F ratio = what?

[NQLD_Newby]

Well folks I have a question about f ratio. I understand how you work out what the ratio of a particular scope is, eg. Focal length/ primary diameter in mm. I believe that the lower the ratio the more light is let through, and to use some terminology, the faster a scope is.

If I am understanding correctly, the lower the f ratio of a scope, the less exposure time is needed for photography. If I am correct in this assumption, it stands to reason therefore, the faster a scope is the more detailed its views will be at a particular magnification when viewing at the ep. (more light in the same amount of time hitting your eye).

My question is, if the above statements are correct, then wouldn't the best viewing scope be the one that is fastest? :shrug:

I understand that there would be a certain focal length that would be too short for any given scope, because then you couldn't possibly get the required maginification to see smaller objects. I guess I am trying to understand why a cassegrain for eg. (hope thats how you spell it) has such long focal lengths for a given arpeture. If my statements above are correct, then wouldn't it be better to have a shorter focal length?

I know there must be reasons for it, but would like to know what they are. I also would like to know these answers because I will be upgrading to a larger scope some time in the near future, and would like to take f ratio into account when i decide which to get.

thanks for listening. hope its not a dumb question.

[Dennis]

Hi Rex

You seem to have it mostly figured out.


A slow F12 refractor is “relatively” cheap as the objective lens is relatively simple to design, manufacture and assemble.
A fast F6 refractor requires a more complex lens design, the use of exotic glass to minimise chromatic aberration and is therefore more difficult to design, manufacture and assemble.



Similarly, a slow F8 Newtonian is “relatively” cheap as the parabaloid mirror is relatively simple to design, manufacture and assemble.
A fast F4 Newtonian requires a lot of grinding and figuring to minimise aberrations and is therefore more difficult to design, manufacture and assemble.



A budget F4 camera lens is relatively cheap as the lens is relatively simple to design, manufacture and assemble and the user knows that using it at F4 will produce images that have false colour, are soft and distorted at the edges. Stopping the lens down to F8 will greatly reduce these unwanted artifacts.
An expensive F2.8 camera lens requires a more complex lens design, the use of exotic glass to minimise chromatic aberration and is therefore more difficult to design, manufacture and assemble. Usually you can use these wide open, at F2.8, and still get a nice, sharp image over most of the frame.



An F12 cassegrain has a relatively large secondary mirror, or central obstruction, in the range 30 to 35% which can reduce contrast in an image.
I suspect that an F6 cassegrain would have a monstrous secondary which would degrade the image to such an extent that it would not be worth building one. It would require a complex design and lot of glass pushing to manufacture the primary and secondary mirrors. Collimation would likely be very sensitive too.

My Tokina 500mm F8 mirror lens is a Maksutov design and it has a whopping central obstruction and the images it produces are consequently a little soft.

Cheers

Dennis

[tempestwizz]

Rex,

While some of you musing is correct, in practice, there are a number of problems with it.
One issue is cost, another is mass.

From an imaging point of view.
A larger aperture does allow more light in, and with a lower F number you can catch more nebulosity ( if thats what you're looking for) before the stars saturate the sensor (too much). But you have got to be able to point your scope in the right direction and get it to track nicely during your exposure. This costs lots of dollars to do with precision.
On the other hand the larger aperture can work against you. If shooting bright objects such as planets or the moon, a smaller aperture can be an advantage, since when you exposure time can be short eg less than 1/10th second, the the tracking problem goes away, and seeing becomes more of an issue. Smaller scopes look through a smaller column of air out into space. Larger scopes tend to average the seeing affect of their colummn of air. While it may not get really bad with a big scope, it probably won't get really good either. The smaller scope will see more fluctuations in seeing, but when it is good, it will be really good. This is why many use a web-cam arrangement to take a series of video pics. During processing, they sort through them and discard those frames with bad seeing, and keep only the ones with good seeing.

From a viewing point of view.
A larger aperture wider F stop scope will mean that you can see much more nebulosity with the naked eye. But Lower F number also equates to a wider field of view (for a given eyepiece).
To look for smaller objects you will still need higher magnification to see them, regardless of your F number. By introducing barlows or powermates you are in affect reducing your F number of the primary lens.

Physical issues come in to play as well. If you have a big wide telescope in a highly light-poluted area it it wasted. You just get a good view of the pollution and a few brighter things that burst through. A smaller scope wold be better, because you can throw it in the back of your car and travel to a dark sky site, and see things from there that you would never see through the pollution.


