Anonymous
Unregistered
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Hi.
I am new to this forum and new to astronomy, so my questions below might seem dumb or naive.
A month ago, my son told me he would like a telescope. Knowing next to nothing, I went to the dept store, and there was one for $30 with high magnification. He said that he wanted it, but I sensed that it was a bad deal.  So I left, and instead I started to search the Internet to educate myself. While I understand the advantage of large aparture, I am still a bit confused about other concepts.
1. if magnification is measured by the ratio of focal length vs eyepiece focal length, then why not purchase an eyepiece with the shortest focal length, like 3mm? I do not see any large-aparture telescope sold with such eyepiece. Why not?
2. I read that a telescope with a low F value is for deep sky observation; a high F value for planet observation. I do not understand why that is the case. Also how bad observations might I have if I do not follow that rule? I do not want to have to buy two telescopes!
3. I read that a telescope with a low F value is more sensitive (less forgiving); a high F value less sensitive -- or might be the other way around  . What does that mean?
I spent a lot of time searching answers, but have not been satisfied.
Anyhow, I studied numerous models and makes, while remaining focused on the most affordable possible price/quality ratio. So the models Deep Space Hunter, SkyMaster, StarFinder, SkyWatcher, StarHopper came to mind. (I decided to by a dobsonian.)
Then the other day, I found SkyMentor that seem (to me) to be the best of the cheapest. But I found *only a couple* of reviews of that model on the Internet. What do you know about SkyMentor? Is it good?
I am willing to spend up to $1000. I mean, I want a telescope that gives sharp details. So a 10-inch or perhaps a 12-inch. I'd be crazy enough to attempt building my own, lets say, 15-inch if it could be for $1000, but that does not seem to be the case.
BTW, a 12-inch scope might be too big to carry in the car, so I had an idea: why don't they sell the big tube in two pieces? Then to assemble, there would be "claws" to attach them solidly. Gee, I should give that a try in my garage. If I had the expertise, I'd build my own custom-scope that way. 
thanks for the advice about my questions.
jean
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Rushwind
Carpal Tunnel
Reged: 03/11/04
Posts: 2137
Loc: Newark, CA
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Jean,
You're asking all the right questions. I'll try to tackle a few of them, and leave the rest to others.
First, a rule of thumb for any telescope is that your maximum useful magnification is going to be somewhere around 50x-60x per inch of aperture. So, a 4" scope can reasonably hit 200x. An 8" scope can go up to 400x. And so on.
Having said that, the atmosphere will often dictate how high of a magnification you can use on a particular night. You're looking through a bunch of air, there, and if it's moving around a lot (a storm just passed through, heated pavement warming the air, etc.), then the images that you see will start to "boil" or look like you're observing something that's at the bottom of a swimming pool. That's called "seeing", and it will often limit you to 200x-300x (sometimes less).
So it's not necessarily about grabbing the highest magnification; it's about grabbing an amount of magnification that will let you see what you're looking at, without the image swimming all over the place.
Also, as you increase the magnification, the amount of sky that you can see decreases. true field of view(TFOV) = apparent field of view(AFOV) / magnification, so 2 eyepieces, both with AFOV of 50°, but one producing 50x and one producing 100x, will allow you to see 1 degree and 0.5 degree of sky, respectively. For something like a planet or the moon, more magnification is nice, because those objects are small (the full moon covers about 0.5 degree of sky, for instance). However, for many other celestial objects (star clusters, cruising the Milky Way, etc.), a wider angle view is nice. There are several star clusters (Praesepe, for instance) that barely fit within a 1° FOV! Wider views are also nice for finding things. You'd be amazed how often I'm looking for something, only to find that I've been running laps around it because my FOV was a little too small... 
The f/ratio of a telescope is its focal length (the length of the tube) divided by its aperture (the diameter of the primary lens or mirror). So, an 8" (203mm) scope with a 1000mm focal length is an f/4.9 scope. Common telescopic f/ratios are in the range f/4 to f/10. f/ratios smaller than f/5 or so are considered "fast", and ones above f/8 or so are considered "slow". These terms come from photography (with a "fast" lens, you expose the film for shorter times than with a "slow" lens).
Shorter f/ratio scopes have a wider maximum FOV than longer f/ratio scopes. There's a simple mathematical reason for this, but it's escaping me at the moment, so someone else answer this question. 
