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Since the mid-1980’s Schmidt-Cassegrain Telescopes (SCT’s) have become a top choice for many amateur astronomers, due in large part to their extreme portability – but also because they’re durable, relatively maintenance free, ideally suited for astrophotography, and offer the flexibility to get excellent views of both planets and faint deep-sky objects. However, the sky is not always clear, alas, for SCT owners. Two problems inherent in the design of most SCT’s provide significant hurdles to quality observing. Fortunately, hurdles are made to be jumped!

First, the corrector plate at the front of an SCT’s optical tube makes it very susceptible to dewing. I did not appreciate this fact when I first bought my 8" Meade 2080 SCT, and I paid for it! It wasn’t until I was at one of BBAA’s Skywatches at NWRP on a wonderfully clear and moonless night, and bright objects like Vega started looking more and more like dim headlights in a fog, that I suddenly realized just how naked my SCT’s corrector plate was. Makeshift dew caps, resistor loops, and hair driers soon followed, and now I no longer fear those humid nights so common in Virginia Beach, North Carolina, and at my current home in Maryland. The fight against dew is well-documented and there are many ways, both commercial and homemade, to conquer it, so I will not dwell on it here.

The second common problem with SCT’s, image-shift, is really the reason why I’m writing this article. Unlike refractors and Newtonian reflectors which focus by moving the eyepiece in or out, SCT’s generally focus by turning a knob which actually moves the primary mirror. Unfortunately, in many SCT’s (mine included) the image shifts position as you attempt to focus the eyepiece, sometimes right out of your field of view! At low powers this is not a problem for an experienced observer; however, at magnifications above 100x, focusing can quickly become a two-handed test of your dexterity as you try to offset image-shift caused with one hand by making declination and right ascension adjustments simultaneously with the other! In this case, someone who’s not familiar with telescopes doesn’t stand a chance. Thus, I found image-shift to be a very inhibiting problem at observing sessions geared toward the general public. No sense trying to show anyone a 160X viewing of Jupiter if I’m the only one that can get it focused reasonably well without losing it! My cheap 12mm eyepiece slowly collected dust.

Certainly, there is a perfectly rational explanation for why telescope makers have opted for this type of focusing. Or so I hope. Truth is, the reason for it utterly eludes me. Perhaps it has to do with the fact that most SCT’s are fork-mounted and have limited space to permit an extended eyepiece when slewing the scope through the fork. But then, SCT’s are supposed to be great for attaching a whole camera there. I don’t know! If someone can enlighten me on this, please do. Anyway, back to jumping hurdles.

Stepping in to save the day is JMI with their SCT-specific focuser, the NGF-S. Complete with a built-in motor, the NGF-S does away with image shift by permitting you to focus by moving the eyepiece. Hurray! It also costs around $250 (back in 1998). Ugh! I decided to give JMI a call to see if they could help me find a less expensive way to rid myself of image shift. Sure enough, there was! JMI produces a variety of different focusers and is willing to help customize as needed. I chose an NGF Mini-2, which is a much less expensive focuser without the motorized focusing of the NGF-S (which I wasn’t interested in anyway). Although their series of NGF Mini’s are actually designed for reflectors, and therefore have some curvature to their base to permit mounting onto an optical tube’s side, for $5 the folks at JMI were happy to machine the base of my focuser flat to permit mounting onto the end of my SCT’s tube. The NGF Mini-2 is made from anodized aluminum and has steel bearings, which are improvements over their NGF Mini-3 and NGF Mini-4. An adapter was also necessary to permit attachment to the visual back of my SCT. Total cost to me was about $140.

The NGF Mini-2 attached to the rear cell of my SCT as shown in the figure. One concern that I had was that it is only held in place with 2 small set screws. Although it's been 2 years and I’ve had no problems, I’d feel better if they had designed it with 4 set screws. The focuser itself moves quite smoothly with no play. My old focuser, I have come to conclude, may have also had a little slop in its motion due to excess grease. The old focuser is unaffected by the installation of the new one; thus, I now have two means of focusing. What I do when I start an observing session is center the NGF in the middle of its travel, and then focus on an object with the old focuser. Then, for the rest of the night, I’ll just use the NGF to focus.

Was it worth it? Well, I got my first chance to really test it out during a partial solar eclipse a couple of weeks later. I brought my newly equipped telescope with me to work, along with a newly acquired solar filter, and e-mailed my co-workers to let them know I’d be outside at lunch showing off the 25% eclipsed sun. Each of the 40+ people that came out were treated to not only great views of the eclipse, but also to crisp views of a couple of sunspot groups which EASILY POPPED INTO FOCUS FOR THESE FIRST-TIME VIEWERS! Image shift was history! I was happy!

Since that day, I have basked in the luxury of easy focusing. I no longer steer away from my 12mm eyepiece because it’s not worth the effort. Heck, now I’ll brazenly toss it into my 2.4x Dakin Barlow if I want the magnification! In just a few weeks I concluded that I probably am able to detect objects of a slightly fainter magnitude than before, simply because it’s easier to focus precisely and I can view them more comfortably. If you own an SCT and have ever been irritated by your subject moving out of the field of view as you focus on it, then I highly recommend you focus on your focuser.