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Imaging and Auto-Guiding: A Primer
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Author: Peter Karboulonis. © Opticstar Ltd.
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Introduction
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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.
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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.
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Auto-guiding
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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.
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M51: 23.03.2006. by Ulrich Gurschen - 3 minutes exposure without and with auto-guiding
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Imaging and Auto-guiding With Two Scopes
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In general to be able to auto-guide with the Opticstar AG-130M
Auto-guiding Kit you will require the following:
- An imaging camera, i.e. Opticstar DS-142C ICE, DSLR etc.
- An imaging scope, i.e. A&M 80mm f7.5 Super APO.
- A secondary scope for auto-guiding, i.e. 70mm f10 achromat.
- An ST-4 compatible mount, i.e. Skywatcher EQ6 or HEQ5 PRO.
- ASCOM software platform V5 or later (ascom-standards.org, free).
- PC with USB2.0 ports running Windows 2000/XP/Vista or later.
- ST-4 guide camera and software, i.e. Opticstar AG130M kit.
- Auto-guiding software, i.e. Guidemaster or PHD Guiding.
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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.
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Imaging and Auto-guiding with a Single Scope
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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.
- An imaging camera, i.e. DS-335C ICE.
- An imaging scope, i.e.10” f10 SCT.
- An off-axis guider.
- A ST-4 compatible mount, i.e. Vixen Sphinx, Skywatcher EQ6 or HEQ5.
- ASCOM software platform V5 or later (ascom-standards.org, free).
- PC with USB2.0 ports running Windows 2000/XP/Vista or later.
- 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.
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Off–axis Guiders
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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.
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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.
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Software and the AG-130M/COOLAIR Guiding Kit
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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.
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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.
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Please visit the ASCOM website for more information. At the time of
writing version 5 is available to download.
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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.
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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.
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