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STI- 4 and STI-201 Autoguider Focusers

                

STI Autoguider Focusers were developed by Stellar Technologies International to specifically  overcome difficulties in focusing the SBIG ST-4 and Meade 201XT Autoguiders.   The ST-4 is a classic autoguider; however, fast focusing with this unit is not one of its virtues.  While parfocal eyepieces have been used to try to achieve proper focus, users have continued to complain about both the difficulty and cost.  The STI-4 Autoguider Focuser represents a low-cost and highly effective alternative that achieves perfect backfocus for the ST-4 in only seconds. The 201XT is also a fine autoguider; however, proper focusing of the unit is very critical to its use and effectiveness. Both the STI-4 and STI-201 Autoguider Focusers are best suited for use with separate guidescopes.

                                How It Works

The STI Autoguider Focuser functions in the same manner as our regular Series IV focuser.  Both the STI-4 and STI-201 employ high resolution 180 line/inch ronchi gratings.  Simply insert the STI Autoguider Focuser and focus until the image is free of dark bars.  Then remove the focuser and insert the SBIG ST-4 or 201XT in its place.  It is that simple!

                           

Features

*  Easy to Use with no calibration necessary.  Place the STI Autoguider Focuser in the 1.25" focusing tube of your guide telescope  and you're ready to go.

*  Elimination of Eye Error is guaranteed. You can be confident the proper backfocus of your autoguider is reached each and every time regardless of poor eyesight.  No parfocal eyepiece approach can come close to its performance  whether your eyesight is 20-20 or not!

*  Inexpensive.  You do not have to dedicate one of your better eyepieces to focusing, or invest in a high-cost guiding eyepiece just to achieve focus for your autoguider.

*  30 Day Clear-Sky Guarantee to prove to yourself the STI Autoguider Focuser is the best device around for focusing your SBIG ST-4 or Meade 201XT.

Note: Installing an STI Autoguider Focuser in an existing Series IV 'Stiletto' body requires the use of  an adapter ring to interface it properly.  These are available in our order section. 

Maintenance

The STI-201 or STI-4 should need very little maintenance and are permanently calibrated to your autoguider. However, the focusing lens is very fragile so utmost care should be taken in handling it.  The ronchi screen itself is sandwiched between two layers of extremely thin optical glass to form the lens.  This assembly is very delicate.                          

Never touch the surface of the screen!  
Should the surface of the glass become dirty from casual eye contact, gently wipe clean with the tip of a cotton stick, or use an air duster spray.  Any undue pressure placed on the protective glass may result in breakage.  If chemical cleaners are used, use sparingly since undue amounts may inadvertently infiltrate and permanently damage the ronchi screen making it unusable.  This will require ordering a replacement screen.

Use

Use of a knife-edge focusing device for critical focusing does not come naturally and requires a short training period.  Normally, our eyes have been trained to try to focus on a pinpoint image and minimize that image as much as possible.  With knife-edge focusing you are not interested in the image that appears at all, rather the light pattern produced.  When out of focus, dark bars will appear across the light pattern.  As you approach focus, the bars will appear to be getting wider but fewer in number.  When in perfect focus, no bars appear at all. 

Since the STI Autoguider Focusers are critical focusers, you should try to obtain a rough focus of the image by traditional means before inserting the autoguider focuser.  The best way to obtain a good rough focus for your instrument is to use a regular eyepiece in the 9-10mm range.  If you are too far out of focus to begin with, it is likely that you will see no bars at all through the focusers since you have far exceeded their practical range of application.  

Users of SCT scopes should also not be surprised to see the central obstruction.  This usually appears as a dark circle or as a donut.  What you must concentrate on is the lighted area surrounding this circle or partial circle and adjust your focus until the bar pattern appears.  If using a separate guidescope in conjunction with the Autoguider Focuser, focus normally.  If using an Off-Axis Guider, it is best to focus with the regular telescope focuser first instead of sliding the eyepiece extension tube in and out of your OAG.  Keep in mind that once you have correctly set the distance of the OAG eyepiece extension tube with the Autoguider Focuser, the positioning of the extension tube may be set or marked for future use.  If you remember or mark this position, you should simply be able to insert your CCD autoguider in the future with no need to refocus! 

As a practical concern when using an extension tube or helical focuser for focusing the Autoguider Focuser, make small changes only between settings.  Your eye has an uncanny ability to adjust for images out of focus and you may convince yourself that there is no bar pattern across the image.  When this occurs, simply focus using your electric focuser until you see the bar pattern re-emerge.  Helical  and pull-out type focusers are notorious for their ability to trick the eye, so keep this in mind.