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Equatorial Platform Autoguiding

written by Del Johnson

Introduction

The combination of the Osypowski Dual-Axis Equatorial Platform (hereby referred to as Equatorial Platform) and the Dobsonian results in a very powerful and stable platform for long exposure astrophotography. This stability and the lack of perceptible periodic error and vibration makes for easy guiding with a crosshair eyepiece. Nevertheless, one tends to tire after a couple of 20-minute exposures. Fortunately, the Equatorial Platform is plug-in compatible with the SBIG ST-4 autoguider. One hour exposures are now practical with the comfort of the ST-4 doing all the work. It is a real hoot to be walking around or relaxing in a chair while the ST-4 carefully guides the next photo with great precision. Unfortunately, there is a learning curve associated with the ST-4 and not everything is explained clearly in the SBIG literature as the majority of the ST-4 user manual covers imaging techniques rather than guiding techniques. It took me several tries to get it right and I would like to pass on the results of my trials to others.

The comments below pertain to prime focus photography with a 12.5" (31cm) f/5 Dobsonian reflector driven by an Equatorial Platform. Guiding is done with the ST-4 mounted on an off-axis guider rather than a separate guidescope. The off-axis guider eliminates mechanical flexure and ensures that the focal length of the guider is sufficiently long compared to that of the camera. It is not necessary or desirable to boost the guiding focal length of the ST-4 with a Barlow in the off-axis configuration, as one might do when guiding manually. My personal equipment list includes an original Olympus OM-2 camera body with the Olympus #1-8 focusing screen (focusing is another topic), the Lumicon 2-inch Newtonian Easy Guider and Coma Corrector, and a generously large 3.1" secondary mirror. Don't be worried about the effect of obstructions on long exposure photography. The size of the star images will be determined by the focusing and the guiding errors, and not by the size of the Airy disc! The large secondary mirror and coma corrector produces an evenly illuminated exposures with tack sharp star images across the entire 35mm film frame.

Orientation of The ST-4 Camera
Here is a picture showing Del Johnson's telescope and Platform set up for a night of astrophotography.

I had previously read both that orientation of the ST-4 guider is important and that it is not important. The fact is that it is important, but that there is allowance for some error. The ST-4 will not calibrate if the sensor array pattern is rotated 45 degrees with respect to the right ascension (RA) direction. It must be close to, but does not have to be exactly aligned with, the RA direction. This is done by noting the RA direction by driving the guide star while viewing it with the reference eyepiece (described later on) using the hand controller supplied with the Equatorial Platform, and then visually aligning the serial number label on the ST-4 camera unit the observed RA direction. Do not use the declination direction as a reference as the pseudo-declination drive built into the platform is not always the true declination direction. The two platform drive directions will be perpendicular near the meridian, but not so at large hour angles (RA is always true). This has not been a problem for me, but could cause calibration to fail in an extreme situation which is probably not a good service position for photography anyway. If orientated correctly then 'X' or '1' will correlate with right ascension and 'Y' or '2' will correlate with declination.

Acquiring And Focusing On The Guide Star

It is frustrating to find insert the ST-4 into the off-axis guider only to see a row of zeros on the console display in the Find and Focus mode. It will happen more often than not unless the user employs repeatable techniques for acquiring and focusing on the guide star. The following tips outline my methods:

Tip #1 : Don't guess. Use a star charting program like TheSky to locate guide stars prior to the photo session using field of view indicators. TheSky does not have an offset field of view indicator per se, but it is possible to construct an offset field box with overlaid circles and rectangles. You will have to determine your own dimensions based on your actual off-axis guider and focal length. In the illustration below I have positioned a suitable guide star in the tracking box above the film frame while keeping the subject centered in the film frame. Keep in mind that the image is flipped in the camera's viewfinder, so the guide star will appear to be opposite the off-axis guider pickoff prism when composing the photo through the viewfinder.

