SkyCruncher

Note · Before you shoot

Alt-az or EQ mode: why the field turns and what it costs

In its normal alt-az mode, a Seestar keeps your target centered but lets the stars slowly turn around it. For most pictures that costs little. For long brightness measurements, star positions and targets that pass nearly overhead, equatorial (EQ) mode on a wedge is usually worth the extra setup.

Updated

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Two ways to follow the sky

Why the field turns

An alt-az mount keeps the target in the middle of the frame, but the camera doesn't turn with the sky. Seen from the camera, the star field slowly rotates around the middle of the frame. That is field rotation, and every alt-az telescope without a rotating camera mount has it, whatever the brand.

How fast the field turns depends on where the target sits in your sky. For a level mount:

turning rate = 15.04″/s × cos(latitude) × cos(azimuth) ÷ cos(altitude)

Here 15.04 arcseconds per second is the rate the whole sky turns (one turn per sidereal day, the day counted against the stars rather than the Sun), and an arcsecond (″) is 1/3,600 of a degree. Azimuth is counted from north, and the sign only says which way the field turns.

Two things follow from the formula. The field turns fastest near the meridian, the north-south line through the point straight overhead (the zenith), where a target is at its highest. And the rate shoots up as a target nears the zenith, because cos(altitude) falls toward zero. Near due east or due west the turning slows almost to a stop.

How fast the field turns, hour by hour (illustrative) Turning rate in arcseconds per second against hours from the meridian, for three targets seen from 40 degrees north on a level alt-az mount. A target peaking at 50 degrees turns at most about 18 arcseconds per second, one peaking at 70 degrees about 34, and one peaking at 85 degrees about 132, all at the meridian. 0 20 40 60 80 100 120 140 −3 h −2 h −1 h meridian +1 h +2 h +3 h hours before and after the target crosses the meridian turning rate, arcsec per second peaks at 50° peaks at 70° peaks at 85°
Turning rate over six hours for three targets, seen by a made-up observer at 40° N with a level alt-az mount: one target peaks at 50°, one at 70°, one at 85°. Worked out from the formula above, not measured (illustrative). At the meridian they turn at about 18, 34 and 132″/s.
One hour of field rotation across the meridian (illustrative) Three panels. Each shows a 16 by 9 frame at the start, middle and end of the hour around the meridian, and the shaded part every frame covers. For a target peaking at 50 degrees the field turns about 18 degrees and about 86 percent of the frame is covered all hour; at 70 degrees, about 32 degrees and 77 percent; at 85 degrees, about 95 degrees and 49 percent. peaks at 50° turns 18° 86% always covered peaks at 70° turns 32° 77% always covered peaks at 85° turns 95° 49% always covered
The same three targets through the hour around the meridian. Each panel shows the frame at the start, middle (solid) and end of the hour, turning about the middle of the frame (the dot); the shaded part is the sky every frame covers. Worked out, not measured (illustrative).

What it costs

Cropped corners

Stacking software turns each sub (one single exposure) back into line before adding it, which removes the turning from one sub to the next. But the turned frames no longer overlap at the edges, so the corners of a stack get fewer subs, come out noisier, and are often cropped away. The longer the session and the higher the target, the more you lose. In the figure above, an hour across the meridian leaves about 86 % of the frame covered all hour for the 50° target, 77 % for the 70° one and 49 % for the 85° one (illustrative).

Smeared corner stars

During one sub, the field also turns a tiny angle about a point near the middle of the frame. Stars near the middle barely move, while stars in the corners sweep out a short arc. The arc grows with the sub's length and with distance from the middle, and stacking can't take it out, because it happens inside each sub.

The smear also fools star-size checks. Many tools grade a sub by its average star width over the whole frame, its FWHM (full width at half maximum: how wide a star is at half its peak brightness). Stretched corner stars raise that average even while the middle of the frame stays sharp, so a sub can be marked down as bad seeing when the cause is the turning field.

I have looked at this on one alt-az night of my own, in an Exploratory Experiment: Stretched stars near the meridian are field rotation, not bad seeing. This Note doesn't carry its numbers: they come from one night and one telescope, and they wait for a repeat on fresh data.

When EQ mode is worth it

Alt-az is fine for a short session, a target that stays low or middling, a quick look, and planets, where frames are short.

What EQ mode asks of you

A wedge, a steady tripod, and a polar alignment every time you set up. A poor alignment lets the stars drift across the frame during the night, so budget some time for it the first few nights.

If you stay in alt-az

Ask the planner

The planner's observing guide for each target says which mount mode suits it, with a one-line reason (implemented). Its rules are our judgment from common smart-telescope practice, not measurements:

What I haven't measured yet

I haven't published a side-by-side of the same target in both modes, or the repeat the smear Experiment needs: a fresh alt-az night with a high crossing, scored on center stars with the method fixed in advance. Both are on my list, with no date (planned). Until then this Note sticks to the geometry and common practice.

Where these numbers come from

The lines beside passages mark how far each claim is proven; see how claims are marked. More Notes and guides are on the Learn page, and dated measurements are in Experiments.