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Imaging and Auto-Guiding: A Primer
Author: Peter Karboulonis. © Opticstar Ltd.

Introduction
This article outlines the process and lists the equipment required for long exposure deep sky imaging. The text refers directly to the Opticstar DS-xxx range of cooled long exposure cameras for deep sky photography and the Opticstar AG series auto-guiders. The same will also apply to cameras and equipment from other manufacturers to a certain degree.

When imaging a deep sky object (DSO) a small number of long exposures (several minutes long) are required to capture the finer detail and faint light emitted or reflected by such objects. The captured images can subsequently be merged by specialised software into a single image of considerably higher detail and quality revealing detail not evident in the individual images. Such software include Paintshop PRO, Photoshop and the more specialised Nebulosity that is bundled with Opticstar’s DS-xxx cameras, AstroArt and MaxIm DL.
Peter Karboulonis Articles
Choosing Your First Telescope
Choosing Your Second Telescope
Powering Your Mount & Telescope
Polar Aligning your Telescope
Improving Dual Speed Focusers
Measuring Optical Quality
Gregorian Maksutov Collimation
Imaging with a Video Camera
An Introduction to Digital Imagers
Camera Focusing - A Primer
CCD Field of View
Image Projection Explained
Apparent Magnitude and Imaging
Imaging and Autoguiding - A Primer
Introduction
Auto-guiding
Auto-guiding with Two Scopes
Auto-guiding with a Single Scope
Off–axis Guiders
Software and the AG-130M/COOLAIR
Monochrome versus Colour Sensors
Assembling an Astro Imaging System
Sensor Dust
USB Fundamentals
Monthly Sky Watch
As the length of the exposures increase, accurately tracking any target becomes necessary. Tracking is affected by bad mount polar alignment, periodic errors in the gears, the quality of the mount, the load, long focal lengths and how close the target is to the horizon.

Auto-guiding greatly improves mount tracking accuracy over long exposures of several minutes keeping the target centred in the field of view automatically making corrections as required. This makes the whole process much easier.


Auto-guiding
There are a number of ways you can guide a telescope but there are mainly two ways of doing so. The most versatile method is to use two scopes mounted on the same mount, one for imaging and the second for guiding. Alternatively you could use a single scope to perform this function. While using a single scope is easier and less costly it will limit your choice of locating a suitable guide star.
Comparison.
M51: 23.03.2006. by Ulrich Gurschen - 3 minutes exposure without and with auto-guiding

Imaging and Auto-guiding With Two Scopes
In general to be able to auto-guide with the Opticstar AG-130M Auto-guiding Kit you will require the following:
  1. An imaging camera, i.e. Opticstar DS-142C ICE, DSLR etc.
  2. An imaging scope, i.e. A&M 80mm f7.5 Super APO.
  3. A secondary scope for auto-guiding, i.e. 70mm f10 achromat.
  4. An ST-4 compatible mount, i.e. Skywatcher EQ6 or HEQ5 PRO.
  5. ASCOM software platform V5 or later (ascom-standards.org, free).
  6. PC with USB2.0 ports running Windows 2000/XP/Vista or later.
  7. ST-4 guide camera and software, i.e. Opticstar AG130M kit.
  8. Auto-guiding software, i.e. Guidemaster or PHD Guiding.
Two telescopes.

Please note that the secondary scope that is used for auto-guiding would further increase your choices for locating a suitable guide star if it was held in collimating brackets, like a huge finder scope. This would allow you to easily locate a suitable guide-star even under very difficult conditions.

Note that the Shoestring auto-guide controller and Opticstar PL-130M guide camera make up the AG-130M kit which also contains adapters, cables and software making an out-of-the-box solution.


Imaging and Auto-guiding with a Single Scope
An alternative setup will involve a single telescope for imaging and auto-guiding purposes instead of two different telescopes. As such you will require the following items to be able to image and auto-guide.
  1. An imaging camera, i.e. DS-335C ICE.
  2. An imaging scope, i.e.10” f10 SCT.
  3. An off-axis guider.
  4. A ST-4 compatible mount, i.e. Vixen Sphinx, Skywatcher EQ6 or HEQ5.
  5. ASCOM software platform V5 or later (ascom-standards.org, free).
  6. PC with USB2.0 ports running Windows 2000/XP/Vista or later.
  7. Opticstar AG-130M or AG-130M COOLAIR auto-guiding kit.
An off-axis guider provides an easy and affordable way to auto-guide with a single scope. It minimises possible tube flexure but has the disadvantage of offering a rather poor selection of guide stars. Also the amount of light that hits the prism/mirror is relatively small making less bright stars more difficult to see and track.

Single telescope.

Off–axis Guiders
Off-axis guiders come in many guises. Units with the ability to rotate the guider port round the main guider body and or adjust the prism are preferable as they are considerably more flexible. Alternatively simpler beam splitters can also be used to divert a part of the incoming light to the guider.

Off axis guiders.

As in the case where imaging and guiding is done with two scopes the Shoestring auto-guide controller and Opticstar PL-130M guide camera can be replaced with a guide camera with and ST4 port. If the guide camera has an ST-4 port the Shoestring guide controller is not required.

Software and the AG-130M/COOLAIR Guiding Kit
The AG-130M includes a PL130M/COOLAIR camera, Shoestring GPUSB interface box and cables that connect the computer to the telescope mount for telescope control and auto-guiding and a CD with Nebulosity Lite, PHD Guiding and plug-ins for MaxIm DL and AstroArt.

The ASCOM platform and auto-guiding software like PHD Guiding are required and have been outlined below.

ASCOM software platform. ASCOM Platform
You will need the ASCOM software platform to be able to auto-guide. Although it is not always necessary it is recommended that the ASCOM software platform is installed. The ASCOM platform is freely available and can be downloaded at http://ascom-standards.org.
Please visit the ASCOM website for more information. At the time of writing version 5 is available to download.

PHD Guiding. PHD Guiding Software
PHD Guiding supports the PL-130 as a Windows DirectShow/WDM device or natively. In Direct Show mode the camera will stream video at high speeds but exposure times are limited to under half a second.
PHD Guiding, AstroArt and MaxIm DL support the PL-130M camera in native (single shot) mode and achieve longer exposures of up to 10 seconds.

In Nebulosity Lite, MaxIm DL and AstroArt the PL-130M also supports the StarView mode that is proprietary to Opticstar cameras. This mode increases camera sensitivity that greatly assists focusing and also makes locating dim stars easier.
StarView window.


Editorial Quick Links
Astronomy Editorial Astronomy and Imaging Articles
John Haunton Astroimaging Tutorials Peter Karboulonis Equipment and Usability Articles



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