RE: [ATM] Tracking Systems for Astrophotography

[Royston_Jeremy@emc.com]

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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