Tip #2: Use a reference eyepiece in the off-axis position to center the guide star prior to inserting the ST-4 camera. The eyepiece field of view should be large enough to allow the user to easily find the guide star, but not so large that it is difficult to center the guide star onto the ST-4 sensor array. I use a 13mm Plossl which covers most of the pickoff prism. I previously found that a 20mm Plossl had too large of a field for centering the star. A 12mm guiding eyepiece with a nonadjustable (always centered) reticle would be ideal. Do not use a wide angle eyepiece. It was necessary to use a one inch extension tube and a locking collar ring on my eyepiece to secure it in the off-axis guider holder in a position parfocal with the ST-4. The parfocal reference eyepieces sold by Software Bisque and others will not work with an off-axis guider as neither the ST-4 nor the reference eyepiece will be fully seated when focused.

Tip #3: The focus of the ST-4 remains fixed relative to the camera with the Lumicon Easy Guider even if one slides the ST-4 holder around to center the guide star. One can preserve the focus location on the ST-4 with a locking collar ring for future reference once focus is found by trial and error. Obviously, one must focus the camera first and then focus the ST-4 by sliding it in an out of the holder. Once marked, it is not necessary to refocus the ST-4 independently of the camera on subsequent photo sessions. In my case, the ST-4 was backed out about 5/8ths of an inch to reach focus. The locking collar ring saves a lot of work. It was also necessary to purchase the 1/2 inch extension tube for the ST-4 to secure it in the OAG holder at the focus position. Both accessories are sold by SBIG and are highly recommended. Now I just focus the camera and drop in the ST-4 up to the collar ring. The ST-4 is now at focus and ready to go.

ST-4 Parameters

The following parameter settings work well for me. The actual displayed brightness value depends on the combination of Exposure Adjust, Boost and Brightness Adjust settings. Strive for a value of 8 to 15 with bA=F (recommended) or a value of 15 to 40 with bA=A while in the Find and Focus mode. I have noticed that the lesser bA=F values are transformed to larger numbers when transitioning from the Find and Focus to the Calibration mode. You must take a new dark frame if Exposure Adjust, Boost or Brightness Adjust are changed.

EA - Exposure Adjust. This is the exposure (integration) time, ranging from 0.1 to 20 seconds. Start with 1.0 seconds. Try to stay below 2 seconds as longer exposures may not allow for quick reaction to guiding errors or disturbances such as wind. Requires a new dark frame when changed.

b - Boost. This is the gain setting, ranging from 1 to 4. Start with b=2. A larger value will increase the displayed brightness value. Requires a new dark frame when changed.

bA - Brightness Adjust. Allows the user to replace the signal from a single pixel (bA=A) with the sum of a 3x3 grouping of pixels (bA=F). The idea is that the star image from a long focal length telescope may not fit into a single pixel, and that the 3x3 summation will do a better job of collecting more of the star light. This works very well and DOES NOT reduce guiding accuracy, as was erroneously reported in another web article on ST-4 autoguiding. I routinely achieve sub-pixel guiding accuracy with the 3x3 summation enabled, typically averaging an adjustment of only 2/5ths of a pixel correction. Setting bA=F not only amplifies the light signal, but also reduces problems associated by the star being located right on the boundary between two adjacent pixels. Set bA=F. If the guide star is too bright then reduce EA, b or use a fainter guide star. Requires a new dark frame when changed.

SA - Scintillation Adjust. This parameter is both poorly named and poorly executed. SA has nothing to do with compensating for star scintillation. What it does is attempt to dynamically improve the drive calibration during tracking. What it does in practice is become confused and drive the telescope away from the guide star. I have verified this behavior by observing one star on a crosshair while the ST-4 guided on another star. Any disturbance, such as a brief moment of wind, and all is lost. The aggressive default setting of SA=2 is a most unfortunate decision by SBIG. Every photo that I have attempted with the default setting was ruined by the unstable SA algorithm. It is STRONGLY recommended that the user set SA=10, which essentially disables the SA feature.

