Advice on focal ratio

[allcart]

Trying to get my head around f numbers and astronomy. I understand f numbers with regard to photography, but what is the impact on telescopes and astronomy. For instance; what is the difference between using a f/5 refractor and a f/12 reflector of similar size. I am thinking about maybe a skymax 127 compared to a Skywatcher startravel 120.

[Moonshane]

for visual use the focal ratio of a scope has no affect whatsoever (in theory - see below), the aperture and optical quality govern the brightness of the image and what you see.

focal length affects the possible field of view (and magnification) with the same eyepiece. so a f5 6" scope gives twice the field of view at 37.5x through a 20mm eyepiece compared to a f10 6" scope at 75x.

In my experience (of newts mainly) a slower focal ratio (say f10) will provide a far better view of bright objects like planets and moon/double stars than a f5 scope. the f5 scope will still show a very good image and the detail is the same broadly but the f10 scope will provide a little more contrast and tighter stars due to less coma.

I think this translates to fast refractors and slow Maks but there will be the added issue (assuming an achromat frac) of chromatic aberration on bright objects.

[Stu]

Hi allcart

To be clear, are you asking about this for visual or imaging? Shane has discussed visual clearly but the answer for imaging will be very different.

Stu

[jgs001]

If the question is regarding imaging, the f numbers on camera lenses are the same as the f numbers for scopes, the difference being that camera lenses have a built in aperture stop, so you can adjust the focal ratio easily, and scopes do not. (i'm guessing a bit, based on having the 5dMkIII quoted in your sig, that you're looking for imaging).

[allcart]

Well both in the long term maybe.

In the short term, I have my dslr and long lenses for the imaging side, and am mounting the 5D straight onto my mount. It occurs to me now that if I am taking the trouble to take photographs, I may as well look at what I am taking photographs of.....So I am looking for a scope that will let me see what it is I am photographing while I take the subs with the dslr piggybacked on top. Having the scope will also allow me to visually confirm that the cam is on target.

in the long term, I may use this scope as a guidescope for the dslr. If were taking pics through the scope with the camera then I would assume the exposures with a f.10 would be much longer than with a f.5

[Moonshane]

sorry I had meant to mention imaging effects too but completely forgot in an attempt to prevent another observer falling into the imaging trap (moohahaha) nah I just forgot.

I'll let others provide the detail but imaging is better with ultra slow for planets and moon and quite fast for most faint targets as the light gathering is governed for imaging by the focal ratio (and to some extent I think by the aperture). as I head towards the cliff of ignorance, I'll leave it there.

[jgs001]

I'm not sure it's a good idea to use a scope for visual on the same mount as a camera... too much chance of wobbling the mount whilst viewing... The exposure time is governed by the focal ratio, and the resolution by the aperture. There's some piece to do with camera resolution too, but I have to be honest, I don't quite get all that bit myself. For imaging time, if you double the focal ratio, you increase the exposure time by 4x to capture an equivalent exposure. That makes your tracking requirements that much more demanding.

[allcart]

So,..on the f.5 an exposure of 60secs would need an exposure of 240secs on the f.10

My main interest is dso objects. I was looking at the Startravel refractors because they are not too big, they will hopefully allow me to see what I am pointing the camera at before I take the subs, thus verifying that the target is in the viewfinder. (sort of like a very big finder ) It would also serve as a guide scope if I decide to invest in a better scope later and take the guiding approach.

I looked at the skymax only because it is of similar size although not the same focal ratio. I just wondered what the visual difference would be.

I am leaning towards the refractors as I think they will be more useful later.

So I suppose the question now is....which refractor. The 80mm, 102mm or 120mm.

[Moonshane]

I doubt that visually the 80 or 102mm scopes will be massively different from the reflector you already have unless for some reason it's not a standard newt?

the 120mm might just edge it visually though although you will have to contend with chromatic aberration which I fear will be even worse in imaging terms.

[allcart]

Its a long time since my old tasco saw any light, but from what I remember, it was hard work and not too impressive. I was hoping that scopes had improved over the years. Maybe I should set it up and see what its like again.

