f/ value vs magnification

[RickNZ]

Heya, after some study on the f/ value im left pondering a couple of unanswered questions and any help would be appreciated.

-Lower f/ means brighter objects which is a good thing in my book, however you have to deal with lower mag. Will a barlow lens counter this drawback without any side effects?

-Ideally id like to get a f/8 newtonian but there relatively uncommon, would i be just better off accepting a f/5 scope?

[hhEb09'1]

For stars, magnification doesn't mean as much as light-gathering--so it's not f/ value so much as aperture. The wider the better. :)

[RickNZ]

So youre saying the lower the F/ value the better as you would gain in brightness?

For more background info, ive decided on a 6" scope as i dont want to lug around anything bigger but am undecided on wether or not to go for a 1200mm focal length or a 900mm one.

[aurora]

For more background info, ive decided on a 6" scope as i dont want to lug around anything bigger but am undecided on wether or not to go for a 1200mm focal length or a 900mm one.

It's probably not a big issue, but...

Do you already have some eyepieces? Or do you plan to get certain ones? If so, you might want to compute what magnifications those eyepieces will give you. Maybe the exit pupil as well.

Do you look mostly at planets, or mostly at deep sky stuff? If planets, you might want to go with the 1200mm since it will be easier to get a higher magnification with a given set of eyepieces. If mostly deep sky, the shorter length will give a richer field.

Does the tube length matter? The 900mm one, being shorter, should be more easily transported and set up. Does the mount support either one well? If the mount is at all shakey, the longer focal length would just make things worse as it would induce more vibration.

Is this a reflector? If so, the longer focal length would be more forgiving for less than perfect collimation.

All things being equal, I'd go with a longer focal length. But answers to the above questions might lead you in a different direction.

[JohnW]

So youre saying the lower the F/ value the better as you would gain in brightness?
Not really. I think you're trying to make an analogy with camera lenses, where you can use a faster shutter speed with a lower F-ratio. But in this case, the focal length doesn't change, and you're changing the aperture. (Remember, F = focal length / aperture)

For more background info, ive decided on a 6" scope as i dont want to lug around anything bigger but am undecided on wether or not to go for a 1200mm focal length or a 900mm one.
In this case, your focal length DOES change, but the aperture doesn't. Both scopes will be equally bright, but the 900mm one will give you a wider field of view - you'll get less magnification from a given eyepiece, but will see more of the sky.

Does that help?

[redshifter]

A barlow lens won't counter the effect of low mag w/o side effects. The side effect being that when you double the mag. (as a barlow does) you get 1/4 the brightness. 6" f/8 newts are very common on dobsonian mounts. Check www.telescope.com, the Orion 6" dob is f/8 IIRC.

[dougreed]

hi, more on JohnW's line of thought. f value means focal length of the objective lens or mirror. for modern telescopes this is not as much an image quality issue as in the past. HOWEVER, do not compare a scopes f value to a camera lens f value. A camera lens does not (usually) actually change the focal length of the objective lens, it changes the diameter of the incoming light path by closing it off with the apeture diaphram.(thereby making a RELATIVE change in the focal length - as the disk of light gets smaller it takes more disk diameters to equal the distance to the film/sensor plane, thus a higher f ratio and the result of less light, because of the smaller and smaller disk of light as you 'stop down' the lens) That is where you get the loss of light with a higher f stop on a telephoto lens. On a scope you don't really LOSE the light with a higher focal length, it just means that the light will focus further from the primary surface. Which will inherently mean some light loss from the density of the air it travels thru.

[randb]

You could always get a telescope with a higher focal ratio and then use a focal reducer for observing DSOs. I think Celestron makes a f/6.3 focal reducer.

[Kaptain K]

The f/ratio is a serious concern only for prime focus imaging.
For visual use, it mainly affects the magnification for a given eyepiece.
Say you have two 6" scopes, one f/5 and the other f/10. All else being equal, a 10mm EP on the f/5 will give identical views to a 20mm EP on the f/10 scope. Of course, all else is never equal! ;)

[Dave Mitsky]

Rick,

I'm assuming by "f/ value" you mean f/ratio, the ratio of focal length to aperture. When it comes to visual observing, f/ratio has absolutely no bearing on image brightness since the eyepiece and the eye work together as a relay lens to transfer the image to the focal plane, i.e. the retina. (Photography is a different story where plate scale is a definite factor on exposure times.) Two telescopes of the same aperture working at the same magnification will produce images of the same brightness no matter what their f/ratios are (see Myth #3 at http://www.televue.com/engine/page.asp?ID=141)

The low power equals deep-sky observing myth is also persistent. Large apertures gather more than enough light to permit the use of higher magnifications when appropriate. Most deep-sky objects are rather small in size and have to magnified enough to activate the receptor cells in the retina. There is, of course, a point of diminishing returns. Roger Clarke, has studied what he calls the Optimum Magnified Visual Angle (OMVA), the angle at which the faintest and lowest contrast objects can be visually detected. He discusses the whole topic in great detail in his book _Visual Astronomy of the Deep Sky_.

http://clarkvision.com/visastro/m51-mag/index.html

http://clarkvision.com/visastro/m51-apert/index.html

I typically use magnifications of 202 to 324x on most deep-sky objects when observing with the ASH 17" f/15 (note the high f/ratio) classical Cassegrain. While in Bolivia in 2002 and 2004, I viewed the great globular clusters Omega Centauri and 47 Tucanae at over 500x through a 22" Starmaster. Their myriad stars filled the field of view of a 35mm Tele Vue Panoptic.

