Telescope Focal Ratio Explained

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Credit:- By Jeff Barton from Garland, TX, USA - Saturn, through 82" telescope, reprocessed, CC BY-SA 2.0, commons.wikime...
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• Telescope Focal Ratio ...

[@TheAlienTechs]

Learn Basics of Telescopes here: 👉 kzbin.info/aero/PLxO7Ri9hh2HGfpx8KZ-rnzSr6_3h9wyim

[@winterpatriot1429]

Very helpful, hit the mark! I’m just getting into astronomy and am researching/learning as much as I can before buying my first telescope. I wish to be able to view planets but also deep space objects, but wasn’t sure which telescope to pay more attention to. I understand aperture is primary, but this has really helped me to understand how focal ratio factors in. Now, I can begin to narrow the playing field of buying options and get closer to being able to enjoy the hobby. I’m recently retired from the Air Force, and my wife tells me I need a hobby. 😃 Thanks for the great data/explanations. Sub’d! 👍

[@TheAlienTechs]

I'm glad my videos helped you. That was the primary aim of making these videos about basic concepts of telescope to simplify and make it easy to understand for new people interested in the hobby. 🤗🤗 🤗

[@69Atho]

So much incorrect information in this video. Varying the f-ratio of a constant aperture has little or no affect on real S/N, except in certain limited circumstances. The relationship of exposure-time and f-ratio only holds true for equivalent focal lengths, which means the aperture must be varied to produce a given f-ratio. The “CCD f-ratio myth” originates in people’s experience with film-based photography, where the exposure-time/f-ratio relation is practically considered a “law of nature”. But that “law” is actually a consequence of some peculiar properties of film emulsions that largely do not affect CCD imaging.

[@primoroy]

The "length" the light travels is incorrect. Slow telescopes are dimmer because the magnification is higher.

[@grugbug4313]

Solid! Top KEK! Peace be with you.

[@rdallas81]

You can stack photos to bright the objects

[@jpastroguy]

One important piece of information is the relationship between focal ratios. How much faster is an F4 vs. F6? There must be a formula. And it can affect decisions on which telescope to buy.

[@MountainFisher]

Difference between an f4 to f6 is the f4 is harder to keep collimated, otherwise, meh. Refractors with a short focal ratio kind of do poorly, needs more distance for the lens I'm led to believe. I've never seen a big refractor less than f7. Even those small APOs with ED glass run f6 or better. You only see f4 in big Newtonians. Mine is f5 and I had the mirror done to a very high spec. It has no coma without a coma corrector. I use it for visual only as it really is nice and I live in the high desert.

[@cl6x533]

​@@MountainFisherabit off topic, my man is looking for how fast f/4 is compared to f/6, he is probably doing astrophotography

[@MountainFisher]

@@cl6x533 It can accept more light and thus image easier, plus a wider field of view. Still field flatteners are needed. It becomes a matter of tradeoffs.

[@UpcomingJedi]

@jp. Do you mean the difference in candlepower in the eyepiece. This, i think will vary depending on the eyepiece which influences magnification. Take an 8 inch dobs with a FL of 8oomm (f4) and 1200mm (f6) and say a morpheus 17.5mm eyepiece giving you 45x at F4 and about 137x at f6. Youll get more magnification but slightly dimmer view. Very negligible to really notice. A camera would but its easy to just raise the ISO rating a wee bit.

[@astrolol904]

great information

[@ronm6585]

Thank you.

[@atluangapachuau]

very helpful...

[@larrywiseman5534]

love your videos. Question? is it okay to leave a telescope outside i. e. overnight in the heat or the cold if its completely covered?

[@UpcomingJedi]

No because it is likely to get stolen.

[@fadzilgoh2411]

Good info

[@stevenholt824]

She says two lenses of similar focal length, not focal ratio, changing the aperture changes the focal ratio, telescopes are fixed Theres no speed of a telscope ie f4 is just a meaningless ratio without any other information. Aperture defines resolution, and light gathering focal length defines field of view. Changing the aperture on a camera dosent change the field of view just depth of field which telescopes don't have. You can only compare camera lenses to telescopes when wide open.

[@valentinotera3244]

Do the obvious party ratio

[@kushalgokhale9114]

0:23 Focal length of what? In a refractor telescope there are two lenses.

[@veridicusmaximus6010]

Of the light. It is where the light comes to a point from where it first starts to bend - which is the first lens of the objective (which can have 3 or more lenses there in Apochromatic scopes).

[@aaaazzzz929]

Osummm ❤

[@Balataone]

Wrong.

[@matthewschardtii1338]

May you elaborate on how they're wrong?

[@igyu6371]

@@matthewschardtii1338 fast scope collect more light but from bigger area, its different size images of saturn should be on 3:49 of this clip. And reasoning about light speed as factor of collection more light simply idiocy.

[@UpcomingJedi]

​@@igyu6371 i disagree because using a camera obscura or pinhole camera to see this, if you put a curtain 5ft from the pinhole, the image is MUCH brighter than if the curtain is moved 10 ft away from the pinhole because as the light travels further, it spreads out. So decreasing the distance of the curtain to pinhole by half, the image becomes 4x brighter.

[@valentinotera3244]

Visually between an f/5 and f/12 there's no difference.

[@ulisessolis3182]

Yes, there is a difference: field of view

[@marcpopick248]

Also at f6 and slower then the image is much sharper and less coma.

[@rdallas81]

Huge difference