The focal ratio is the ratio of the focal length to the aperture. So, a telescope with a 2500 mm focal length, and 250 mm aperture is a f/10 scope. A scope with a smaller focal ratio always benefits the users in imaging as it shortens the exposure time. Some people will call it photographic speed.
Why it takes more time to have the same result using the same imager but a slower telescope? Let’s say I have a f/5 and f/10 telescopes and both of them have the same aperture. Let’s make them 8″. Now, I move from f/5 to f/10. Since the aperture is fixed, focal length is the only increasing factor. In this case, it is doubled. As the image you see is in 2 dimensions, it needs a least 2 rays of lights to form an area. Therefore, the lights are spreaded into a factor of 4. In conclusion, you take 4X more the exposure time to get the same S/N.
*S/N refers to Signal/Noise ratio
Below is the formula to calculate the exposure factor,
Exposure Factor = (Focal ratio 1)^2 / (Focal ratio 2)^2
Once you figured out the exposure factor, you know how much is A scope faster than B scope or vice versa.
To master your imaging platform, you’ll have to know the resolution of your setup. The setup mentioned is consisted of a telescope optical tube assembly (OTA), a mount and a camera.
Let’s say, if your setup’s resolution is sub arcsec/pixel (less than 1 arcsec/pixel), you need a very stable and precise mount, usually a high end German Equatorial Mount (GEM), for this. As the resolution is higher (the lower the value of arcsec/pixel), your setup is more sensitive to vibration, atmostpheric turbulence, wind, flexure between optical tube and GEM head and the mis-polar-alignment. In a simple phrase, the margin of error is limited.
Therefore, before you start taking image, you should know the highest possible resolution of your setup so that you do not end up with a imaging resolution that exceeded the conditions allowed. Nobody likes poor quality photo. Well, let’s get into the math part.
To calculate the resolution, follow the simple formula:
Resolution (arcsecond) = [CCD Pixel Size (in micron) ÷ Focal Length (mm)] * 205
Credit to Webfoot at Cloudy Nights forum for providing this formula. 
Isn’t it a GoTo telescope is capable of tracking celestial objects? Why do I need the autoguiding? ![:-\](http://www.astronomynotes.net/smilies/yahoo_question.gif ":-\")
At first, tracking is just similiar to guiding. For instance in the Autostar Suite from Meade, tracking allows you to define a bright spot (centroid) that it (Autostar Suite) will use to anchor images when combining (stacking) them into one image.
Continue reading ‘Autoguiding By Imager’
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