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