Focal lenth

[richard2600]

Hi can someone tell me whats the difference if you have 2 telescopes both 5 inch in diameter one has fl of 650 and the other fl 1250. Dose this mean the one with bigger focal lenth has more power or better focus

[Camelopardalis]

Neither. Just a longer focal length :)

[Amaranthus]

If by "power" you mean magnification, then the longer focal length telescope will yield higher power with the same eyepiece.

Magnification = Focal Length of Telescope / Focal Length of Eyepiece

Focus is how sharp you can get the image - this is unrelated to FL.

[mental4astro]

As Dunk says, the difference is focal length. However, this actually does not mean one has more "power" than the other. While with the same eyepiece will give more magnification in one (the longer focal length), the actual limiting magnification is actually IDENTICAL in each. Barry alludes to what I'm noting below.

The highest practical magnification is determined by the aperture of the scope, not the focal length. The rule of thumb I use is 50X per inch of aperture. So for these two 5" scopes, the highest practical magnification is 250X for both. Exceed the practical limit, and the image begins to degrade and you actually don't gain anything. Magnification is theoretically limitless, but this is not the case.

Now, take a 10" aperture. With the rule of thumb, the highest magnitude is 500X. Again, BUT, there is another limiting factor - the atmosphere. Typical conditions allows only 120X. Good conditions allows 250X. Exceptional conditions and 400X is possible. Go high up in elevation above sea level, and "I've just won Lotto" conditions will allow 800X.

Note, if the instrument you have is of exceptional quality, the 'rule of thumb' upper limit can actually be pushed to 70X per inch of aperture. But, man, this is a rare, rare fish...

[barx1963]

The other thing to remember with focal length is the size of the field of view. A longer focal length, all other things being equal, will give you a narrower field of view. And a shorter on gives a wider field. That is why short scopes are sometimes referred to as rich field telescopes.
Longer focal lengths have other effects as well. In newtonians a longer fl scope will display less coma, in a refractor it will display less chromatic aberations, and in both scopes it will result in a longer less convenient OTA.

Malcolm

[Camelopardalis]

And just to confuse the issue further, some of the longer focal length scopes on the market, notably the Cassegrains (Schmidt and Maksutovs of this family are quite popular) typically have long focal lengths yet provide relatively compact packaging for their aperture...at least the smaller ones :lol: this is due to the folded light path of their nature and the curvature of their mirrors.

[multiweb]

The first one is F/5 so it gathers light faster. You'll get a brighter picture. The second one is nearly F/10. Your view will be magnified but very dark. Cropped compared to the F/5 FOV. It's likely the F/10 view will look blurrier as well.

[barx1963]

Are you sure that is right for visual use Marc?

[multiweb]

Visual or imaging in effect what you are doing is barlowing x2 an F/5 if you double the FL.

[richard2600]

Ok I think I get the idea. Yeh I was just curious as mine is f 650 and I see same size scopes but with 1000 or more and was curious Thanks everyone

[Steffen]

No, not quite, only as far as the final magnification is concerned.

The f/10 scope will generally be more pleasing to look through, most aberrations will be diminished compared to the f/5 one, and it will have better apparent contrast due to the darker sky background.

IMO fast scopes are "only" good for astrophotography, or when you can't handle the longer OTA size for some reason (large OTAs can be an issue in windy conditions).

Cheers
Steffen.

[barx1963]

Yes that was the point I was trying to make. For visual use (and it is entirely different for imaging) the "speed" of a scope makes no difference to the overall brightness of an image. How can it? It's the aperture which determines how much light is collected hasn't changed. BUT the magnification has changed. So a scope with a fl of 650 with a 10mm ep will give 65x while a scope of fl 1250 and the same ep will give 125x. The available light from an extended object will be spread over a larger area so its surface brightness will appear lower. But overall it is just as bright, the same number of photons are being collected. And logically a point source such as a star will exhibit no change in brightness.

In imaging you are accumulating signal over time. A "faster" scope will concentrate the available photons on your sensor from an object on a smaller area of the sensor obviously so it will produce enough signal to create an image in a shorter period of time, hence "faster" means something to an imager.

The above is how I think of it and explained in simple terms. I am not an optician or trained expert, so I have this wrong I am happy to be corrected.

There is an interesting discussion involving Les and Alex on this thread a few years back http://www.iceinspace.com.au/forum/showthread.php?t=55495&highlight=faster

Cheers
Malcolm

[barx1963]

Actually I have just realised an important proviso to my earlier assertion about brightness. If a scope produces a larger exit pupil, some light may be lost if the exit pupil is larger than the pupil of the observers eye. So if I was using a 40mm ep and a scope of f10 the exit pupil would be 40/10 or 4mm. If I went to a scope exactly identical but at f5 the exit pupil would be much larger at 40/5 or 8mm. At my age my pupil can maybe dilate to about 4 or 5mm max so potentially I am wasting light with the faster scope so in this case the "faster" scope would actually produce a dimmer overall image.

I think!!

Malcolm

[multiweb]

Well I've owned a Celestron 130SLT for 5yrs which is exactly what the OP is talking about. It's a 5" F/5 system. I imaged with it and observed with 32/25/9/6 & 5mm eyepieces. I barlowed it x2, x3, used an AO on it as well. The view at native F/5 is brighter than at F/10. The contrast is also much better at F/5.

