The Alias Field

The ALIAS field is normally used for cross referencing one catalog to another. An example is any of the Messier objects. These objects are ALIASed to their corresponding NGC or IC numbers. The Galaxies are using the more accurate PGC catalog data which has precedence over the other datasets but the cross reference also exists to the NGC/IC, VCC, UGC, MCG, CGC, Arp, IRAS, and other galaxy catalogs. Please refer to the following for more details.

Galaxy Questions and examples

Take the HorseHead nebula which is normally cross referenced to IC434. However, for the HorseHead Nebula I have chosen to NOT use the ALIAS field to cross reference IC434 because the coordinates from the IC catalog for the object 434 are not as precise as they could be.

Please see the following related links

A list of all non-ALIASED objects

Catalog Data Explained

Explanation for not using the ALIAS

In TheSky if you enter Edit | Find "HorseHead Nebula" without the quotes, TheSky returns the following.

Object name: Horsehead Nebula
Other ID: no find or alias in SDB
Equatorial: RA: 05h 41m 21s   Dec: -02°27'21"(current)
Equatorial 2000: RA: 05h 41m 02s   Dec: -02°27'36"
Horizon: Azim: 275°33'12"   Alt: -10°27'19"
Visibility: Rise 16:10,  Set 04:02
Transit time: 22:04  
Source catalog: Sky Database: Common Non-Stellar
Hour angle: 06h 46m 35s
Sidereal time: 12:28
Click distance: 2.0000
Celestial type: 20
Index: 35
Julian date: 2453749.9953
Catalog: 1

As opposed to using Edit | Find IC434 which returns the following.

Object name: IC 434
Magnitude: 30.0 <------ What is this??? Magnitude 30 what gives?
Equatorial: RA: 05h 41m 19s   Dec: -02°26'25"(current)
Equatorial 2000: RA: 05h 41m 00s   Dec: -02°26'40"
Horizon: Azim: 275°34'15"   Alt: -10°27'07"
Size: 60.0 x 60.0
Visibility: Rise 16:10,  Set 04:02
Transit time: 22:04  
Object type: Nebula
Source catalog: Revised IC
Blue mag: 11.0
Constellation: ORI
Remarks: Horsehead nebula
Dreyer description : Nebula, 60' long, south from zeta Ori.
Position angle: 00°00'
Hour angle: 06h 46m 37s
Sidereal time: 12:28
Click distance: 0.0000
Catalog number: 434
Celestial type: 14
NGC/IC: 0
Julian date: 2453749.9953
Catalog: 2

Why?

I chose to use, in my humble opinion, very accurate coordinates for the dark nebula found within IC434 AKA Barnard 33 when a search is made using Edit | Find Horsehead Nebula as opposed to Edit | Find IC434.

NOTE: This does not violate any catalog altering issues either since you are searching by the common name HorseHead nebula which is exactly what you are looking for not IC434 which has its' own entry found in the untouched IC data. It just so happens that there isn't an accurate data entry for this small portion of IC434 available. So again right or wrong I chose to I make my own position.

Coordinate comparison

Object ID

Epoch 2000 coordinates

IC434	RA: 05h 41m 00s Dec: -02°26'40"
HorseHead Nebula	RA: 05h 41m 02s Dec: -02°27'36"
Barnard 33	RA: 05h 34m 36s Dec: -02°32'00"

Justification for this!

The above 3 sets of coordinates plotted against the DSS photograph

You be the judge!

The label HorseHead Nebula uses the coordinates that I feel best represent the center for the dark nebula.

How can this be?

Are you saying that there are 3 different sets of coordinates for this object? Yes, that is exactly what I am saying! For an extensive detailed explanation see the following on-line article by Stuart Goldman of Sky and Telescope here Understanding Catalog Capriciousness

For those interested in how and why TheSky6 does what it does with the data please see my explanation on the following page.

Catalog Data Explained complete details!

Edit | Find HorseHead Nebula

Common Non-Stellar names list

I have intentionally removed the ALIAS to IC434 in the Common Non-Stellar SDB file and used coordinates for the HorseHead label which more accurately reflect the exact position of the dark nebula, AKA Barnard object 33. This is demonstrated below. Click on the image to go to the Barnard Dark Nebulae page for complete details and the data download!

Barnard 33 - The HorseHead nebula

Interesting, also referred to as IC434!

So even the data entry found in Barnard Dark Nebulae catalog for this nebula also differ slightly from the exact position of the so called famous HorseHead nebula as demonstrated above! The above link to the Barnard Dark nebula catalog has more details and includes a download to the Barnard data in an SDB file that can be downloaded and used with TheSky.

Now YES the above correction I have made to the size and position of Barnard object 33 is probably "against" the rules but you can see the obvious reasons for making the slight change. Of course anyone that wants the RAW Barnard Dark Nebulae Data with the original offset coordinates can certainly revert back to it OR of course you can always use the coordinates for IC434 too.

UPDATE: In spot checking the current position for IC434 (see above) in TheSky6 using that latest IC/NGC data when compared to earlier versions of the IC catalog I have found that the newest position more accurately reflects the HorseHead position then it did previously. Most likely for all of the above stated reasons for me choosing to make an entry that I felt was more accurate.

Now instead of being a difference of many arcminutes off it is only 1 arcminute. For most poor pointing telescopes the entry for IC434 could just be used. However, when pointing a telescope at the 15 to 30 arcseconds RMS like the Paramount ME this catalog entry for IC434 instead of using mine searching for HorseHead instead will be noticed.

List of all objects currently not using an ALIAS in the Common Non-Stellar text

18.31499587 -13.84719346 Eagle Nebula
 5.68391213  -2.45998142 Horsehead Nebula
18.04187778 -22.95508333 Trifid Nebula
17.39465278 -23.67686111 Snake Nebula

NOTE: The Snake Nebula Barnard object #72 does not have a cross reference to any other of the datasets found in TheSky6 therefore no ALIAS is being used.