Your third question regards collimation. For some reason, this is a subject that really freaks people out. I don't know why. It's the same concept as tuning a guitar. In order for everything to work its best, you have to tune your scope. This means getting out your "tuning tool" once in awhile, and using it to line everything up. The telescopes that need collimation the most often are reflectors. Telescopes with a short f/ratio (call it f/5 or faster) are more sensitive to being out of collimation ("out of tune") than are faster telescopes. So, you'll tweak the tuning on your f/5 telescope more than on your f/10 telescope. Given that they're the same type of scope (refractor, reflector, whatever), the f/5 telescope will have a tube that's half as long as the f/10 telescope, however.
$1000 for a first telescope. I'm sure the rest of the CloudyNighters are champing at the bit for a chance to help you spend it.
I heartily suggest that you go to a telescope store and *look* at these scopes before you buy one. A 10" or 12" telescope is bigger than it looks in the picture.
I think that a 10" Dobsonian reflector is probably a great first scope for you. You'll see lots of stuff, have plenty of $$ left over for accessories (you'll need it), and have a great time. The Hardin Deep Space Hunter and the Orion/SkyWatcher Dobs get high marks from all.
Good luck, and dark skies to you.
Jimbo
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Order of the Unblinking Eye
NJP 300D SSAG 8"f/5 (Rig)
Guidescope? What guidescope?
I used to shoot Nikon DSLR.
Before that, I shot film.
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 13646
Loc: southeastern Nebraska
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Hi there Jean. Well, lets take your questions by the numbers:
Quote:
1. if magnification is measured by the ratio of focal length vs eyepiece focal length, then why not purchase an eyepiece with the shortest focal length, like 3mm? I do not see any large-aparture telescope sold with such eyepiece. Why not?
There are many factors in what eyepiece to get, and there is a limit to the amount of power that a given telescope can provide. The limit on power is roughly 50x to 60x *per inch of aperture*. This would mean that a 4.5 inch aperture telescope like Orion's Skyquest XT4.5 (highly recommended for youngsters BTW) with its aperture of 4.5 inches would be capable of giving about 225 power to perhaps as much as 270x. Going beyond this level will produce an image which is rather dim and fuzzy. Powers from 150x on up are best used mainly on the moon and planets, but again, there is a limit to how much power can be used. However, much of the time, you will want to use only a small fraction of this power, especially for larger objects like the so-called "Deep-Sky" objects (open and globular clusters, nebulae, and galaxies). Longer focal length telescopes merely allow you to use longer focal length eyepieces to get to moderate to high powers. Some of these longer focal length eyepieces have better "eye relief" which makes them easier to use with glasses. Inexpensive eyepieces also tend to perform just a bit better with longer focal length telescopes than they do with shorter focal length ones (unless you get into the very complex and expensive 'wide-field' eyepieces).
Quote:
2. I read that a telescope with a low F value is for deep sky observation; a high F value for planet observation. I do not understand why that is the case. Also how bad observations might I have if I do not follow that rule? I do not want to have to buy two telescopes!
This is not necessarily so, but low f/ratio telescopes do allow you to get to the wider fields of view needed for some deep-sky objects a bit more easily. They also make the telescope a bit more compact than with the longer focal length instruments. You can "make" a short f/ratio telescope into a longer one by using a device known as a Barlow Lens, which can boost the focal length by 2 or 3 times what it normally is.
Quote:
3. I read that a telescope with a low F value is more sensitive (less forgiving); a high F value less sensitive -- or might be the other way around . What does that mean?"
Well, all Newtonian telescopes have to be aligned or "collimated" from time to time, as vibration or rough handling tends to knock their optics out of alignment (easily readjustable to proper alignment though). Short f/ratio Newtonians are more sensitive to miscollimation than longer focal length ones, so the longer ones are said to be "more forgiving" of being a little out of alignment. Also, precisely grinding and figuring the mirrors for a short f/ratio Newtonian is harder than it is for a longer focal length one, so some of the longer "budget" Newtonians might have better mirror performance than the shorter ones, although this isn't necessarily true all the time.
On a general suggestion, I would start small (no larger than 6 or perhaps 8 inches) and get some experience before deciding to commit a lot more money into equipment. The Orion SkyQuest series of Dobsonians are pretty good buys for the money, so you probably won't go wrong there. Also, find the nearest Astronomy Club and take a look at what some of their people use before buying one yourself. Clear skies to you.
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David W. Knisely . . . . . . "If you aren't having fun in this hobby, you aren't doing it right."
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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wilash
Fairy Godmother
Reged: 09/30/03
Posts: 5746
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I will add a few more points to these good answers.
Aperture also affects angular resolution. The larger the aperture, the finer the detail can be seen.
Focal length determines the field of view. The longer the focal length, the smaller the field of view. Eyepieces change the magnification and so the field of view can also be change in a particular telescope, but since there is not an infinate number of eyepiece focal lengths, each telescope has a practical range. Magnification is calculated by dividing the telescope's focal length with the eyepiece's focal length.
