Overcoming Challenges in Astrophotography

Star Bloating and Star Trailing problems

star bloating and star trailing

What is Star Bloating?

The Horsehead Nebula with bloated Stars
The Horsehead Nebula with bloated stars

In astrophotography, star bloating refers to stars appearing larger and less defined than they actually are. It is typically caused by factors such as optical aberrations and camera settings.

One common cause of star bloating is spherical aberration, which occurs when light rays pass through the outer parts of a lens or mirror focus at a slightly different point than those passing through the centre. This can result in stars appearing larger and softer, with their light spreading outwards.

Another factor that can contribute to star bloating is poor focus. If the camera is not properly focused on the stars, they may appear larger and less distinct. This can happen if the lens or telescope is slightly out of focus, or if the autofocus system is not accurately locking onto the stars.

Additionally, camera settings such as high ISO values and long exposure times can contribute to star bloating. Higher ISO settings amplify noise in the image, which can make stars appear more diffuse. Long exposures can also cause the stars to appear larger due to the accumulation of light over time.

To minimize star bloating in astrophotography, it is important to use high-quality optics that minimise spherical aberration. Ensuring accurate focus through manual focusing or precise autofocus calibration is also crucial. It can be helpful to experiment with different camera settings, such as lower ISO values and shorter exposure times, to achieve sharper and more defined star images.

Star bloating can also be attributed to various factors related to equipment, image processing, and environmental conditions.

Let’s talk about this in more detail:

The Mount Type

One crucial consideration is the type of mount used during your astrophotography session.
An Alt-Azimuth (AZ) mount lacks the capability to rotate to compensate for the Earth’s movement, which can result in no field rotation.

Types of mounts

This means that as the Earth rotates, the stars in the field of view will lead to blurry and elongated star images. To overcome this, multiple photos need to be taken and stacked, while post-processing techniques like ‘Morphological transformation’ can be employed to reduce the elongation and dim the stars in postprocessing you will find that it will require a lot of time and effort, to get your desired result.

On the other hand, an Equatorial (EQ) mount, when properly levelled and aligned, moves the camera at the same rate as the Earth’s rotation. This applies field rotation and allows for long exposures, which gives you detailed and sharp images of stars.

The Type of Telescope

Different Types of Telescopes

The telescope used also plays a crucial role in star bloating. For reflector telescopes, proper collimation is essential to ensure accurate focusing and minimise aberrations. Refractor telescopes, which can range from single-lens to quintuplet designs, offer varying levels of performance in star acquisition and overall image quality.

Your Image Processing

When it comes to image processing, extreme stretches applied to astrophotos can lead to star bloating. Stars have a Gaussian profile, and as the brightness of dimmer areas is increased during processing, the saturation of the stars also increases. This results in larger and bloated star appearances.

PixInsight
Image processing in Pixinsight

It’s important to find a balance in stretching to preserve the natural appearance of stars while enhancing the desired features of the overall result.

Colour and Temperature of the Stars

Coloured stars
Field of colourful stars.

Additionally, the colour and temperature of stars can impact focus and contribute to bloating effects. Refractor telescopes, designed primarily for visual observations, may not bring the entire spectrum recorded by a CCD camera into a common focus, especially in extreme wavelengths such as violet or near-infrared. As a result, stars emitting in these wavelengths may appear bloated or defocused. Using infrared cut filters can help eliminate interfering wavelengths and enhance focus accuracy.

Atmospheric Conditions

‘Seeing’ also plays a significant role in star bloating. Seeing refers to the stability of the atmosphere and is measured in arc-seconds. Higher values, around 4-5 arc-seconds, result in larger star diameters, while better-seeing conditions, around 2 arc-seconds, lead to smaller and more defined stars. Good seeing conditions are often found at elevated locations like mountains or islands, where atmospheric turbulence is reduced.

Filters used

Comparing different elements in astrophotos requires careful consideration to avoid misleading conclusions. For instance, the Ha filter, which filters out most of the visible spectrum, reduces the light reaching the camera and produces smaller stars. However, the red channel of the same object may transmit more light for each star compared to the Ha filter, leading to blooming effects.

Characteristics of the Object

The relative brightness and characteristics of the objects being captured can impact star bloating. For example, in comparing the Orion and the Eagle nebulae, the Eagle Nebula is brighter and located in an area densely packed with stars.

Elements of an Objects

This high stellar density results in an abundance of starlight, making it easier to overcome read noise. In contrast, (M42) the Orion Nebula is situated in a region with larger dust clouds and lower overall signal strength. Excessive stretching of (M16) the Eagle image to make it appear brighter than the Eagle nebula can cause the stars in the Orion nebula image to appear bloated due to the stretching of their faint halos. It is essential to carefully adjust the stretch to maintain a balance between preserving details and avoiding bloating effects.

