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A deep sky atlas for electronically-assisted observing


Martin Meredith

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As much as I like star atlases, it became apparent to me soon after getting into this electronically-assisted observing lark that printed (or printable) charts are 5-10 magnitudes too shallow compared to what a sensitive camera and modest scope can reveal in under a minute. There are good software solutions such as Sky Safari, but even these are not quite deep enough, and don't contain all the objects that I'm interested in. Online options such as Aladin are deep and informative but not entirely practical for use in the field (not in my internet-free field at least…).

So perhaps there is room for a new deep atlas.

For me, such an atlas should support planning of observation sessions, assist at the point of observing (e.g., to identify the field), and permit post-observation analysis of captures. In addition it should be complete to at least mag 18, rich in information on the map itself, and portable to many devices with a low processing overhead.
Thanks to one of the wettest winters for years, I've had plenty of time to crack on with constructing such an atlas. Now that galaxy season is upon us I'm making available a set of deep maps of the Leo region. My aim is to improve the maps with your feedback over the coming months and release additional constellations (Virgo/Coma next) on a regular basis prior to an entire sky first release later in the year.
The charts themselves are PDFs. Each is square and covers 3.5 degrees in declination, with 0.5 degree overlap between charts. At this small scale I use gnomonic projection in order to match the view on a planar CCD sensor.
They are not designed to be printed and bound. At the current scale of 14 arcsec/mm, charts for the entire sky would take up a little short of an acre and still not be readable without a magnifying glass….  :smiley:
On each chart, interesting objects or groups of objects are marked using fonts designed to be readable 'at a glance', but most of the time the charts are meant to be explored via zooming. Some charts contains thousands of galaxies, all of which are identified; in as many cases as possible they also contain supplementary information on magnitude, type, distance, surface brightness and the like. Font sizes for much of this information have been chosen to be practically invisible until zoomed, so as not to overwhelm the spatial impression of plotted objects. 
Finding one's way around such a set of charts could be a challenge. As a first solution, I've created constellation-scale low-resolution finder charts (limiting mag 7 stellar and somewhat deeper for DSOs) and an intermediate scale set of medium-resolution charts (mag 13 stellar). I've been selective on which objects to plot in these finder charts since the font size has to be readable. Different types of objects are grouped by colour. 
The finder charts are hyperlinked to the main deep charts. As you'll see with the Leo maps, the low resolution chart covers 27 degrees of dec and is made up of a 3x3 grid. Clicking on one of these squares leads to the intermediate maps, which are also made of 3x3 grids. Choosing one of the grid squares leads in to the deep charts. This means that there are 81 such maps for Leo alone. Clicking again on the deep charts takes you back to the intermediate charts, making it possible to navigate reasonably easily. Clicking on the Leo link takes you back to the main constellation chart. 
Since the charts are PDFs they will work on all platforms, but you might need to set up your viewer to allow it to reload within-document links into the same viewer window. I've done things this way so that the processing overhead is low -- only a single chart is loaded at any one time. I've found that Acroread works well on my laptop, but other viewers might do a better job on tablets. An App called Documents for the iPad works well, for instance.
You can download the collection of Leo charts here (68Mbyte):
Just unzip and open the file Low.pdf and have a play.
In the next post I'll say a bit more about what's in the maps. Meanwhile, I'm open to any and all constructive suggestions for improvements.  :smiley:
cheers
Martin
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What's plotted?

The stellar backbone of the atlas is the PPMXL database of around 600 million stars down to magnitude 20 (although at present I'm cutting off at mag 18), while the galactic content is from the LEDA database (approx 2.4 million galaxies and galaxy candidates down to mag 22). In addition, the following catalogues are present in their entirety based on the most recent releases:

Stellar

XHIP (Extended Hipparcos; 100000+), which provides information on brighter stars (around mag 12)

Washington Double Stars Catalogue (approx 110000 systems)
American Association of Variable Star Observers catalogue (AAVSO; approx 322000)
Nebulae
Dark nebulae: Barnard and Lynd catalogues (1903)
Planetary nebulae from various sources (2414)
Van den Bergh Reflection Nebulae (158)
Bright nebulae: Lynd and Sharpless catalogues (1248)
Clusters
Globular Clusters from Harris (157)
Dias' catalogue of optically visible open clusters and candidates (2167)
Galactic
- Clusters
Abell Galaxy Clusters (including the southern catalogue and its supplement; 5250)
- Compact groups
Shakhbazian (377)
Hickson (100)
Palomar (487)
- Interacting galaxies
Arp (338)
Arp-Madore (5973)
Vorontsov-Velyaminov (2015)
Quasars from the Veron catalogue down to mag 20 are also plotted
More complete acknowledgements will come later, but at this point I want to say a huge thanks to the professional and amateur astronomy communities -- and to the people behind the Vizier data server in particular -- for making all this data freely available. It has been a real pleasure, not to say an (ongoing) education, to attempt to make sense of it all in compiling the atlas.
Martin
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This is a monumental work, Martin! Really impressive!  :shocked:   :smiley:

It is clearly beyond the limits of my scope!

However, for people (like many of us!) appreciating star charts, it is always interesting to look at them!  :smiley:  :smiley:

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Thanks very much everyone. Mark, creating a mag15 version is on the agenda…. 

