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This is an article I did for our society's
newsletter. It's a bit of a read but maybe there is some useful information here
for others.
Regards
Monte
Auto Guider Troubles
I thought the following tale might be of interest
to anyone using an SBIG STV or any other type of Auto-guider. The STV is a
digital CCD camera and Auto-guider built into one very neat box. Don't get me
wrong, contrary to what the title I'm very happy with it and so glad I could get
my hands on one. It is truly amazing to see what SBIG have been able to fit
into a small box and how useful and entertaining it can be.
For some months now I have been having a problem
with the unit when it is in the guide mode. It first happened one
night when I left it guiding and went into the house to get some sleep. I know
this sounds a bit hard to believe but the STV and the legendary ST4 were
developed so you could get a bit of shut eye (or at least that's my take on it).
When they are working properly and to be fair there are so many things that can
go wrong beyond the control of these machines, you can go to sleep and set
the alarm for when it is time to finish that shot, start a new one and go
to bed again.
The problem has been that at some time, normally
about 20 minutes into the shot while the unit appears to be happily guiding, the
DEC drive starts to run constantly and within a few seconds the shot
is ruined as the scope slews off the frame. Sooner or later I wander out to find
the mount pointing to the wrong part of the sky and the STV beeping its error
tone.
The first thing I did was turn off the guide
feature which should have stopped the DEC motor from running. Not so. I even
turned off the STV and the motor still kept running. However when I pulled out
the cable from the STV to my drive controller the DEC drive finally stopped. At
first with all big problems, you assume it is something simple and with a bit of
poking around you will solve the problem. I assumed the problem was a stuck
relay in the STV and carried the unit into the house for a closer
inspection.
The build quality and complexity of the innards is
to be admired and probably best not touched. That is all well and good when you
have a service centre near you and plenty of time. However the sky was clear and
everything was set up and ready to go so I decided to see what could be done. To
my relief SBIG had wisely made the relays replaceable. A relay is a little
switch that uses an electromagnet to pull a piece of metal towards it which in
turn closes the contact. It is just like an electric finger to press the "on"
button.
Cars use a large number of relays. They are a
great way of switching large amounts of current without it actually going
through the on/off switch itself. The ignition key is a prime example -
starting a car requires a few hundred amps. If you were to run this current
through the key mechanism, smoke would probably appear and the key would get
very hot. In the case of the STV these relays are designed to switch up to one
amp. This is a fair bit of current in telescope mount terms so I was a bit
puzzled as to how my controller (which on the line in question pulls 0.04 of an
amp could be locking up the relay).
Back to the story, I switched the positions of the
four relays so each was then doing a different drive direction. When I put the
STV back to work, it all went OK and satisfied I was victorious and
convinced of my genius, I started a new exposure and went to bed. You can
already guess what greeted me when I got back an hour later. The error tone and
my scope wandering lost in the Virgo cluster.
The night was a write off but I had a month
to do something so I ordered a few more relays from the states. They came to me
at light speed but for the convenience I was charged the deluxe ultra bucks
rate by Fed Ex. I wasn't given an option to get it sent by a cheaper
carrier.
I replaced the four incumbent relays with my
freshly imported upstarts and set off for the next new moon weekend thinking I
had won. Of course I was kidding myself, after twenty minutes of guiding the
problem was back. This time I wasn't silly enough to start a photo so no film
was wasted.
After packing the STV up, I carried it down to see
Mike Kerr. I told him my problem but asked if he would he be interested in
trying the STV's CCD on the prime focus of his 25" f5? I didn't have to ask
twice. Initially it wouldn't focus but we found the 2x Barlow solved the
problem. The light grasp was incredible and within a short time we had the
central band of Centaurus A filling the frame. The STV has a 5"
screen and it makes CCD imaging so much fun and very easy. The clouds
closed down our techno peep show and I trudged back to the house
and with the nights lesser moments still playing on my mind. The problem
with the mount had me stumped.
In the morning before the water for the
coffee was boiled I already had the cover off the drive
controller. Although the symptom was in the STV I was convinced
the cause was in the drive controller. This unassuming scratched
black box is over twenty years old and houses the circuits to drive
the synchronous RA motor and the stepper motor DEC drive as well as their four
relays, a voltage regulator and a rat's nest of wires. To be fair the rat's
nest is my work. When I bought it from Joe Cauchi, it was just an
RA drive controller and the mess was an integral part of
getting the mount ready to take auto-guided pictures and fit it all in the
same space.
