Foto-Gorilla · Knowledge
Checking, organising and archiving astrophotos
Select good data, compare processing versions and find your projects again.
Assessing Image Quality – FWHM, Eccentricity and Subframe Selection
Not every individual exposure has the same quality. Even when a telescope automatically photographs a celestial object over several hours, conditions can change during that time. Atmospheric turbulence, wind, thin clouds, tracking deviations and increasing background brightness can affect individual images to different degrees. Assessing and selecting individual exposures is therefore one of the most important steps in careful astrophotography processing.
In astrophotography, an individual exposure is often referred to as a subframe or light frame. During stacking, many of these individual images are aligned and combined into a single final image. The quality of the finished stack depends not only on the number of images used, but also on their sharpness, signal-to-noise ratio, star shape and background brightness.
An important measurement of image sharpness is FWHM – Full Width at Half Maximum. Put simply, it describes the width of a star’s brightness profile at half its maximum intensity. Because distant stars appear approximately point-like, their images are useful for assessing optical and atmospheric image quality.
In comparable exposures, a smaller FWHM often indicates more compact stars and better representation of fine details. A higher value, by contrast, may indicate atmospheric turbulence, inaccurate focus or other influences. However, the measured value also depends on the software used and its calculation method. FWHM values should therefore only be compared directly under comparable conditions.
In addition to FWHM, eccentricity is an important quality parameter. It describes how strongly the shape of a star deviates from a circle. A circular star image has low eccentricity, while elongated stars may show higher values. Such elongated star shapes can be caused, for example, by tracking problems, wind or other movement during the exposure.
It is important to note that eccentricity cannot clearly identify every possible cause of poor star shape. Optical aberrations or unusual star profiles can also influence the measured value. Several quality characteristics and the actual individual exposure should therefore always be considered together.
Organising Multi-Night Projects Effectively
Many deep-sky objects benefit significantly from more total integration time than can reasonably be collected in a single night. Particularly faint nebulae, galaxies and fine dust structures often require several hours or even several nights before the desired signal stands out clearly from the background.
A multi-night project means imaging the same celestial object on several evenings or nights. The individual sessions are later combined into a single dataset and stacked. This increases the total integration without requiring all of the exposure time to be collected in one night.
This is particularly practical with my Seestar S50 Pro. The compact system makes it easy to return to the same target on several evenings and collect additional image data. A project can therefore grow step by step.
Good organisation is important. Each session should be documented as clearly as possible. This includes the date, imaging time, individual exposure duration, total integration time, filter used and any special conditions during the night.
Information about Moon phase, transparency, seeing and background brightness can also be useful later. If two nights had noticeably different conditions, this makes it easier to understand why the raw data differ.
For later processing, it is useful to keep the individual exposures from each night separate at first. This makes it clear which files belong to which session.
Common Astrophotography Mistakes with the Seestar S50 Pro
Even a modern smart telescope cannot prevent mistakes from occurring during an astrophotography session. The Seestar S50 Pro automates many processes and makes it much easier to get started. Nevertheless, careful preparation, suitable imaging conditions and reviewing the captured data remain essential for a high-quality result.
A common mistake begins with the setup itself. An unstable surface, an insufficiently secured tripod or an unfavourable position can cause movement during imaging. Especially on balconies, wooden terraces or in stronger winds, even small vibrations can affect star shapes. The telescope should therefore stand on a stable surface and be protected as far as possible from wind and accidental contact.
Inaccurate alignment or levelling can also make reliable operation more difficult. The setup should therefore be checked using the guidance in the Seestar app. A stable setup is more important than trying to compensate later for motion blur with overly aggressive sharpening.
Another common mistake is choosing an unsuitable celestial target. Not every subject is equally appropriate for every night and every season. Imaging a very faint galaxy under a bright Moon or a low nebula above a heavily illuminated city will inevitably reduce data quality.
The combination of target selection, Moon phase, transparency and target altitude is therefore especially important. A carefully planned image of a well-placed target can produce better results than a longer session under unfavourable conditions.
The available observing time is also often misjudged. A target may remain above the horizon for several hours, but be high enough and free of obstructions only during a smaller time window. Houses, trees and mountains can shorten the actually usable imaging window even further.
FITS, TIFF, JPEG and the Right Bit Depth in Astrophotography
In astrophotography, the quality of the exposure is not the only factor that determines how much detail can later be revealed. The file format and bit depth also play an important role. Especially with faint nebulae and galaxies, very small differences in brightness often need to be processed. If image information is reduced too early or altered by unsuitable storage, later processing can become more difficult.
A particularly important format is FITS – Flexible Image Transport System. It was developed for astronomical data processing and can contain extensive information about an exposure in addition to the actual image data. This may include details such as exposure time, image dimensions or astronomical coordinates, provided this information is available when the file is saved.
FITS is widely used in professional astronomy and is well suited to processing astronomical raw data and stacked images. It can store different numerical data types, including integer and floating-point values. This allows image information with a wide range of values to be processed without prematurely restricting it to the typical display range of a finished photograph.
It is particularly important to distinguish between the file format and the processing state of an image. A FITS file is not automatically linear, and a TIFF file is not automatically non-linear. What matters is which image data has actually been stored and which processing steps have already been applied.
In a typical deep-sky workflow, largely linear image data is initially used after stacking. These data often contain a wide range of faint brightness values that are barely visible on the screen without additional display adjustment. Only through later stretching do these details become clearly visible to the human eye.
If an opened FITS image initially appears almost black, this does not mean that the exposure is empty or unsuccessful. Often, only the screen display needs to be adjusted. It is important to distinguish between a temporary preview and a brightness transformation that is actually applied to the image data.
Archiving Astrophotographs Properly – Raw Data, Master Files and Backups
A successful astrophotography image is often created over several nights. Especially with faint nebulae and galaxies, many hours of total integration time may be required. Added to this are the preparation, stacking and sometimes extensive image processing. Long-term archiving of the images is therefore an important part of astrophotography. Anyone who stores only the finished image may lose valuable opportunities for later reprocessing.
The foundation of a good archive is the raw data. This includes the individual light frames, meaning the actual exposures of the astronomical object. They contain the original image information and make it possible to register, analyse and stack an entire imaging session again later. Especially when your processing skills improve or new software becomes available, older raw data can suddenly become particularly interesting again.
For my Seestar S50 Pro, this means transferring the available original exposures to a computer or suitable storage medium as soon as possible. Which files are provided depends on the imaging and storage mode used. The important point is not to delete the original data too quickly simply because an attractively processed result already exists.
In addition to the light frames, other files may also belong to an imaging session. These can include calibration frames, provided they are supplied by the device and workflow being used or have been created separately. Information about the imaging settings, the filter method used and special conditions during the night can also be helpful for later processing.
Another important component is the master stack. It is created by registering and combining the selected individual exposures. A carefully created linear master stack makes it possible to restart the image-processing workflow later without having to process all the raw data completely again. It should therefore be stored in a suitable format with sufficient numerical precision.
A master stack that has not yet been permanently stretched is particularly valuable. With such linear image data, background corrections, colour calibration and other suitable processing steps can be repeated later. A result stored only as a JPEG does not offer this flexibility.