F-ratio of a telescope

[Putthi Cheat Lim]

Hi all! I'm thinking of upgrading my telescope. I own a 10" dobsonian, I love it but I want to get a Catadioptric telescope instead because it is much more mobile than the dob. My worry is that the Catadioptric's f-ratio is f/10 compare to f/4.7 of my dobsonian, does it mean that the images through the catadioptric telescope would be much darker than those through the dob? (like the f-ratio in camera lenses?)

[jambouk]

For visual, the f number/value (whatever you want to call it) is less noticeable than when imaging.

I've never seen a direct comparison but i suspect with two identical scopes with very different f values, visual would be very similar.

But i'm still new to the world of optics and i've had some wine so take what i say with a pinch of salt.

I have a 127 mak.

James

[ronin]

Not really easy to say Yes or No.

If you got a 10" SCT then at the same magnification the image should be the same brightness, you are collecting the same amount of light and putting it into the same size image.

If you stuck a camera at the prime focus then yes it would be dimmer, also bigger. However since you seem to be visual this is (maybe) not applicable.

If you got a smaller SCT then 10" - a likely scenario - then it all change with the magnifications you choose to use but in general the same magnification would be dimmer, simply because you are collecting less light via the objevtive. If you went for a 8" then you would have to have a lower magnification fot the same brightness. However you may be taking the smaller scope to darker sites.

[Jordan2013]

Magnification is what determines the brightness of the image not the f ratio

[Stu]

I have f20, 8" mak. Although I've not directly compared them, for a given magnification, visually the images should be very similar to an 8" newtonian of much shorter focal length.

The benefits of the mak are the compact tube, and being able to use longer focal length eps to get high magnifications. An 18mm BGO for instance gives x222. It also has great contrast due to the small secondary obstruction. The downsides are cool down time, plus dewing of the front plate. It also has a very small field of view, around 0.7 degrees at maximum so is really a planetary and lunar scope although it is fab on globs and smaller DSOs too.

Imaging would be a different story though!

Stu

[Oblong]

Both magnification and aperture play a part in determining brightness. One way to look at it is in terms of exit pupil. The brightest image is obtained when this equals the pupil size of your dark-adapted eye. This is upwards of 5mm (how much upwards depends on age, amongst other things). So, this pretty much dictates the longest focal length of eyepiece that can be used successfully (because exit pupil diameter = eyepiece focal length / f-ratio. At f/4.7 it will be somewhere around 25-35mm; At f/10, longer focal length eyepieces (55mm Plossls, for example) become usable.

Another way of looking at it is that for the same aperture and eyepiece, a change to a longer focal length scope will darken and magnify the image (just like putting a barlow in the original scope) - but you could use a barlow + longer eyepiece to get the same effect as a short eyepiece.

Of course, overall brightness is not the same as contrast - and higher magnification can often improve contrast, even if the absolute level of brightness is less. Furthermore, well-corrected optics can brighten up an object by briging it to focus across more of the field.

Faster f-ratios are more about imaging and keeping tubes short and fat for good aperture in a reasonably convenient package, I think. Even the HST is f/24, apparently, so I wouldn't say focal ratio is an obstacle (but it might need some tweaks to the eyepiece collection).

[ollypenrice]

Mods, I keep getting double posts. I don't know why...

Olly

[ollypenrice]

Start again. A 10 inch Dob (by which you mean a 10 inch F.7 Newt) is physically longer than a 10 inch SCT optical tube assembly because the light path of the SCT is folded within the tube making it physically shorter. So much for the tube itself. Physically smaller, yes.

Now, problem. What are you going to mount the 10 inch SCT short tube on? You can't use a Dob mount for an SCT so you need either a fork or a GEM. These add either a lot of bulk (fork mounted SCT) or an even greater amount of bulk (GEM with counterweights). So at what point do you really reduce the bulk compared with a 10 inch Dob? In my view you don't. The smallest total 10 inch aperture setup you can have is the Dob.

In visual use the brightness of the image for a given aperture is determined by the Effective Focal Ratio (which is the another way of describing the magnification). It's the combined FL of scope and eyepiece together. It really doesn't matter whether you arrive at this effective focal ratio by having a short FL scope and long FL eyepiece or a long FL scope and a short FL eyepiece. If the effective focal ratio is the same the brightness will be nearly the same for a given aperture. Yes, the size of the secondary obstruction has an effect but let's not die of over-excitemement over this...

The real thing about having a much longer FL for a given aperture is that you will massively crop your field of view. That may or may not matter to you. For me it would be a disaster, but that's me.

Olly

[ollypenrice]

Sorry, the 'net was a pain when I was typing the reply above. I meant to say F4.7, of course.

Olly

[E621Keith]

Olly, I think you meant you need short FL eyepiece for short FL scope and long FL eyepiece for long FL scope.

Long fl eyepiece on short scope produces small magnification, short fl eyepiece on long scope produces big magnificaion

[ollypenrice]

Olly, I think you meant you need short FL eyepiece for short FL scope and long FL eyepiece for long FL scope.

Long fl eyepiece on short scope produces small magnification, short fl eyepiece on long scope produces big magnificaion

Of course! I'm getting worse...

Thanks for putting it so nicely.

Olly

[Luke]

I spent a bit of time comparing views in an 11 inch SCT (F10) versus a 12 inch dob (F5). They gave similar views of faint DSOs (when using different eyepieces to get similar magnification), with the dob edging it.