For imaging I thought aperture was king?

[swag72]

I thought that aperture was probably one of the most important things in imaging and that the faster the scope the quicker the data gathering. So, how come I look at scopes, such as the Celestron Edge HD series, that says how they are really good for astrophotography and yet it's an f/10 scope.

The Tal 100RS is f/10, yet I read nothing but good stuff about it.

How can it be that aperture is so important, for example, I have read that the SW 100ED is too slow really for imaging yet it's an f/9 - Faster than the other two mentioned scopes.

Look forward to reading the answers on this.

[johnrt]

Aperture isn't king for DSO imaging. The f ratio is more important. Most folks for DSO work want to be around f4-6 ish, anything above this would take very long exposures to capture the faint signal. That's why many people use short fast small refractors.

Planetary astro photography is another ball game and not one I can comment on, but I don't think such a fast f ratio is required due to the much brighter subject.

The Celestron you mentioned at 2032mm focal length and f/10 would make a great planetary imaging rig, but without a reducer be pretty unsuited to DSO. The Green Witch website mentions something to do with a removable secondary for fast f/2 imaging which would make the scope suited to DSO.

[swag72]

So many of the scopes then that are said to be ideal astrophotography rigs, at f/10 are really more suited to planetary work? How about then for example the Tal which is a 4" refractor - That's no good for planetary work - So how is it strong at DSO's?

Aperture isn't king for DSO imaging. The f ratio is more important

Aperture is the f ratio in normal photography though - How can the f ratio be more important when it's the same thing? I assume that aperture is the same in astro and normal photography?

[johnrt]

f ratio is not aperture.

The f-number is the focal length divided by the "effective" aperture diameter of the lens/scope.

[johnrt]

For example my telescope has an aperture of 72mm and a focal length of 440mm.

440 / 72 = f/6.1

[ollypenrice]

Firstly the people saying their F10 scopes are good for imaging are the people who make them. What do you expect them to say?? They are, bluntly, being mendacious. Celestron have been saying since the launch of the Edge series that a focal reducer will follow. Where is it? It seems others now make suitable ones...

DS imaging at F10 other than on selected objects is nuts. I say that because I am an imager who does not sell telescopes. It is just plain crazy to do DS imaging at F10 on a regular basis. You will need 4 times the exposure required by F5. That is not going to change. I did an M42 at F5 with 11 hours of data. Very nice. How do you feel about getting 44 hours on M42?

Aperture and f ratio are absolutely not the same thing. This is very simple.

Aperture = diameter of primary optics.

Focal length is distance from objective to focal plane.

Focal ratio = focal length divided by aperture.

Take the same scope, a standard F10 SCT. At native it has a focal length of 2500 mm. Convert it to Hyperstar at F2 and it now has a focal length of 500mm. The aperture has not changed but its usefulness as a DS imaging tool has been transformed.

By far the best thing is to believe not one single word of what the manufacturers say and listen to what the users say. Also look at the images posted. Bear in mind when you do so how large they are. You can do an impressively 'close up' image in a big slow telescope but the pic is so full of noise that you can only post it at 25 percent of full size. Meanwhile, at a shorter, faster instrument you have a 'smaller' image of a quality that allows you to show it at full size. So... well... why use the big slow instrument in the first place?

Olly

[brantuk]

An f-ratio of 10 is slow where as f-5 and less are fast.

Aperture is king for observing wich is why observers tend to like big dobs.

Fast scopes like short tube appo's are good for AP because they capture faint light fast and have wide fields - both good for the large, faint objects that dso's tend to be.

Mak's and Sct's are good for planetary imaging cos they are slow - you don't need fast scopes cos planets are very bright - and cos they have comparatively narrow fov's (a planet isn't wide), and long focal lengths for sharper focusing giving crisp, contrasty images

[Glider]

From reading other threads I have gathered the following is true.

If you have 2 OTA with focal ratio of 5 then they will both gather the same amount of light. So subs of the same duration will appear to be equal brightness.

So 200mm aperture/1000mm focal length is equivalent to 80mm aperture/560mm focal length. This is approximatly a SW 200P reflector compared to an ED80 refractor.

