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Cataloged Data
Explained in detail
Following is a very detailed explanation of how
TheSky
Astronomy
Software displays cataloged data for stellar and non-stellar objects including
the objects magnitude, position angle, coordinates, etc.
This document should/will clarify why there are times when a magnitude is
being reported as "0.0" for an object when searched for as
displayed in the object information dialog,
and also why some objects are labeled with a magnitude of "30.0", and
then some. For the most part the magnitudes for cataloged objects are not
being computed by TheSky but simply reported
by TheSky as
found in the dataset.
So if
TheSky
or any other desktop planetarium program for that matter had access to only one single
"PERFECT" catalog
everything would be extremely simple and very straight forward. For
example, let's say that TheSky
used only the common NGC 2000 catalog (assuming again it was perfect - big if!) for non stellar objects and nothing else.
If this were the case then whenever you search for an object
it would find it by NGC number "only NGC number" and the magnitude
reported would be the magnitude as contained in the NGC catalog, right or wrong.
Of course this holds true for the RA and DEC coordinates for the object, size,
and position angle as well. Done deal! Life is not so simple.
NOTE: There are three exceptions to reporting
the RAW data from the catalogs. One is
the magnitude that is displayed for the USNO stars. Here the missing visual magnitude
is computed using the Red and Blue magnitudes contained in the data. Secondly, the CURRENT EPOCH field contains the corrected coordinates (RA/DEC) for the current date and time
set.
TheSky is computing the new entry by correcting for precession, nutation, and
aberration. The "CURRENT EPOCH" position is more accurate then the EPOCH
2000 position, unless of course it were the year 2000 ;). The third entries are the computed Rise, Transit, and Set
times. Because these change for location on earth and date and time they
must be computed for the site information supplied!
First coordinate entry is current Epoch
Current date/time coordinates highlighted
Note there is only a small difference in the
current Epoch highlighted versus Epoch 2000
since we are only a few years away from Epoch 2000 during this writing. Over time the Epoch 2000
positions versus the current epoch position will change appreciably. To
see that set the date and time in TheSky
to something like 1950 and note how much of a change there is in the two
positions.
For those stars that have a high proper motion
(they are actually moving a small amount over a large amount of time) you can have TheSky
compute the effects of proper motion and see exactly how far the star moves and
in what direction.
Click here for details on how
TheSky
handles Proper Motion using
Barnard's Star as an example.
And as Greg Crawford has pointed out on the
Software
Bisque Users List when Software Bisque redistributes cataloged data whether it
be stellar, non-stellar, Quasars, Radio Sources, etc. the information contained
in the datasets is left untouched, with the exception of the newly computed additional
current EPOCH entry reported for the current date
and time along with the computed Rise, Transit, and Set Times, and any other similar
fields that do not exist in the
original catalog data to begin with.
Software Bisque makes no attempt to fix the errors in the
datasets, including, but not limited to errors in object size, object position,
position angles, object magnitudes, object descriptions, etc. This is even true for errors that are
blatantly obvious or well known errors as found in many of the very popular and well accepted
catalogs like the NGC 2000! To put this into perspective this would be like editing a book that
contains many grammatical and or spelling errors and taking it upon one's self
to fix the errors and then reprint the text. Of course there are copyright/legal and
moral issues here.
Also refer the following
Catalog Capriciousness article for more details.
NOTE: TheSky does have an extremely easy to use and very powerful
Ellipse Tool for creating new object entries
(in TheSky6 Tools | Ellipse Tool)
for existing objects or new database entries based on actual photographic or CCD data, the Palomar 102 CD's of data, or the
compressed version of the Digitized Sky Survey known as RealSky. Fortunately, there is currently an undergoing
project working on revising the NGC/IC
Project catalogs in an attempt to make them more
accurate! Here is a link to the NGC/IC
Project. Rest assured when the newly corrected data is available Software
Bisque will make an attempt to acquire the rights to distribute the
data for inclusion with TheSky.
Use Tools | Ellipse Tool to access the VERY HANDY tool. This Ellipse Tool can be used to "clean-up" thousands of
errors in any datasets in an efficient and reasonably accurate manner very
quickly. Many of "TheSky"
users have already created their own extensive databases using the RealSky
data
or the very own CCD images linked in
TheSky with
TheSky's
ellipse tool.
