Some basic questions

I have some questions which I hope the community at large can help me
answer:

1. I see no mention made to the reference point used in the computation of
the orbits and positions of stellar objects. I am curious to what the "start
time" is, and how or where we obtain the positions of every object at that
start time?

2. I read that there are about 100 billion galaxies each with about 400-500
billion stars. And I see star catalogs, but I know that none of them are
near 10E+20 bytes large. My first question is a) who decides which stars
get cataloged. Or does every visible star get cataloged? My bigger
question is b) how do two people know that they are talking about the same
star? Given the distances, the angular differences must be too little to be
useful, I assume.

3. I see fantastic telescopes with lots of gadgetries in Sky & Telescope.
I can do ray tracing for a theoretical Newtonian or Cassegrain (learned that
in high school). However, what are those short stubby gleaming telescopes
with coiled cables and a keypad? There are lots of advertisements on these,
so I assume that many people must own them. Maybe because they are so
common, the advertising copy is, so as not to offend the majority,
deliberately snobbish to leave out any clues on how they work or what they
are supposed to do.

4. I bought myself a 4" Meade Newtonian with an equatorial mount. I did
not invest a lot of time in it because I got frustrated with not
understanding how the equatorial mount would ever allow me to track an
object with any precision. I mean, how is it possible to align with the
accuracy that would match the earth's axis exactly! On that issue I always
wonder how people take pictures. How could a motor move exactly in step
with the earth's rotation and so smoothly that it would allow a perfectly
stationery picture to stay in view for a few minutes?

I hope I will be enlightened shortly. I have never attended any astronomy
classes but am not challenged in areas of general science or technology, but
sometimes I am just too lazy to inquire sufficiently to derive answers by
myself.

Thanks in advance.

1 = dunno!
2 = we can only catalog that which we can see!
3 = if you can raytrace a newt or cassegrain then you should also know that
a short tube = cassegrain/maksutov(generally)!
4 = because the planet turns at roughly the same speed as a clock and drift
alignment allows us to get the RA axis in line with the rotationary axis of
the planet!

Ok?

Inexperienced Rob

"John Smith" wrote in message
...
I have some questions which I hope the community at large can help me
answer:

1. I see no mention made to the reference point used in the computation
of
the orbits and positions of stellar objects. I am curious to what the
"start
time" is, and how or where we obtain the positions of every object at that
start time?

2. I read that there are about 100 billion galaxies each with about
400-500
billion stars. And I see star catalogs, but I know that none of them are
near 10E+20 bytes large. My first question is a) who decides which stars
get cataloged. Or does every visible star get cataloged? My bigger
question is b) how do two people know that they are talking about the same
star? Given the distances, the angular differences must be too little to
be
useful, I assume.

3. I see fantastic telescopes with lots of gadgetries in Sky & Telescope.
I can do ray tracing for a theoretical Newtonian or Cassegrain (learned
that
in high school). However, what are those short stubby gleaming telescopes
with coiled cables and a keypad? There are lots of advertisements on
these,
so I assume that many people must own them. Maybe because they are so
common, the advertising copy is, so as not to offend the majority,
deliberately snobbish to leave out any clues on how they work or what they
are supposed to do.

4. I bought myself a 4" Meade Newtonian with an equatorial mount. I did
not invest a lot of time in it because I got frustrated with not
understanding how the equatorial mount would ever allow me to track an
object with any precision. I mean, how is it possible to align with the
accuracy that would match the earth's axis exactly! On that issue I
always
wonder how people take pictures. How could a motor move exactly in step
with the earth's rotation and so smoothly that it would allow a perfectly
stationery picture to stay in view for a few minutes?

I hope I will be enlightened shortly. I have never attended any astronomy
classes but am not challenged in areas of general science or technology,
but
sometimes I am just too lazy to inquire sufficiently to derive answers by
myself.

Thanks in advance.

Wasn't it John Smith who wrote:
I have some questions which I hope the community at large can help me
answer:

1. I see no mention made to the reference point used in the computation of
the orbits and positions of stellar objects. I am curious to what the "start
time" is, and how or where we obtain the positions of every object at that
start time?

Astronomers call this start time the "Epoch". For day-to-day purposes
the differences due to precession and the proper motions of stars are
usually negligible. Different catalogues can use different Epochs.

