In deep-sky photography, the "back focus" directly determines whether stars remain sharp across the entire frame. If the back focus is incorrect, no matter how good the camera or how expensive the telescope, the stars in the image will be distorted. Today, we'll use the SC571CC as an example to guide you through understanding and adjusting the back focus step by step.

What is Back Focus?

Definition of Back Focus

Back focus refers to the optical distance from the mounting surface of the telescope (usually a threaded interface) to the camera's sensor plane. More precisely, it is the exact distance required to ensure optimal image quality from the optical system.

For most refracting telescopes and Newtonian reflectors, the standard back focus is typically 55mm. This value originates from the era of film cameras and has been widely adopted by modern astronomical accessories.

Why Must It Be Precise?

The optical design of a telescope assumes that light, after leaving the tube, needs to travel a specific distance to converge into a sharp image. If:

• Back focus is too short: Light converges too early, and the sensor is positioned after the convergence point, causing stars to appear "divergent" and blurred.
• Back focus is too long: Light converges too late, and the sensor is positioned before the convergence point, also resulting in blurred stars.

This leads to stars at the edges of the frame no longer being round, exhibiting issues like:

• Astigmatism (stars appearing elongated)
• Coma (stars appearing comet-like)
• Field curvature (inability to achieve focus simultaneously at the center and edges)

Simply put: if the back focus is incorrect, the entire image will not be sharp simultaneously.

SC571CC Back Focus Parameters

Before starting calculations, we need to know the SC571CC's inherent back focus.

Camera Body Back Focus

According to the SC571CC mechanical drawings:

• Camera body back focus: 17.5mm
• Thread mount: M42 x 0.75

This means the distance from the front threaded surface of the camera to the sensor plane is 17.5mm.

Included Adapter Rings

The SC571CC comes standard with:

• M48-M42 adapter ring (thickness: 16.5mm)
• M42 extension tube (thickness: 21mm)

These accessories help users achieve the standard 55mm back focus.

Calculating for Standard 55mm Back Focus

Basic Formula

Total Back Focus = Camera Body Back Focus + Adapter Ring A Thickness + Adapter Ring B Thickness + ... + Other Accessory Thicknesses
Our goal is: Total Back Focus = 55mm

Standard Configuration for SC571CC

Calculating outward from the camera:

• SC571CC Body: 17.5mm
• M42 Extension Tube: 21mm
• M48-M42 Adapter Ring: 16.5mm

Total: 17.5 + 21 + 16.5 = 55mm

This is the standard 55mm back focus configuration!

Practical Tips for Adjusting Back Focus

Tip 1: Calculate Outward from the Sensor

The most common mistake is calculating from the telescope towards the camera. The correct method is to calculate outward from the sensor:

• Determine the target total back focus (usually 55mm).
• Subtract the camera body's back focus (SC571CC: 17.5mm).
• Subtract the thickness of fixed accessories like filter wheels, OAG (Off-Axis Guider), etc., sequentially.
• The remaining distance is filled using various adapter rings or extension tubes.

Tip 2: Utilize Spacer Rings for Fine Adjustment

Even with accurate calculations, factors like thread tolerances or filter thickness in the actual optical path might require fine-tuning. It's recommended to have a set of spacer rings (e.g., 0.5mm, 1mm, 2mm) for precise adjustments.

Tip 3: Verify with Actual Imaging

After theoretical calculation, validate by taking actual images:

• Focus on a distant terrestrial object (or a bright star).
• Capture images, paying attention to stars at the edge of the frame.
• Magnify and check the shape of stars in the four corners.
• If stars are elongated in a consistent direction, the back focus needs adjustment.

Diagnosis Guide:

• Stars elongated radially (pointing toward the center): Back focus is too long.
• Stars elongated tangentially (perpendicular to the radial direction): Back focus is too short.

Summary

• Define the Goal: Determine the required back focus for your setup (usually 55mm).
• Calculate from the Sensor Outward: Start with the SC571CC's 17.5mm, then add the thickness of fixed accessories sequentially.
• Fill the Remaining Distance: Use adapter rings or extension tubes to reach the target total.
• Verify and Fine-tune with Actual Imaging: Check edge stars and use spacer rings for precise adjustments.