This page is meant for amateurs who want to take astrophotographs with a small telescope and a camera. Here is a rough field guide to determining the kind of exposure you will need to capture the target object
The first thing you need to know is the concept of a stop ratio or the f – number. Yes these are those funny looking numbers you might have seen on the lens of your camera… they look something like
f/1.4, 2, 2.4, 4, 5.6 etc
Officially speaking the f – number is the ratio of the focal length to the diameter of the optical system. SO, before we do anything, this is an important piece of information you need to know before you go out in the field.
To determine the f number (f/#) of your system, you need to know what kind of optical system is going to be used. Astronomers typically use two methods for photography. The more easier method is called the AFOCAL PROJECTION method, and the more advanced method is called the PRIME FOCUS method. Most professionals with high end gear prefer to use the prime focus method due to the superior image quality produced by it.
AFOCAL projection is much easier, requires simpler & cheaper equipment and most amateurs begin with this method, although it has more optical components compared to the prime focal method
Determining the F-number
If you’re using Afocal projection method, your effective stop ratio is influenced by two optical components. The first being the optical tube and the other being the camera
To calculate the effective f/# of your afocal system
f/N ‘ = (f/N) * Camera FL/Eyepiece FL
So lets say we have a 114 mm f/11 Celestron scope with a 25 mm eyepiece and a 50 mm Pentax SLR at the eyepiece
We will see an effective f/# of (f/11)*(50 mm / 25 mm) ~ f/5.5
Hence you can see afocal projection makes the system ‘faster’ than before
In the prime focus projection system, there are no intemediary lens systems. The focussed wavefront of the incoming starlight is made to fall directly onto the film or CCD/CMOS sensor
So there are no calculations involved here. If I choose to have a prime focal setup for the same Celestron 114 scope, I will have the same stop ratio as the tube… i.e. f/11
Exposures and Stop Ratios
So what is all the relationship between stop ratios and exposure times.
If you notice the f/# on your cameras, they are each successive number is 1.414 ( which is square root of 2) times bigger than its previous one. DECREASING the stop ratio by one step i.e 1.414 times allows twice the light exposing the detector
So a system with f/4 is twice as fast as a f/5.6 system, requiring only half the exposure time as the latter
Note that modern cameras have stop ratios in half-steps. What that means is that between a f/4 and f/5.6 there will be a median value of f/4.8 also.
This information is handy while trying to extrapolate exposure information of an object using different optical equipment. You might want to take a picture of Andromeda galaxy using your setup but the only exposure information may be from somebody else’s page
Absolute Exposure Scales for different objects.
Here is a crib sheet on guessing the right ball-park exposure settings for your imaging project. Choose an appropriate ISO and a stop ratio you are using, and look up the exposure times for the appropriate target
|DEEP SKY OBJECTS, time given in minutes
|Open and Glob. Clusters
|SOLAR SYSTEM OBJECTS, time given in seconds>
|Jupiter with Moons
|Naked Eye Comet Nucleus
|Sun w/ Filter
|Total Solar Eclipse – Corona
|Moon 50% Phase
|Moon Full Phase
* Double the exposure if object is only 8-14 degrees above horizon.
* Quadruple (4x) exposure if 5-8 degrees high, or for thin clouds.
* If ISO is higher, decrease exposure time by same proportion, i.e. if ISO is doubled (or if film is hypersensitized) reduce exposure time by 1/2. ( God people take so much pain with film!)
* Keep a record of your own exposure data, it will save time energy & film
* Use the f/11 exposure data if you are using an f/10 SCT.
The data presented above is (c) 1991, 1997, Jeffrey R. Charles
An AAAD & TWAN Presentation