Last updated on
4/11/2006

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Astrophotography requires an equatorial wedge for the scope, and also requires that the equatorial wedge�s polar axis be accurately aligned with the celestial pole.  Otherwise the object you are photographing will drift during the exposure, leading to streaking or smearing of the photographic image.  If there is only a slight drift during the exposure, you may not readily see smearing but the result will be elongation of star points into ovals and what in effect will look like a loss of focus or crispness of the image of the object you are shooting.  So in astrophotography an accurate polar alignment, like accurate focus, is critical.  This is a major issue for the long exposure times required for deep-sky objects on 35mm film, but is still an issue for dim objects with a CCD camera.

Unfortunately an accurate polar alignment isn�t very easy to do.  You can�t simply aim the polar axis at Polaris because Polaris is about 0.8 degrees away from the true celestial north pole.  Alignment with Polaris is is close enough for visual observing but that 0.8 degree offset is enough to cause drift during CCD photographic exposures and especially during the long exposures needed for film astrophotography.  So you need to find a way to align the scope with the true celestial north pole.

Celestron has an accessory that makes this alignment easier - the 7x50 Polaris Finderscope.  (This finder scope is the one Celestron provided for the U2K but it can be purchased from Celestron for use on other scopes.) It makes use of the fact that Polaris makes a complete circle around the Celestial Pole, as all stars do, every 24 hours. If you know how far and in which direction Polaris is off-set from the Celestial Pole for a specific date and time, you could off-set the scope�s polar axis opposite to that and achieve alignment with true celestial north. The 7x50 Polaris Finderscope has a reticle with the standard finder scope cross-hairs, but the reticle also includes a special ring around the center.  The ring is divided into 24 slices and has a radius of 0.8 degrees within the optical field of view for the finder. (The view through the reticle is shown here on the right.) So if the finder scope is aligned well with the main scope and if you place Polaris in the correct one of the 24 slices your main scope will be aligned with true north   (You need to be sure to set your scope for a declination of 90 degrees and move the mount, not the scope, to align with the true celestial north axis.)

So how do you determine which direction Polaris is off-set for a particular date and time? For that you need a special circular calculator that Celestron calls the �Polaris Setting Plate�, shown here on the left.  It�s a simple cardboard pair of circles (like a circular slide rule if you�re old enough to remember them) - you rotate the inner circle to the current date and time and the pointer indicates which direction to place Polaris (and already accounts for the inverted image of the finder scope).  There is also a free software application that can be used for this, called PolarFinder by Jason Dale, which you can download from his Web Site. 

For some bizarre reason the Polaris Setting Plate was not included with the U2K. Celestron sells it separately for a whopping $7.  Even more bizarre is that this item is not shown in any Celestron catalog - they have placed it in some kind of witness-protection program.  However, the Celestron part number is #60121 , and you can get it from the major scope vendors. Also, another thing Celestron didn�t include with the U2K was the Illuminator you need to light up the reticle in the Polaris Finder Scope. The illuminator is part number 60011 and sells for about $50; it screws into a threaded hole just in front of the finder scope�s eyepiece. It contains a red LED light, a battery, and a switch, and will illuminate the reticle so you can use it at night.  (It�s the same Illuminator that comes with any of the Celestron or Meade Illuminated Reticle Eyepieces - both companies clearly get that Illuminator from the same supplier.)   Since eyepiece illuminators use a standard 8mm thread, you could also consider the PulsGuide Illuminator made by Rigel Systems.  That illuminator can be set to slowly pulse on and off which can make finding stars easier, and it sells for $35 which is less expensive than the Celestron unit. However, it is physically very large and made of plastic, whereas the Celestron is made of machined metal and feels a lot sturdier. 

If you purchase a 7x50 Polaris Finderscope from Celestron you will receive the Polaris Setting Plate and the Illuminator. But if you have a U2K you have the Polaris Finder Scope - you only need to purchase the Setting Plate and the Illuminator.
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You should be aware that most folks find the standard mount for Celestron finder scopes, including the 7x50 Polaris Finderscope, to be inadequate. It has set-screws in the front ring but the rear of the finderscope is held in place with an O-ring in the rear ring, which is a stupid arrangement that doesn�t work.  It doesn�t retain alignment and if you commonly remove the scope and place it in a case - the finderscope will usually fall away from the mount because the o-ring doesn�t provide enough mechanical rigidity. Typically, folks replace the standard 7x50 finderscope�s mount with a quick-release finder mount. Celestron makes a Quick Release Finder Bracket (part #51149, shown here), that sells for $60; Orion sells a similar unit (Part #05040) for half that price. Either of these units provide two advantages: they support the finderscope with set-screws at both ends and they have a quick-release bracket so you can remove the finderscope when you don�t need it or when you are placing the SCT in a hard case.
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The most accurate method of polar alignment is the Drift Alignment process, and if you plan to do long -exposure astrophotography you�ll need to use this method for fine-tuning your polar alignment.  There is more information on drift alignment (and polar alignment in general) at Starizona�s Web site, and another good explanation of the drift alignment process at Bruce Johnston's Astronomy Pages.

Back to Astrophotography Accessories
 

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