Full details he
http://en.wikipedia.org/wiki/Epoch_%28astronomy%29

2. I read that there are about 100 billion galaxies each with about 400-500
billion stars. And I see star catalogs, but I know that none of them are
near 10E+20 bytes large. My first question is a) who decides which stars
get cataloged. Or does every visible star get cataloged?

Each team that creates a catalogue will have their own criteria for what
gets included. There's no overall set of rules that told, say, the
Tycho2 team not to include stars dimmer than a certain magnitude. They
just decided to catalogue the 2.5 million brightest stars.

My bigger
question is b) how do two people know that they are talking about the same
star? Given the distances, the angular differences must be too little to be
useful, I assume.

The angular information is sufficient for current purposes. If the
pointing accuracy of a particular telescope isn't sufficient, then the
users would look for nearby brighter reference stars in the
neighbourhood and work from those positions. If you take a picture that
includes the area of the target object and also includes reference
stars, then you can measure the distance between them with very high
precision.

--
Mike Williams
Gentleman of Leisure

"John Smith" wrote in message
...
I have some questions which I hope the community at large can help me
answer:


Some replies, maybe not a complete answer:

1. I see no mention made to the reference point used in the computation
of
the orbits and positions of stellar objects. I am curious to what the
"start
time" is, and how or where we obtain the positions of every object at that
start time?

If you are referring to the proper motions of stars, a catalogue that
provides this data usually quotes a reference date; for example, the
Hipparcos catalogue of positions and proper motions has an epoch 1991.5 (or
something like that--I may have the date off a bit). Thus to calculate the
position 10 years later, add 10 x annual proper motion to the coordinates.
Some older catalogues have an epoch for the equator and equinox date, such
as 1950.0, but individual stars may have other dates which are the zero
points of proper motion for that star. These will be stated next to each
star's data.

Orbits of binaries are usually referenced to a convenient epoch of
periastron passage for elliptical orbits, or to some other standard epoch
for circular orbits.


2. I read that there are about 100 billion galaxies each with about
400-500
billion stars. And I see star catalogs, but I know that none of them are
near 10E+20 bytes large. My first question is a) who decides which stars
get cataloged. Or does every visible star get cataloged? My bigger
question is b) how do two people know that they are talking about the same
star? Given the distances, the angular differences must be too little to
be
useful, I assume.


All catalogues are compiled to be complete only down to some manageable
fainter magnitude limit. For example, the Bright Star Catalogue is complete
to about mag 6.5, the HST Guide Star Catalogue to about 16 or so, etc.

Fainter than that, the stars are uncatalogued except for those in special
objects where individual catalogues are created for study purposes, e.g.,
the stars brighter than the limiting magnitude in a globular cluster.

3. I see fantastic telescopes with lots of gadgetries in Sky & Telescope.
I can do ray tracing for a theoretical Newtonian or Cassegrain (learned
that
in high school). However, what are those short stubby gleaming telescopes
with coiled cables and a keypad? There are lots of advertisements on
these,
so I assume that many people must own them. Maybe because they are so
common, the advertising copy is, so as not to offend the majority,
deliberately snobbish to leave out any clues on how they work or what they
are supposed to do.


You are thinking of the usual Schmidt-Cassegrain design, in which a
corrector plate removes aberrations and the field is flat rather than curved
as in ordinary Schmidts. Or possibly the Maksutov design, which is
different but does a similar job.

4. I bought myself a 4" Meade Newtonian with an equatorial mount. I did
not invest a lot of time in it because I got frustrated with not
understanding how the equatorial mount would ever allow me to track an
object with any precision. I mean, how is it possible to align with the
accuracy that would match the earth's axis exactly! On that issue I
always
wonder how people take pictures. How could a motor move exactly in step
with the earth's rotation and so smoothly that it would allow a perfectly
stationery picture to stay in view for a few minutes?


If you are taking it out at night and putting it away each time, you will
never achieve the alignment of the axis that you want. Getting to within 1
arcmin of the correct alignment is difficult even for a permanently mounted
scope. But computer controls on telescopes allow you to at least find
objects, though it is true that photography will still be a challenge due to
poor alignment.

Motors are usually pretty good, though there are issues with "periodic worm
errors" that make the drive oscillate around the correct position in the
sky.

