Signal to Noise Ratio and Exposure Times

When planning an observing session at the Sierra Stars Observatory you need to consider many variables to get the optimum results for your project. Are you doing quantitative photometry or astrometry or are you striving to get the highest quality images for esthetic reasons (high quality artistic 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).

At SSO we take high-quality calibration frames each night to give you the best image data possible. Therefore, the ultimate quality of your image data is primarily determined by the transparency and seeing of our observatory sites during the time your images are scheduled and taken.

Scientific Data Considerations

If your project is to do photometry of object(s), your goal is to obtain a minimum (or optimum) SNR with specific filters appropriate for the goal you have in mind. If your project is to do astrometry, then you are likely trying to achieve a least the minimum SNR of the intended object that will enable you to measure an accurate position (right ascension and declination) and time (if the object is an asteroid, comet, or spacecraft).

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 submit your data 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 work. Basically 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 and being able to use stars in the image with an appropriate magnitude and color index for comparison in your 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 the more stars you use for comparison in the same field, the more precise your measurements will be. 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 standards so that measurements can be compared equally. During the past several decades optical filters were devised to only allow light in a narrow part of the spectrum to pass to a photometer (used less and less today) or CCD chip. 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. SSO uses 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 the SSO network check out the Internet references below.

Note: Click Here for a convenient online SNR calculator you can use to determine appropriate exposure time to attain your required SNR for the images of your photometry project.

Astrometry Projects

Astrometry is the technique of measuring the position of objects in the sky. Most astrometry projects using the SSO network are measuring the positions of asteroids and comets. To get an accurate astrometric measurement of a moving object such as an asteroid or comet you must determine its exact right ascension (RA) and declination (DEC) coordinates and the precise time. RA and DEC coordinates in images are computed using the World Coordinate System (WCS) method. The WCS method matches the pattern of stars in your image with the pattern of stars in either the Hubble Guide Star Catalog (GSC) and/or the USNO Catalog and, based on the image scale of your image, enables you to measure the exact RA and DEC coordinates of any point in the image. You can use one of the many available image processing software programs to perform these measurements.

Exact time measurements of when an image was taken are critical for determining the ephemeris (orbital calculation) of asteroids and comets. The SSO software records the exact time of the end of the exposure in the FITs header of each image. The SSO control computer keeps time accurately to a fraction of a second using an NTP server.

SNR is much less critical than photometry work when doing astrometry work. Primarily you are measuring the centroid of an object in your images. Even objects with a relatively low SNR (< 10) can be measured accurately.

Some of our customers do astrometry work discovering new asteroids and follow up measurements of recently discovered asteroids and comets. If you decide to do an astrometry project and want to submit your data to the Minor Planet Center (MPC), you will have to use the SSO MPC Observatory Code: G68 for your submissions.

Note: Click Here for a convenient online SNR calculator you can use to determine appropriate exposure time to attain your required SNR for the images of your photometry project.

Imaging for Art and Fun Projects

If you are interested in imaging objects for their beauty and unique characteristics, the SSO network offers huge opportunities for you to explore. SNR is for quantitative analysis is not generally a concern for this type of work. 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 color bands (B, V, R and possible I 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 achieve. 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 fascinated with 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 and 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 SSO telescope offers 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!

SSO will provide you with good 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 the image data you take. 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. See the references below for some of the astronomical image processing software that is available.

References Available on the Internet

Below are some links to places on the Internet to get started. These are only a few places to start. Doing web searches will enable you to find much more information.

Signal to Noise Ratio Calculator

A convenient SNR calculator for planning photometry and astrometry projects.