f/5 versus f/8

[Scott]

can anyone tell me the difference btween f5 and f8. is one easier or better for photos, is a low number better? i haven't a clue

[Peter Drew]

The F5 has the shorter focal length and will be "faster" photographically, the F8, having a longer focal length, will not be as demanding on eyepieces and will be ideal for planets and any object that requires high powers.

[rowan46]

focal length divided by apparture gives focal ratio (the f number) the lower the f number the wider the view and the less magnification but the better for photography. so f5 wide view, not really a planet killer. good photo scope. f8 not so wide view more magnification better on planets , not such a good photo scope. Also fast scopes (those with lower f numbers need to use better eyepieces) the comparison about planets only really holds for scopes of the same design and apparture. a 5inch apo f6 is perfectly able to hold its own against a 6 inch mak f12 on planets

[Parus major]

I'm willing to guess you're looking at the 150P and the 150PL?

[umadog]

The brightness of the image at the sensor is proportional to the angle subtended by the mirror when viewed from the focal plane. So an 8" f/5 will produce a brighter image than an 8" f/8. The image will also be of lower power.

[sabana]

The lower the F stop the more 'light gathering capabilities' it has. (you'll see brighter images).

The higher the F stop the more depth of field you will have. (you'll see more further).

What you really want are really bright images as far away as possible (because thats how astronomy excites us). But its going to cost a lot to have your cake and eat it. The physical dimensions of telescopes also increase dramatically for higher powered, large aperture scopes.

[umadog]

I know you know this, Sabana, but just to be clear for the OP: The effect of higher F stop doesn't carry over to telescopes and astrophotography since depth of field is irrelevant. You always are focused at infinity, after all! It's therefore a little hard to draw direct parallels between terrestrial photography and AP.

The other thing to remember is that you don't need a large a telescope to do astrophotography. You may want a fast one to allow for wide fields of view, but a large aperture instrument is less important. Bigger aperture buys you two things: shorter exposures and improved resolution. By about 8" you're hitting the seeing limit on most nights so going above that isn't going to yield sharper images, it's just going to decrease your exposure time. Compared to the days of film, CCD astrophotography allows relatively short exposures to still produce nice bright images. You can the get the brightness from the exposure not the aperture.

[Moonshane]

do also bear in mind that at the same magnification and aperture, the image is NO brighter for visual observing. the above comments only relate to AP.

[ollypenrice]

Yes, divide this into three camps.

Planetry imaging, long focal length essential because targets are tiny, fast focal ratio not needed because targets are bright.

Deep sky imaging, fast focal ratio essential. (F4 is four times faster than F8). Targets are often large and always faint.

Visual observing; doesn't matter because it is the effective focal ratio which matters, ie that of the eyepiece and telescope combined. For a given budget an inherently longer F ratio scope might give a marginally better result at high powers but the number of variables means you can basically forget about it.

Olly

[Macavity]

If we're talking Newtonian, F8 / F5 is about four times less demanding on collimation requirements. I know collimation is "easy", but given the (default) mechanical construction of some faster Newts, I begin to feel that "slower" has it's advantages.

But then, F8 is longer (has a bigger "turning circle") than F5, and HEAVIER too (more tube metalwork). If you're thinking of a smaller, shed-like, observatory for your "old age", such things might begin to matter...

Slow Catadiopteric systems (SCTs, MAKs) are inherently going to have smaller true fields. This may be and advantage or disadvantage. TBH, I can think of few disadvantages of a large aperture, well-corrected, FAST refractor - Except perhaps price?

[Squagnut]

I was going to say, a faster 'scope needs more collimation. That's not a very difficult thing to get the hang of, but if you're a beginner then it's something else to learn on top of making the 'scope point in the right direction. The main use of a faster 'scope is astrophotography, and if that's your interest then getting the right mount is more important than the OTA (in the sense that it doesn't matter what OTA you put on an unsuitable mount, all images will be bad regardless).

[Moonshane]

I don't agree that less collimation is necessarily something which would attract me. there are certainly advantages to slower scopes (Newts I mean) in terms of contrast image scale etc but even with my f11 6" newt, I always check collimation each time and tweak it if needs be. someone knowing little about collimation needs to get to know how it all fits together so something with a requirement to check and make a slight adjustment each time (like an f5 newt) and this way they will soon pick it up. it's not any more difficult to collimate an f4 newt than an f11 newt; in both cases care and accuracy are needed (although I accept that if you get it wrong with an f4 newt the images are a lot worse).

obviously with more aperture comes longer focal length and this can quickly become cumbersome, hence the proliferation of faster newts on the market. I'd not like to observe through a 16" f11 (15 feet off the ground!). Then again, maybe I would........

[Scott]

thanks everyone, starting to get a the idea. just another thing to concider. lol. i think evntually its gunna be dso so am i right in saying f5 over the f8?

[Moonshane]

when you say it will be DSOs do you mean visually or imaging? if visual it makes no difference if imaging then yes for the wider fainter stuff (like galaxies) but no for the brighter smaller stuff (like double stars and planetary nebulae).

[sabana]

A good way to learn a wise old astronomer taught me many moons ago is.

Start at the nearest object (the moon) then work your way from brightest to dimmest. You will progress naturally and will understand so much more about resolving light.

[Scott]

moonshane, i'd like to do some imaging eventually