Focal ratio is the aperture divided by the focal length. The smaller the number, the brighter the image. The image brightness also changes with magnification, but like the field of view, each telescope has a practical range. Dividing the eyepiece focal length by the telescopes focal ratio gives you the size of the exit pupil. The larger the exit pupil, the brighter the image.
You notice a 10" f/5 telescope has the same focal length as a 5" f/10 telescope. They will both have a similar field of view, but the 10" scope will have brighter images (it will also have higher angular resolution).
I suggest you get a book on this subject. Star Ware by P. Harrington covers equipment very well. Not just scopes, but accessories also.
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lighttrap
Reged: 02/06/04
Posts: 3833
Loc: cloudy, foggy, humid NC, US
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Jean, you've received some excellent answers and input from Jim, David and Will. But, before you get too absorbed in the numbers, there's another number that might be relevent. How old is your son? Since he initiated the request for the scope, I forsee him wanting to use it on his own. Using the larger scopes you're contemplating might be pretty difficult for him, depending on age and size. I'd suggest not getting anything that he can't setup and use by himself when the time comes. There are several good reasons why Dobs in the 4.5-8" range are often recomended. I'd suggest that you concentrate your search on a 6-8" scope. Save some of your money for books, star atlases, and decent eyepieces.
Mike Swaim
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18" Starsplitter II f/4.5
8" Hardin Dob f/6
C5 workhorse mini SCT f/10 or f/6.3
70mm TV Ranger dual purpose birding/astro
77mm Leica Televid APO
16x70 Fujinons on UA Deluxe Mt.
12x50 Nikon SE
8x30 Nikon E2s
and many others
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wilash
Fairy Godmother
Reged: 09/30/03
Posts: 5746
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Good point Mike. I would leave one third (or more) of your budget for accessories. And I also agree 6" to 8" is a good place to begin. The easier the scope is to move and set up, the more it is used.
BTW, there is a dob which has a tube that can be separtated in the middle for transportation. It is sold in Japan by Schwartz Trading. Also, I have never heard of SkyMentor. Can you post a link?
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Jarad
Postmaster
Reged: 04/28/03
Posts: 5732
Loc: Atlanta, GA
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I will emphasize 2 of the many excellent points above:
1 - Get Starware. It is easy to read, covers all of your questions in depth, and has a lot more info on accessories, etc. as well as some sections on starhopping and seasonal charts listing some of the nicer objects to see.
2 - I would aim for an 8" f6 dob. An 8" is big enough to see a lot of stuff, small enough to move fairly easily and use without a ladder, and slow eough f-ratio to make collimation not quite so critical and not need a coma corrector. Bigger scopes tend to have faster f-ratio's because it makes the tube shorter, making them more portable and avoiding ladders for observing (or at least making the ladders shorter). It will also leave some money left over for accessories like eyepieces and filters. I would recommend budgeting no more than $600 for the scope, and the rest for a few good eyepieces, a nebula filter, a star chart and a few goodies like red flashlights, etc.
Finally, find out about astronomy clubs in your area. Take your son to some of their events, look at and through the scopes (astronomy clubs are usually happy to help out new people interested in the hobby). They may even have a loaner scope you could try. That will give you a much better idea both of what you can see through the scope and how big they are. They will also know what local astronomy dealers are in your area, and may have someone looking to move up who will sell you their old scope.
Hope this helps,
Jarad
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jrcrilly
Refractor wienie no more
Reged: 04/30/03
Posts: 30716
Loc: NE Ohio
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I think the gang has pretty much covered the material. I can add one thing; there is a commercially available Dobsonian telescope with a split tube but it's larger and more expensive than than your target figure. Check out:
http://www.discovery-telescopes.com/PDHQ.html
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John C
Battle Cry of Reno
http://www.wadsworthobservatory.com
My Cloudy Nights gallery
AT12RC
AT65EDQ
QSI683WSG-8
Roper Scientific Quantix 6303E "project" camera
mystery EQ mount on the way
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MMICKELS
Aluminum Knight
Reged: 01/20/04
Posts: 36050
Loc: Land of Shake and Bake
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Quote:
Good point Mike. I would leave one third (or more) of your budget for accessories. And I also agree 6" to 8" is a good place to begin. The easier the scope is to move and set up, the more it is used.
BTW, there is a dob which has a tube that can be separtated in the middle for transportation. It is sold in Japan by Schwartz Trading. Also, I have never heard of SkyMentor. Can you post a link?