FL - Focal Length. This sets the size of the tracking box. Setting FL= S results in a 9x9 box, while FL=L results in a larger 33x33 tracking box. This has no effect on tracking accuracy. It simply allows more movement before declaring the star lost. The readout overhead with the larger tracking box is very small, so I always use the larger box at FL=L.

C1, C2 - Calibration 1, 2 . Amount of time in seconds that the drive is driven in the RA and Dec directions to establish a calibration. A larger value will improve the calibration accuracy, but may run the star off of the sensor array. Suggested values are C1=10 and C2 = 10.

H1, H2 - Hysteresis. Tom's great platform drives have virtually zero backlash so there is no hysteresis that requires compensation. Suggested values are H1=0 and H2 = 0.

AA - Average Adjust. Will average multiple readings before making an adjustment. This is useful to minimize unnecessary corrections if the exposure is less than one second. Otherwise, set AA=1 so that corrections are made after every exposure.

AL - Alarm. Will trigger an alarm after the specified number of missed readings. This might occur if a passing cloud dims the guide star. The default value of AL=4 is fine.

Final Comments

Don't forget to switch the hand controller cord with the ST-4 controller cord after the star has been centered with the hand controller. You will fail calibration if the hand controller is still plugged into the drive panel. I have done this several times. Watch out for the end of declination travel. The Equatorial Platform has only a limited range of declination travel, so it is best to center the declination cam prior to each exposure. This is easily done marking the movement extents of the lifting azimuth pad on a reference block of wood. Having said this, if you are consuming large amounts of declination travel during the exposure then your polar alignment needs to be improved. Running out of declination travel will cause the ST-4 to abort and reset. Use the star drift method for polar alignment for long exposure photography. Most references erroneously state that the reference stars must be near the equator. This not true. Star drift is surprisingly insensitive to declination as long as one avoids the polar regions. The altitude adjustment star should be +/- 6 hours from the meridian, which means that it will be far removed from the equator for users in temperate latitudes in order to maintain a reasonable air mass. Put a piece of tape on the ground and mark the azimuth adjustments as you go along. Adjust the azimuth until the star actually reverses the drift direction so that you positively know that you have bracketed the correct position. The final adjustment should be within a millimeter or two of platform movement. Use similar techniques for the altitude adjustment, but always make the final adjustment in azimuth as this is more important if the subject is anywhere near the meridian. Be extra careful with stray light sources. The flashing LED display on the ST-4 console spoiled several of my photos. I now cover the console with a towel and use a viewfinder cover on the camera (the Pentax viewfinder cover fits on the Olympus OM-2). Sometimes the ST-4 corrections are limited to 0 or 5 with nothing in between (whole pixels). Reducing the brightness signal with EA or b and taking a new dark frame will put things back to normal. It has been stated on another web page that relay sticking is a problem, though I have not experienced this. SBIG claims that this has not been an issue. The original IDEC RD2N-1U-12V relays are out of production and have been replaced by Omron G5V-2-DC12 relays. Final tip. It is real pain to connect and disconnect the power and data cables for each session. I keep the ST-4 system is a sufficiently large box which allows me to keep the cables connected all the time.

Other Sources Of Information

http://www.sbig.com/sbwhtmls/softpage.htm

The CCDTRACK guiding software for DOS is now a free download from SBIG.

http://www.astrocruise.com/st4tips.htm

Great source of information but I disagree with James Janusz's comments regarding Brightness Adjust and Scintillation Adjust. I find that bA=F guides better than bA=A and that Scintillation Adjust should always be set to '10' (disabled) when using an off-axis guider and the Equatorial Platform.

http://www.astrocruise.com/guide.htm

I do agree with most of Philip Perkins' statements, except that the 50% Star Brightness problem has been fixed by SBIG in version 4 of the firmware ('Fr 4' is displayed on power up). The guider will sound an alarm if the brightness drops below 50% of the original value, but will no longer abort.