If you can permanently mount your scope you can get the alignment very close
using the drift method. I like to use Polaris for this. If it drifts
horizontally in the field with the drive on, the elevation angle needs
correcting; if vertical, adjust the azimuth. A little trial and error will
tell you which direction improves things. There are some alignment tools
you can buy that let you set up quite close to Polaris--see Sky & Tel ads.
Not much help in the southern hemisphere!

I hope I will be enlightened shortly. I have never attended any astronomy
classes but am not challenged in areas of general science or technology,
but
sometimes I am just too lazy to inquire sufficiently to derive answers by
myself.


If you live in the London area, perhaps you would like to look into the UCL
Diploma in Astronomy (evening course)

http://www.phys.ucl.ac.uk/part-time/PTAstro/index.html


--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

Telescope Buyers FAQ
http://home.inreach.com/starlord


--

The Lone Sidewalk Astronomer of Rosamond
Telescope Buyers FAQ
http://home.inreach.com/starlord
Astronomy Net Online Gift Shop
http://www.cafepress.com/astronomy_net

"John Smith" wrote:

I have some questions which I hope the community at large can help me
answer:
[snip]
4. I bought myself a 4" Meade Newtonian with an equatorial mount. I did
not invest a lot of time in it because I got frustrated with not
understanding how the equatorial mount would ever allow me to track an
object with any precision. I mean, how is it possible to align with the
accuracy that would match the earth's axis exactly! On that issue I always
wonder how people take pictures. How could a motor move exactly in step
with the earth's rotation and so smoothly that it would allow a perfectly
stationery picture to stay in view for a few minutes?

There are techniques for polar alignment of an equatorial mount, the
usual method for precise alignment is to initially align using a polar
alignment scope (a small telescope inside the axis, which you align
using Polaris as a guide - cheap mounts don't have one), then use
'drift alignment' to refine the position. There is detail of the
techniques on the web, but in principle you watch stars at different
positions in the sky to see whether they drift in the eyepiece
(indicating misalignment) and from the direction of drift can deduce
the corrections you need to apply to the mount alignment. For visual
use though, I just plonk my tripod down on some reference marks and
it's close enough (this works because I took the time to ensure good
alignment on those marks in the past and I don't adjust the mount
between uses).

The motors staying exactly in step really comes down to the quality of
the electronics.

Even with perfect polar alignment and an atomic clock driving your
motor you'll still likely have detectable mechanically-induced errors
in tracking. Periodic errors induced by the drivetrain can be
'memorised' and a map of these errors played back as the mount turns,
compensating for the errors. This is know as PEC - Periodic Error
Correction and is a feature of mid to high-end computerised mounts.
Any remaining errors can be minimised by guiding during a photographic
exposure (drift is usually small enough for visual use with fairly
casual alignment). Guiding involves something (an astronomer or a CCD
+ processor) watching a star through a smaller telescope attached to
the main one, or by 'stealing' a bit of light from the main telescope,
and feeding corrections back to the drive system. Guiding alone isn't
enough though - you need excellent polar alignment first, as any
misalignment will have a rotational component not compensated for by
guiding.


Tim
--
You are being watched. This gives you power.

Hi John, you might like to have a look for your local AS (if you don't
already know them) in this list at the FAS site. All societies will
help beginners get set up.
http://cgi.fedastro.force9.co.uk/pub...as/society.php
Book explanations are ok to get going but a practical demo is much
easier to comprehend :-)
The OU do some excellent starter courses in astronomy.
jc

"Mike Dworetsky" wrote in message
...
"John Smith" wrote in message
...


4. I bought myself a 4" Meade Newtonian with an equatorial mount. I
did
not invest a lot of time in it because I got frustrated with not
understanding how the equatorial mount would ever allow me to track an
object with any precision. I mean, how is it possible to align with the
accuracy that would match the earth's axis exactly! On that issue I
always
wonder how people take pictures. How could a motor move exactly in step
with the earth's rotation and so smoothly that it would allow a
perfectly
stationery picture to stay in view for a few minutes?


If you are taking it out at night and putting it away each time, you will
never achieve the alignment of the axis that you want. Getting to within
1
arcmin of the correct alignment is difficult even for a permanently
mounted
scope. But computer controls on telescopes allow you to at least find
objects, though it is true that photography will still be a challenge due
to
poor alignment.


Eh?