So it is all a matter of horses for courses. What is best for you desired viewing and circumstances is likely quite different from many others - who in turn have their own circumstances to consider.

HTH

So there are

That

[Blue Skies]

ok, my two cents worth:

Your assumption is false, as I take it - it is aperture that determines the resolution of the image, not the focal length. If you want a more detailed image you need to go for a wider mirror. While the people above have made some valid comments i.e. that a larger aperture can collect too much light on bright objects like Jupiter, for detail in low surface brightness objects such as nebulae and galaxies aperture is king.

When choosing focal length, you need to take into account what you are going to use the scope for the most. If it is imaging then shorter can be better. But if you going to image the planets or moon most of the time then a longer focal length may be better.

The rule of thumb I learnt was that if you are going to do comet searching or variable star observing or imaging then a short f-ratio/ wide field view is usually the way to go.

If you mainly want to do visual viewing somewhere inbetween would be good, say F/5 to f/8.

Unfortunately there is no perfect scope for both, you usually need to make a sacrifice somewhere (even if it's your wallet when you pay for two different scopes). It's horses for courses, as tempestwizz says!

[NQLD_Newby]

Thanks for the help people.

ok to answer the question of what interests me, (viewing DSO's, viewing planets, or photography). well the true answer is, everything. :P

But the answer that actually means something is, I am absolutely gobsmacked with DSO's. Following is a rundown of a typical night of viewing for me.

Ok I always look at venus first. obviously because its theonly thing to see soon after the sun dissapears.

Then have a look at saturn.

I look at omega century, (only seen it 4000 times now but it still amazes me). occasionally the seeing (as you people call it) allows me to see much more than I had previously. This is very exiting for a newby like myself.

I look at the jewel box, which is exceptional. always stunning.

I then move onto the double star (really a triple) of the left hand pointer. In my scope this is probably the best binary viewing I have found so far. (although I am working through Vings Seeing double posts, so I may find better).

While I am in the area I always look for NGC5128 (the hamburger galaxy). Have a lot of fun trying to see this galaxy, and although I am not sure one night when the seeing was very sharp, I swear that with overted vision I could see a slight glo ( no detail but its a start).

I then proceed to jupiter. Which for me is directly overhead. painful ep position but generally ok views. Very occasionally i actually see some detail, and always like to see which positions the four moons are in.

I then move to Scorpoi, and look at some of the open clusters like m7, m6 and others. Had a look at m8 and m20 (Sagitarious), not sure exactly which i found but was fantastic.

I've looked down around where eta carinae is and found some form of neb, not sure which one because there is a few in that area, but thats part of the fun. Also found a star cluster that reminds me of the jewel box. Did look it up and found what is was called but cannot remember now.

Igot up early morning (3am) a few times to view certain things because I knew they would be around then. eg.Sword of orion, pleiades, the SMC and LMC are too low on the horizon for me to see earlier. Had a good look at 47 tuc and also found some neb not far away in the SMC, but have failed to locate what it is called. Anyway the viewing so far has been fantastic. Can't wait to see what its going to be like through a 12" scope. M42 is fantastic, as is the pleiades.

Then after I have viewed the things I know and can quickly locate, and enjoy, I generally look for something new that I haven't seen. I have found out when andromeda rises, so have planned to have an early morning again soon to view it. At the moment though there is a huge big bright ball in the sky that seems to drown everything out lol.

Oh and of coarse I have looked extensively at the moon.

So I guess the short answer is, I like DSO's most. As for photography, I have no plans really to get into it, I don't think I have the patients to take hunreds of photo's to combine into one image. I much prefer looking around and seeing.

Thanks again for your advise Dennis, Tempest, and Blue Skies. I guess the outcome of all this is that I need to buy 4 scopes. One huge one to build a million dollar observatory out in my horse paddock, one that I can leave set up ( I already have this one) and steal 10 minutes here and there. one medium size one to pack up and take to darker sky sites and star parties. and one that i can let other people look through so that i don't get called a telescope hog. :D Hmmmmm. wonder what the chief loan officer (my lovely lady) would say to all this?????

[erick]

The same as all our spouses say, Rex - "Go ahead dear, if it makes you happy, spend all that money!!" :rolleyes: :whistle:


Please give us a report when you see M31 (Andromeda galaxy) - down here it doesn't rise high enough for the best viewing.

[NQLD_Newby]

Thanks Eric,

I will tell her when it arrives that Eric talked me into it and said that you wouldn't mind. :whistle: Think she may get suspicious when 2 semi trailer loads of stuff arrive :rofl: