I love my Lunt LS50 H-Alpha scope, but I've never been happy with the supplied helical focuser. The focuser drawtube sags with just the weight of the blocking diagonal, and there is some slight back-and-forth "play" in the drawtube. The sag and play can be mitigated by locking the focus knob on the focuser, but it means more fiddling with the focuser than I would like and it's inconvenient to have to lock and unlock the focus knob all the time. Also, due to the sag in the drawtube, the scope's pressure tuning sweet spot is positioned off-center in the field of view (slightly to the lower left of the view). Again, locking the focus knob straightens out the drawtube and helps center the sweet spot, but I find it annoying. When I told Lunt about the focuser issues, they told me the behavior was considered "normal".
After talking it over with Scott at Starizona, we settled on the Borg 7315 non-rotating Helical Focuser as a replacement. There really wasn't much choice since the scope doesn't have enough back-focus to go to anything like a crayford-style focuser such as a Feathertouch. Starizona made me a custom threaded adapter that allows the Borg focuser to thread into the Lunt scope. Total cost: Borg 7315 focuser $109 and custom threaded adapter $50. Some may consider this to be an extravagant expense, but I am so happy with the Borg focuser that I consider it money well spent.
Here are details on the Borg 7315 focuser: http://www.sciencece...s/html/7315.htm
It is a very smooth operating focuser, with no play in the drawtube. I immediately noticed that the scope's sweet spot was more centered in the field of view, for both visual and imaging use. No more fiddling with the focus lock! The Borg focuser has the additional advantage of having a slightly shorter light path (just a few millimeters) versus the supplied focuser. I can still focus with my Lunt zoom eyepiece across it's entire zoom range. And as a bonus, the Baader Mark III zoom eyepiece also comes to focus without having to adapt it to use a slightly shorter light path (as I did with the supplied focuser). (See my post here for the Baader zoom modification: http://www.cloudynig...4#entry6476959)
The major drawback to the Borg helical focuser is that it is much more of a light-duty focuser than the supplied focuser. With the Lunt zoom, which is a fairly lightweight eyepiece, the focusing action with the Borg is not as smooth as when there is a lighter-weight fixed focal length eyepiece being used. The focuser also operates fairly smoothly with my small Skyris 132m camera mounted to the blocking diagonal. The much heavier Baader zoom eyepiece puts more demands on the Borg focuser, but I love using the Baader zoom so I can live with lifting the eyepiece end of the blocking diagonal a little bit to take some weight off the focuser while I'm focusing. I can switch back to the supplied focuser whenever I need to use a heavier accessory, so I'm OK with this.
Here is the Borg Helical Focuser 7315 mounted on the scope, with my Skyris 132m camera attached to the blocking filter. To adapt the camera and achieve focus, I use a Starizona T-to-C adapter mated to a 10mm T-thread extension tube.
Starizona has several of the Borg custom threaded adapters for the Borg focuser, so they are available if others are interested in going this route. If you are interested, it's best to ask for Scott.