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It's important mostly for astrophotography. If you aren't doing that (not recommended for newbies) then the only things to remember about it is that the faster the f-rating (f-rating is described as speed, like in photography), the wider the "Actual FOV" is (somewhat important in eyepiece selection), and in the case of Newtonian telescopes, it will be harder to "collimate" (essentially align the optics) to get your mirrors ligned up well, and they will likely need more frequent re-alignment. In the case of Refractors or *-Cassegrains, there isn't much to worry about.
posted
It's also important in order to see fainter deep sky objects, like galaxies and faint nebulas. For any given aperture telescope, the lower f-number also means that you will have less magnification (which is a different way to consider "Actual Field of View"), but a brighter, usually higher contrast image. Less magnification means it gets fuzzy if you try to magnify it too much.
When buying a telescope, you really have to consider what you want to look at, because big "fast" telescopes, especially newtonians, are not really very good for looking at planets, but great for looking at deep sky objects. To look at planets, a fairly small refractor telescope with limited diffraction is likely better than a big newtonian.
For the best of both worlds, Cassegrain telescopes do both pretty well, but they are expensive.
Posts: 3735 | Registered: Mar 2002
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posted
yeah, I was thinking mostly in terms of deep field viewing... the only planets I was thinking of were Jupiter and Saturn... but I'd like to do some viewing of nebulae and near galaxies in particular... basically I'm trying to decide between a 6" equatorial mounted with F/5 and a dobsonian 8" with f/6 which are roughly the same price, and a dobsonian 6" with F/8 which is about $100 cheaper than either of the others...
Bok, if I'm reading you right, faster means more maintenance, yes? that *is* a factor...
and thanks both of you...
Posts: 3846 | Registered: Apr 2004
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posted
Well, only because deep sky objects tend to be relatively spread out in the sky, Glenn, and therefore the faster scope, having the larger TFOV (Total, or actual, field of view, as opposed to Apparent FOV), can get the entire object in the eyepiece. An 8" f/8 will make a star cluster brighter than a 6" f/5... Incidentally, low f-number scopes are more common the larger your primary surface is, because of practicality. And larger mirrors/lenses means more light-gathering power, which means diffuse objects like galaxies and star clusters are more easily seen.
SIDENOTE: You can compute the length of a telescope tube(*) by multiplying the f-number by the diameter of the primary lens/mirror. So when you get to 10-12 inch mirrors (in Newtonians, for instance), and the f-size is 10, you have an 8 to 10 foot long tube. A bit unwieldly. Hence, the low f-number .
For the record, my 6" f/8 (got it for 100 bucks on clear out, I had a thread about it...) does a great job of Jupiter (to me) at around 175 magnification. I see all the Galilean moons, and 4 (sometimes 6, depending on the atmosphere) cloud bands.
The general consensus in amateur telescope circles is a 6" or 8" dob to start. They are both more or less transportable by one person, they are cheap for their size, and their size allows them to be fairly versatile. Also, an EQ-mount, while ultimately more accurate for tracking an object over time, can be confusing, due to making sure it is polar aligned, and the fact that it doesn't move according to the horizon as a person sees it.
In any event, Newtonians (EQ-mounted or dobs) have more upkeep as far as collimation are concerned. f/6 is the boundary it appears for where the collimation starts going from "rarely" to "every-time-out". Collimating itself, aside fro mthe first time or two, usually takes 5-10 minutes, tops.
As a beginner, you could probably just go out without collimating for a while; as you get more experienced, you will want to collimate your scope (which, while taking some time, doesn't cost much in equipment) to save you from thinking an expensive barlow or eyepiece will make a difference.
CloudyNights rocks as a resource for just about everything, including how to do your own collimating. I recommend going to the beginner forum.
-Bok
* Cassegrains usually require you to divide by 2 to get the length, because they fold the light cone (which is another reason for their popularity). A 10" f/10 Cassegrain is half the length of a 1)" f/10 refractor or newtonian.
