Focusing

The Focus screen gives you direct control over your motorized focuser with real-time star analysis, and provides an automated V-curve autofocus routine that finds optimal focus position reliably.

Manual Focus

Open the Focus screen from the bottom navigation bar or from the Imaging screen toolbar. The screen shows a live camera view with focuser controls overlaid.

Focuser Controls

• Step buttons — Move the focuser in or out by the selected step size. The current position displays as a step count at the top of the screen.
• Speed selector — Choose between coarse, medium, and fine step sizes. Use coarse steps to get close, then switch to fine steps for precision adjustment.
• Position readout — Shows the current absolute focuser position in steps. Tap it to enter a specific position and move the focuser there directly.

Star Analysis

While focusing, ARIS continuously analyzes the image to provide real-time feedback:

• HFR (Half-Flux Radius) — Displayed prominently on screen. This is the primary metric for focus quality. Smaller HFR means tighter stars. Adjust the focuser until HFR reaches its minimum value.
• Peak value — The maximum pixel brightness of the selected star. A higher peak with a lower HFR indicates better focus. If the peak is saturated (maxed out), reduce the exposure time.
• Star profile — A 3D surface plot or cross-section of the selected star, showing the intensity distribution. A well-focused star shows a tight, symmetrical peak.

Tap any star in the view to select it for analysis. ARIS highlights the selected star and centers the star detail overlay on it.

Tip

Select a moderately bright, isolated star for focus analysis. Avoid stars near the edge of the frame (which may show coma or tilt) and saturated stars (which give unreliable HFR readings).

Loop Mode

Toggle loop capture to take continuous short exposures while you adjust focus. Each frame refreshes the HFR and peak readings, giving you immediate feedback as you step the focuser. A 2-3 second exposure is a good starting point for focus loop.

Autofocus (V-Curve)

Tap the AF button on the Focus screen to enter the autofocus view. ARIS runs an automated V-curve routine that sweeps the focuser through a range of positions, measures HFR at each step, and fits a curve to find the optimal focus point.

Running Autofocus

1. Set the exposure time for autofocus frames. Use 2-4 seconds for most setups. Longer exposures give more reliable HFR measurements but take longer to complete.
2. Tap Start to begin the routine. ARIS moves the focuser to the starting position and begins capturing frames at each step.
3. The V-curve graph builds in real time as measurements come in. You will see HFR values plotted against focuser position, forming a characteristic V shape.
4. When the sweep completes, ARIS fits the curve, identifies the minimum, and moves the focuser to the calculated best position.

Interpreting the Graph

The V-curve plot shows:

• X axis — Focuser position in steps.
• Y axis — HFR in pixels.
• Data points — Each measured position. Points on either side of focus show increasing HFR.
• Fit curve — The calculated best-fit line through the data points.
• Best position — Marked on the graph and displayed numerically. This is where ARIS moves the focuser after the routine completes.

A good V-curve has:

• A clear, symmetrical V shape with a well-defined minimum.
• Consistent measurements (points close to the fit line).
• Enough range on both sides of focus to define both arms of the V.

Caution

If the V-curve is flat or asymmetric, the step count or step size may be too small. Increase the range so the routine sweeps farther on both sides of focus. If measurements scatter wildly, increase the exposure time or check for wind and vibration.

Autofocus Settings

Configure autofocus behavior from the focuser settings (gear icon):

• Step count — The number of positions to measure on each side of the starting point. More steps give a better-defined curve but take longer. 7-9 total steps is typical.
• Step size — The number of focuser steps between each measurement position. This determines the total sweep range (step count x step size). Adjust based on your focuser’s critical focus zone.
• Initial direction — Whether to start by moving inward or outward. This rarely matters unless you have significant backlash.

Temperature Compensation

Temperature changes cause the optical path length to shift, pulling the system out of focus over the course of a session. ARIS can compensate automatically by adjusting the focuser based on temperature readings from the focuser’s built-in sensor.

Configure temperature compensation in the focuser settings:

• Enable/Disable — Toggle temperature compensation on or off.
• Steps per degree — How many focuser steps to move per degree Celsius of temperature change. This value is specific to your optical system. Run autofocus at two different temperatures during a session and divide the position difference by the temperature difference to determine your coefficient.
• Threshold — The minimum temperature change (in degrees) before a compensation move is triggered. Setting this too low causes unnecessary adjustments; too high lets focus drift. 0.5-1.0 degrees is a reasonable starting point.
• Direction — Whether focus moves inward or outward as temperature drops. Most refractors move inward (negative coefficient); reflectors vary.

Note

Temperature compensation pauses automatically during capture sequences to avoid moving the focuser mid-exposure. Moves are applied between exposures.

Backlash Compensation

Mechanical backlash causes the focuser to lag when reversing direction. If your focuser exhibits backlash, configure compensation in the focuser settings:

• Enable/Disable — Toggle backlash compensation.
• Backlash steps — The number of extra steps to overshoot when reversing direction. The focuser moves past the target by this amount, then moves back to the target, ensuring the final approach is always from the same direction.
• Direction — Set whether backlash compensation applies when moving inward or outward. Typically set to the direction opposite your most common focusing motion.

To measure your backlash: move the focuser outward to a reference point, note the position, then reverse direction and move back. The difference between the expected and actual position is your backlash in steps.

Best Practices

• Focus at the start of every session after the telescope has thermally equilibrated (typically 20-30 minutes after setup).
• Use a Bahtinov mask for visual confirmation if you have one. ARIS can analyze the Bahtinov diffraction pattern to confirm focus.
• Run autofocus after filter changes if your filters are not parfocal. Save per-filter focus offsets in your equipment profile to automate this.
• Enable temperature compensation for sessions longer than an hour. Refocus manually if you see HFR creeping up in your quality badges on the Imaging screen.
• Check focus periodically during long sequences by monitoring the HFR trend in your capture frames. A gradual increase indicates thermal drift that temperature compensation should be catching.