I checked the current flowing in the relay lines
and while three of them showed a predictably low current flow, one was showing
an enormous 2 amps! I quickly pulled off the battery clips and relieved there
was no smoke coming of the circuit board - I sipped my coffee convinced
(for not the first time in this story) I had found the problem. But
the taste of victory lasted about as long as the taste of the coffee.
While I had indeed found a problem it wasn't the subject of
this tale one but one I manufactured at the time by allowing a
resistor on the stepper motor driver to break through the insulation and
short to the casing. It burnt out a component and left me high and dry
as far as auto-guiding went for the rest of the weekend. Because I had a
defective drive, the sky cleared up (there is a correlation) and so I took
some piggy back shots with a 200mm lens on my Pentax 6x7 camera. You can see the
result in the member's pictures section of www.asnsw.com . I used the STV as a kind of
guiding eyepiece and made manual correction based on the drift shown on the
screen. The pictures are great so I didn't feel I wasted the
weekend.
On returning to Sydney I found a replacement for
the damaged component (for those who want to know it was a $4 Darlington
driver IC to step up the power from the driver IC). The DEC drive kicked back
into life and I counted myself lucky I hadn't damaged anything else. Never
wanting to believe I still had no answer to the problem I convinced myself this
episode with the insulation was the culprit and the failure was just a chance
thing that happened when I was pulling the cover off. Every test I did showed no
problems. Every value with the multimeter was normal.
I was able to get up to Ilford the following
weekend at first quarter moon. It's not a bad time to go up because you can set
up during the first hours of the night bathed in helpful moonlight and
when you are ready, the moon exits stage left. I run two
batteries in my set-up. One is for the mount, heaters and drive controller
and the greedy STV gets it's two amps from the second one. After I dragged
them out and set everything up, I was ready to see if my theory was
correct. Twenty minutes into the guide the dreaded error tone started. It
was so cold and windy that going to bed defeated but warm wasn't such
a bad idea.
You have to have a good sense of humour to go
anywhere near astrophotography (even typing the word is difficult). Someone once
told me there is a fine line between a hobby and a mental illness and standing
out there in the dark and cold with gremlins climbing all over my mount and
laughing at me I decided I must have been barking mad. I suspected the
observers who always ask me why I don't just look through an eyepiece and enjoy
myself might have been right. Normally I would have thought that was a good idea
but I had been given a great honour and I was obliged to perform, a friend from
the US lent me his Takahashi FSQ-106 refractor as he wanted a series of
southern sky photographs for a planetarium production he is making. It is
probably one of the best 4" telescopes in the world. Having lost a
few months of clear skies and photo opportunities the
pressure to perform was mounting as the scope is going back to its owner in a
few months. The drive back home was made better by the fact
that my girlfriend drove and I watched the scenery and pondered the problem. For
a while, I developed a theory in my mind that the relays were getting some
interference from a transformer below them in the tight confines of the
box. There is a whole school of theory on inductance that I wont go into now but
for a while it seemed to be a possibility.
I spoke to my father later in the day who knows
more about this than me and he suggested the best thing to do would be to
isolate the two protagonists (the controller and the STV) and use optical
signals to pass on the instructions from the STV. The problem only ever occurred
when they were connected so if I could get the information passed on but have no
actual connection maybe that would solve it.
I also finally decided to contact SBIG and they
were quite helpful, so much so I felt a bit silly for struggling on solo for
such a long time. I thought the problem was not in the STV so I hadn't bothered
them with the details. They told me a charge can build up in the coils
of the controller's relay and that can cause a spark between the contacts
in the STV's relays making them hot enough to weld shut. This problem is well
known in electronics and is easily fixed with a diode parallel with the coil. A
diode is a like a one-way valve for electricity. Current can only go in one
direction and would stop the energy in the coil getting back to the STV.
I did this and with much help from my
father made the opto-isolator. Now the signal is sent between the two units
by light instead of electricity. One interesting spin off is the lights in the
opto-isolator use such an incredibly small amount of power ( less than
40 micro amps ) that the single D size battery will last for about
four years of constant operation. Given that I will use it for no more than
twelve hours per new moon weekend with a 50% duty cycle - it should last me for
around 520 years! For some strange reason, I still have a spare battery in
my tool box.
The result? Problem solved. I have run it for
hours in my garden at home and no error tone rings in my ears. I'm not
sure if there is a moral to the story. I guess it might be that you
shouldn't be afraid to ask other amateurs if you are being challenged by
some piece of equipment, chances are someone else has suffered from it
too.
You can see one of my Milky Way photos in the
society website - go to the members photos section. The picture has a "NEW" next
to it.
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