Aperture size Vs Focul Length - astrophotography

[NewToStars]

hope iceman doesn't mind me posting his reply in here. but i thought i'd get the wider community's feedback on the topic.

"Normally people would start with an ED80 instead of an ED100, because the ED80 has a shorter focal length and a faster focal ratio."

i know when i bought my dob years back everyone said apature is everything. but with these refractors your saying that the 80 is better than the 100 because of the focal length. how does this work. wouldn't the larger size allow more light to be captured. ??????

[NewToStars]

never mind... just googled it :D

[iceman]

I don't mind :)

Aperture is good, especially for visual observing.
For photography, aperture is also good, but more aperture usually means a longer focal length.

For someone starting out in astrophotography, a longer focal length means the polar alignment has to be more accurate, and guiding is essential and has to be accurate too.

The faster focal ratio for astrophotography means you can use shorter exposures to capture the same amount of light/detail. An f/7.5 scope is better than an f/9 scope because you can get more detail in a 1 minute exposure (for example).

I hope that helps explain it.

Others will chip in i'm sure.

[Terry B]

Your correct sort of.
A 80mm f5 scope will have less resolution and less light gathering power than a 200mm f5 scope all other things being equal. The problem is that most larger diameter scopes have a longer focal length so produce a more magnified image. This spreads the light over a greater area resulting in a less bright image so often the smaller shorter focal length scope is recomended.
It depends what you want to image. If you like widefield images of nebulas then get the smaller scope. If you want to image galaxies (with the exception of the 4 or 5 bigger ones) you will need a longer focal length and hence a larger diameter scope to collect enought light. Same goes for planetary imaging.
If you aim to search for supernovas or measure the brightness of variable stars then the f ratio is not important and aperture wins.

[iceman]

This site has a fairly good explanation and the pros/cons of short vs long focal length scopes.

http://www.telescopes-astronomy.com.au/telescopes018.htm