Earlier today I posted links to a series of gallery pics and schematics for a 2 inch double targeting setup for use on the Ealing. This post explains the optional "index error wheel" shown those pics. The index error wheel (my term) is a cheap substitute for a reticule and acts as an aid to center a target in the small TFOV eyepiece or on a small camera chip. It provides a reference frame for positioning a target in a search eyepiece, adjusted for targeting error, such that the object will then appear centered on a camera chip or small focal length ep TFOV. Athough the context of the following discussion involves the Ealing and a two eyepiece search and target setup, the principle applies to imaging with a parafocal eyepiece, a flip mirror and a camera with a small chip: Ealing - visual - double targeting setup - 2 inch version http://gallery.utahastronomy.com/main.php?g2_itemId=11558 Ealing - visual - double targeting setup - 2 inch version - index error wheel view 2 http://gallery.utahastronomy.com/main.php?g2_itemId=11562 Whenever a flip mirror is placed in an optical train to select different optical paths, a small index error is created between the two fields-of-view. Imperfections in the manufacture of the flip mirror surface and/or its mounting will cause an object to appear at different apparent locations in the two eyepieces or in one eyepiece and camera. Simply put, an object centered in a search and targeting parafocal eyepiece will not be centered on a camera chip in the other optical path. I term this "targeting index error". On equatorial mounted scopes, the targeting index error can be addressed by using a Meade reticule with a 12mm focal length. For longer focal length scopes like the Ealing or an SCT without a focal reducer, the TFOV of a reticule eyepiece is extremely small (7 arcminutes in the Ealing and around 15 arcminutes in an SCT). That small TFOV presents a target acquisition problem in itself. With a reticule, a bright star is centered on the chip and then in the targeting parafocal eyepiece, the object's error offset from the center of the eyepiece and its position angle is noted. For further DSO targets like faint galaxies, you just put the faint fuzzy at the same location as measured in the reticule and then it will be somewhere on the camera chip. The index error wheel is a cheap alternative that performs the same function. It is something I whipped up and not something generally seen in amateur astronomical literature (although I am sure someone else already came up with it in the past). The index error wheel is simply a circular protractor that is glued to a CD or a DVD. The CD is drilled out with a 1 1/4 inch or 2 inch drill bit. Then the protractor can be trapped in place between the eyepiece and eyepiece holder. Use the index wheel like a reticule. First, you slew a star in the eyepiece back and forth using the right ascension buttons. Then rotate the protractor so a convienent 90 or 180 degree mark lines up with this line. You can pull back from the eyepiece and see both the target star and the protractor scale at the same time. It helps to use a finger to point from the protractor scale to the center of the eyepiece. You'll only need to make a rough adjustment. The protractor and eyepiece are trapped in a fixed position using the eyepiece screw. The protractor and eyepiece remain fixed in that position for the rest of your observing session and mark the right ascension and declination sight lines in your eyepiece TFOV not matter how the orientation of the eyepiece changes as the scope if slewed or meridian flopped. Second, center a bright star in the camera or small TFOV observing eyepiece. Flop your flip mirror to look through your large TFOV search ep. The target will not be centered in the ep. Note the position angle of the offset target using the index error wheel. Estimate the distance of the target from the eyepiece's center, noting that generally eyepieces have a 50 degree apparent field of view (AFOV). Third, slew to your faint fuzzy target and position the target using your search eyepiece and the index wheel. Position the faint fuzzy in the wide angle search TFOV eyepiece at the same position angle and distance from the ep's center as was measured for the bright star. Odds are, the faint fuzzy will be in your small TFOV observing eyepiece or will be seen on the camera chip. The effect of small TFOV on the level of difficulty in acquiring a target is significant. On the Ealing, a 32mm ep has a 20 arcminute TFOV. The circumference of the ep TFOV is about 63 arcminutes. If a faint 4x5 arcminute galaxy is hovering just outside the ep's field of view, your odds of finding it are not good (1 in 15). The search area covered by a larger 52mm ep is disproportionately large. The area of a circle varies by the square of its diameter. Therefore, the relative areas of two ep TFOV's is proportional to the square of their ratios. That means on the Ealing for a 52mm ep with a 32 arcminute TFOV and a 25 mm ep with a 15 arcminute ep, the ratio of the search areas is ( (32/15^2) or about 4.5 to 1. When slewing the two AFOVs around, you can see how the 52mm will quickly cover 20 times the search area of the 25mm ep. Testing the above setup on the Ealing last night under a near full Moon, I had no difficulty getting M13 centered exactly the middle of a 32mm ep with a TFOV of 20'. Using the above double search and observing eyepiece setup and the index error wheel are aids to make using the Ealing a much more enjoyable experience. I also have found it a useful kludge for centering DSO and lunar targets on a camera chip. Peace - Kurt