Reflecting Telescope or Refracting Telescope?


by pravdt
Tags: reflecting, refracting, telescope
pravdt
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#1
Jan31-12, 11:11 PM
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Which one is better for general non-professional star gazing, Reflecting Telescope or Refracting Telescope?
I live in sub-urban area, not that much light/air pollution.
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Chronos
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#2
Feb1-12, 12:24 AM
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Depends on personal preference. There really isn't a perfect telescope. Each has its plus and minus features. A refractor suffers from chromatic aberation and reflectors suffer from coma. Certain designs minimize these effects, but, at a price. Refractors quickly become unwieldy as aperature increases - mainly due to their relatively longer focal lengths. Anything over about 3" is pretty awkward. Reflectors tend have shorter focal ratios are are comparatively light, but, the central obstruction negatively impacts resolution. They are also less expensive for the same aperature. They usually start around 4", which would be large for a refractor [and considerably more expensive]. Dobsons, a type of reflector, are very popular. They offer good aperature, portability and are fairly inexpensive in smaller [8" or less] sizes. A scope like this would probably be a good choice. You also have enough aperature to see a lot of things in a scope like this. If there are any clubs in your area it would be a good idea to visit a star party and take a peek through some of the different offerings. This would go a long way in helping you chose the best scope for you. Get there early so you can see how much effort is involved in setting them up.
Topher925
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#3
Feb1-12, 02:31 PM
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If you're new to astronomy, I think the best option is for a large aperture dobsonian reflector. You get a lot of light for the buck.

turbo
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Feb1-12, 03:17 PM
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Reflecting Telescope or Refracting Telescope?


I prefer refractors, though they tend to be a bit pricey. Dobsonian reflectors are very good at bang-for-the-buck performance, but they tend to be a bit larger than some people can house or transport easily. You really need to get out to a few star-parties with local astronomy clubs, so you can see what these scopes entail in transportation, set-up, adjustments, etc. This hobby is not cut-and-dried!
davenn
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Feb1-12, 08:41 PM
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Hi Pravdt
firstly welcome to the forums :)

You really have to also consider what you mainly what you want to use the scope for and what sort of budget.
Personally I would go for a 8 to 10" Newtonian f5 Dobsonian. A respectable light bucket and still easily transportable in all but the smallest of cars. They are easy to set up and get observing. And as one other said they really are the best bang for buck!!

Refractors or Schmitt-cassigrain reflectors with their higher f ratio, f 8 or higher means they are great for planets, but not as good for faint deep space objects as the Newtonian with an f4 to f5. The higher the f ratio the fainter the light getting to the eyepiece. That doesnt mean you wont see them, they just wont be as bright.

There's an old saying amongst amateur astronomers along the lines of....
"The scope that is easiest to use will be the scope that gets used the most."

Some people have bought monster lightbuckets 12" or larger reflector only to find that was so bulky that it was difficult to setup to use and as a result it didnt get used very much.

If you have a decent budget and would like to buy something that is computer controlled ... it can be made to go to any object.... then something like this...
http://www.telescopesdirect.com.au/C...&category=-289 may " float your boat"

at 9.25 inch primary mirror its a good middle of the road scope fully computer controlled (tho you can use it manually) its still small enough to put into a car and take out to a real dark site. I purchased this one ~ 4 weeks ago, now only if the rain would stop ( been 3 weeks of rain now) I mite actually make some more use of it. haha

