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Some Useability Improvements for Celestron 8” EdgeHD and Similar Telescopes
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Some Useability Improvements for Celestron 8” EdgeHD and Similar Telescopes
Everyone who has worked with the Celestron 8” EdgeHD telescope, and I’m sure other 8” SCTs, knows their useability problems: (1) the scope is difficult to focus, and (2) the screw-on diagonal provided by Celestron is difficult to tighten enough to prevent eyepiece rotation, and when it is tight enough, very difficult to loosen.
I worked on these problems for some time, and have come up with what I believe are very worthwhile improvements. Perhaps the best part about the improvement is that they do not utilize any parts made in China!
First, let’s start with the focuser. Over several years I’ve experimented with electronic focusers, one by Orion and the new one by Celestron. They work, and the Celestron focuser has the advantage that it allows you to record the position of the focuser. However, I always preferred the feel of directly controlling the focus with my fingers, and I found that having to rely on a separate control pad or several commands on the hand control to even get to the focus panel made the whole process very cumbersome. So I decided to try the FeatherTouch focuser, model FTM-CPC8, which fits the Celestron CPC800, NexStar 8SE, 8” Edge HD, and some other scopes. See Figure 1. This focuser is rather expensive at $315, but in my experience with it, definitely worth the money. Plus, made in USA!
Figure 1. FeatherTouch Focuser for SCT
The focuser is extremely smooth, with the fine focus knob making very small adjustments possible. The general impression is that it makes focusing the scope very refractor-like, as opposed to the somewhat clumsy focus knob that comes with the scope. I found it very easy to get extremely good focus on all objects, better than ever it seems. Everything appears sharper than it did before—whether that is an illusion on my part or not, I cannot say; but the ability to do the kind of fine focus that this device enables is a huge improvement and makes using the scope a real pleasure.
Installing the focuser is very easy. It comes with a CD-ROM with complete instructions. You just need to remove a screw and retaining washer, unscrew the old focuser shaft, screw on the new one, and screw down a plate. Total time is about 10 minutes.
I did one other thing which may be of interest. I found the most eyepieces focus in a fairly narrow range of shaft rotation. I copied a 360 degree circle from the Internet, printed it, laminated it with clear package sealing tape, and attached it to the back of the SCT around the focuser (see Figure 2). Then I picked one eyepiece as the standard (a 40mm Pentax XW, but any one would do), focused it, and marked the 0 degree point on the black focus knob with a piece of tape. Then I took other eyepieces and focused them, noting how many turns (usually less than one) were needed relative to the standard, and the final degree position on my circle. This I recorded so next time I can just dial in the focus. Same can be done for cameras or other accessories.
Figure 2. 360 target for recording focus position
This works well, as well as the numerical value on the Celestron electronic focuser, which always varied a bit from one session to the next. Note that FeatherTouch makes similar focusers for other SCT models, so you can look up the one for your scope.
Next I worked on the diagonal problem. With the SCT design, unlike refractors, there is an optimum position for the eyepiece (or any device), one which gives the flattest field and least distortion. Celestron gives this for the EdgeHD as 133.25 mm from the back of the projecting threaded shaft. (On this scope, Celestron is emphatic that the large black nut on the shaft should not be removed. On the C9.25 and C11, the design is different and the nut can be removed). The trick therefore is finding a configuration to replace the screw-on diagonal that is close in overall light path distance to the optimum 133.25 mm. There are several screw-on visual backs available that allow you to use standard 2” accessories such as diagonals, but they tend to be rather long. Celestron even sells one, about 2” long. These I found were all too long, i.e., they made the optical path much longer than 133.25 mm. Then I hit upon the Lumicon Short adapter, # LA1066, by FarpointAstro.com. See Figure 3. This allows for a shorter opticaI path length.
Figure 3. Lumicon Short SCT Visual Back Adapter, With Larger Thumbscrews
I purchased one from B&H photo, about $20, also made in USA!. This works right out of the box, though I did replace the included 8-32 thumbscrews with larger ones, available for about $1.50 from Home Depot (must be ordered, 8-32 x 3/8). This works well, attaches easily, and holds a diagonal firmly. Just one problem: it is so short that the nose on many diagonals is too long to fit completely into it! However, that problem is fixable as well. I use a Baader 2” BBHS click lock mirror diagonal (made in Germany), which has a short nosepiece. This fits perfectly and goes all the way into the visual back, and is a superb piece of equipment in its own right, especially the click lock feature. See Figures 4 and 5. This gives an overall optical path length from the end of the projecting shaft to the top of the diagonal of about 136 mm, 2.75 mm greater than the optimum. Interestingly, the Celestron screw-on diagonal gives an optical path length of about 128 mm, or -5.25 mm from optimum, so the Baader configuration is actually closer to the optimum. Whether this is reason for the improved performance, or the better optics of the Baader, I cannot say.
Figure 4. Comparison of Diagonals Showing Baader on Left with Short Nosepiece, and Standard Diagonal on Right with Longer Nosepiece
Figure 5. Baader Diagonal in Lumicon Short Visual Back
I tested the overall configuration by turning the diagonal 90 degrees so that it was parallel to the ground, and inserted a heavy eyepiece (TV Panoptic 41mm). Everything held quite firm. I was afraid that the visual back might unscrew, but it did not. And the clamps on the visual back held the eyepiece well. On slews I never had any problems, which I often did with the screw-on diagonal. The only issue with the shorty visual back, as you can see from Figure 5, is that you cannot rotate the diagonal more than about 30 degrees to the right before it hits the focuser. I never found this to be a problem—if I needed to rotate the diagonal, I just did so to the left.
Of course, if you are going to use the Celestron 0.7x focal reducer, you will have to remove this visual back. But that reducer is exclusively for imaging, so you won’t be using a diagonal either.
One caveat: Celestron claims that there is some tolerance on the optical path length, about 20mm. I did not attempt to test this.
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