Download Autoguiding - Thrush Observatory

Guiding
By: 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
• Con’s
•
•
•
•
• Will not find a suitable
guide star of all objects.
• Will need to guide from
various positions.
• Harder to frame objects.
Easy setup.
Simple design.
No tube flexure or sag.
Eliminates mirror shift
problems.
Guide Scope
• 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 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 StarlightXpress MX series.
Auto Guider Alignment
Before starting any kind of
guided exposure, it is highly
recommended that the tracking
CCD is aligned with the axis
of the telescope so that motions
in R.A. and DEC. cause the
guide star to move parallel to
the 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 Combination
Formula
Arcseconds/pixel = 206*(F.L. in mm / pixel size in microns)
AstroArt Guider Setup
AstroArt Guide Window
Guiding Errors
Questions?