GuidingBy: Jeff Thrush

Why is Guiding Required?

• If you follow a star with your telescope at high magnification, you will probably notice that the position of the star changes in the field of view of your eyepiece.

What Causes Star Drift ?

• Star drift is caused by three main factors:1. A poor polar alignment - which causes a slow drift

and a slow rotation of the star in the field of view.

2. Periodic error in the mount’s tracking rate - this error results from a worm gear.

3. Random errors - caused by dirt, dents and variations in the drives gear train. If these random errors are large and fast enough, they can make unguided exposures impossible.

Fixing Problem Mounts.

• Align your mount with the NCP.

• Periodic Errors

• Random Errors

How to correct for this drift.

• Off-Axis Guider

• Guide Scope

• Guiding Eyepiece– Manual Guiding

• Auto guider

• Self guider

Off-Axis Guider

• For Beginners, the OFF-AXIS GUIDER is a major life saver. A novice is immediately capable of getting some great photos without having to deal with the flexure problems that are sometimes inherent when one uses a separate guide scope.

Off-Axis Guiders

Off-Axis Guider Set Up

How do they work?

Off-Axis Guider

• Pro’s

• Easy setup.

• Simple design.

• No tube flexure or sag.

• Eliminates mirror shift problems.

• Con’s

• Will not find a suitable guide star of all objects.

• Will need to guide from various positions.

• Harder to frame objects.

 

• A second telescope which is used to guide the main telescope during an imaging or photographic session. • Guide scopes must be at least ½ the EFL of the main imaging scope for CCD work and at least 2.5x the photographic power of the main photographic scope for manual guiding.

Guide Scope

Guide Scope Guidelines

• Contrary to popular opinion, guiding with a guide scope is no more difficult or complicated than using an off-axis guider if one follows a few simple rules.

• The most fundamental mistake is to attempt to mount the guide scope directly to the primary tube. A guide scope is just too heavy and will bend the main tube in all sorts of random ways when in use.

• The most convenient way to mount the guide scope is with a set of adjustable rings. These must however, be attached to the mounting in some direct way that will not influence or be influenced by the primary telescope.

Guide Scope

• Pro’s

• Easy setup.• Easier framing of main

object.• Better guide star

symmetry.

• Con’s

• Possible tube flexure or sag.

• Possible mirror shift with SCT’s.

• Requires the purchase of an additional scope and mounting rings.

• Guide stars will be dimmer.

Guiding Eyepiece

Guiding eyepieces offer a myriad of options:

•Single set of crosshairs •Dual crosshairs •Target style reticles •Diopter adjustment for crosshairs •Variable illumination •Blinking illumination •Wireless battery •Multicolor options •Movable reticles

Guiding Eyepiece Setup

Auto Guiding

• The use of a CCD camera (or a WEBCAM) to guide the telescope for another instrument.

ST-4 Auto Guider

Self Guider• Integrated imaging and auto guiding using a single CCD

camera with a built-in auto guider like the SBIG ST7, ST8 and ST9 cameras or a CCD which can download a part of the image without deleting all the pixels, like the Starlight-Xpress MX series.

Auto Guider Alignment

Before starting any kind ofguided exposure, it is highlyrecommended that the trackingCCD is aligned with the axisof the telescope so that motionsin R.A. and DEC. cause theguide star to move parallel tothe CCD array.

ST-4 Guider Head Alignment

How to Select a Guide Star?

• The star must be at least 1000 ADU’s over the sky background.

• The star must not exceed 50% of the detectors saturation point.

Saturation point = 50 * (well depth / gain)

                    • The selected guide star must be the brightest star in the

detector field.

Adjusting Calibration Times

Focal Length (mm) Suggested Calibration Time (Seconds)

100-300 30

300-500 20

500-1000 10

1000-2000 7

2000-3000 4

Camera/Telescope CombinationFormula

Arcseconds/pixel = 206*(F.L. in mm / pixel size in microns)

AstroArt Guider Setup

AstroArt Guide Window

Guiding Errors

Questions?