Jean, this is good advice! Do not buy a scope that is so big it becomes a chore to set up. I would also suggest hooking up with a local astronomy club and going to some star parties. You can look through many different scopes and make a better determination of which one will fit your lifestyle.
Mark
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Mark
I'm going outside to stand, so if anyone asks I am outstanding.
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Anonymous
Unregistered
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Jus thought I'd add in I spent a little over $300 just in must have accessories. 2 EP's, a Barlow, some filters, couple of books, etc.. so if your budget is $1K your telescope budget really is $650 or so.
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lighttrap
Reged: 02/06/04
Posts: 3833
Loc: cloudy, foggy, humid NC, US
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Quote:
so if your budget is $1K your telescope budget really is $650 or so.
That's about right. 1/3 to 1/2 of the budget should be set aside for accessories. And some of that should be set aside for things not envisioned at the outset.
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18" Starsplitter II f/4.5
8" Hardin Dob f/6
C5 workhorse mini SCT f/10 or f/6.3
70mm TV Ranger dual purpose birding/astro
77mm Leica Televid APO
16x70 Fujinons on UA Deluxe Mt.
12x50 Nikon SE
8x30 Nikon E2s
and many others
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Anonymous
Unregistered
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I haven't seen it pointed out yet, and possibly it goes without saying, but by adding a barlow, you effectively change the focal ratio. I've heard that there are also ways to change the ratio in reverse (that is, to "quicken" a long ratio telescope). I'm not sure how that's done or how common it is -- I suppose if you use a really long focal length eyepiece, one would see a similar effect?
This is ignoring the question of whether you'd degrade your image quality by introducing too many extra lenses/mirrors/whatever. On that subject, I think you have the right idea by setting aside a good chunk of money for accessories. Without accessories equal in quality to the scope, any "extra" money you'd spend on the scope may be a waste.
Pay close attention to apparent field of view and eye relief. If you're like me, you probably want a lot of both, and remember that any given eyepiece will work slightly differently on nearly any telescope.
Don't only take my word for it, though; I'm still relatively new, myself, and these guys have already answered all the easy questions.
I have been considering (for when I have the money) the purchase of a medium aperture Maksutov-Cassegrain scope. You might consider something like that if portability is a concern. They're more expensive than the standard newtonian per unit of aperture, but the design allows them to fit a wide aperture and reasonable focal length in a short tube.
Chris
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BluewaterObserva
Postmaster
Reged: 05/18/04
Posts: 5748
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I will add that magnification, is NOT an indication of a telescope's power. Low Magnification / wide fields of view are every bit as important, if not more so than 200x / narrow fields of view.
Only in a few quality telescopes, does the f/ratio matter all to much. Most people / most scopes get great views and can attain low magnification and high magnification using the right accessories weather the scope is F/10 or F/4.5
The F/10 will allow you to use larger eye pieces for moderate to high magnification observing. The f/4.5 will usually allow lower magnifications and widerfields of view than an F/10 will. So that is a trade off, one is not better than the other.
Just come back and let us know what you buy!!!!
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AJTony
sage
Reged: 04/17/04
Posts: 379
Loc: Hamilton Square, NJ
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You received nothing but good advice, so I will just add a comment.
I have students who ask me; if the high magnifications listed on dept. store telescopes are essentially worthless, how can they legally advertise them. The answer of course is they list the highest theoretical power based on objective lens/eyepiece, which always exceeds the practical power (~50 X inches.) My own rule I pass on to my students; divide dept. store scope high power by 10, and that will likely be your best practical power for these low quality scopes. However, the best rule is don’t buy these scopes.
It is good that you went with instinct at the store. I always felt that many a young (or older) amateur lost out on a great hobby, because of the frustration they found with a poorly made telescope.
Stay with Cloudy Nights, and scan the Net. I personally think that Orion Telescopes is a good site to check out, but you will be directed to many other good suppliers by our CN members.
http://www.telescope.com/jump.jsp?itemID=0&itemType=HOME_PAGE
AJ
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Oberwerk BT100-45 Binos
Apogee 25 X 100 Binos
Canon 15 X 50 IS Binos
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bierbelly
Postmaster
Reged: 01/23/04
Posts: 6179
Loc: Sterling, VA
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My $.02. The f/ratio of a scope is more relevant to photographic uses, and almost irrelevant to visual use, except as to the length of the tube, difficulty in collimation, etc, raised above. I'd be surprised if you could really detect, visually, a brighter image on an f/6 than on an f/10 scope of the same aperture.
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12" DSH
8" f/4 Vega MakNewt
6" MN66
TV85
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half meter
Postmaster
Reged: 05/05/04
Posts: 13026
Loc: Great Lakes
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You're right, Bierbelly.
