Diagnosing Odd Discrepancies in Astrophotography Images: Tilt or Collimation?

As astrophotographers, we strive for the sharpest, most pristine images possible. However, sometimes, even after meticulous polar alignment and guiding, we encounter peculiar artifacts that mar our celestial captures. Recently, a fellow astrophotographer shared an image exhibiting star discrepancies in the corners – fine in one, but noticeably out of focus and exhibiting slight star trails in another. This prompts a crucial question: what's causing these anomalies?

Identifying the Culprit: Tilt vs. Collimation

The symptoms described – uneven focus and star elongation in specific corners – strongly point towards either sensor tilt or a collimation error. Let's break down what these mean:

Sensor Tilt

Sensor tilt occurs when the camera's sensor is not perfectly perpendicular to the incoming light path. This misalignment can cause stars to appear sharper in some areas of the frame and softer in others. Many experienced astrophotographers lean towards tilt as the primary suspect when issues are localized to specific corners, as observed in the shared image.

• Diagnosis: Software like NINA's "Hocus Focus" can be instrumental in diagnosing sensor tilt by analyzing star shapes across the frame.
• Solutions: Addressing tilt often involves careful adjustment of the camera's mounting or the use of shims to ensure the sensor is perfectly aligned.

Collimation Errors

Collimation refers to the alignment of the optical components within your telescope. If the mirrors or lenses are not perfectly aligned, it can lead to distorted star shapes and a general loss of sharpness across the entire image, though it can also manifest as corner issues.

• Distinguishing from Tilt: While collimation errors can cause oblong stars, they tend to be more prevalent across all regions of the image and in a consistent orientation. However, a minor collimation error can be exacerbated by tilt.
• Potential Contributors: Reducers, especially, can be sensitive to collimation and can magnify existing tilt issues. The thumb screws that secure the camera can also introduce slight tilt if not properly tightened or if there's play in the mechanism.

Other Considerations

Beyond tilt and collimation, a few other factors could be at play:

• Dew Heaters: In some cases, Celestron dew heater rings have been known to potentially introduce optical pinching, which could lead to such artifacts. It might be worth testing by temporarily removing the dew heater.
• Tracking/Guiding Errors: While the user double-checked guiding, significant guiding errors (even around 0.5-0.8 arcseconds at a given focal length) can cause star elongation that might be mistaken for tilt or collimation issues.

A Practical Approach

It's important to remember that minor tilt issues are often manageable. As one comment suggested, a bit of processing magic with tools like BlurX can sometimes correct subtle imperfections without needing to chase every minute adjustment. However, for more pronounced issues, a systematic approach to diagnosing and resolving tilt or collimation problems is key to achieving stellar results.

This exploration highlights the intricate nature of astrophotography optics and the importance of understanding these subtle issues to improve your celestial imagery.