However, the 200P will have the better spatial resolution.

I'm still not sure why though. Is it to do with FOV and camera chip size?

[FraserClarke]

The bigger the telescope, the more light it will gather. So in that sense, aperture is king. However, the longer the focal length of the telescope, the more it spreads that light out in the image (giving you bigger image and hence better resolution). It turns out these two effects cancel, and image *brightness* is only affected by f/ratio (faster being better).

image brightness though is only really interesting if you're looking at big extended objects like nebulae and nearby galaxies. Most people do that of course, so it's a fair assumption to make. If you want to look at faint point sources (i.e. stars) though, f/ratio is not the be all and end all -- you need aperture.

Similarly, saying an f/10 SCT is no good for DSO imaging is making an assumption (probably quite fairly) about your camera and the kind of objects you're interested in. If you pick a camera with very large pixels, you can make an f/10 telescope just as sensitive as an f/4 telescope with smaller pixels. The disadvantage of course is that you'll cover a much smaller area of sky (not a problem if you're interested in faint galaxies, say) -- or have to spend much more to get a physically bigger detector.

[swag72]

Thanks for clearing that up folks.

Interesting that the word aperture changes with telescopes and camera len's -I am sure that aperture in camera lens is the f ratio.

Mmm, well that all gives me some food for thought as I may have a purchase choice to make in the summer, but that will be a different thread!!

[brantuk]

Aperture is allways the diameter of the objective lens in both scopes and cameras.

F-ratio is the speed of light gathering. I had the same problem understanding it when I first got into astronomy - it's strange but eventually it clicks

Think of it this way - a high f-ratio will demand less exposure time than a low f-ratio cos there's more light available on a bright object and less light available on a dim one

[martin_h]

Thanks for clearing that up folks.

Interesting that the word aperture changes with telescopes and camera len's -I am sure that aperture in camera lens is the f ratio.

Mmm, well that all gives me some food for thought as I may have a purchase choice to make in the summer, but that will be a different thread!!

Apature is the "hole" which light has to pass through. In a camera lens you can change the "hole" by changing the apature...a fast lens is one with a bigger apature -"hole". A picture taken at f2.8 will need a lot more exposure time taken at f16.

[Photosbykev]

Apature is the "hole" which light has to pass through. In a camera lens you can change the "hole" by changing the apature...a fast lens is one with a bigger apature -"hole". A picture taken at f2.8 will need a lot more exposure time taken at f16.

Lens aperture is the ratio between the diameter of the main objective lens and the diameter of the adjustable aperture internal to the lens. As a lens gets 'faster' the diameter of the adjustable aperture approaches the diameter of the front objective lens which is also why fast lens have a big front objective lens.

[JamesF]

I don't think the f-ratio really is any different in photography than astronomy, but because it's a useful way to think about the effects of changing the aperture size perhaps it's become synonymous with aperture size in photography. You can't *actually* change the f-ratio, because it isn't a physical thing. You have to change one of the things that it's a ratio of.

By way of comparison with DSO photography, from what I've read planetary photographers are keen to work at focal lengths of f/30 to f/40 or even greater if the seeing is very good.

Some of the Celestron SCTs are f/10-ish with an f/2 or f/2.3 primary mirror. Their "fastar" system allows the secondary mirror to be removed and replaced with some corrective lenses and a camera, allegedly giving a well-corrected f/2-ish imaging system suitable for DSOs. They don't seem to make the fastar lens bits themselves any more though, which makes me wonder if they're either not as good as Celestron would have you believe, or not as popular as they thought they would be. I have wondered what effect the cabling in front of the lens might have during exposure, and how one goes about taking dark, flat and bias frames in such a setup.

James

[ollypenrice]

Yes, the difference between scopes and lenses is that scopes are always used 'wide open' (OK sometimes they are stopped down for sun and moon) but camera lenses have a means of restricting the light cone, hence the use of the term 'aperture.' This allows the photographer to control the depth of field, something of no interest to astronomers since everything is at infinity. Camera lens makers will sell lenses which are are optically very 'imperfect' when wide open because, say, portrait photographers actually want an incredibly narrow depth of field in order to have only the subject in focus and the rest blurred. In effect they are making the lens itself large not to gather more light, nor to increase resolution, but really just to contrive a shallow depth of field. (I think? I'm not a daytime photographer.) In astronomy you would never want this effect and will probably have to stop down to use a camera lens if you want point stars across the chip. Again, in astronomy this is not something you want but you will have to put up with it! But astronomers have always liked portrait lenses. E E Barnard and others pioneered the use of Petzvals in early astrophotography.

