F-Stops and focal lengths

[marcusk]

Hiya

I'm fairly new to this astronomy lark, and am just trying to get some correct information on focal lengths and f-stops before I decide on which first telescope to buy as I'm mildly confused.

Now I understand photography and how cameras work; ideally for lots of light I would increase my aperture down to something in the region of f2 (if possible) that would open the aperture, increase the light as well as teh extra depth of field (and hence blurred background) IF i put my aperture up to f10 it would let less light in although the image woudl be sharper throughout the image.

I now turn my eye towards telescopes and I'm told a high f number is better - how? As in my mind that means letting less light in. On top of that how do you account for the focal lemgth in mm?

For example: A Celestron Omni XLT 102 has a smaller mirror, hence less light and is measured at 1000mm at f10 where as the Omni XLT 120 larger mirror and s 1000mm at f8.3. Makes sense to me - large, more light, lower f-stop.

BUT (and one of the below is my intended purchase)

The Skywatcher Startravel 120 - 600mm F5

The Skywatcher Evostar 120 - 1000 F8

Both have the same size mirror so the same light gathering power, but people rate the Evostar because of the 1000mm bit (and it is more expensive) but it has a LOWER Fstop so less light?

Can some please enlighten ME?! (think i'm about f22 and very dim

Cheers

Marcus

[rowan46]

the f stop in photography is analogous to the f number in telescopes but it is not exactly the same. In cameras the f number is changed by altering apparture via iris in the lens. In scopes it is usually changed by altering focal length both have the same effect as the the f number is the relatonship between objective and focal length. that is focal length divided by apparture. The effect in visual astronomy is that a larger f number gives a narrower field of view and a higher magnification for a given eyepiece. conversely because of the steeper light cone in fast scopes they make better imaging instruments. visually it's a question of preference, purpose and economics. fast scopes require better optics along the whole visual train to get the best views out of them. Slower scopes make better planetary scopes as they are fine on bright objects which take a lot of magnification also slow optics don't need to be as complex (expensive)

[acey]

Only thing I'd add to Rowan46's very full answer is that in astronomy you generally want maximum aperture, subject to cost and convenience. Convenience with large aperture tends to mean a fast scope, so that the tube isn't too long. The Startravel has a shorter tube, hence it's easier to travel with. But since faster optics tend to mean greater aberration (edge-of-field coma, and false colour in an achromat refractor), you may find the Evostar performs better. In choosing between them a main consideration should be portability - if the longer tube is not an issue then go for that.

[x6gas]

the f stop in photography is analogous to the f number in telescopes but it is not exactly the same. In cameras the f number is changed by altering apparture via iris in the lens. In scopes it is usually changed by altering focal length both have the same effect as the the f number is the relatonship between objective and focal length. that is focal length divided by apparture. The effect in visual astronomy is that a larger f number gives a narrower field of view and a higher magnification for a given eyepiece. conversely because of the steeper light cone in fast scopes they make better imaging instruments. visually it's a question of preference, purpose and economics. fast scopes require better optics along the whole visual train to get the best views out of them. Slower scopes make better planetary scopes as they are fine on bright objects which take a lot of magnification also slow optics don't need to be as complex (expensive)

I don't think this is quite right. It is focal length that gives rise to magnification in telescopes, not f number. For any given focal length, the greater the aperture, the more light gathering power and the lower the f ratio (and the faster the scope).

[rowan46]

Only thing I'd add to Rowan46's very full answer is that in astronomy you generally want maximum aperture, subject to cost and convenience. Convenience with large aperture tends to mean a fast scope, so that the tube isn't too long. The Startravel has a shorter tube, hence it's easier to travel with. But since faster optics tend to mean greater aberration (edge-of-field coma, and false colour in an achromat refractor), you may find the Evostar performs better. In choosing between them a main consideration should be portability - if the longer tube is not an issue then go for that.

Thanks i forgot to answer that bit

[rowan46]

I don't think this is quite right. It is focal length that gives rise to magnification in telescopes, not f number. For any given focal length, the greater the aperture, the more light gathering power and the lower the f ratio (and the faster the scope).

I was right you just said it in english . I was rather unclear

[Stu]

Hi marcusk. Just to clarify, are you considering this purchase for imaging or visual? I'm assuming visual. I guess first thing to say is that a long focal length is not viewed as better than a short focal length necessarily. It is a complex topic and there are many different aspects to consider.

Aperture may well be very important for visual use, but with all the challenges of guiding properly, larger aperture, longer focal length scopes are not always well suited to imaging.