Especially if the true field of view (FOV) is made the same. If you have the same aperture and FOV in the eyepiece the images will look identical.
To achieve the same FOV requires different eyepieces for different f/ ratio scopes, though.
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Gary
Collins I3 (Thin Film) Image Intensifying Eyepiece
Coronado Maxscope Double Stacked 90mm <0.5A w/BF30
6" f/8 TMB/A&M Carbon Fiber APO; f/5 with 4" Borg ED Field Flattener/Reducer
8" f/5.6 Starmaster Versa V8/Zambuto Mirror/ServoCat Jr dob
30" f/4.5 Obsession/OMI Mirror/ServoCat/Argo Navis dob
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Jarad
Postmaster
Reged: 04/28/03
Posts: 5732
Loc: Atlanta, GA
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Where it makes a difference visually is in what TFOV's are achievable. A 10" f10 SCT isn't going to see wide fields, even with a 2" focuser. A 10" f5 newt will be able to achieve twice the TFOV of the f10. Same goes for long refractors - an f15 achromat isn't going to deliver the wide fields possible with an f5 scope with any eyepiece.
Before getting too deeply into the details and confusing Jean, I think the best bits of advice have been given. For the rest she needs to do some reading, and preferably get to a public star party with her local club. Then she will be in a better position to ask good questions, which will let us give her useful answers instead of a deluge of details that may or may not help her.
Jarad
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 13646
Loc: southeastern Nebraska
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Wilash posted:
Quote:
Focal ratio is the aperture divided by the focal length. The smaller the number, the brighter the image.
This is true for photography, but not necessarily for visual use. If two telescopes of the same aperture are used at the same power, they will provide the *same* image brightness. You will have to use a longer focal length eyepiece with a longer focal length telescope to achieve that lower power and wider field, but again, the brightness is the same if the powers are the same. Clear skies to you.
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David W. Knisely . . . . . . "If you aren't having fun in this hobby, you aren't doing it right."
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 13646
Loc: southeastern Nebraska
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Protheus posted:
Quote:
I haven't seen it pointed out yet, and possibly it goes without saying, but by adding a barlow, you effectively change the focal ratio. I've heard that there are also ways to change the ratio in reverse (that is, to "quicken" a long ratio telescope). I'm not sure how that's done or how common it is -- I suppose if you use a really long focal length eyepiece, one would see a similar effect?
This reduction is done with a device known as a "focal reducer", which is most commonly used with long focal length telescopes like Schmidt-Cassegrains. In the case of the Celestron Focal Reducer/Corrector, it takes an f/10 telescope and makes into about an f/6.3 telescope. This yields a somewhat wider field of view at lower power, but these reducers are more commonly used in photography rather than for visual use.
Quote:
This is ignoring the question of whether you'd degrade your image quality by introducing too many extra lenses/mirrors/whatever.
There really is no question here. Modern Barlows of halfway decent quality do *not* significantly degrade the image. In fact, sometimes they can make things look even a little better, as some of the simple eyepiece designs which don't perform very well with shorter f/ratio instruments can be made to function adequately when using a Barlow. The Barlow can also preserve the eye relief of a longer focal length eyepiece while at the same time providing higher power.
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David W. Knisely . . . . . . "If you aren't having fun in this hobby, you aren't doing it right."
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Rusty
Postmaster
Reged: 08/06/03
Posts: 19246
Loc: Brooker, FL
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You may also wish to consider a GOTO scope. Once aligned (on a couple of stars), you can point to various objects simply by punching them in the hand control.
See: http://www.buytelescopes.com/product_list.asp?m=1&d=1&pricelow=&pricehigh=&sort=prlh&go=Go
for a selection - IMO the sweet spot is the 130GT - a 5" Newtonian reflector. Dobsonians (also Newtonian reflectors) offer the most aperture for the money, but have a simple mount that requires star-hopping and nudging the tube to keep the object in the field of view (FOV). This feature may not appeal to children.
Much of any scope's perfomance is derived from the quality of the eyepieces, in addition to the quality of the primary optics.
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N11GPS Fastar//TOA-130S//MK66 Std//AT6RC//Vintage C5//Megrez II 80mm APO//SolarMax 40
NJP Temma II//Sirius EQ-G
Too Many Astro-Cameras//Mallincam Color Hyper Plus
Two not-spoiled Golden Retrievers - Casey and Nelson
Lot 19 Deerlick Astronomy Village (Canis Major)
Sometimes I think we're alone in the universe, and sometimes I think we're not. In either case the idea is quite staggering. - Arthur C. Clarke
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