seconds (for example) looks like in your guidescope.

Find your typical setup. Let's say you use a LXD75 8" SN.
This scope about 800mm F/L (812mm to be exact).

- Now you do the calculation and find that you can be off by no more than 6.5 seconds of arc.

- 6.5 seconds of arc is .4 seconds of real TIME.

- Look in your guidescope and watch 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: The guiding error will usually not show up opn your final image, if you catch it fast enough and 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 you start with the star in the lower left of the box, keep it there. Do not decide it will be easier now to switch and move the star to the upper right side or whatever in the middle of your exposure! Wherever the guidestar is at the beginning of the exposure, that’s where it shall remain!

Note your error in DEC. After 5 to 10 minutes you may see if your mount is not perfectly aligned. As you correct the DEC error (see I told you I would get back to this!) you should make a mental note of how much error you have ACCUMULATED.

For example. You see ¼ of guidebox worth of error. You correct it. Then 5 minutes later you make another ¼ guidebox correction. That’s now ½ 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 ½ 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.

Understand that after some experience this will be easy. Since you now know exactly how much drift you are getting in DEC, you will be able to quickly make the proper polar alignment correction.

Let's say you note an error after 20 minutes of guiding. That should require only the tiniest amount of polar alignment adjustment vs. if you saw the error after only the first 3 minutes!

--- NOTES on guiding magnification ---

The more magnification you guide at, the larger tracking errors appear in your guiding reticle. This boils down to comfort vs tolerance. Higher magnification allows you to allow small errors occur (what we call guiding tolerance), but it's not a good habit to get into. On the other hand, I do not guide as accurately when piggybacking a 135mm lens vs prime focus at 812mm. There's no need.

Some try to find a guiding magnification to let them stay comfortably within the guiding reticle's box, allowing any movement of the guidestar within the box to go without correction. At the 11X ratio with my setup, I try to keep the guidestar behind a line the whole time.. never letting it peek out for more than a split second. I probably could go 1/4 of a box without the error showing up, but I like to err on the side of over-correcting vs under.

Adding more magnification also means less comfort at the eyepiece. You end up with a dimmer star, and of course a narrower field of view - which makes finding a guidestar that much more difficult. Also, eye relief suffers. Like I said.. it boils down to comfort and tolerance.

Seeing plays an important part too. You can only guide as well as seeing will allow. When the star is jumping all over the place, give it up and try later in the evening or another night.

The argument to support higher magnification is that the better your guiding, the finer the resolution of your image will be. Small tracking errors destroy resolution your tiny stars blur and very fine detail is lost.

I like to think of my guidescope as a MICROSCOPE. I am peering down into a microscope at my film, watching the tiny white spec of light burning an image into a single grain of film. The more the star is allowed to move, the more blur I am allowing.

The 11X ratio works for me. Each astrophotographer has to find the ratio that works best for him/her.

---- GUIESCOPES VS OFF-AXIS GUIDERS ----

For beginning imagers, which means those using 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 film 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 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'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 w.r.t. 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 debugt 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 are highly comatic. Which means your guidescope can be a nasty little seagull which makes guiding or autoguiding a judgement call - "where did I decide the center is???"

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 side by side: http://64.246.48.14/~siliconv/lee.suk/root/equipment/M250.html
or over/under: http://www.astropix.com/HTML/I_ASTROP/I07/I0705/I0705.HTM

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. Nooo.... 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, and as the focal length of the imaging scope gets larger.

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!!!!!

I had an extremely nice, extremely expensive OAG that was simply too much work/too frustrating to use. Why don't I sell it in Shop and Swap? I smashed it to smithereens in frustration. (who me, violent?

)

Some people use OAGs very successfully, but they're clearly manlier men than me!!!

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

Cheers,
Suk + Clownfish

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