Normalization techniques such as aligning the median or mean values of images and applying the same Stretch Transfer Function (STF) can help achieve a fairer and more accurate comparison. Aligning the red channel of the Eagle Nebula and employing the same STF can provide further insights into the contrast and bloating differences.

The blue channel is known to exhibit more star bloating compared to other channels, and this should be taken into account during image processing.

Polar Alignment accuracy

Lastly, accurate polar alignment is crucial to minimise elongated stars caused by misalignment. Ensuring a levelled tripod and precise alignment of the mount with the South Celestial Pole (S.C.P) contribute to sharper and well-defined star images.

In conclusion, star bloating in telescope imaging is a complex issue influenced by factors such as mount type, telescope design, image processing techniques, star colour and temperature, atmospheric conditions, and object characteristics. By understanding these factors and employing appropriate techniques, astrophotographers can effectively mitigate star bloating and capture clearer, more detailed, and accurate astrophotos.

Star Trails

Star Trails
Star trails

If you’re tired of those pesky star trails appearing in your night sky photographs then I hope I can help you in this blog.
There’s no denying that capturing the beauty of the night sky can be challenging, especially when it comes to avoiding star trails. But fear not! In this blog post, I’ll share my own tips and techniques to help you capture clear, crisp images of the celestial wonders above without those pesky streaks.

What are Star Trails and their Causes

Before diving into the solutions, it’s important to understand what causes those unwanted star trails. Star trails occur due to the Earth’s rotation during long exposures. As the Earth moves, the stars appear to move across the sky, resulting in streaks in your images. To avoid star trails, we need to take measures to counteract this rotation.

Earth Spins Around An Axis

Essential Equipment for Star Trail-Free Photography:

To capture stunning night sky images without star trails, make sure you have the following equipment:

Star Trail Photography Equipment

Sturdy Tripod:

A solid tripod is essential to keep your camera steady during long exposures. Invest in a tripod that can handle the weight of your camera and lens, providing stability for crisp, shake-free images.

Saxon Heavy Duty Tripod – $229.95

Wide-Angle Lens:

Opt for a wide-angle lens with a low aperture (e.g., f/2.8 or lower) to capture a broader view of the night sky and allow more light into your camera.

Remote Shutter Release or Intervalometer:

Using a remote shutter release or intervalometer helps eliminate camera shake caused by manually pressing the shutter button, resulting in sharper images.

Mastering DSLR Camera Settings for Star Trail-Free Photography:

Camera settings

To avoid star trails, it’s crucial to nail your camera settings.
Here’s what you need to consider:

Aperture

Set your lens to a narrower aperture (e.g., f/4 or higher) to reduce the amount of light entering your camera. This helps maintain shorter exposure times, minimizing the chances of star trails.

ISO

Use a lower ISO setting (e.g., ISO 100-400) to decrease the camera’s sensitivity to light. This reduces the risk of overexposing the stars and creating star trails.

Shutter Speed

Opt for shorter exposures, typically between 10 and 30 seconds, to limit the Earth’s rotation from causing star trails. Experiment with different shutter speeds to find the perfect balance between exposure and avoiding trails.

Proper Focus Techniques

Achieving accurate focus is crucial for star trail-free photography.

Follow these tips:

Manual Focus

Switch your lens to manual focus mode (M) to have full control over the focus point.
Autofocus can struggle in low-light conditions.

Infinity Focus

Set your lens to the infinity (∞) symbol, ensuring distant stars are in focus.
Use the Live View mode or focus peaking to fine-tune the focus accurately.

Eliminating External Factors:

Sometimes, external factors can introduce unwanted star trails.
Consider the following:

1. Wind

Wind can cause vibrations that result in blurred images. Shield your camera from wind or use a windbreak, such as a jacket or cloth, to minimize movement.

2. Camera Stability

Ensure your tripod is securely set up on stable ground. Avoid accidentally nudging the tripod or touching the camera during exposure.

3. Post-Processing Magic

If star trails still manage to sneak into your shots then post-processing software can come to your rescue. Use photo-editing tools like Adobe Photoshop or similar software to remove any remaining star trails and enhance the overall image quality.

Conclusion

With the right techniques and equipment, you can capture breathtaking night sky photographs free from star trails.

If you’re passionate about capturing the beauty of the cosmos, investing in an Equatorial Mount, will undoubtedly elevate your astrophotography game. Say goodbye to star trails and hello to stunning, crisp images of the night sky. Get ready to embark on a captivating journey through the universe, one breathtaking image at a time. Happy shooting!

saxon AZEQ6 GT Equatorial Mount

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