Software
Bisque does not consider themselves in a position to be the
"catalog" police so-to-speak, that is best to left to the
professionals.
Anyway, most of us already know that the NGC 2000 catalog
containing a mere 7,840 non-stellar objects is filled with inaccurate input
coordinates as well as inaccurate magnitudes for many of the objects in the
dataset (yes even the biggest and brightest!),
although there is an on-going concentrated effort to clean this data set up.
Details are found at http://www.ngcic.org <--
Check this out.
However, keep in mind that magnitudes for extended objects like nebula and galaxies are much more difficult
to determine (very subjective/more difficult to accurately measure) when compared to the stars or other point sources
like minor planets, variable stars, double stars. etc. Also keep in mind that The NGC 2000 data is a
very small dataset when compared to the GSC stellar data containing 15 million
stars (magnitudes only as good as 2/10ths) along with another 3 million questionable objects, or the USNO A-2.0 data with
either 54
million or 526 million stars, the
USNO-B1 with 1 billion stars, or the SAO catalog of
258,996 stars, but you get the idea. By
sheer volume the larger datasets are more prone to input errors.
Even the
Hipparcos/Tycho and Tycho-2 data which claim extremely good positional data
along with very accurate magnitudes (magnitudes are accurate as 100th or
better) after careful scrutiny it has been shown that these datasets
is not quite as "near perfect" as originally thought. Again, I point
out the on-line article reprint from Sky & Telescope that explains catalog
errors in general in grave detail.
Now consider this example. Here is an image created from
the 102 CD ROM version of the Palomar Sky Survey (POSS). It shows the
entries for M104 the Sombrero Galaxy from the NGC 2000 catalog and the PGC
catalog along with the actual photographic data in digital form. You can
easily see which of the two is technically more correct! The Green
outline is the PGC entry and the Yellow outline is the NGC 2000
cataloged position. The magnitude
for M104 from the PGC catalog is reported as 8.56 and the NGC 2000 reports the
magnitude as 8.30. Again the question becomes which is more accurate and
which one should be reported in the Object Information dialog box in TheSky?
PGC versus NGC data version
5
Older NGC 2000 - Image
RealSky/DSS Copyright
If you do not see both catalogs plotted in TheSky you can choose
the option to plot the PGC data and NGC 2000 data simultaneously using
Data-Options as shown below.
Data-Options
Turn on both NGC and PGC
Or in TheSky version 6 use View |
Non-Stellar Options see below.
Plot both the NGC and
PGC
Enable the NGC plotting here
Updated NGC 2000 entry is
much better!
Newer version of the NGC 2000 data!
RealSky/DSS Copyright
So back to the issues at hand. In an attempt to make TheSky as accurate
as possible Software Bisque does its' best to redistribute the most accurate
stellar and non-stellar catalogs available at the time like the newer NGC2000
with many corrections supplied by the NGC/IC
project. Unfortunately, this is a
major struggle because there are corrections being made on a continuous basis
and new catalogs are being introduced quite frequently as well. To that end
TheSky has included a powerful and easy to use Database Compiler for years.
The
database compiler allows the end user to easily create a database of objects to
plot by TheSky, either not
found in TheSky or one that has been updated to be more complete and or more
accurate. Any ASCII text data that contains coordinates whether they be
Altitude/Azimuth or RA/DEC can be used! It should be noted that it is simply impossible for
TheSky to contain
every single catalog that exists because there are literally thousands of
catalogs available containing literally billions of stellar and non-stellar
objects. CD-ROMS and even DVD's can only hold so much data so the line must be
drawn as to what data will be included as "standard" datasets with
TheSky, what to leave in what to leave out, Bob Segar, "Against the
Wind". And of course this applies
to all other Desktop Planetarium programs, as do the inaccuracies found in
nearly all catalogues. TheSky's easy to use database compiler is detailed here,
It should be noted that
TheSky's database compiler
should
not be used with catalogs that contain more than about 150,000 objects because
the non-optimized plotting will be EXTREMELY slow. Please see TheSky's
user manual for more details.