Could you let us know if it is visual or imaging you are considering, and, if imaging then whether you are interested in planetary or deep sky. The answers will be different for each case

Cheers

Stu

[Stu]

Apologies, I mean't I'm assuming imaging!<br /><br />Stu<br /><br /><br />

[marcusk]

Thanks everyone - plenty to wade through there

In response to BigMakStutov: ultimately deep sky imaging, but possibly not just yet (got a lovely Canon 7D so I have the photography kit and knowledge side sorted!).

I am learning, I have some very nice 10x50 bins so its all a learning curve, so happy just to get a decent scope around £350 I can use to view deep sky first *then* look at imaging later and step up to a larger more imaged based scope. If I *can* get one that is possible edging towards one suitable for imaging then all the better. I will get an EQ3-2 base without a motor or goto drive initally to get used to navigating the sky on my own, then get a drive later for imaging (if possible on this scope).

Also size and portability isn't an issue - If it helps I'm thinking a refractor due to removing the issue of calibration/maintinance/dewing etc but open to suggestions...

It is focal length that gives rise to magnification in telescopes, not f number. For any given focal length, the greater the aperture, the more light gathering power and the lower the f ratio (and the faster the scope).

Just re-reading your comments and the ace summery above; If I could get a larger focal length (1000mm?), as big a lens as feasible (120mm in my case) and a low a F as possible then my priniciples of a lower F are principly correct. I think. Actually...don't judge me on that, going to analyse all your comments a bit better.! Lots to take in.

[Stu]

That's where it all starts to get tricky

For deep sky visual, you really need a reasonable aperture. Focal length and ratio, although relevant is not so important as you can use different focal length ep's to change the magnification and field of view. An 8" reflector such as the 200p would be ideal.

For deep sky imaging, the aperture is less important, focal ratio is key in terms of how long your exposures need to be to get the necessary image brightness. Focal length is also important in terms of how easy it is to guide the scope ie shorter is easier.

As has been mentioned, shorter focal lengths are more demanding on eyepieces. Short focal length achromatic refractors such as the Startravel 120 mm will suffer from quite severe chromatic abberation which make them less suitable for imaging. Short focal length reflectors tend to be quite fussy over collimation.

Many people choose something like an ED80 as a first deep sky imaging scope as it is fast and well colour corrected. It's relatively short focal length makes it easier to guide which is key. Planetary imaging benefits from long focal lengths, and larger apertures to get the resolution, and uses stacking of hundreds of short exposures so tracking is not so important.

So it is more tricky as the best first visual dso scope wouldn't be the best first imaging scope. A 200p would need a fairly hefty mount to use for imaging at a later date but I think plenty of people so use then successfully.

Hope some of that is helpful

Cheers

Stu

[x6gas]

Just re-reading your comments and the ace summery above; If I could get a larger focal length (1000mm?), as big a lens as feasible (120mm in my case) and a low a F as possible then my priniciples of a lower F are principly correct. I think. Actually...don't judge me on that, going to analyse all your comments a bit better.! Lots to take in.

It does indeed get tricky... So if you have a big aperture and a short focal length (thus having an ultra fast set-up) then you have to bend the light quite a lot and that introduces various sorts of abberation... and in order to avoid that abberation things tend to get quite expensive.

So different types of scope have f ratios around the same value - Newtonian reflectors (including Dobs) tend to be around f/5, refractors tend to be around f/7, and SCT's tend to be around f/10. To faster each of these is over those values the more challenging... To put it another way, Newts don't tend to have very long focal lengths for any given aperture (so not much "natural" magnification); you can always increase magnification by using a shorter focal length eyepiece but you need a high quality one because that stretches the optical capabilities of the scope more than an SCT...

[Naemeth]

One thing you do need to be aware of with imaging is the need for a (usually expensive) accurate mount: A HEQ-5 / NEQ-6 are usually the minimum most people use, as the extra Payload (and accuracy) help a great deal. But, these are expensive mounts even second hand (around £500 for the HEQ-5, around £700-£800 for the NEQ-6).

Sorry to throw that into the mix...

[acey]

Also size and portability isn't an issue - If it helps I'm thinking a refractor due to removing the issue of calibration/maintinance/dewing etc but open to suggestions...

Refractors are just as prone to dewing as any other kind of scope - the reason you don't hear so much comment from refractor users about dewing is that most are using apertures of 6" or less, compared with reflector users who are usually using more, hence are more vulnerable to dewing. And don't be put off reflectors by all the collimation threads you may have read. If size and portabiltiy aren't an issue then most people would probably suggest you think of a reflector which would potentially give you more aperture, though refractors are popular with imagers, and for visual observing they can excel in planetary views. For deep-sky viewing it's aperture that counts, regardless of what kind of scope it is.