If you are just learning your ST-4, I recommend that you practice using it at home or at some close local site before going out to that cold dark site you love. It's amazing how much easier it is working where its reasonably light and somewhat warmer and how many tools, etc., you may need that are handy at home. This paper assumes that you understand the basics of ST-4 operation and does not necessarily go through all the steps necessary to perform an operation.
Remember to calibrate for the current sky location. If the seeing is excellent, setting this to a 2 or 3 should result in improved guiding as the ST4 will learn and correct for under/over corrections. In fair to poor seeing the ST4 will confuse seeing errors with guiding errors so this parameter should be set high.
I usually use 2 for excellent seeing, 5 for average seeing, 7 for poor
and 10 for those days when everything is moving. SBIG recommends
increasing this parameter on "longer focal length telescopes or poor nights."
Assemble your telescope and drive and make all necessary ST-4 connections.
Polar Align as accurately as possible, a few more minutes spent here drift aligning will help the ST-4 to reach its full potential. The idea is to require the ST-4 to correct only when necessary.
If possible, start your first attempt at using the ST-4 by using a guide
scope, or maybe practice with the main optics of your photographic instrument.
This is many times easier than starting with an off axis guider.
I will discuss both as I progress. Whatever you choose, try to use
a guiding focal length of less than 100 inches (2500 mm) this will make
learning easier. Compose your photo, focus the camera and select your guide
star, or a temporary focusing star, in the guide scope or off
axis guider. (Or the main optics if this is just a practice session) Use an illuminated reticle eyepiece to center the star. For a practice session or for focusing, select a reasonably bright star in spite of the tip below.
TIP. The ST-4 loves dim guide stars, the dimmer the better. The cause of this is a CCD characteristic called pixel saturation or blooming. If your guide star is too bright it will bleed to adjacent pixels. The ST-4 will then guide less accurately because it may incorrectly determine guide star movement based on changes in pixel saturation rather than actual drift. We are attempting to keep the star image in a single pixel. In operation just how dim a guide star you use will be a function of the guiding focal length and more importantly the ST-4 exposure time that your drive can tolerate without excessive drift.
Insert the ST-4 in the guide scope or off axis guider and orient the CCD head with the Right Ascension and Declination axis of your mount. Use the label on the CCD head for orientation as referenced in figure 1.
TIP. I believe that the orientation function is very important to guiding performance. This is because, if the ST-4 makes a correction, in RA for example, and the CCD head is incorrectly aligned it will also move in Dec. This will cause an error that also needs correction. The ST-4 is smart and will learn during calibration that this is happening and attempts to compensate, but why force the ST-4 to make unnecessary corrections. Always align the CCD head as accurately as possible.
Start the ST-4 and allow the CCD to cool down for about two or three minutes. Begin with all ST-4 parameters in their default start up settings.
Cover the optics and take a dark frame. Remember that without taking the dark frame the ST-4 will be "seeing" hot pixels that your scope doesn't. The ST-4 electronics causes them. The dark frame subtracts these pixels from the ST-4 exposure.
TIP. I will repeat this again
later because it is important to remember. Take a dark frame any
time you change any of the following ST-4 parameters. Exposure Adjust,
(EA Exposure Time) Boost Factor, (B) or Brightness Adjustment (BA Faint
Star Mode). When in doubt take a dark frame.
Fact. Focusing the ST-4 is THE most important function to become proficient at. The ST-4 will not track at sub pixel accuracy if it is using a bloated, unfocused, star. Again, the star image must be a pinpoint that is smaller than an ST-4 pixel.
The first time you focus for a given set up, guide scope, off axis guider, etc., will be the hardest. This first time you will most likely have no idea of where the ST-4 focus is (unless you have a parfocal eyepiece) and it will be a case of trial and error to find it. I recommend you start with focusing a guide scope or the main optics and keep off axis focusing for later. The ultimate goal of this practice session will be to develop a par focal eyepiece, preferably the illuminated reticle, which will allow you to get the star centered and close to focus each time.