Now back to our example with the NGC catalog. As far as
galaxies, TheSky currently uses the NGC catalog that contains many galaxies as
well as the more accurate and now widely used PGC galaxy catalog AND that data
is also cross referenced to the UGC, VCC, MCG and CGCG galaxy catalogs as well. When you search for M51 The Whirlpool Galaxy what information
should TheSky display????? Should the size, coordinates, magnitude come from the
PGC, NGC, UGC, MCG, do ray me, etc. As pointed out this is actually a very
complicated topic.
Example what happens if you Edit-Find
"M51" in TheSky? It returns the following,
Edit | Find M51 (The Whirlpool)
Object Information from TheSky
Next, Edit-Find "NGC5194" in TheSky. Again the
same entry is shown.
Now search by the
common name "Whirlpool Galaxy" and
the ALIAS (cross reference to) NGC5914 is also returned.
Edit | Find NGC5194
Whirlpool Galaxy search
Yep one more finally! Edit | Find PGC47404 returns the following entry which
differs from the NGC data!
Edit | Find PGC47404
PGC magnitude entry shown
Actually following is all
the data returned for the above search!
Object name: NGC 5194
Other ID: PGC 47404
Magnitude: 9.0
Equatorial: RA: 13h 30m 09s Dec: +47°09'22"(current)
Equatorial 2000: RA: 13h 29m 52s Dec: +47°11'40"
Horizon: Azim: 51°34'12" Alt: +48°08'53"
Size: 9.8 x 6.8
Visibility: Rise 06:16, Set 23:34
Transit time: 14:55
Object type: Spiral Galaxy
Source catalog: PGC (Principal Galaxies Catalog)
Type: Sbc
Multiple: M
Morphological code: 4.038
Radial velocity: 539.531
Alias 1: UGC8493
Alias 2: MCG8-25-12
Alias 3: CGCG246-8
Alias 4: (ARP85)
Alias 5: KCPG379A
Alias 6: (VV1)
Alias 7: VV403
Alias 8: IRAS13277+4727
Position angle: 162°00'
Hour angle: -03h 20m 55s
Air mass: 1.34
Screen x,y: 611.00, 333.00
Sidereal time: 10:09
Click distance: 13.0000
Catalog number: 47404
Celestial type: 8
NGC/IC: 5194
Catalog: 0
Note the other catalog entries
too. Like the ARP85 ID, UGC8493, MCG, CGCG2, and
so on. For a rose by another word would still smell as sweet!
Wew!
So hopefully you can see the information reported is different
depending on how you searched for the object. NGC 5194 or PGC 47404. So how can this be one might ask? The reason is because
TheSky is searching for the object in a different way each time. FYI: TheSky attempts to report the data contained in
the most accurate catalog first. For example, if a star is in the Hipparcos
catalog and you search for it by name or say GSC number the entry displayed for
position and magnitude comes from the more accurate Hipparcos data then the
object is cross referenced to the other catalogs. This is true of
the galaxy positions and magnitudes as well.
Example, "Find Arcturus" in TheSky. You will see
these entries,
Arcturus
SAO 100944
GSC 1472:1426, HIP 69673, PPM 130442, HD 124897, B+19 2777
This is how it was decided that
TheSky should handle multiple entries for stars and other objects with more than
one set of data. With the galaxies in the PGC
and NGC catalogs by default the position, size/rotation, and magnitude are
displayed from the PGC catalog NOT the NGC! This is because the PGC data is more
complete and also more often than not (almost always) more accurate. To see the
difference between the entry for the PGC catalog versus the NGC entry you can
use Data-Options "Show Overlapping NGC and PGC galaxies. When this is done
you will see two galaxy sizes, shapes, and rotations for one galaxy. If you spot
check enough of them you will see why the PGC catalog is TheSky's default.
Version 6 of TheSky will incorporate the latest version of PGC containing entries for 1
million galaxies!
