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An autoguider is an automatic electronic guidance tool used in astronomy to keep a telescope pointed precisely at an object being observed. This prevents the object from drifting across the field of view during long-exposures which would create a blurred or elongated image.
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Astrophotography Equipment - Selecting an Auto Guider - Part 5
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Welcome to Selecting Astro Photography Equipment Part 5 Selecting a Guide Scope. First lets discuss the purpose of a guide scope and camera. Most telescope mounts after they are properly aligned will do a good job of tracking the stars, for a few seconds. However, to achieve nice round stars you need a Guide Scope. The purpose of a Guide Scope is to LOCK on a star and sends fine adjustments to your mount. With a guiding system youll track the night sky more accurately. With auto guiding youll be able to take longer exposures and capture fantastic astro images! As with all astrophotography equipment, there are tons of choices. Basically there are two categories of Guide Scopes, a dedicated guide scope, or an off axis guider. Also known as an OAG. The OAG style is for advanced users who have time to fiddle with fine adjustments. An OAG takes a “tiny” part of your optical train and reflects this “tiny” section to your auto guider. Your imaging camera shares the same optical path as your auto guide camera. The reward of using an OAG is you can use the same scope you use for imaging and it has the potential to lighten the load of your AP setup. Some people claim an OAG helps reduce flexure. Flexure is when your “main scope” moves without your “guide scope”. Im telling you right now…even under the “best” of conditions, youll run into some kind of challenge, and the last thing you need to mess around with is your guide scope. Since these videos are designed for the first time astrophotographer, Ill be discussing the dedicated guide scope. I believe the dedicated guide scope is the easiest and most reliable option for beginners. There seems to be a popular movement towards 50MM Finder Scopes for auto guiding. If you already have 50mm finder scope, you can easily modify it to accept a guide camera. Orion offers the 50mm Guide Scope package for $350 and if you already have a guide camera, Orion sells the 50mm guide scope for $100. Orion also offers an 80mm guide scope package for $400, but stay away from “set screw” mounts. “set screw” clamps will introduce the dreaded “flexure” to your imaging train. Instead of using “set screws” use Solid Ring mounting brackets. Solid Mounting Rings Firmly grab your scope and reduces flexure. Dont worry too much about the guide scope, almost any scope with a 1.25” draw tube will work. If you decide to use a scope for guiding, grab a refractor, and when you find focus, you can mark the draw tube, so the next time out youll have perfect focus. The Orion Star Shoot Auto guider is a great guide scope camera and @ $280 its a good entry level guide camera. The Star Shoot Auto Guider is a little bit noisy when you compare it to other high end auto guide cameras. The Star Shoot Auto Guider has a little problem locking on the guide star in MaximDL. However, Using Push Here Dumby (PHD) the Star Shoot Auto Guider works flawlessly. If you decide to look at higher end guide cameras youll be looking to spend $600 or more. It important to support the cables attaching to your auto guider so you dont run into connector problems later down the road. The Star Shoot Auto Guider, is the best value, and you will enjoy how simple it is to use. With PHD software, its literally, PUSH HERE DUMBY and away you go! I recommend the Star Shoot Auto Guider camera for beginners. An auto guider camera needs to connect to your mount via an auto guider port. Syncing your mount with an auto guider camera will help you to take longer exposures. Taking longer exposures is the key to successful imaging and an auto guider is required for astrophotography. Thanks for watching and please visit Myastroimages.com. Stay tuned for the Next Video Selecting Accessories.
Usage
Imaging of dim celestial targets, usually deep sky objects, requires exposure times of many minutes, particularly when narrowband images are being taken. In order for the resulting image to maintain usable clarity and sharpness during these exposures, the target must be held at the same position within the telescope's field of view during the whole exposure; any apparent motion would cause point sources of light (such as stars) to appear as streaks, or the object being photographed to appear blurry. Even computer-tracked mounts and GoTo telescopes do not eliminate the need for tracking adjustments for exposures beyond a few minutes, as astrophotography demands an extremely high level of precision that these devices typically cannot achieve, especially if the mount is not properly polar aligned.[1][2]
To accomplish this automatically an autoguider is usually attached to either a guidescope or finderscope, which is a smaller telescope oriented in the same direction as the main telescope, or an off-axis guider, which uses a prism to divert some of the light originally headed towards the eyepiece.
The device has a CCD or CMOS sensor that regularly takes short exposures of an area of sky near the object. After each image is captured, a computer measures the apparent motion of one or more stars within the imaged area and issues the appropriate corrections to the telescope's computerized mount.
Some computer controlled telescope mounts have an autoguiding port that connects directly to the autoguider (usually referred to as an ST-4 port, which works with analog signals).[3] In this configuration, a guide camera will detect any apparent drift in the field of view. It will then send this signal to a computer which can calculate the required correction. This correction is then sent back to the camera which relays it back to the mount.[4]
An autoguider need not be an independent unit; some high-end CCD imaging units (such as those offered by SBIG) have a second, integrated CCD sensor on the same plane as the main imaging chip that is dedicated to autoguiding. Astronomical video cameras or modified webcams can also serve as an autoguiding unit when used with guiding software such as Guidedog or PHD2,[5] or general-purpose astronomical programs such as MaxDSLR.[6] However, these setups are generally not as sensitive as specialized units.
Since an image of a star can take up more than one pixel on an image sensor due to lens imperfections and other effects, autoguiders use the amount of light falling on each pixel to calculate where the star should actually be located. As a result, most autoguiders have subpixel accuracy. In other words, the star can be tracked to an accuracy better than the angular size represented by one CCD pixel. However, atmospheric effects (astronomical seeing) typically limit accuracy to one arcsecond in most situations. To prevent the telescope from moving in response to changes in the guide star's apparent position caused by seeing, the user can usually adjust a setting called "aggressiveness".[6]
Notes
- ^ Lodriguss, Jerry. "Polar Aligning". Retrieved 15 November 2013.
- ^ note: This can be accomplished by manually adjusting the orientation of one or both axes of an equatorial mount to hold either the target itself or a nearby guide star at a certain position, but such a method is difficult and time-consuming.
- ^ "Richard's Ramblings – the Blog Formerly Known as McWiki".
- ^ "What is an autoguider?". BBC Sky at Night Magazine. Retrieved 2021-12-29.
- ^ "PHD2 Guiding". Open PHD Guiding. Retrieved 26 December 2020.
- ^ a b Covington, Michael (2007). Digital SLR Astrophotography. New York: Cambridge University Press. pp. 109–110. ISBN 978-0-521-70081-8.
References
- Lodriguss, Jerry. "Catching the Light." http://www.astropix.com/HTML/I_ASTROP/TRACKED/AUTOG.HTM.
- Covington, Michael. Digital SLR Astrophotography. New York: Cambridge University Press, 2007.
This page was last edited on 30 July 2022, at 02:07