Star cameras, particularly those that use CMOS technology, are bursting with features and adjustments that all, in one way or another, impact the picture data they create. Given what it accomplishes, one might consider sensor gain (or ISO on DSLRs) one of the most potent and significant adjustments on any camera offset.
But again often has a partner unfamiliar to many people called offset. For many people, offset seems tied to gain somehow, but how it operates and its authentic connection to gain is unclear. This article tries to clarify offset and its relationship to achieving and identify the ideal offset setting for using the camera offset.
Jon Minnick of Cloud Break Optics (Seattle’s most extraordinary astronomy emporium) published a very excellent post on acquisition and processing as part of a multi-part series on astronomy if you’re genuinely unfamiliar with the idea of sensor acquisition and processing. I advise you to begin with that before moving on to this.
Many people find offset weird, and for a good cause. None of the typical performance graphs for camera offset define or quantify them in any manner; camera offset and sensor statistics are at most limited to serving as a reference.
On the other hand, the “Offset” slide or entering the field is always placed adjacent to, or at the very least near, the gain. What is it, then, and why do I need to know?
Offset does precisely what its name suggests: it applies a counterbalance to the histogram curve that causes it to move a particular amount farther to the right on the histogram. Doing this ensures that the least effective possible recording pixel is not revealed and is instead covered in black.
This is crucial in astronomy because specific sensor pixels might be positioned to concentrate on a dark region and will, as a result, catch considerably fewer photons than pixels illuminated by light from a star, nebula, or another source—an object into which light is emitted.
By staying away from black-clipped-black pixels, you can prevent pixels from having a negative signal or, at the most, a value of 0. A picture with white-clipped pixels is this reverse. No data is present in any successfully chopped pixel that can be processed in any significant way.
Choose what to ignore.
We will learn how to assess the impact of the offset in the technique employed and identify what constitutes a “good” offset setting to use, independent of camera offset or model since I prefer to teach by example. First, however, I want to bring you one thing concerning offset terminology before we continue.
The term “optimal” offset is a misnomer. In our industry seeking the ideal or ideal setting for a certain parameter in a particular circumstance is instinctual, but editing is not really one of those fields. Nevertheless, as certain restrictions exist, you should consider whether you have an adequate offset.
You’ll need camera offset control software that can operate the camera using native camera drivers to do this configuration properly. The camera settings may be managed directly and interactively, thanks to this.
I use the excellent sharp cap software for this, but it can be done with any app that can capture images in real-time using local drivers and offer a histogram that can be critically examined. One such application is Fire Capture.
Working on your connections is an excellent way to get money. What about the offset, though? What value—0, 100, or 42—should I enter? Here is how I will respond to this query.
As with dark or bias calibration photographs, first shut the camera offset to ensure no light may enter the sensor. This does not need refrigeration; therefore, it may be closed if necessary. Set the following settings after connecting the camera to the software of your choice:
- Display with the lowest display duration feasible
- Gain is adjusted to the desired level for the camera.
- 0 is the offset value.
ZWO users (and maybe others): short cap refers to the default option for ZWO cameras as Brightness and places it in the Image Controls section rather than the Camera Controls section as it does for QHY cameras.
Other kinds of camera offset could also experience this. If you know that your camera has an offset knob for rotating but are having trouble rapidly locating it in the short cap, look around (or ask).
Users of OSC cameras: It is often advised to do this task in a short cap with the camera output set to RAW16 and debayering enabled. It may be perplexing since this causes the histogram to appear in the monochromatic region without displaying the different R, G, and B channels. After all, the hard pixel array is what we are interested in.
Then, open the histogram display and enlarge it as much as possible (in sharp cap: Tools > Histogram). It may seem to be vacant, but it is not. This is because a significant portion of the pixels in the histogram that are cut out in black is likely to push the curve to the left side of the histogram. With offset, we hope to resolve this.
The connection between acquisition and surrender
We’ll only cover what the offset accomplishes and how to calculate the application’s value. What transpires if we raise the profit without increasing the compensation? The quick explanation is that we run the danger of clipping the blacks once again since raising the gain will make the curve near the end of the histogram thicker or larger. The low-value pixels will be clipped off when the income rises to the point where the left side of the curve once again touches the left edge of the histogram.
Concerns about calibration frames
Like when you modify the gain or operating temperature of the sensor, changing the offset necessitates the production of a fresh batch of frames. This is because altering the compensation also affects the image’s pixel value.
It’s a good idea to ensure you capture the set order and the gain and temperature in the file names if you alter your photographs using master calibration frames (master dark, master bias, master flats, etc.). Get confused and lose your way. A filename format token and the OFFSET key, or something similar, are sometimes used by apps that provide camera drivers.
To modify the application’s offset.
Changing the offset in different astronomy applications might be a little complicated. This is because these applications often operate cameras either “natively,” using a vendor-supplied software interface (“SDK”) to access the camera offset, or via a driver system like ASCOM or INDI.
Because native camera offset access relies on a vendor SDK, programs that provide this option often let the offset be set up and altered within the program.
However, if you are using a program that connects to the camera via the ASCOM driver, you may not have access to the removal control through the program; instead, you would need to do the removal through a different notion utilizing the vendor’s ASCOM driver settings.
A driver architecture for controlling the camera offset from the astrophotography application was added to ASCOM 6.5, which was released in July 2020. However, camera manufacturers and ASCOM client application developers may need to update their ASCOM drivers and applications to make this functionality available to users.