Then again, it has no tube rings so I would have to buy some in order to mount my camera. If I have to spend money to use it, I'd rather spend a little more and get something more usable.

Whats you opinion of the skymax 127. Would that serve the purpose for visual imaging. I could piggyback this one without any problems.

[pixelsaurus]

Don't fall into the trap of Aperture Confusion. Unfortunately most general photographers think that aperture and f/ratio are the same thing and swap the terms without batting an eyelid. In astronomy, Aperture is King. Telescopes are generally described by aperture (clear diameter of the objective be it lens or mirror) followed by the f/ratio eg 150mm f/5. So a 150mm diameter lens/mirror with a f/ratio of 5 has a focal length of (150 x 5) = 750mm.

Most photographers will interpret the data as a 150mm focal length system operating at f/5. In this case the aperture will be 150/5 = 30mm. Not the same thing. When I started astronomy in the 60's, I came from a photographic background and it took me a while to get my head around the difference in interpretation of aperture. Needless it say it has annoyed me ever since and I'm sad to say, the general photographic community will perpetuate this confusion.

[allcart]

Speaking as a general photographer...you are almost right. I do understand that aperture relates to objective size, but what I am struggling with is the f/ratio. I see many scopes of different types with the same aperture but vastly different f/ratios. I am trying to understand which f/ratio and type of scope is best suited for which purpose. Its all a bit mind boggling at the moment.

Your simple bit of maths actually helped in my understanding.

I now know that I want a better quality refractor than I have been looking at. I must now save for at least ED optics to control abberations. The budget is rising.

[Moonshane]

one thing to consider looking at your kit list is that for many objects (especially the extended ones) all you need is excellent tracking and a good telephoto lens with your camera. see this web page http://www.koenvangorp.be/deepsky/galaxies.html where some of the objects are taken without a telescope. you may find that initially at least you don't need a scope depending on what you want to image.

[pete_l]

... but what I am struggling with is the f/ratio. I see many scopes of different types with the same aperture but vastly different f/ratios. I am trying to understand which f/ratio and type of scope is best suited for which purpose.

Let's see if this helps.

A lower F ratio (F/2, F/4 etc.) makes your DSOs appear brighter. In photography this manifests as allowing you to use shorter exposure times. However, in astrophotography, images are always underexposed, so the longer the exposure time, the better the image.

However, low F ratios also make the target object smaller in the frame. The other way of considering this is that you get larger fields of views with low F ratio telescopes. Some people like widefield views (me included). However it does mean that if you image a small DSO it only takes up a small portion of the image and you end up cropping (or wasting, depending on your opinion) a great deal of the image. So with a 6 MPix camera, your target might only require a cropped image of 600x400 - webcam size!

So what does this mean? It means that for small, bright objects such as planets a small F ratio will result in a very bright but very small target on your image. Using a slower F ratio (up in the 10s, 20s or 30s) gives you more magnification but reduces the brightness. So you get a larger, more detailed image - but it's dimmer. These are about the only astro images that won't be exposure limited.

The question you have to ask yourself is where's the tradeoff between the size of objects (or scenes, if you really want wide fields of view) you intend to image and the length of exposure you are able to take? For astro work, people take multiple sub-exposures ("subs") and digitialy combine them into a more-or-less equivalent longer exposure. The length of each sub will be determined by factors such as the quality (read: cost) of your mount, how few clouds there are and ultimately how much your local light pollution will "fog" the sub image. With these factors, most individuals try for subs between 1 minute and 20 minutes and try for total exposure times in the hours.

Personally I find I spend more time after the imaging session, getting the image processed than I spend outside actually taking the images.

[allcart]

That was a useful link Moonshane. It shows me what I can achieve with some practice. It was interesting to note that his pics of Andromeda were 20x360s subs @ iso 1600. My subs the other night were 20x 180s @ iso 800 and were completely washed out and overexposed. It must be the light polution in my garden.

[allcart]

Thanks for that explanation pete. I think finally the lights are coming on.