Dave
Chronos
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Feb2-12, 01:17 AM
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Personally, I think anything larger than 8" will be problematic from a portability standpoint [especially if you are just getting into the hobby]. Catadioptrics are nice, but, somewhat pricey. If you are just beginning, a modest [<10"] f6 [or lower] reflector is a good choice. Goto is nice, but, certainly not essential for basic stargazing. Learning the sky is half the fun. Even with goto, if you don't know the sky the thing will frustrate you to no end. A decent set of setting circles and a basic star atlas is all you really need to get started. You should also ensure the mount is appropriate to the scope. A cheap, cheesy mount will spell disaster, even with a fine optical tube assembly [OTA]. You should expect to spend roughly as much for the mount as you do for the tube. Some off the shelf mounts that come with the OTA are OK to good, but, that is unusual in my experience. It is normally better to buy separate. You can get some really good deals on preowned scopes and mounts on places like ebay. It is worth checking out. Astronomical equipment suffers little from wear.
jnorman
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Feb2-12, 10:27 AM
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chronos - inre: "the central obstruction negatively impacts resolution" - i dont think this is correct. resolution is determined by the diameter of the objective. the spider in a reflector can, of course cause a certain amount of diffraction, and lessen the amount of light hitting the primary mirror, but it does not limit resolution.
Chronos
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Feb2-12, 02:31 PM
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The secondary does not so much cause diffraction [like a spider], it mainly just reduces the amount of light reaching the eyepiece which reduces contrast and brightness of the image.
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Feb2-12, 02:50 PM
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Quote Quote by jnorman View Post
chronos - inre: "the central obstruction negatively impacts resolution" - i dont think this is correct. resolution is determined by the diameter of the objective. the spider in a reflector can, of course cause a certain amount of diffraction, and lessen the amount of light hitting the primary mirror, but it does not limit resolution.
yes thats correct, the resolution isnt affected.
A reason I like Schmitt-cassigrain reflectors is that there is no spider and you dont get the huge spikes from bright stars ( of course particularly noticeable with photography)

Dave
Drakkith
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Feb2-12, 06:49 PM
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According to my book, Telescope Optics for the Amatuer Astronomer, the secondary may not affect resolution, but it does affect contrast. For example, looking at Jupiter through a 5 inch apochromatic refractor would allow you to see more detail than a 5 inch reflector simply because the low contrast areas on the planet stand out more. How much better the image is depends on sky conditions and quality of each scope. But this is mainly an issue for the planets and moon.
Chronos
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Feb5-12, 05:04 AM
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It's an issue for all observations.
Drakkith
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Feb5-12, 08:18 PM
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Quote Quote by Chronos View Post
It's an issue for all observations.
Maybe. But there's so much more fine detail on the planets and the moon that are readily visible to smaller telescopes. At least that's what I've been told. I could be wrong.
chemisttree
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Feb21-12, 11:49 AM
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The resolution IS affected by the central obstruction. The modulation transfer function clearly shows that an obstruction diminishes spatial resolution at some frequencies and enhances it at others. For extended objects which contain all spatial frequencies, there is a loss of image quality with an obstructed instrument vs an unobstructed one. Contrast is the metric used to determine resolution so I'm not sure how a central obstruction could affect contrast without affecting resolution.
chemisttree
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Feb21-12, 11:59 AM
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Quote Quote by Drakkith View Post
Maybe. But there's so much more fine detail on the planets and the moon that are readily visible to smaller telescopes. At least that's what I've been told. I could be wrong.
That is likely due to a smaller instrument's being a bit more resistant to poor seeing relative to a larger aperture... obstructed or not. I've never seen the same amount of lunar or planetary detail with my 60mm or 80mm APO as I've seen with my 9.25" SCT, for example. I have noticed from time to time that my 80mm performs as well as my 6" SCT on Jupiter or Mars but it's no contest when I compare it to the 9.25".
Drakkith
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Feb21-12, 04:59 PM
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Quote Quote by chemisttree View Post
That is likely due to a smaller instrument's being a bit more resistant to poor seeing relative to a larger aperture... obstructed or not. I've never seen the same amount of lunar or planetary detail with my 60mm or 80mm APO as I've seen with my 9.25" SCT, for example. I have noticed from time to time that my 80mm performs as well as my 6" SCT on Jupiter or Mars but it's no contest when I compare it to the 9.25".
By small telescopes I mean the size range available to most amateurs, not the multi-feet sized ones at major observatories where you can start to see a lot more detail on faint deep sky objects.
russ_watters
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#16
Feb21-12, 05:23 PM
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Quote Quote by davenn View Post
yes thats correct, the resolution isnt affected.
A reason I like Schmitt-cassigrain reflectors is that there is no spider and you dont get the huge spikes from bright stars ( of course particularly noticeable with photography)

Dave
Unless you want those spikes, in which case you lay wires across the front of the objective.

http://www.russsscope.net/images/Horsehead-HaRGB.jpg
davenn
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Feb21-12, 10:13 PM
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Quote Quote by russ_watters View Post
Unless you want those spikes, in which case you lay wires across the front of the objective.

http://www.russsscope.net/images/Horsehead-HaRGB.jpg
hahahaha well true I guess there are those that think is looks photogenic

Dave


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