Everything you always wanted to know 'bout "Air Mass" (but were afraid to ask) Planning WHAT targets you want to observe and find out WHEN they are visible in the sky helps to make the most of the precious time when the weather is just right. Stargazers also have to be weather-(wo)men : /stargazer95050/500315
Another factor that determins the WHEN has to do with the atmospheric distortions caused by the "air mass" : /stargazer95050/21434571
And of an even bigger impact are the seasons -- the Earth "wobbles" around it's axis and the view of the sky changes throughout the year. This fact is usually overlooked up until you start diving into astronomy.

STELLARIUM & Starry Night SE are more or less free -- Starry Night SE is bundled with many useful items from Orion. Ask friends for spare copies. Stellarium is free and has no strings attached. Both can simulate the view of sky -- past, present & future. Both tools lack the ability to plot the future position of objects and that makes them less useful to PLAN OBSERVATIONS, but you can manually advance time to see the future trajectory.

Let's say you are interested in imaging M51, there are a number of factors to be considered (and I exclude weather & light pollution)
  1. the object has to be above the horizon while at he same time, the sun has to be at least 3°..12° degree below the horizon (dark "night", not twilight)
  2. to be visible, the object also has to rise above any fixed, local obstructions (hill or mountain ridges, nearby houses & trees)
  3. for better quality, you may want to wait till the target is high enough to have less than 3x air-mass ==> 18° min. altitude, higher than most obstructions
  4. ideally you want no moonlight, it is useful to know the distance from the moon and the moon's brightness to improve contrast


One useful tool I have discovered is ASTROPLANNER -- and it is far more centered around numbers & tables than visual GUI & photo-realistic representation of the sky. That approach helps in sorting & selecting observation targets. ASTROPLANNER also can link photos with each object -- you can choose to download from different online database(s) & resolutions and also add your own shots.
ASTROPLANNER is not a replacement but a good addition to my "Holy Trinity" -- filling in some of the blank spaces.
  1. of course it can calculate twilight and the position of celestial objects
  2. you can sort objects and filter out those below the horizon
  3. you can draw the horizontal obstruction and AP will show that in its charts
  4. Air-mass also can be displayed and shown in the sky map
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  5. provide useful technical detaqils about each objects -- from size, brightness, sample images to "splitability" of double-stars
  6. current position, 1-day graph and also a 30/60/120-day graph of future altitude position as well as observability
  7. these graphs incorporate additional details (twilight, moon, obstructions)


And unlike Stellarium, AstroPlanner is very much capable of sending commands to an ASCOM-controlled telescope mount. And with a proper setup of hub, AstroTortilla, APT & AstroPlanner, it is very easy to choose a target in AstroPlanner and send the SLEW-command. Use AstroTortilla & APT to capture the sky's image, run plate-solver and issue commands to correct the position & sync the mount: see a realtime video of this /stargazer95050/28668501
Stellarium isn't helpful when you use ASCOM-commands, Starry Night SE is a good alternative to observe the movements of the telescope on a photorealistic sky map.


For planning purposes, AstroPLANNER has a wealth of useful features -- surprised ? Not all are easy to decipher, especially to newcomers. The graph in the center combines the visibility of the target "M51" (dark red). You also see the time when moonlight can interfere (yellow) -- that long-term graph doesn't show the actual impact as it neglects the moon's phase and distance to the target. The short-term "DAILY" graph on the left is more accurate.
I am not sure how accurate the (dark red) graph is -- there is a drop in observability with the moon rising but I have the feeling the current implementation neglects factors like the moon's phase & the distance between the moon and the selected observation target.

My suggestion is you try it out yourself -- the shareware terms are very generous. There is no expiration date for the tryout-period and the one limitation of the free version is the initial "nagging screen" that you have to click to continue.
  • You can customize what columns to show and omit details you are not interested (at the moment).
  • SORT the objects based on the value of any of the columns
  • hide currently invisible objects, less clutter
  • the table can automatically update & sort itself throughout the night since not all values are static
  • Astroplanner can be a "diary" and log your observations incl adding notes & images (I haven't used that yet)
  • with a build-in scripting language (an extension of the Real Studio REALbasic scripting language RBScript), you can do a lot more (I haven't used that yet but Astroplanner's author Paul Rodman wrote one for me to import locations from Google Earth KML file


In addition to Astroplanner, there are other useful tools and I want to mention the website CALSKY because it provides information Astroplanner doesn't. For example satellite transits, weather & terrestrial maps (via Google) -- you can enter your location manually (e.g. a remote site) or let it pick your current location automatically from (your GPS). Just be sure to save that data to a file / screenshot or hope to have a reliable reception for your phone out there. CALSKY too appreciates your support (kind of "shareware")

If aligning stellar views with terrestrial ground is your thing, Astro-planner's longterm preview can help but you still need to (manually) get all the bits & pieces together. It is a team effort, with Stellarium, Astroplanner, TPE & Google Earth all playing a role. And sometimes creatively combining tools in unexpected ways is a great way to visualize a result -- e.g. approximate the startrails on a certain location and direction.
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There are more tools -- and I am not claiming AstroPlanner is the best. I mention it because it works for me and it isn't a bulky all-in-one colossus. Some features I have not seen elsewhere :
  • the observability graph in the 1-day view and the different long-term views (Altitude & observability, 30...120 day)
  • comparing the mount's settings (date, time, RA/DEC, GPS) with those of the computer ==> spot clock offsets & avoid pointing errors, sync the mount's clock
  • define the visual obstruction of the horizon and mask stars below that threshold
  • define a list of GPS locations, each with it's own obstruction pattern
  • enter the tech-specs of your equipment and, like in real life, you can mix & match them and get simulated views of the result
  • scripting language & manual of that language included ==> DIY extension and further automation


It is a bit of a preview on the "HOW". Based on the combination of telescope, eye- piece & cameras, both AsroPlanner & Stellarium also facilitate the preview of expected star image. Stellarium provides a photo-realistic view, AstroPlanner's view is far more abstract but sufficient enough for planning purposes. Switching different combinations is easier in AstroPlanner than in Stellarium or Starry Night. Furthermore, AP support the fixed position of an off-axis guide-camera

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