The meaning of fast scope - scientific explanantion required.

[anth]

Hi there

as a relative newcomer to this site I was wondering if anybody could tell me the meaning of the term 'fast scope'. I am aware that a short focal length would give a fast scope but why is it called this and why is a fast scope more desireable for AP? Is it something to do with how long it takes the light to travel the length of the scope? Given the speed of light what diference would a longer Fl make?

Thanks

Anthony

[pvaz]

What matter for photography is the focal ratio. The exposure time varies by the square of the focal ratio. This means an f/10 scope, needs 4x more exposure time then a f/5 scope.

A "fast scope" usually means anything with a focal ratio of f/6 or less. I guess the name was coined cause it takes less time for light to travel inside a "fast scope".

[Photosbykev]

you can take images faster with a fast lens A fast telescope might be f3.6, which in general photography would be considered a slow lens, where a fast lens is more like f1.4 or f1.2

[ollypenrice]

It has nothing to do with lightspeed inside the scope nor is it to do with focal length. It is, as stated above, to do with focal ratio.

Take a given focal length, say 1000 mm. If the aperture is 100mm the f ratio is f10 (1000 divided by 100 = 10.) Slow. If the aperture is 200mm the f ratio is f5. Fast.

The area of the objective in the 200mm scope is four times as large as that of the 100mm scope. (Pi r squared) So the fast scope gets 4 times as much light as the slow one and images appear on film or chip much faster. 4 times as fast assuming reciprocity.

Olly

[anth]

Excellent , thanks for the replies

I understand now the - F/R on my cpc925 if F/10 ....so slow.

Ok now my final question, what eyepieces are NOT recommended for my slow scope. I was having a look on the F1 Telescopes site at some celestron eyepieces as some were being advertised as being good on fast scopes......not sure why

[acey]

Good on fast scope doesn't mean bad on slow scope. You can use pretty much anything, within the highest and lowest useable powers.

Fast scopes can offer wider fields of view but may be prone to edge-of-field distortion, hence some EPs might be better than others for a fast scope.

As said before, "fast" and "slow" refer to photographic exposure times and don't have any literal meaning in relation to visual use of a telescope.

[pvaz]

Low focal ratios usually bring out the problems in EPs. With high FR, such as f/10, you'll have trouble to see the differences between a decent EP and a more expensive one, which means: You don't need to spend too much money on EPs.

[sixela]

Excellent , thanks for the replies

I understand now the - F/R on my cpc925 if F/10 ....so slow.

Ok now my final question, what eyepieces are NOT recommended for my slow scope.

Anything good on fast scopes is even better on slow scopes.

[James4]

Hey Anth, If you do decide to do Astrophotography you can easily turn your slow f/10 into a fast (ish) f/6.3 with a focal reducer. It's a lens that screws on to the back of your SCT reducing the focal length of the scope. The are about $99 (Antares) and $149 (Celestron). One thing though, I can only bring one of my eyepieces to focus with my Celestron F/6.3 reducer. Photographically it makes a huge difference allowing you to capture deep sky objects with much less exposure time.

[jj1]

You can get the FR down to about 5.2 ish with those reducers by altering the spacing.

[ollypenrice]

Other photographic advantages of using a reducer;

- you speed up exposure so making fewer damands on your tracking.

- you reduce the focal length so making still fewer damands on it. (Long focal lengths 'magnify' so they they magnify errors, if you like.)

- they are also field flatteners which reduce the edge of field distortions and they help illuminate the chip more evenly across its whole area.

BTW, I should have added in my original contribution that the focal length in my explanation has to be a constant or you are not taking the same picture. If you widen the field of view by shortening the focal length you are using light from a wider source - the full moon rather than a quarter of the moon, for example.

Olly

[Breakintheclouds]

I think the terms fast and slow derive from photography and primarily refer to the exposures you'd use with a given lens. Shooting the same scene, a 'faster' lens means you can use a shorter exposure. For example, if you have an f/5.6 lens and find the best exposure time for a certain scene is 500ms, then reducing down one f/ stop means you can halve the exposure time to 250ms - a faster photograph.

[CGolder]

I found it harder to get a widefield eyepiece for my slow sct, but got the 36mm hyperion and its very good. If you do not have a two inch diaganol, a good 32mm plossl comes in handy. Generally you can get away with cheaper eye pieces with a longer focal and better eye releif due to the slow optics and get the same mags

Sent from my ZTE-RACER using Tapatalk