CHAPTER 13: Digital Astrophotography — What’s New?Chapter_13__Digital_Astrophotography.htmlChapter_13__Digital_Astrophotography.htmlshapeimage_1_link_0
 
Tips on Using an Autoguider

CCD autoguiders are great but can cause some consternation in learning how to get one going. Here are some tips we’ve learned.

Powering the Guider
Older stand-alone autoguiders such as the ST-4 and STV operate off 12 volts but drew 1 to 2 amps of power. A heavy-duty battery or power pack is essential to power those units. Newer units like the SG-4 are much less demanding of power. 
The same applies to any laptop used to run autoguiding software — it must be powered in the field off a high-capacity 12-volt source by using a 12-volt power supply of the type sold for use on airliners.

• Keep power cables short, and use heavy gauge cables to avoid power loss. If the voltage drops, a stand-alone autoguider may behave oddly, or simply shut off. 
• Always bring spare fuses, car lighter plugs and power cables. In our experience, $10 lighter plugs are the weak link in a multi-thousand dollar astrophoto setup. 

Acquiring and Focusing a Guidestar
Whatever the method, focusing an autoguider is critical. They can behave erratically if the guide star is swollen or blurry.

• The autoguider camera replaces the eyepiece in an off-axis guider or guidescope. To calibrate, some systems need a single bright star in the field of the guider. Other systems simply guide better on a decently bright star. The trick is getting a suitable star well-placed on the tiny CCD chip. Some sort of eyepiece is still needed for the initial centering of a guide star 
• This eyepiece must focus at the same point as the autoguider. To do this first find and center a bright star in the eyepiece of the guidescope. 
• Then swap in the autoguider head. Now refocus the star onto the camera chip using the focus routines of the guiding software (it usually takes a rapid series of exposures).
• Now lock the focuser down. Swap in the eyepiece again and slide it in and out of the holder so it, too, is now in focus. 
• Scribe that position on the eyepiece’s barrel. 
• In the future, use that eyepiece at that focal position, find a guide star, focus it, then insert the autoguider. The guider will then be pre-focused and ready to go. 
• An alternative is a flip-mirror viewer: flip the mirror down and the light beam goes to the eyepiece, flip the mirror up and it goes to the autoguider. 

Calibrating a Guider
Once a guide star has been located, all autoguiders must be put through a calibration routine. The guider moves the mount north, south, east and west for a set period of time — say, 10 to 60 seconds. Select a longer calibration time if you are guiding through a very short focal length guidescope like a small finderscope, but use a short calibration time if your guidescope has a long focal length, In that case, a long calibration time can drive the guidestar off the chip. At the end of each move the guider takes an image (or, as with PHD Guiding, a series of images during each move), noting in its memory where the star was at each extreme. This tells the guider software how much to pulse each motor in order to bring a drifting guide star back to its original position on the chip.

The image here is a screen shot of Craig Stark’s PHD Guiding (http://www.stark-labs.com/index.html) software doing a calibration.

• A dual-axis drive with similar motors and gear ratios on each axis is required.
 When performing the calibration set the mount to a suitable guide speed, perhaps 1x or 0.5x: too fast and the star will be driven off the chip; too slow and the star may not move enough to satisfy the guider, giving you an error.
If you are using a short-focal length guidescope (such as a small finderscope), increase the Calibration time (the length of each move. Or, with PHD Guiding, increase the length of each Calibration Step to 1000ms (1 second) under the Advanced settings (the Brain button). 
• Exposure time is also important: too short and the star won’t record well enough for a solid fix of its position; too long and the star might be overexposed and bloated.
• Some autoguider systems must be aligned so the camera’s rows and columns of pixels align with RA and declination in the sky. Most systems work even if the camera is turned at an angle, but check your instructions or help files.
• Calibration moves should be performed after going to each new object, if they differ greatly in declination (i.e. you move far to the north or south) OR if you flip to the other side of the pier of a German equatorial mount. Doing a pier flip usually means the camera turns upside down, so what was north is now south, throwing out any previous calibration.


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