The ST-4 has a "find and focus" function. This function takes repeated exposures, and displays the results on the ST-4 display panel. It displays the location of the brightest object on the CCD in x and y coordinates and it displays the brightness of the object expressed as a percentage of pixel saturation.
The x coordinate is RA and the y coordinate is Dec. if you correctly aligned the CCD head, as in figure 1. The brightness value is a percentage of the maximum pixel value of 256, so if it displays a 50, it is indicating that the brightest pixel it has identified has a value of 128.
Start the find and focus mode by pressing its key and watch the display. At this point the most likely thing that will happen is you will see the display report all zeroes. Any, or all, of three things can cause this. The star is not on the CCD, the star is badly out of focus, (very likely) or the star is too dim for the 1 second exposure time, which is the default. Hopefully you removed the cover used for the dark frame.
If you are reasonably confident that your mount is accurate and stable and that the star remains centered on the CCD, check it again anyway. This will eliminate this possibility. If it is off the CCD, the cause may be periodic error or you may have inadvertently moved the mount. If periodic error is the cause, lowering the guiding focal length may help but, my best advice is to get rid of the error. If the error is large enough to cause the star to drift off the CCD you will have consistent problems working with the ST-4. Substituting the ST-4 head for the reticle eyepiece may cause some fork mounts to deflect, this will cause the star to appear centered when it really is not. You will never be able to easily obtain a guide star unless you correct or adjust for this.
You should have selected a reasonably bright star, one that looks bright even with the reticle light turned all the way up. If you think it may be too dim stop the ST-4 by pressing interrupt, select menu, increase the exposure time using the adjust key, to four or five seconds, press interrupt again, take a new dark frame and restart find and focus mode. If it still reads all zeroes reset the ST-4 to the one second exposure time, take another dark frame and continue.
There are two ways to focus the ST-4, trial and error or with the use
of a computer and the SBIG software provided. If you are lucky and
have a computer, it will display an image of the de-focused star and allow
you to quickly focus. Use the computer to assist in obtaining critical
focus. See the ST-4 manual for instructions. I will not discuss computer
focusing in this paper. The trial and error method usually works
best with a power focuser such as those available from Jim's Mobil, but
can do it manually.
Focusing by trial and error is painful, but it is also the only way to begin and need only be done once if you follow my advice. Try moving the focus out first if you used a 12.5 mm reticle. Move the focus a small amount each time.
Allow the ST-4 two or three cycles after each focus adjustment to be certain it obtained a steady exposure. Eventually you will begin to get value and x y readings on the display. When you do, this is a good time to center the star on the CCD using your hand controller. Try for a reading of 50 50 plus or minus 10.
TIP. Schmidt Cassagrains and off axis guiders are the hardest to focus. The SCT when out of focus presents a diffuse ring of concentric circles with a dark hole in the middle that the ST-4 does not see. The off axis guider prism can split the image into two half circles causing the ST-4 to try and guide on alternate halves depending on which was brightest on any given exposure. Refractors have the best chance of showing a value when somewhat out of focus.
Once you have obtained a value readout, continue to adjust the focus to obtain the maximum value. If you get a value over 75 your star is too bright to obtain critical focus. Reduce the exposure time or select a dimmer star and start over again. Optimally you are looking for a maximum value of about 50 when accurately focused on a test star.
At this point I recommend that you identify a par focal eyepiece.
It may require using extension tubes or stop rings to accomplish. (SBIG
recommends simply scribing a line on the eyepiece barrel.) This will
eliminate trial and error focusing for subsequent ST-4 sessions.
I have addressed this problem by securely installing a short extension
tube to my ST-4. (Adding about one inch to its overall length.) This
did three things for me, it protects the end of the ST-4 from abuse, it
addition of standard eyepiece filters to the ST-4, such as a moon filter, to reduce the value of bright guide stars, (my early ST- 4 used a non standard thread) and it allowed me to set my 12.5 mm reticle eyepiece par focal using an extension and stop ring. Update: I am now using a Software Bisque focusing eyepiece.
