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It was when I had attended my first star party that I got the bug to make changes to my scope. The biggest bug which had to be stamped out was the focuser. Orion provided a 1.25" helical-action focuser with my 8" Dob. The focuser was fundamentally sound for me so I went about my business during the first five months of ownership of my scope.However, it was in November, 1996, that I first tasted the ultra-smooth focusing of a rack & pinion focuser. Granted it was only a Meade model, but it put my helical focuser to shame for easibility and speed. Prior to that I had never tried any other kind of focuser and therefore did not know what I was missing. It was again a star party which sent me in the right direction.
In the following weeks, I set out on a mission to find myself the best focuser I could for my Dob. I talked to many people on the subject and settled upon a Crayford design. That design combines the smooth action of a rack & pinion and the low-profile design of a helical. Next it was the decision of going with either 1.25" again or moving up to 2", which ended up being a very easy choice. I was told to go with 2" all the way, no doubt about it. Then I could go out and try all sorts of eyepieces and filters not to mention that most of the GREAT Crayford focusers come in the 2" size only.
With that in mind, I surfed over to Astromart in order see what kind of deal I could get on a used focuser. I placed an ad that I was looking for the reasonably priced JMI NGF-3 model. The very next day I got a response and ended up purchasing the focuser in LNIB (like new in the box) condition. In addition, the seller even sent me the focuser to inspect prior to my sending of payment! Good deal all around.
While the Crayford design is among the best, do not disregard a helical or rack & pinion from your choices of focuser. The big reason why I went with the Crayford instead of the rack & pinion (which I had so fallen in love with) was that both the helical and Crayford stood about the same height off the tube. Therefore I could simply put the new Crayford focuser over top of where the old helical sat on my tube. If I were to have purchased a rack & pinion focuser, which usually would sit 2" higher off the tube than my helical, I would have had to move the primary mirror closer to the focuser or vice-versa in order to allow for my eyepieces to focus (pictured to the left is a homemade Crayford focuser; not only do they work great, they are relatively simple to construct).
By installing a rack & pinion focuser over top of the hole where the helical sat and not moving the primary mirror closer to it, for the most part none of my eyepieces would have reached focus. The extra 2" of height of the rack & pinion focuser creates a longer path for the light to travel and therefore pushes all eyepieces 2" out too far from the tube.
So when would a rack & pinion or a helical focuser be good? The rack & pinion would be ideal when used with a homemade large-aperture Dob while the helical could be used to some benefit on a planetarily-optimized Dob. The large-aperture Dob would already have a slightly large secondary mirror which could handle the extra distance of the light path for a rack & pinion focuser. Generally the rack & pinion focusers require a larger than usual secondary mirror but those light-bucket Dobs already have large secondaries, so the focuser and secondaries would work in concert very well.
As long as the helical focuser is quality, it would be of great help on a planetary scope. Such a fine helical would be that made by Gary Wolanski in Canada (focuser pictured to the right). A helical focuser provides finer focusing when minute adjustments are needed, therefore ideal for those long focal length Dobs. It also helps to have parafocal eyepieces when using a helical focuser since no matter how well it is made, it will still be a pain to crank and crank between eyepieces.
What, after collimation, will reak havoc on images in a scope? Tube currents and lack of proper ventilation of the primary mirror. Amateurs out there still using newtonian telescopes with closed, or even semi-closed, mirror cells will know what I mean. When I was still using my Orion particle-board cell I had a mean cool-down time somewhere in he neighborhood of two hours. Absolutely horrible! I hated it!
In order to cure this problem, I made yet another innovation to my stock Orion Dob and invested in a University Optics aluminum mirror cell. For under $30 I received a mirror cell that was sure to allow proper ventilation. And that it did. With this new cell in place, air can move more freely around the primary, through the tube and out the focuser-end. This set up permits from between a 30 to 45 minute wait before using the scope. Not at all bad considering the 2 hour wait prior!
Decreasing the cool-down time even further, I have added a 12V muffin fan behind the mirror cell. This fascilitates a cool-down time of nearly 15 minutes! The scope is taken outside on my back porch, the fan is turned on and is left on all night long. I prefer the fan on all the time since images seem to become fuzzy if the fan is on and is then turned off while observing. No tube currents or vibrations are seen. The fan is the single best addition to the scope; believe it or not!
Collimation is the number one killer of bright, pin-point star images in newtonian telescopes like mine. One must take great care to fully learn and understand the process of collimation to insure the best possible images through his or her scope. Unfortunately, for me this lesson was learned the hard way when trying to collimate Orion's single-stalk secondary holder which they originally provided with their Deep-Space Explorers. I hungered to have the ease and stability of a spider and adjustable secondary mirror holder. Not to mention that also by purchasing a Crayford-style focuser I was forced to upgrade to a spider. Unlike Orion's 1.25" Helical focusers, commercial Crayford focusers do not contain a holder for a single-stalk secondary mirror holder.The secondary system of choice was one from Astrosystems. I purchased everything all at once; secondary mirror (1.83" Minor Axis), secondary mirror holder (with four-point adjustment) and a 10" four-vane spider. Astrosystems offers enhanced coatings on their secondary mirrors, so I went for every bit of bang for my buck and sprung for the 98% coated mirror. The recommendation to purchase Astrosystems equipment came from a friend of mine, Rick Singmaster, owner of StarMaster Telescopes. He uses Astrosystems spiders, secondary mirrors and holders in his large Truss-Dobs.
The entire set up is a breeze to collimate and once set, stays set. Collimation is made even easier with the addition of a Laser Collimator. Simply place the Laser Collimator in the focuser and with an allen-wrench turn the adjustment screws on the back of the Astrosystems secondary holder until the laser is 'dead-center' in the primary mirror. I would definately suggest doing business with Astrosystems or a dealer since their products are outstanding.
