RE: [ATM] Tracking Systems for Astrophotography

[artbianconi@blast.net]

Thanks Jeremy

I need to sit down and read this a few times to make sure I get 
it.

Art

From:           	Royston_Jeremy@emc.com
To:             	artbianconi@blast.net
Copies to:      	atm@atmlist.net
Subject:        	RE: [ATM] Tracking Systems for Astrophotography
Date sent:      	Thu, 23 Sep 2004 18:42:18 +0100

Hello Art,
 On the various points:
Atmospheric refraction
 The apparent altitude of the star differs from the geometric
altitude (calculated from RA, dec, local siderial time) according
to the zenith distance, temperature and site altitude. It is zero
at the zenith and about 30 arc minutes at the horizon. In general
there is a refraction correction in declination and hour angle.

Misalignment of the polar axis
 We need a tracking accuracy in the order of 1 arc second. It is
 very
difficult to achieve this accuracy in the polar alignment.

Flexure
 The flexure of the OTA and the whole mounting is certainly more
 than
1 arc second. It changes with the pointing direction. We can
almost eliminate the effect by using the same optics for guiding
and photographing, but it doesn't help with wind flexure.

Seeing
 Image blurring is not the only seeing effect, tere is also image
motion.

I didn't say that it's impossible to produce an autoguider, but
that it's a non-trivial project. One has to have a detector which
measures the xy position of a very faint star image, and also
solve the servo control problems for driving the telescope. It's
not so difficult for the hour angle, the drive is always moving 
in
the same direction. The declination correction can be in both
directions, so you have to construct it to be entirely free of
play, or compensate for play. 

The comparison with aircraft autoguidance isn't quite valid. It
doesn't matter if a plane is a few meters off course, or the
heading is a bit wrong (except on landing). If we had the same
accuracy in astronomical photography the photos would be very
blurred.

Best wishes, Jerry

-----Original Message-----
From: artbianconi@blast.net [mailto:artbianconi@blast.net] 
Sent: Donnerstag, 23. September 2004 17:51
To: Royston_Jeremy@emc.com
Cc: atm@atmlist.net; Herbjohnson@comcast.net
Subject: RE: [ATM] Tracking Systems for Astrophotography


I am not sure I understand the things you say need "compensation"

How does one compensate for "atmospheric refraction"

How does one compensate for "seeing"? I am not even sure I 
understand what that means.

If, by "misalignment" you mean deviation from the polar axis, 
that
does not apply for this reason:  the telescopes axis of the
rotation only need be coaxial to the Polar axis in a single motor
system. I made it clear that I was discussing a system that would
track independently of assumed polar rotation or location and
strictly on the displacement of the viewed object from x1,y1 to
x2, y2.

If flexure is as substantive as you suggest, me thinks perhaps
that the OTA is not properly built or adequately designed. Surely
you are not intimating that serious astrophotography is being
undertaken with a $4.oo cardboard tube with 50 cents worth of
shellac on it!

Not that it matters because if the sensor is part of the same
architecture as the mirror, then flexure of the telescope and of
the mount have already been compensated for and both are seeing 
or
should be seeing the same result. Or, to put it in simpler terms,
you wouldn't mount your sights on your neighbors rifle and expect
to hit anything with yours.

You can, for a modest amount, equip even the most mundane of
todays small aircraft, with a  2 or 3 axis autopilot that will
track the aircraft to a given destination across 3D space, while
maintaining a preset altitude. And, you can do it in spite of
enormous inconsistencies and variables that are not present in
astronomy. Turbulence, changes in wind velocity and direction and
ground speed being just a few. We do that sort of thing every day
and have done so with remarkable precision even before the advent
of Global Positioning Satellites. When EE's and other 
professional
tech types commented on the fundamental nature of the problems 
and
the solutions needed, the consensus was "piece of cake"

I am well aware of the fact that the pavement of an idea can be
seriously altered when theory meets application. I've been there
more times in my career than I care to count.

I will also acknowledge that there are things I don't know and
which I don't know that I don't know. 

That's why I am brainstorming with those who have played here 
more
than I and why I addressed this question to Mel, whose experience
in such matters is recognized. . . . so I can learn. 

Merely asserting that something can not be done, easily or at 
all,
is just a position, unsupported  by any logic. I want the logic.
That was one reason why I opened this string by requesting a book
to read, a plain english one that explains things.

At the risk of overstating the issue:

If a light sensitive device sees that an object in it's field is
moving and there is firmware designed to prevent movement and 
that
logic consists of applying equal and opposing forces, and there 
is
a feedback or "loop" circuitry between the sensor seeking "null"
and the driving force(s), the object should, at some point,  
cease
it's relative movement on the detector and achieve a motionless 
or
null state. We've used similar systems in manufacturing for at
least 30 years to achieve plant automation and, we consider them
to be dumb!

I do not see this as being a problem but, as I acknowledged 
earlier, I don't know what I don't know and I am trying to learn.

Please, . . . . .  don't just throw judgements or assertions at 
me
without explaining their evolution. Teach me please . . . that's
why I am here.

Art Bianconi

From:           	Royston_Jeremy@emc.com
To:             	artbianconi@blast.net
Copies to:      	atm@atmlist.net
Subject:        	RE: [ATM] Tracking Systems for Astrophotography
Date sent:      	Wed, 22 Sep 2004 13:48:24 +0100

Hello Art and all,
The rotation of the earth is well known, but we have to
compensate for misalignment of the mount, atmospheric refraction,
flexure of the telescope and seeing. This means that we have to
track a guide star accurately, and correct the drive
correspondingly. Suitably bright guide stars are seldom. It is a
non-trivial project!

Best wishes, Jerry

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On
Behalf Of artbianconi@blast.net Sent: Mittwoch, 22. September 
2004
14:04 To: atm@atmlist.net Subject: [ATM] Tracking Systems for
Astrophotography


Mel, in a recent response to my earlier question, you intimated
that the problems associated with tracking a moving object are
difficult to resolve and costly. 

While they are not astronomers, others, EE's for the most part,
are suggesting that the circuitry needed for accurate optical
tracking in an x and y axis does not require a novel solution.

They based their responses on what I told them. They understand
the motions of the earth. However, it may well be that I left out
some piece of information that distorts the problem.

What makes this effort appear so formidable?

Thanks

Art Bianconi

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