I now quickly focus my ST-4 by combining focusing and centering of the guide star into one simple operation.
Focusing an off axis guider is very similar to what we have discussed.
The differences are that you can't change the telescope focus and your
guide stars are usually very dim. (Which, as I said is good.) It
will require extension tubes of various lengths to focus an off axis guider,
such as the Lumicon Giant Easy Guider. Depending on whether you are
guiding at equal or longer focal ratios (as compared to the main optics)
will determine the length required. Equal ratios require shorter
extensions. I recommend developing the par focal eyepiece before attempting off axis focusing as it will quickly get you close. To obtain critical focus requires sliding the ST-4 in or out of the extension tube.
TIP. Remember to document and save the extension tube length you used. I also recommend using double set screws on all extension tubes to eliminate any slippage. If your particular system requires a very long extension to reach focus, be particularly aware of any possible flexure the ST-4 head weight may cause. This will cause the star to deflect off of the CCD. Brace the system to eliminate any flexure. A star diagonal may reduce the required length.
TIP. Recheck focus before each exposure. Conditions change, particularly if you move to a part of the sky with different seeing conditions.
You now should have the star in focus and your camera ready to begin an exposure. If you selected a bright practice star to focus on and want to move the system to the photo object and a new guide star, do so now as the next steps are to set up for tracking.
I define optimal ST-4 performance as tracking at an average correction rate of less than two ST-4 units, when using equal photographic and guiding focal lengths, over an extended period of time. Since the ST- 4 averages errors using signed numbers, an average of two is usually plus or minus two, or a total of four ST-4 units. One ST-4 unit is equal to .2 pixels, or 3 microns, (.003 mm). 4 units times .2 pixel equals .8 pixel. This is the sub pixel accuracy I referred to earlier. It is also close to the resolution limit of Technical Pan 2415 film and below the 15 micron resolution of many color films. Compare this to the 40 micron best case manual guiding.
Optimal performance will result from a balanced ST-4 and drive system. Each system will have a performance limit determined by the inherent accuracy of the drive, stability of the mount, weather conditions, polar alignment, ST-4 calibration and ST-4 performance. The ST-4 will correct for many of these conditions, but, in doing so will very likely not perform optimally. You should address each of the factors under your control to increase performance.
Assuming that all other parts are performing optimally we can adjust the ST-4 parameters which effect performance. Remember to select a dim guide star or use a lunar filter. With an 8 inch SCT as a guide scope and a one second exposure, this would mean about a ninth magnitude star. As discussed earlier, getting the star image to fit within a pixel is critical. Do this by using the EA, B, BA and AA parameters in conjunction with critical focusing.
Using these parameters, adjust each to obtain a value between 10 and
25 as your system allows. There is not a single solution to the problem.
I attempt to use the exposure time and boost at first. Select a star
and in individual steps, increase boost up to three and EA up to four or
five seconds (or as long as your drive will allow). I use BA on faint
only if a brighter guide star is not available. I currently try to
optimize corrections at around 1 to 3 seconds either by long exposures
a dim star or short, averaged exposures of brighter stars.
TIP. Remember to take a dark frame any time you change any of the following ST-4 parameters. Exposure Adjust, (EA Exposure Time) Boost Factor, (B) or Brightness Adjustment (BA Faint Star Mode). When in doubt take a dark frame.
TIP. If the system seems to act strange when tracking, re-calibrate. If the ST-4 reports any error conditions after calibration, refer to the manual under problem solving. The most common error is inadequate correction in one or more axis. This should not happen if you increase the C1 and C2 settings.
TIP. If large tracking errors alternate with periods of normal tracking, you may have more than one star, of similar magnitude, on the CCD. This will cause the ST-4 to guide on one star until it becomes fainter than the other, then detect the second star and move to it as if it was the first star. The solution is to be certain only one star is on the CCD. This may require placing the selected star off of the CCD center
The following list is a summary of the items we have discussed.
Clear skies and success with your ST-4.
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© Chris Cook 2001