[Steffen]

So, you turned your f/5 scope into an f/10 scope by inserting a Barlow? You could as well have changed from a 20mm to a 10mm eyepiece ;)



You mean the sky background looked more washed out? And the contrast was better at f/5 at the same magnification? Or at half the magnification?

We all know that views get dimmer when you plop in a Barlow lens, but that's because the magnification doubles (or triples, or whatever).

Try using an f/5 and an f/10 scope of same aperture (like what Richard was asking about) at the same magnification.

Cheers
Steffen.

[brian nordstrom]

Another thing not mentioned about is the light cone at f5 is very steep and its harder to find the perfect focus point and even harder with a Barlow in place.
Brian :)

[multiweb]

Yep. No I imaged with it. Here's the native FOV (https://skydrive.live.com/?cid=6930D3AD12D5044C&id=6930D3AD12D5044C%21863#cid=6930D 3AD12D5044C&id=6930D3AD12D5044C%211354&v=3)and barlowed x2 (https://skydrive.live.com/?cid=6930D3AD12D5044C&id=6930D3AD12D5044C%21863#cid=6930D 3AD12D5044C&id=6930D3AD12D5044C%211127&v=3)


I found that with the barlow x2 a 32mm eyepiece was big enough magnification.



Visually it was much darker and less contrast at F/10. 5" is too small of an aperture for visual use at F/10 I think. Imaging at F/10 through it was a bit of a struggle too. 15min subs at least and lots of them to get a decent SNR.


I used a Baader FCC and it has very little light scattering. You don't even know it's there. So it doesn't impair on the view at all. It's just that 5" at F/10 is still very dark to look through.


There was a bigger Celestron Newt at the time that was a 10" F/9 if I recall but that was out of my paycheck. That would have been nice to look through this.

I also experimented through a 90mm refractor at F/20 (https://skydrive.live.com/?cid=6930D3AD12D5044C&id=6930D3AD12D5044C%21863#cid=6930D 3AD12D5044C&id=6930D3AD12D5044C%211351&v=3). But you need pretty good optics at that FL. Worked ok but you'd be hardpressed to see anything through visually.

[ausastronomer]

That's because you halve the exit pupil diameter when you insert a 2x barlow.

The f-ratio has no effect whatsoever in terms of image brightness to a visual observer. It has an effect for imaging purposes only.

100mm f10 telescope used with a 20mm eyepiece will give 50x with a 2mm exit pupil.
100mm f5 telescope used with a 10mm eyepiece will give 50x with a 2mm exit pupil.

in each scenario the visual image will be of equal brightness.

The advantages of an f10 telescope:-

Greater depth of focus
Collimation accuracy is less critical for a newtonian
A newtonian will have a smaller secondary mirror which improves contrast
A newtonian will have less coma and field curvature and a refractor will have less chromatic aberration and field curvature.
Because of the shallower light cone the f10 scope will work a lot better with cheaper eyepieces and simple eyepiece designs like plossls and orthoscopics.
Higher optical quality in cheaper telescopes is sometimes likely because it is a lot easier to make f10 optics well than it is to make f5 optics well.

Advantages of an f5 telescope:-

Greater field of view
A lot easier to mount, particularly on an equatorial mount.
Easier to store and transport.

For visual astronomy image brightness and contrast are not a consideration in deciding between an f5 and an f10 telescope. The f ratio can be a major determining factor in selecting a scope for imaging as it affects the exposure time significantly.

Cheers
John B

[cometcatcher]

Cheaper optics don't barlow well. Neither do fast achro refractors. Under those conditions it's better to use something at native focal length.

[barx1963]

I think the upshot of this entire discussion is that the terms "fast" and "slow" are misleading when applied to scopes used for visual purposes.
We use them as a shorthand to indicate the physical properties of a scope structure or to give an indication that it may have certain aberrations or focusing properties but they are quite meaningless as far as the brightness of an image in the eyeball is concerned.

Cheers

Malcolm

[Steffen]

Also, in vacuum all scopes fall at the same speed :P :D

Cheers
Steffen.

[PSALM19.1]

Hi malcolm, so my scope being an F6 (8" dob) would produce a comparable image to a slower scope of equal aperture? (Say Nexstar 8SE?) :shrug:

[Camelopardalis]

At the same magnification, yes :)

But there may be other mechanical or optical factors involved that might introduce differences...

[barx1963]

The point about brightness was that the assertion was made that " a scope at f5 gathers light faster so it produces a brighter image". I suggest that is incorrect for visual use for the simple reason that your retina doesn't act like a sensor or a film in that the time you look at something doesn't make it brighter so the "speed" of a scope is irrelevant.

That is not to say that the focal ratio is not an important consideration in selecting a scope and there are factors that need to be considered and John (ausastronomer) has done quite a succinct summary in his post.

But what you are saying is essentially correct. At the same magnification, the images should be comparable. Of course a dob and an 8SE are different in a couple of ways. The 8SE is an SCT so it has a corrector plate at the front which will slightly reduce the light throughput and also has a larger secondary obstruction so that will also slightly reduce the light throughput and contrast.

Malcolm

[Camelopardalis]

With modern coatings and mirrors, the difference between your average 8" Dob and SCT will be practically indiscernible.

The different levels of coma is a lot easier to see...

The main difference is the maximum FOV, but even that is not so straightforward if you consider maximum exit pupil, in which case you'd probably shy away from the longer focal length eyepieces with the faster scope.