BTW, in talking about the shortcomings of slow F ratio I was talking about amateur imaging and thnking about the kind of targets we really want to image with the cameras that are available to us. Professionals have other priorities, of course.

Olly

[iwatkins]

Indeed Olly, as someone who ran a pro photo studio for two years, same kind of questions for camera lenses as for telescopes. In the photography world there is the same kind of discussions.

It might be worth noting that most camera lenses don't actually work very well wide open. I.e. a f2.8 lens usually gives much sharper or better controlled colour abberations at f4 or even f5.6. Kind of unwritten rule that any lens will be at its sharpest a stop or two down from the max. Obviously there are exceptions.

Cheers

Ian

[dph1nm]

I don't really want to get involved in this argument again, but please note that what really matters for speed of imaging is not the f-ratio of the scope but the effective f-ratio of scope + camera. If you double the pixel size in microns on your camera you, in effect, halve the f-ratio. So a 4" f/10 scope with 8um pixels is the same 'speed' as a4" f/5 scope with 4um pixels.

NigelM

[Shibby]

BTW, in talking about the shortcomings of slow F ratio I was talking about amateur imaging and thnking about the kind of targets we really want to image with the cameras that are available to us. Professionals have other priorities, of course.

Indeed, Hubble is f/24 I believe. Who knows what FL the Ultra Deep Field was taken at..! But that's just confusing matters

I don't really want to get involved in this argument again, but please note that what really matters for speed of imaging is not the f-ratio of the scope but the effective f-ratio of scope + camera. If you double the pixel size in microns on your camera you, in effect, halve the f-ratio. So a 4" f/10 scope with 8um pixels is the same 'speed' as a4" f/5 scope with 4um pixels.

QFT, but if we are only changing one variable at a time (the F ratio of the scope) then a smaller number is better.

[swag72]

I don't really want to get involved in this argument again ............

If this argument has already been had, do point me in the direction of the thread so I can learn something

[FraserClarke]

Indeed, Hubble is f/24 I believe. Who knows what FL the Ultra Deep Field was taken at..!

About 31 meters... WFPC2 has a f/ratio of 12.9 (the instrument re-images the telescope's f/24 beam onto the detectors).

But that's just confusing matters

Oops..

[ollypenrice]

31 metres... Guiding with an ST80 and PHD????

Olly

[JamesF]

31 metres... Guiding with an ST80 and PHD????

Olly

Oh, be fair Olly, this is NASA we're talking about. It'll be an 80ED.

James

[FraserClarke]

The fine guidance sensors for HST are things of beauty -- pointing stability is about 1/1000th of an arcsecond or so (~1/50th of a pixel). Really quite an impressive control loop, when you consider it's whizzing round the Earth at 27000 km/hr

[antimorris]

Firstly the people saying their F10 scopes are good for imaging are the people who make them.

Olly

Hey, I use my CPC800 (which is f/10) to image (the recent Ring Nebula I posted was using this scope and I think its a pretty darn good scope for imaging things that are small like planetary nebulas, smaller galaxies, etc.

On the other hand, if it were faster than f/10 I probably wouldn't have to do 30min subs to get the faint halos on M57 that I did ....

Also, with that said, I also have an 80ED piggybacked that I generally use to image just for the wider FOV with the small-chip ccd I am using (and it is a bit faster at f/6.25)

[NickK]

Indeed, Hubble is f/24 I believe. Who knows what FL the Ultra Deep Field was taken at..! But that's just confusing matters

Ahh but it has 24 hr nights

What is fun is that using Speckle imaging earth based observers have passed Hubble's resolution. This differs from using centroid alignment to stack.

Using Speckle imaging have a look at these surface shots of Betegeuse