by James J. Janusz
Anyone who has learned to effectively use the ST-4 as an autoguider would agree that the documentation provided leaves much to be desired. There is little information regarding basic setup of the ST-4 with a guide scope or an off axis guider. Performance variations caused by the interaction of factors such as, the effective guiding focal length, the guide star brightness, the CCD head orientation, the sky conditions, the drive performance, and the film resolution, are non existent. It's a wonder anyone ever gets the perfect results the ST-4 is capable of. Santa Barbara Instruments Group (SBIG) indicates sub pixel guiding accuracy is possible and I concur. I will discuss how this is possible later.
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.
Our goal is to allow the ST-4 to perform at its maximum capabilities.
To do this, we should understand all of the menu parameters, when to adjust
each one, and the effect each adjustment will have. Be sure to read and
understand the SBIG documentation on tracking and all of the parameters.
The following discussion of the parameters is subject to revision, as it is
based on my personal experience with the ST-4.
This allows you to select the ST-4 exposure time from 0.1 to 20 seconds.
This is one of the key parameters used to control the image of the guide star
on the CCD. The tracking accuracy of your drive will determine how long
an exposure you can take on faint guide stars. If the exposure goes beyond
the drive's capability to track accurately, excessive errors will occur and
be visible in the photograph. Assuming an ST-4 exposure begins concurrent with
periodic error movement, the exposure must
be completed in time to correct it without noticeable error on the film.
This controls the gain of the ST-4 internal amplifier to boost the signal of
faint stars. Settings are 1 to 4. Use the boost in conjunction with Exposure
Adjust (EA). to obtain a minimum value of 15 and a maximum value of 40 for long
photographic exposures. (I have successfully used the ST-4 below a value
of 10 but you risk losing the guide star.)
Avoid using this parameter unless no usable combination of Exposure Adjust
and Boost will result in a satisfactory value, or your system has a guiding
to photographic focal length advantage. This parameter will effect the
sub pixel accuracy you desire as it replaces each pixel with the sum of a 3
by 3 box centered on the pixel. It does work well on very faint stars,
so I recommend using it if you understand its effects. For example, I
use it when a five second Exposure Adjust and a Boost of
4 result in a value under 8.
This parameter determines if you will allow the ST-4 to modify its calibration settings on the fly during an exposure. Set it to the number of pixels error, between successive exposures, required before the ST-4 assumes the calibration factors are in error and modifies them. A setting of 1 indicates you would like this done often, a setting of 10 effectively turns it off. Remember, 1 pixel is equal to 5 error units.
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."
This parameter controls the size of the track box. S for short uses a
9 by 9 pixel box and L for long uses a 33 by 33 pixel box. Here again
the accuracy of your drive controls the use of this parameter. If your
system moves more than 9 pixels between ST-4 exposures, because of guiding focal
length or drive error, use the long setting. I have successfully guided
at 2000 mm and 2800 mm using the short setting. Selecting the long setting is
acceptable as it will not effect guiding accuracy.
SBIG recommends using the long setting for guide telescope focal lengths over 50 inches (1250 mm).
This parameter controls the time the ST-4 moves each drive axis during calibration.
Calibration is necessary for the ST-4 to learn the correction speed required,
for your particular system to move a star n number of pixels. You should
calibrate each time you begin a photograph as calibration is sensitive to the
sky location. I recommend that you experiment with how long a calibration
time you can use without moving the star off the CCD or beyond 30 units.
Start calibrating with the star close to the CCD center. I recommend using whatever
time produces at least a 10 count movement, because it seems to produce more
accurate guiding. This is just intuition on my part.
Same as C1 for the other axis.
This is provided to compensate for backlash in you drive system. If you
require a setting other than zero get your drive tuned up. A good drive
has close to zero backlash. This setting is critical to those mounts with declination
worm gear systems as their is usually backlash to account for. Friends
of mine who have these systems prefer to let the ST4 adjust for this rather
than use the drive corrector.
Same as H1.
The number of exposures the ST-4 should average before making a correction.
This is a great parameter for good drives and or bright guide stars. I
do not recommend making a correction each time, if your exposure time is less
than 1 second. Averaging corrections allows for minimizing "seeing"
induced errors and eliminates over correction caused by short exposures.
My system typically allows me to correct within 5 seconds and obtain excellent
photographs. I normally set AA to allow a
correction between 1 and 5 seconds.
If the ST-4 begins to lose a star, usually because a cloud drifts by, it will sound its alarms after n number of cycles are unsuccessful. I normally set this to the default value.
I will assume that the ST-4 is compatible with your current telescope drive. I use a German Equatorial mount but, I don't think there are any operational differences when using other common types of mounts.
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.
Focus the ST-4. It seems simple doesn't it, lets move on. Not!
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 you
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 allows the
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.
When the ST-4 powered on during focusing you used the default parameters. Now it is time to fine tune these parameters for optimal tracking performance.
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 of
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.
Before attempting to track a star, you must calibrate your system on the guide star. Use the longest time in each axis that your system will support, without exceeding 30 units as mentioned in the parameter section. Always start with the star close to center and the CCD head oriented accurately.
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.
After setting all the parameters, start the tracking mode and observe the system performance. Typically the ST-4 will settle into a rhythm after a few corrections. If the average error is beyond what you feel is acceptable, stop tracking and repeat the fine tuning process. Remember the average error will be high even if the ST-4 is perfectly tuned if the entire system is incapable of delivering optimal performance
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.