[Moonshane]

it's all in the processing. I know nothing about imaging but most start off as faint versions of the final image.

[allcart]

While trying to work out the best setup for me, I have noticed that my dslr could be connected directly to the scope. If I went for the 80ed ds-pro I could have a 600mm f/7.5.

I can manage 520mm f/8 with my dslr and 400mm+extender and 1120mm if I add a 2x extender as well, but then it gets really dark. Shooting at 600mm f/7.5 seems very attractive. I always wanted a 600mm lens.

This is perhaps when guided might be better than unguided.

So many options and so much fun to be had.

[jgs001]

The 80ED is a great scope and works really nicely with an SLR for widefield. On my HEQ5, unguided, 2 to 3 minutes was normal. You can use one as a 600mm Telephoto, and it does work, but good luck if you're wanting to shoot wildlife getting focused in time ... it's hard... I often use a 2xTC with my 80ED for the moon and sun (properly filtered), from a camera tripod, and it works a treat.

[ollypenrice]

Imaging...

F5 is four times faster than F10 for the same sampling rate in arcseconds per pixel.

Focal length determines the size of the image on the chip. Focal ratio has no effect on this whatever.

In visual use the F ratio that matters in terms of image brightness is the effective focal ratio of the EP and telescope combined so the F ratio of the scope on its own has little effect.

The budget fast achromats like the ST series are totally unsuited to imaging despite their fast F ratios because they have very poor colour correction. On a budget, for imaging, go for a small apo refractor or a larger reflector where colour issues don't arise. Don't buy a fast achromat for imaging.

Or so it seems to me...

Olly

[allcart]

Just driving home from work and thinking about what I had just posted, I think I had a revelation. I already have a top class telescope on my camera. If my 400mm f/5.6 lens costs almost twice as much as the ED80 and already has top quality optics then why am I looking at a possible downgrade. if I get the ED80 it will just be for visual observing and to verify the target is centred. I could do this with the ST80 and save almost £500, although the view would not be as nice.

I have been shown many pics of great subjects that were taken with shorter focal lengths than 400mm so I already have the equipment I need to take such pictures. All I need is practice and patience and advice from all you knowledgable folks.

I think I have just stepped back from the brink. Moonshane was right all along, I just couldn't see the wood for the trees.

I will order an ST80 tomorrow, strap my dslr to it, stick it on the mount and start taking pics. I'll leave guiding and big scopes until I really need them.

[allcart]

Thanks Olly, thats just the conclusion I have come to. I already have a fast CA controlled telescope on my camera. It may only be 400mm focal length, but that would appear to be long enough for a vast array of targets. I also have many lenses from f/1.2 upwards. How fast do you need to be?

I have probably got enough equipment now to keep me busy for some time. The only thing I lacked was a good mount, so I got one. I will only get the ST80 to use a a visual aid. If I later decide I want to see more and better, I can buy the right tool for the job.

I do think the ED80 would have been the right choice if I did not have the lenses I already have, but for the moment I don't think I really need it..

[Moonshane]

some people use the Astrotrac with cameras and if you have the budget I think this would be a good purchase. http://www.flickr.com/groups/569839@N21/

[allcart]

Now that looks like a very portable setup for grab and go.

[Interested]

Allan,

A few thoughts:

I would say "Aperture is King" for visual observation only. Big aperture = more light getting to the eye = you can see more over the short exposure of an eye blink. In asrto-imaging, small aperture is compensated for by longer exposures. The short tupe apochromat at f5 will give wider FOV's which is what (most) astro-imagers want.

I'm not sure a small refractor will be ideal for 'checking framing' if your interest is DSO's. Many nebula/galaxies that are easily imaged would not be visible to the eye with a small refractor.......DSO's are not called 'feint & fuzzies' for nothing! Unless you have clear skies, something like M 13 will appear as a grey patch!

If you don't already have it, you might like to Dowload CCDCalc, it's free, and it will allow you to see what can be imaged using different scope/camera set-ups:

http://www.newastro.com/book_new/camera_app.php