Posts: 7021 | Registered: Nov 1999
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posted
thanks again... while I missed out on the $99 dobbie you got (I wanted to get one of those) I am going to try to get something similar here and introduce the kids to something beyond the moon, venus, and the pleadies... (which are about all I could find regularly with the cheap thing we had back in dallas)
Posts: 3846 | Registered: Apr 2004
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posted
Another alternative is a sturdy tripod and a pair of binoculars... Cheaper, and easier for kids to use (due to the image being right-side up and not mirror-imaged). I have a pair of 8x56 and 20x80s. The former are handholdable, the latter need a tripod. You can see all sorts of stuff. One of my favorites is in the area of Cygnus and Lyra where there is an unnamed asterism (non-constellation collection of stars) that looks like a coathanger.
posted
My 10" f/4.6 most certainly does not need collimating every time out. However, it is really hard to collimate compared to my old 4.5" f/8.7, and needs it with some frequency.
So I bought a laser collimator.
Maybe it's just me, but I'm really glad my first scope had an eq. mount. It's very helpful in learning about the movement of the sky, the ecliptic, and so forth. Now I think like a german mount, even when I'm using my Dob.
quote:An 8" f/8 will make a star cluster brighter than a 6" f/5...
No it won't. With the same eyepiece the f/5 will be brighter. Bear in mind that the f-ratio only applies at prime focus. Once you put an eyepiece in, the effective f-ratio changes. With a camera and eyepiece projection, the effective f-ratio is way different than you'd expect.
Here's an example calculation:
quote:Suppose you have a half-inch focal length eyepiece located 8 inches from the camera plane in a 10-inch diameter, f/7 telescope. Its focal ratio would then be:
f ratio = [(8/0.5)-1]x(70/10) = f/105
The effective focal length would then be:
105 x 10 inches = 1050 inches.
When you look through the scope, you're using eyepiece projection. You have to think of your retina as if it was the film in the camera. The light on the retina is more spread out when the magnification is higher, so the image is dimmer. The focal ratio determines how intense the light is per unit area. But a larger objective allows you to magnify the image more, and still have it bright enough to see.
Posts: 3735 | Registered: Mar 2002
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Quick synopsis: It has to do with the size of the Airy disk. But it seems to only really matter with stars (which are actually Airy Disks). He still concludes that for "faint fuzzies", aperture (the diameter of the primary mirror/lens. So a fast, large aperture scope is best, in the page's writer's opinion.
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I guess it all depends on what you hold constant.
I'd be curious to see the article "you can't make them brighter"
also, I never knew where the word "Airy" came from in Airy disk. I thought it was someone's attempted description of the appearance of the dark and light rings. Makes more sense now.
Posts: 3735 | Registered: Mar 2002
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posted
I like good equitorial mounts... but the one I had was cheap and one of the gears broke and now the thing's just about useless... I like the idea of just being able to rotate the scope on an axis perpendicular to the earth's... makes tracking really easy... but I still find it easier to originally point a dobbie...and I saw one of those new unmagnified sights that would seem to make pointing even easier...
Edit: since the 8" F/6 is slightly cheaper than the 6" F/5, and we're talking about nearly twice the light gathering area vs. a slight speed difference, that seems like it might be the way to go, for me Posts: 3846 | Registered: Apr 2004
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I didn't mean to imply that speed is more important than aperture. It's not. As has been pointed out, fast optics are more finicky in terms of maintentance. They also tend to have poor resolution and limited magnification. The 8" f/6 would be the better all around scope. Especially if it's cheaper.
Posts: 3735 | Registered: Mar 2002
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posted
about $60... but I can get one of those nifty sights or maybe an electric eyepiece for that...
I have a couple of acres in the country and am looking forward to taking advantage of the lack of light pollution...
Did I mention that the local science center has the old Princeton Refractor that used to be at Mt. Wilson? it's pretty cool... they have viewing every Friday night... the last one I went to was a double-binary star somewhere... I forget now which one... pretty cool Posts: 3846 | Registered: Apr 2004
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posted
An electric eyepiece? Haven't heard of them...
On my honeymoon in August, we went to the observatory in Victoria, BC, Canada that was the worlds largest. Well, for 6 months, until Mt. Wilson was completed It was cool. We went the day before the Perseids peak, but still saw a lot of meteors.
posted
It's a CCD chip that has a standard video out... you hook it to a TV or other monitor and view without looking through the eyepice... it's like $60-65... only black and white, but still kinda cool.
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