When you are searching for an object with a common name,
i.e. Horse Head, Crab Nebula, The Mice, Whirlpool, etc. note the SDB used to
resolve the search is displayed in the Object Information dialog. As was
the case when searching for Whirlpool Galaxy above. In this case the
SDB listed is the "Common Non-Stellar" SDB (Sky Database). Since the Sky Database is listed as Common Non-Stellar
here in the object information dialog that is the key. The data does not contain the information for Magnitude only
the coordinates for the label, name to search and a Description. These
descriptions are also different from the Dreyer Descriptions you see if you Find
NGC 5194! See the following possible descriptions from three different
sources.
Note too that there are different
descriptions available for the same object in many cases! These
descriptions come from three different sources. But again, which one is correct
or technically better than the others?
- Common Non-Stellar Description: Two galaxies, possibly
interacting; telescopic object.
- Dreyer description: A magnificent (or otherwise
interesting) object!, great spiral nebula M51.
- Other description: Spiral galaxy structure attached
companion.
Common Name Search
Found by Common Non-Stellar SDB
Now, if you have other SDB's active like your own, Tom
Lorenzins' 2000+', the Caldwell catalog, the Hubble Gallery, and or others, you
may see even more information for a given object! The SDB's that TheSky uses allowing one to search
for objects
by something other than a less popular and harder to remember cataloged number
like NGC 5194 or SAO 1254. If you can "EDIT" the text file named
Common Non Stellar.TXT found in the folder ..\TheSky\Auxiliary Files to see
this. Here is the Whirlpool entry M51, NGC 5194, in the Common Non-Stellar SDB,
Whirlpool Galaxy
NGC5194 13.498333 +47.200001
Two
galaxies, possibly interacting; telescopic object.
The Common Non-Stellar database is of object type
"LABEL". This SDB allows you to search/label the galaxy with the name
Whirlpool Galaxy as opposed to NGC 5194 or PGC47404, etc.. The same is true of the Messier Object
Label database named "LabMess.txt" also found in the ..\TheSky\Auxiliary
Files folder. Of course you can make the labels be anything you like, add to
the existing non stellar SDB text file, and then recompile the new version using
the database compiler or just make your own!
When searching an SDB you must have a field that is
defined as a "Find" field when creating the database. Then this
field is used with TheSky's Edit- Find option. For example, Find Caldwell
Catalog "search string". See below. This way you are
telling TheSky exactly where to search for the data. If the SDB is not
listed under the Database in the Find dialog there has not been a Find field
created.
Finding data in an SDB
Must specify a "Search" field first!
The above shows how to search the
SDB named Arp Peculiar Galaxies. Because it was compiled with a SEARCH
field the individual entries are displayed under Edit | Find | Sky Databases
(SDBs) | Arp Peculiar Galaxies.
Note: The very hand
use of Frame Object and Object Information on the Find dialog. Using these
allows you to scroll down the list of objects and each one is Centered and
Framed for you. The Object Information dialog will also show for each if
it selected.
It should be noted that the LabMess.TXT file
that labels and allows searching of the Messier objects by M number works
correctly. This is why when M51 is searched for and the match is returned
by the LabMess.SDB file the reported information is correct in that it contains
the magnitude and other cross referenced data! See here.
Cross Referencing Data
Using the Alias field!
In TheSky there are times when a magnitude is reported as
30.0. In these cases the NGC/IC objects there was no magnitude contained in the catalogs for these
object so they default, right or wrong, to a magnitude of 30.0. The magnitude in
many cases for very large and or bright nebula is listed as non-existent in the
catalogs, and that was chosen to be set to 30.00.
You can use TheSky6's
DATA WIZARD to query the NGC/IC
data for only those objects with a magnitude of 30.00 (magnitude = 30). That query is shown below!
617 NGC/IC objects with no magnitude?
Can that be right?
If you have access to the RAW
NGC/IC data and searched it for all objects with a non-existent magnitude you
would see this is in fact the case.
The NGC 2000 like many other catalogs (the GSC
data has 3 million questionable objects) has many objects with a high degree of
uncertainty as to the object type, coordinates, magnitude, or all of the above.
In the original data they are distinguished from the other data with a "?" in
the data.