Dave Mitsky

Here's something apropos that I posted on the IAAC board back in 1998:

I'd like to present some evidence favoring the use of high magnifications when observing small deep-sky objects (very small open clusters, globular clusters, planetary nebulae, and galaxies). Of course, one must take into consideration the aperture of the telescope being used and the quality of the seeing. Small instruments quickly "run out of light" as magnification is increased and poor seeing makes high power counterproductive.

1. Sky & Telescope's Backyard Astronomy Article "Secrets of Deep-sky Observing" by Alan MacRobert (reprinted as a pamphlet)

"Using high powers. Another point made by (Roger) Clark involves the use of high magnification on faint objects. The conventional wisdom is that low power works best for deep-sky viewing. ...The tremendous boom in low-f/ratio Dobsonian telescopes has been fueled in part by the belief that low power shows deep-sky objects best.*

Clark demonstrates that this assumption is usually false. ...

The essential point is that the retina, unlike photographic film, has very poor resolution in dim light. That is why you can't read a newspaper at night - even though you can see it and your eye lens theoretically resolves the letters just as sharply as in daylight.

As Clark reports, the eye can resolve details finer than 1 arc minute in bright light but can't make out features smaller than about 20 or 30 arc minutes across when the illumination is about as dim as the dark-sky background in a telescope. ... So details in a very faint object can be seen only if they are magnified to such a large apparent size - which can require using an extremely high power!

The explanation lies in how nature has adapted the visual system to cope with low-light conditions. ... In dim light, the retina compares signals from adjacent areas. A faint source covering only a small area - such as a small galaxy in the eyepiece - may be completely invisible at the conscious level. But it is being recorded in the retina, as evidenced by the fact that a larger galaxy with the same low surface brightness is visible easily. In effect, when receptors see a doubtful trace of light they ask other receptors nearby if they're seeing it too. If the answer is yes, the signal is passed on up the optic nerve. If it's no, the signal is disregarded.

When the image is magnified, its surface brightness does indeed grow weaker. But the total number of photons of light entering the eye remains the same. ...It doesn't really matter that these photons are spread over a wider area; the retinal image-processing system will cope with them. At least within certain limits. A trade-off is needed to reach the optimum power for low-light perception: enough angular size but not too drastic a reduction in surface brightness. ...

What does all this mean for deep-sky observers? Simply that it's wise to try a wide range of powers on any object.** You may be surprised by how much more you'll see with one than another."

2. Mr. MacRobert goes on to reprise the above on pages 55-57 of his fine book _Star-Hopping for Backyard Observers_.

3. On page 47 of _The Guide to Amateur Astronomy_ by Jack Newton and Philip Teece the authors state, "Faint galaxies require a different approach. Although they may be most easily found by sweeping with a very low-power eyepiece, they often appear brighter when studied at high magnification. A power of about 2x per centimetre of aperture is useful for sweeping, but 5x or 6x per centimetre will probably show a better contrast between the galaxy's pale surface and the surrounding sky."

4. John Sanford says in his _Observing the Constellations_(page 8), "A higher power eyepiece will make the sky darker and frequently helps see fainter objects better."

5. On page 90 of _The Backyard Astronomer's Guide_ Terrence Dickinson, when discussing Telescope Myth #2. Images Appear Brighter in Fast Telescopes, comments, "... Therefore, contrary to popular belief, long-focus telescopes can be used for deep-sky viewing, especially when equipped with a low-power eyepiece (such as one with a 40mm-to-55mm focal length). The drawback is that it is difficult to reach extremely low powers and wide-fields with f/10 to f/16 telescopes which hampers views of deep-sky objects that extend over a large area.*** This is the real reason that deep-sky fans are advised to stick with fast-focal-ratio instruments.****"

6. And finally Phil Harington asserts on page 41 of his new book _The Deep Sky: An Introduction_,***** "Base your choice of magnification on what you are trying to see. Widely scattered star clusters and large nebulae are best viewed with low-power, wide-field eyepieces, while smaller deep-sky objects (e.g., planetary nebulae and most galaxies) require higher power."

* that and the fact that the tubes can then be made conveniently short
** no argument there
*** exactly, a 40mm ocular in the ASH 17" f/15 clasical Cassegrain yields a "low power" of 162x!
**** and perhaps the fact that it is much harder to locate (by star-hopping) those DSOs that will fit into the field of view
***** http://ourworld.compuserve.com/hompages/pharrington/Sw12.htm (Note: http://www.philharrington.net/sw12.htm is the current URL for information on this book.)

So what should we conclude from the above? Simply to use the appropriate magnification when doing deep-sky observing, low power for large, extended objects and high power (if your telescope is large enough to compensate for the light loss at higher magnifications) for small, dim ones. Don't be afraid to experiment!

Dave Mitsky
ASH, DVAA

[RickNZ]

Sweet!
Cheers for all the info guys, youve convinced me to go for a longer focal length scope.

Thanks to all!
(now all i have to do is resist the urge to purchase a wider app scope, man those large dobs are so dirt cheap)

[Dave Mitsky]

The advantages of a faster f/ratio Newtonian are a shorter tube (less weight and easier portability) and a wider field of view with a given eyepiece. The disadvantages are a larger central obstruction due to the larger secondary mirror, less depth of field (an exact focus is more difficult to obtain), the need for more critical and frequent collimation, the need for short focal length eyepieces* (and/or the use of Barlow lenses) to achieve high magnifications, the need for highly corrected** eyepiece designs if wide-field eyepieces are to be used, and the increased chance of the mirror being of poorer optical quality since a fast mirror is more difficult (i.e., expensive) to grind.

* which typically have limited eye relief
** correction for field edge astigmatism, which is the major optical aberration in play, requires complex and expensive designs such as the Tele Vue Nagler

Dave Mitsky