Signal to Noise Ratio and Exposure Times


When planning an observing session using the Sierra Stars Observatory Network telescopes you need to consider many variables to get the optimum results for your project. Is your goal to schedule images for high-precision photometry or astrometry measurements or to acquire the best quality images for an esthetic image project (high quality art work)? Your intended purpose determines how you schedule your exposure times to achieve your intended goal. If you are taking images for scientific use (photometry and astrometry), then you are most likely concerned with the quantitative value of your data (how accurate and precise it is). If this is the case, then the signal to noise ratio (SNR) is likely to be most important to you. If you are taking images for the highest quality esthetic value, then you are likely to be striving for high-contrast images that show interesting detail of the object (probably using an LRGB or RGB color combination using various filters).


Scientific Data Considerations

If your project goal is to do photometry of object(s), your objective is to obtain a minimum (or optimum) SNR with specific filters appropriate for the object you want to image. If your project is to do astrometry, then you want to achieve a SNR of an object good enough to measure an accurate position (right ascension and declination) and, if the object is an asteroid, comet, or spacecraft, a precise time that the image was taken.

The following discussion is meant to give you general guidelines on how to proceed with your project. There are many books and sources on the Web that you can refer to for a more detailed explanation on how to be most effective and how to submit your data measurements to scientific organizations and institutions. These guidelines are only to point you in the right direction.


Photometry Projects

For photometry projects the goal is typically to achieve a SNR that will give you a desired precision for measuring the magnitudes of objects using specific filters. If your target object is relatively bright then you can obtain high SNR data and precise magnitude measurements in relatively short exposures. There are two major methods for taking photometric measurements: all-sky photometry and differential photometry. All-sky photometry is much more complicated, involves many computations, and requires pristine conditions. Differential photometry is much easier to learn (and more forgiving) to use for doing photometry projects. Using the differential photometry technique you compare the magnitude of the object being measured to other non-variable stars within the same image. Measurements in the same field of view in images cancel out the air mass and atmospheric disturbances that must be figured into the calculations of all-sky photometry measurements.

Ultimately the accuracy of your photometric measurements depends on the SNR of the object you are measuring. In addition, your images need to have a few non-variable stars in the image with an appropriate magnitude and color index for comparison in your differential photometry measurements. The absolute error in your measurements is a direct function of the SNR. The higher the SNR you achieve the more accurate your photometric measurements become. If you want to achieve an accuracy of 0.02 magnitude or better for your photometric measurements, then you must achieve a SNR of 100 or greater. Also using two or more stars in the same field of view enables you to achieve more precise measurements. You can use photometric measurements of fainter objects in images with a SNR as low as 10 or 20 (for example, in the case where the object may be so faint it is near the limit of practical exposure times of the telescope). However, the inherent accuracy of your measurements will necessarily be less.

Scientific photometric measurements are often based on standard optical filters so that measurements taken by different people and instruments can be compared equally. The filters are referred to as band-pass filters because they only pass light in a restricted part of the spectrum and cut off light outside of that “band”. The standard series of photometric filters used for scientific research with CCD cameras by research observatories today is the Johnson-Cousins UBVRI standard, where the filters pass light in the Ultraviolet, Blue, Visual (green), Red, and Infrared parts of the spectrum respectively. Some of the telescope in SSON have a BVRI subset of the UBVRI filters and a Clear (no filter) position to allow all ambient light to collect on the CCD chip.

There are many opportunities for doing photometry projects: variable stars, cataclysmic variables, asteroid light curves, and so on. If you are interested in learning more about doing photometry using SSON you can find many valuable resources searching the internet.


Esthetic Imaging for Art and Fun Projects

If you are interested in imaging objects for their beauty and unique characteristics, the SSON network offers great opportunities for you to explore. What you are most likely striving to achieve is a high-contrast final image that shows interesting detail and, if you are doing color composite images, a suitable saturation of the colors using available filters.

There is no fixed “formula” for exposure times for taking images for esthetic projects. If the subject is faint and extended then, in general, the longer the total exposure times the more contrast and detail you will attain. However, if there are bright stars in your composition, then combining (stacking) shorter exposures that do not overexpose (bloom) the bright stars might give a more appealing final result than combining longer exposure images. Often it’s simply a matter of experimentation to see what works best.

When you first start out taking images you are likely going to be excited about imaging large, brighter objects (such as the well-known Messier objects). These are good objects to try first if you have little or no experience imaging astronomical objects. The resulting images produce rewarding results that you can compare with many examples you’ll find on the Web. But why stop there? Beyond the dozens of such objects that are commonly imaged there are literally thousands of interesting objects that are rarely, if ever, imaged well. The SSON telescopes offer a relatively large image scale and wide field of view that will enable you to image objects (and multiple objects in a field) that are too small and/or faint to image well with smaller and shorter focal length telescopes. For example, there are many smaller galaxies, galaxy clusters, planetary nebula, and so on that would be interesting and attractive subjects to image. The opportunities for doing this type of work are wide open.

SSON provides you with excellent raw imaging data for you to work with. To produce excellent high-quality final compositions, you’ll have to use image processing software that enables you to stack images, combine images for color composites, and other techniques that will bring out the interesting details in your image data. Several companies sell excellent astronomical image processing software you can use and most of them offer trial versions for you to try out before you buy. There is also software available for free that you can download from the Internet. Search the internet to see what is currently available.