Like many (most?) amateurs, I have no permanent mount but setting up every
session has never been an issue preventing me observing/imaging. Alignment
takes less time than the cool down time. Polar aligning sufficently well
for visual use is trivial. Just set the elevation to your latitude and plonk
the mount down roughly facing Polaris. The motor drive will keep the object
in view of a wide field eyepiece for many minutes - long enough for an
observation.
A decent mount with a Polar scope and graticule can be aligned sufficiently
well to take exposures up to say one minute on a typical CCD field.

Robin
----------------------------------------------------------------------------
-
Robin Leadbeater
54.75N 3.24W
http://www.leadbeaterhome.fsnet.co.uk/astro.htm
----------------------------------------------------------------------------
-

"Robin Leadbeater" wrote in message
...

"Mike Dworetsky" wrote in message
...
"John Smith" wrote in message
...


4. I bought myself a 4" Meade Newtonian with an equatorial mount. I
did
not invest a lot of time in it because I got frustrated with not
understanding how the equatorial mount would ever allow me to track an
object with any precision. I mean, how is it possible to align with
the
accuracy that would match the earth's axis exactly! On that issue I
always
wonder how people take pictures. How could a motor move exactly in
step
with the earth's rotation and so smoothly that it would allow a
perfectly
stationery picture to stay in view for a few minutes?


If you are taking it out at night and putting it away each time, you
will
never achieve the alignment of the axis that you want. Getting to
within
1
arcmin of the correct alignment is difficult even for a permanently
mounted
scope. But computer controls on telescopes allow you to at least find
objects, though it is true that photography will still be a challenge
due
to
poor alignment.


Eh?

Like many (most?) amateurs, I have no permanent mount but setting up every
session has never been an issue preventing me observing/imaging. Alignment
takes less time than the cool down time. Polar aligning sufficently well
for visual use is trivial. Just set the elevation to your latitude and
plonk
the mount down roughly facing Polaris. The motor drive will keep the
object
in view of a wide field eyepiece for many minutes - long enough for an
observation.

No arguments, that's what the fancy computer star alignment procedures are
for.

A decent mount with a Polar scope and graticule can be aligned
sufficiently
well to take exposures up to say one minute on a typical CCD field.


OK, but you are a very experienced observer with all the right equipment and
knowledge to get within a few arcmin of the pole very quickly. The OP needs
both experience and the right equipment to get where you are. And a one
minute exposure is not very long, even for a CCD camera.

Robin
--------------------------------------------------------------------------
--
-
Robin Leadbeater
54.75N 3.24W
http://www.leadbeaterhome.fsnet.co.uk/astro.htm
--------------------------------------------------------------------------
--
-






--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

"Mike Dworetsky" wrote in message
...
"Robin Leadbeater" wrote in message
...

"Mike Dworetsky" wrote in message
...
"John Smith" wrote in message
...


4. I bought myself a 4" Meade Newtonian with an equatorial mount.
I
did
not invest a lot of time in it because I got frustrated with not
understanding how the equatorial mount would ever allow me to track
an
object with any precision. I mean, how is it possible to align with
the
accuracy that would match the earth's axis exactly! On that issue I
always
wonder how people take pictures. How could a motor move exactly in
step
with the earth's rotation and so smoothly that it would allow a
perfectly
stationery picture to stay in view for a few minutes?


If you are taking it out at night and putting it away each time, you
will
never achieve the alignment of the axis that you want. Getting to
within
1
arcmin of the correct alignment is difficult even for a permanently
mounted
scope. But computer controls on telescopes allow you to at least find
objects, though it is true that photography will still be a challenge
due
to
poor alignment.


Eh?

Like many (most?) amateurs, I have no permanent mount but setting up
every
session has never been an issue preventing me observing/imaging.
Alignment
takes less time than the cool down time. Polar aligning sufficently
well
for visual use is trivial. Just set the elevation to your latitude and
plonk
the mount down roughly facing Polaris. The motor drive will keep the
object
in view of a wide field eyepiece for many minutes - long enough for an
observation.

No arguments, that's what the fancy computer star alignment procedures are
for.


Hi Mike,

I must admit I would be lost without the Skysensor ;-) particularly Periodic
Error Correction for imaging work but its is not really needed for tracking
for a lot of visual work, just an RA motor drive. Worst case, even with say
a 5 deg error in polar alignment, a star at the celestial equator drifts
less than a moon's diameter in 20min. Of course, I do normally align using
the GOTO computer, but that is mainly so I can get from one object to
another as efficiently as possible, not for visual tracking.

Robin