Meridian flip explained

Why it happens

Stars rise in the east, climb to their highest point and set in the west. The meridian is the line running north to south overhead, and a star crosses it at its highest. A German equatorial mount follows the star by turning one axis once per sidereal day. As the star passes the meridian, the telescope, camera and counterweight bar swing toward the tripod and pier. Past a point, they would hit it, so the mount has to move the telescope to the other side.

The flip does two things at once: it turns the mount about halfway round on its polar axis, and it flips the telescope over on the declination axis, so that it points at the same star as before, but from the other side of the pier.

What changes afterwards

• The image rotates 180°

  The camera is now upside down relative to the sky. Stacking software usually aligns the two halves for you, but if yours doesn't, rotate one set of frames by 180° before stacking. A camera rotator can avoid it by turning the camera back.

• Guiding runs the other way

  After the flip, the declination axis moves in the opposite direction, so a guider calibrated before the flip will push the wrong way. Most guiding software can reverse the declination output or re-calibrate automatically. Check the setting before you rely on a flip overnight.

• Framing shifts slightly

  The mount never returns to exactly the same point. Plate-solve and re-center, then check the framing before you carry on.

• Focus can change

  The telescope has been moved and the temperature has dropped since you last focused. A quick refocus after the flip is cheap insurance.

• Flats may not match

  Dust spots sit on the camera sensor and filters, so they rotate with the camera. If your flats were shot in the pre-flip orientation, shoot a second set after the flip, or keep the two halves of the night separate through calibration.

• Balance and cables matter

  Check the balance on both sides, and leave slack in every cable. The most common flip failure is a cable catching on the mount, tripod or pier as the scope swings over.

Planning around it

1. 1

   Pick targets that match your window

   A target that crosses the meridian near the start or end of your session needs no flip, or only one. Dark Window shows when each suggested target crosses the meridian tonight, marked with a ⇄ flip note.

2. 2

   Start in the east, finish in the west

   Begin a few hours before the target crosses and finish a short time after, and you'll image across the meridian with at most one flip. If you only have a few hours, put the crossing in the middle or avoid it.

3. 3

   Image past the meridian within your mount's limit

   Most mounts can track a few degrees, often 5 to 15 minutes, past the meridian before they must flip. Set that limit in your mount or capture software so the flip happens at a time you choose.

4. 4

   Run the first flip while you're watching

   Do a test flip in daylight or early evening with the cables routed. Then you trust it overnight.

Questions

What is the meridian?

An imaginary line running from due north through the point directly overhead (the zenith) to due south. Every star crosses it once a day, at its highest point, when it is said to transit.

Which mounts need a meridian flip?

German equatorial mounts, the common type with a counterweight bar. Fork mounts and alt-azimuth mounts handle the sky differently and don't flip in the same way. Small tracker mounts vary: some have a go-to flip feature, others you stop and turn by hand. Check your mount's manual for its own flip limit and flip feature.

Why can't the mount just keep tracking?

Past the meridian, the telescope, camera or counterweight bar can hit the tripod or pier, cables can snag, and some mounts have a hard stop. Flipping puts the scope on the other side of the pier so it can keep following the target.

How long does a flip take and how much imaging do I lose?

The slew itself is under a minute, but re-centering, restarting guiding and checking focus usually costs about 5 to 10 minutes. Automated capture software does all of it for you.

Is the flip the same as the target 'transiting'?

The crossing is the transit. The flip happens a little after it, at the limit you set, because many mounts can safely track a short way past the meridian.