So for an object like NGC 4170 that is labeled as "Nonexistent"
in TheSky
this is why. When using the 102 CD-ROM data for the same field you can
clearly see that the NGC 2000 catalog distributed by Sky & Telescope is
simply in error. Or now TheSky version 6 as shown below. It should be
noted that the very accurate PGC catalog with
900,000 galaxy entries does not contain this obvious error. For this reason
the PGC data has precedence over the NGC/IC entries for galaxies.
From the NGC/IC project
page here is a complete synopsis of the object's history along with others in
the same field. By using this page you can enter any NGC/IC object in
question and get a very detailed answer. Thanks to Robert
E. Erdmann are in order.
NGC 4170 and 4171. Found by d'Arrest near the group of
four galaxies NGC
4169, 4173, 4174, and 4175, these two objects are probably stars
picked up on
a night of below-average seeing. D'A's entire observation
(translated to
colloquial English by a Latin teacher, and relayed courtesy of Steven Dick and
Brent Archinal at USNO) reads in full: "In addition, I think I see
two other
nebulae very close to this one [NGC 4169]; a clearer sky would help."
His
note for the night (10 May 1864) reads: "Wind; not perfectly
clear." The
approximate positions in the NGC apparently come from Dreyer. And that is
the
extent of the original "data."
Holes around brighter stars
Why are there areas void of stars
around the brighter stars?
Hole around Arcturus!
What gives??
Why are there two stars near
Arcturus? There is an entry from the Hipparcos/Tycho data and another from the
USNO-B1 data. Generally the more accurate will be the entry from the
Hipparcos/Tycho data. Note the Hipparcos and Tycho data is extremely accurate in
comparison because it was created by using a satellite to gather the stellar
data not by scanning the photographic plates. However, this is only a mere 1
million stars so it is not very complete when compared to the
UCAC2 and USNO-B1
subset included with TheSky.
Sirius example - Missing stars
You can be serious!
Actually it could be worse!
DSS gen 1 image from 102 CD's
of the Palomar Sky Survey
Where did the other stars come from then?
In this case Sirius lies in the
area covered by the very accurate UCAC2 data.
It is only good to +40 degrees declination but one can see that there was a
better effort put into filling in the hole here. The UCAC3 data does not exist
yet, completing the sky, but when it becomes available TheSky will support it.
And for more on this and other related
topics please read the following well written article.
Now the following animated GIF
demonstrates the common GSC errors. Note when the GSC data is enabled the star
Aldebaran has stars plotted that appear to be the diffraction spikes of the
telescope's secondary spider.
Easy enough!
Just use View | Stellar Options |
Catalogs/Astrometry and remove the check mark next to Guide Star Catalog.
GSC data error around
Aldeberan
How can that be??
Here is exactly how and why this
happens.
By looking at the original
photographic plate from the DSS data (GSC data was created with) one can see how
this sort of thing can happen.
102 CD's the DSS 1 Sky Survey
Yuck!
It should be noted that even after
plotting 1 billion stars from the USNO-B1 full 80 gigabytes, the Hipparcos/Tycho
and UCAC2 there are still only 3 stars being plotted in the halo around the
star.
Still missing stars?!?
What next?
The next step would be to have
someone concentrate their efforts on the brighter stars and fill in the holes by
accurately performing astrometry/photometry on all stars in the missing areas.
In fact that may be an ongoing effort already!
M67 Open
Cluster one more example defining holes in globular clusters.
Another example of "ratty data" is
the following coordinates in the sky when using only the GSC data.
The Monolith
Note when the GSC data was created
(by scanning the photographic plates and estimating stars positions and
magnitudes) something had blocked a large portion of the photographic plate
resulting in missing stars. No this is not 2001 a Space Odyssey!
GSC data error!
Pen on the photographic plate?
I have even found several place in
the DSS images where the National Geographic copyright is stamped in the plate.
This too wreaks havoc on the pattern recognition scanning process.
Oddly enough the only dataset that
actually fills in the monolith is the UCAC2 dataset.
What is the UCAC2 dataset?
Will the
UCAC3 dataset lay to rest the
halos? We will have to wait and see! Better yet the Nomad stellar data coming to
TheSkyX Professional soon.
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