In the absence of documentation here's some more information on the representation of objects in the maps. 

In general, label font sizes decrease with magnitude.

Brighter stars (in black, from Hipparcos) are named and have some information on magnitude, spectral type, B-V colour, distance, estimated age and number of exoplanets. 
Multiple stars (in green). The designation shown is the discover code plus number from the WDS. The Struves and S.W. Burnham have sigma and beta codes for consistency with the Cambridge Double Star Atlas, amongst others. Systems are displayed in different ways dependent on the separation. Stars that are separated by a sufficient amount to see the individual components are joined by lines. For very wide separations these are dotted if the pair is non-physical. If the secondary is too close to show the line it is drawn 'conventionally' as partially-overlapped. If the secondary is closer still, then an oriented information line lists the separation, orientation and magnitude of the secondary.  Other information displayed might include the number of components if more than 2, spectral type(s) etc. Actually, this all needs a bit of work to make coherent. Probably I will add the WDS code too. Any thoughts from double star fanatics would be helpful!
Variable stars (yellow annulus): the annulus width indicates the range of magnitude variation. The saturation of the label colour indicates variability range (the yellower the colour, the more variable the star). The infoline shows the range, the period (where known), the type of variability, and the AAVSO object identifier.
As a result, stars that are bright, variable and multiple have 3 designations and sets of information. This might seem cluttered but it helps to identify the different types.
The non-Hipparcos stars come from PPMXL. The way PPMXL was constructed means that there are quite a few artefacts (e.g., stars representing parts of galaxies). You'll notice an absence of stars around bright stars as well as the occasional representation of spider vanes (many have been removed though).  
Galaxies are plotted in the correct position angle and size where this information is available. Only Messier, NGC, IC and UGC are named, with shortened prefixes. The rest go by their PGC number. Maximally, the infoline contains magnitude, surface brightness, morphology and distance. Galaxy extents have been adjusted quite a bit (reduced) since LEDA lists the mag25 isophote, which is unlikely to be visible in most of our scopes/cameras. Even so, they are still a little on the large size compared to what you might see in your captures. You'll also see stars peeping out from under the skirt of the galaxy ellipse… These are often (but not always) artefacts from the PPMXL catalogue.  Colour saturation maps to magnitude, and uncertain galaxy candidates have a very pale interior.
Abell galaxy clusters (blue): mag10 is (red) magnitude of 10th brightest member, N = number of members, Dclass = distance class, Rich = richness; if available, the brightest member magnitude is also provided as is the redshift and Bautz-Morgan type (see original Abell paper for details).
For the various interacting galaxy catalogues (also in blue) the individual components are indicated by numbers or letters (keeping to the standard conventions). So, Hickson components have letters a, b, c etc. I've dealt with overlapping designations to some extent (the alternative names are listed at the end of the infoline) but many have slipped through, so occasionally you'll see the same object labelled with both a number and a letter, usually superimposed and unreadable. This will be improved in the next version. For the Palomar Compact Groups, since they're identified by RA+Dec I've simply labelled them 'PCG' and placed the RA/Dec designation in the infoline.
Quasars (red crosses): the infoline gives the magnitude, B-V colour info if available and redshift.
The other object types (open and globular clusters, nebulae of all sorts) are almost completely absent in Leo so I'll leave it to later releases to say more about them.
Martin
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Finally, some known limitations and plans for the next release:

The various dark, bright/reflection nebulae need to be outlined. This will take a while but I plan to start with the Barnard objects in time for summer.

Dense stellar regions in the Milky Way are problematic for maps of this scale (purely in terms of file size) and may need some kind of density map approximation. This shot of B72 (the Snake) and friends gives an idea of the problem:
post-11492-0-53925800-1426242892_thumb.p
I'd like to include further catalogues of more exotic objects (e.g., quasar pairs, gravitational lenses). I'm open to suggestions here (and I may have missed some obvious types of objects too).
Better handling of multiple star systems.
Better handling of overlapping designations (eg amongst the Arp and V-V catalogues).
Believe it or not :smiley: , an awful lot of work has gone into label placement but it still needs some refinement…. 
Martin
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This is spectacular.  As a committed cartophile (both terrestrial and celestial), I'm amazed and eager to see the next editions.  This will be very helpful on those faint-fuzzy expeditions.  My only suggestion might be to add more RA and DEC lines on the hi-res charts, and label the intersections with larger (but grey instead of black) font.

Thanks for doing this!

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Thanks Philip, aparker and Don. I also like all sorts of maps, even for places I don't intend to visit…. something about the imagination I think.

Yes, the intersection labels are rather too small at present. I had major and minor RA/Dec lines on an earlier version and may add them back. There's more of an overhead than you might think (since the Decs are curved), but it isn't excessive.

By the way, I've created a mag15 version too although it needs a lot of tuning for visual use. The coverage is 45, 15 and 6 degrees (i.e. 5 degree + 1 of overlap).

https://www.dropbox.com/s/3luw7nsf8sb4oa2/Leo15.zip?dl=0

Pretty soon I'll get the Coma/Virgo maps done. I just need to create the top-level constellation browser infrastructure.

cheers

Martin

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