Constellation Overlay Helper Tool

The Constellation Overlay helper tool works out the settings that line the Allsky Website's constellation overlay up with the stars in your images, so you don't have to align it by trial and error.

It needs to know three things about your images: where the zenith (the point straight overhead) is, how big the sky is, and how the image is rotated. It gets them from the stars: it compares the bright points in your image with where the bright stars were at the time the image was taken, fits your lens, and then picks the overlay settings that match it best.

Using the tool

• Night image

  Pick a clear, dark night image: no clouds, and ideally without the Moon, which washes out the fainter stars. Use an image as Allsky saved it, because the tool reads the time from the file name. Allsky's Latitude and Longitude settings must be correct.

• Update the Website

  Leave this on to have the tool write the new settings into the configuration of each enabled Website (local and/or remote) and upload the remote one, as the WebUI does when you change a Website setting. A weak fit is never written. Turn it off to only see the suggested settings.

• Run

  Press Run. The tool looks for the stars by itself, which can take a minute or two. It uses another image from the same night, about half an hour apart, to tell stars (which move) from the text on your image (which doesn't).

  A box above the tabs then says what happened and what to do next. If the stars were found, the Images tab shows your image with the bright stars circled and named: check that the circles sit on stars. The Output tab shows how well the stars fit, the suggested overlay settings next to the ones you have now, and how far from the stars the overlay should be over most of the sky. A second image marks every bright star with a green circle where it is and a yellow cross where the overlay will draw it.

• If the stars aren't found automatically

  This can happen with few stars, clouds, or a lot of text on the image. The section Only if the stars aren't found automatically then opens, the Output tab lists the bright stars (and planets) that were at least 20 degrees above the horizon, with their altitude and direction, and the Images tab has a copy of the image with a pixel grid.

  Pick two stars from the list that are well apart and not too low. Enter each name in First star and Second star, press Pick next to each and click the star in the image. Use 100% to zoom in. The click moves to the brightest spot nearby, so it doesn't have to be exact. Then press Run again.

Info

Remember that your image may be rotated: if north is at the bottom of your image, a star listed as being in the south is near the top. A star chart for the same time helps.

The time in an image's name is the camera's local time. If the stars don't fit in the Pi's time zone, the tool tries the time zone nearest to the camera's location and says so. If you see that note for an image from your own camera, check the Pi's time zone.

With Update the Website on, the box above the tabs says which Websites were updated; reload the Website to see the new overlay. With it off, nothing is changed: to use the settings, edit the Website's configuration.json in the WebUI's Editor page and enter the suggested projection, overlayWidth, overlayHeight, overlayOffsetLeft, overlayOffsetTop, az, overlayLean and overlayLeanAz.

A camera that isn't level

The tool also works out how far the camera leans and which way, and sets the overlay to lean the same way (overlayLean and overlayLeanAz), so the constellations still sit on the stars. A camera leaning 11 degrees, for example, is matched to about 1 degree.

If it doesn't work

• "No consistent fit": check that the names match the stars you clicked, that the image is clear, and try another pair of stars.
• "East on the RIGHT": your image is a mirror image of the sky. The overlay can't be mirrored, so flip the image in Allsky's settings and use a new image.
• "The fit is weak": few stars were found. Try a darker, clearer image.