OPTEC TCF-S3 3-inch I.D. Temperature Compensated Focuser

[Benny L]

http://www.optecinc.com/optec_067.htm

I am considering purchasing one of these for my 14" SC... does anyone have any personal experience or info on this little gadget.

From what OPTEC says it looks pretty cool.

[jase]

They're an excellent focuser Benny. Speak with Fred (Bassnut) about his TCF-S. There are pros and cons you should consider when place items in your optical train. TCF will change your focal length/ratio depending on its length. Generally, this isn't an issue because you will have ample backfocus on the LX200R.

Compared to a rich field refractor, you want to really minimise what is in the path to keep those wide views. This is where something like a RoboFocus is good (its also temperature compensating). Its "non-intrusive" and attaches to your existing focuser - be it an SCT course knob or crawford drawtube type focuser. You can also get a RoboFocus set up to focus camera lenses with extreme precision for wide field work. I'm more of a RoboFocus person myself, but you can't go wrong with either. There are also plenty of others on the market such as the FLI Precision Digital Focuser (PDI) etc list goes on and on.

[Bassnut]

Benny, Im happy with my TCF-S, what can I say, it always works ;-). Temp compensating is a bonus, Ive heard robofocus is also good and reliable, although Im not sure if it is an absolute focuser (as opposed to relative, which is a small detail, ie requires homing on power up, Jase would know). I have no trouble at all with back focus on my LX200R OTA, in fact the AP 0.67F reducer I have fits inside the focuser (TCF-S is 2"), so adds nothing to back focus at all!. OPTEC drivers are included in all serious astro apps (as are Robofocus I think). I dont have any experience with the 3" version.

I suspect theres not much in it apart from travel count resolution and weight bearing capacity, the 3" would handle the ST11K no probs

[jase]

Robofocus (http://www.robofocus.com) also provides absolute focus measurements (so performs similar to an Optec), though there are a few things to consider. A Robofocus system is only as good as the focuser you attach it to. If you’ve got a Taiwan branded Crawford focuser the exhibits focus shifts with heavy attachments and show backlash, you’re wasting your time. You need to have good mechanics to start with. I purchased a JMI NGF a few years ago and modified it with a Robofocus set up. It was a strong performer and I would recommend it for those on a tight budget. A Robofocus system needs to be trained so it knows the start and end points of focuser travel – once complete it will provide absolute measurements within this range. Both TCF and Robofocus provide identical step sizes (2micron – 0.002mm), though this is configurable. I usually operate at around 6 to 10 micron considering the critical focus zone (CFZ) on the FSQ is 55 microns wide. The faster your focal ratio the smaller your CFZ and the more difficult it is to obtain precise focus manually. Robofocus support is very wide spread as the stepper motor can also be adopted to other functions such as instrument rotators (http://www.astrodon.com/documentation/documentation.cfm?DocID=10). Obviously support for ASCOM are available in both TCF and Robofocus.