Telescope Advice: 6" vs 8" Dobsonian

[Sheneron]

Hi
I had posted here for some advice a couple of months ago in the past, and it was very helpful. I have yet to buy a scope; however, I am on the presently on the verge and wanted a little more advice. I have basically come down to a decision that I will bet getting one of two dobsonian. Either a 6 inch of 8 inch.

The 6 inch is 167 dollars (nearly half price) while the 8 inch is 300 dollars. This will basically be my first telescope, but I want to do some nice observing. I have clear skies and I would like to be able to see galaxies and nebula as well as planets and what have you.

How much of a difference will there be between the 6 and 8, and in particular would that difference be worth the difference in money?

http://www.telescope.com/control/pr...nians/~pcategory=telescopes/~product_id=09964

http://www.telescope.com/control/product/~category_id=classicdobs/~product_id=09707

 

[James R]

The difference will be in how bright things look through your scope, and therefore also in what are the dimmest objects you can see.

The light collecting power of a scope depends on the cross-sectional area of the tube.

The cross-sectional area of a 6-inch scope is $36\pi$ square inches.
The cross-sectional area of an 8-inch scope is $64\pi$ square inches.

So, basically, the 8-inch scope has 64/36 = 1.8 times the light-gathering power of the 6-inch scope.

Therefore, in terms of the power alone, you'd expect the price of the 8 inch scope to be about 1.8 times the price of the 6 inch scope, which works out at almost $300. However, I'm not sure that the extra cost in materials justifies this increase in price, so I'd advise you to shop around a bit. At first glance, I suspect that of the prices you have quoted, either the 8 inch is too expensive, or the 6 inch is too expensive, for what you're getting.

Personally, I would go for the 8 inch over the 6 inch, just because you'll get almost twice the light-gathering power. You won't regret it. But, it depends on your budget, of course.

 

[turbo]

Get the 8" with the full-sized Dob mount. One of the best deals in the market.

 

[Chronos]

I agree with posters above. An 8" for $300 is very reasonable. The practical differences, mainly portability, are not that much for the same f ratio. If the 8" has a shorter f ratio, however, it is definitely a better buy.

 

[Sheneron]

Therefore, in terms of the power alone, you'd expect the price of the 8 inch scope to be about 1.8 times the price of the 6 inch scope, which works out at almost $300. However, I'm not sure that the extra cost in materials justifies this increase in price, so I'd advise you to shop around a bit. At first glance, I suspect that of the prices you have quoted, either the 8 inch is too expensive, or the 6 inch is too expensive, for what you're getting.

I can't find any telescopes 6 or 8 inch cheaper then those that I listed...

Thanks everyone for your advice, I will probably go with the 8 inch telescope. More then likely the one I listed. For now I will keep browsing though.

 

[turbo]

The 8" that you listed is widely-recognized as a great bang-for-the-buck scope and Orion has a great reputation for customer service. Drawbacks of the 6" include small aperture, single-sided alt-azimuth mount, and the need for some sort of stable platform (heavy picnic table, for example) to set it on. The 8" dob is sturdy and stable and the only accessory you might want to start observing with is a step-stool or some other "seat" that can put you at various heights. A very important part of visual observation is your personal comfort. If you can observe in a relaxed, comfortable position, you will see more detail and will spend more time with each object.

 

[Sheneron]

Oh excellent.

After reading that one post I was a little worried I wasn't getting the right price. However, I am certain I am going to be getting the 8 inch now. In fact I am just about to order it. Another thing that helped my decision is the 6 inch is now 250 dollars.

I can't wait to get it and start observing.

Thanks for all your help.

 

[turbo]

Oh excellent.

After reading that one post I was a little worried I wasn't getting the right price. However, I am certain I am going to be getting the 8 inch now. In fact I am just about to order it. Another thing that helped my decision is the 6 inch is now 250 dollars.

I can't wait to get it and start observing.

Thanks for all your help.

You're very welcome. If you have a dark-sky site that you can get to easily, you will find that 8" of aperture will pull in a LOT of objects. If you don't already have an atlas, I heartily recommend Wil Tirion's comb-bound Sky Atlas 2000. The chart size is generous, and it lays flat on the observing table. Objects are displayed with symbols appropriate to their size/magnitude and you will soon find yourself pushing yourself to observe faint/difficult objects. Just look at what field is near the zenith, find the corresponding chart in the atlas and start star-hopping from one object to another. It's the best way to learn your way around the sky, IMO.

 

[B. Elliott]

The difference will be in how bright things look through your scope, and therefore also in what are the dimmest objects you can see.

The light collecting power of a scope depends on the cross-sectional area of the tube.

The cross-sectional area of a 6-inch scope is $36\pi$ square inches.
The cross-sectional area of an 8-inch scope is $64\pi$ square inches.

So, basically, the 8-inch scope has 64/36 = 1.8 times the light-gathering power of the 6-inch scope.

Therefore, in terms of the power alone, you'd expect the price of the 8 inch scope to be about 1.8 times the price of the 6 inch scope, which works out at almost $300. However, I'm not sure that the extra cost in materials justifies this increase in price, so I'd advise you to shop around a bit. At first glance, I suspect that of the prices you have quoted, either the 8 inch is too expensive, or the 6 inch is too expensive, for what you're getting.

Personally, I would go for the 8 inch over the 6 inch, just because you'll get almost twice the light-gathering power. You won't regret it. But, it depends on your budget, of course.

Just wondering, but how did you get those numbers for the cross sectional area and why couldn't you just calculate for the available area?

6" diameter: Area = 28.26 in2
8" diameter: Area = 50.24 in2

 

[russ_watters]

Yeah, that post had a few things wrong with it. Something else in there is that there is a central obstruction from 10-15% and usually the fraction is bigger for a smaller aperture.

That said, the focal ratios on the two scopes are different - the 8" scope has a higher focal ratio, so things will actually appear dimmer than in the smaller scope. But the magnification is much higher.

 

[Chronos]

The ratio russ gave is correct- the square of the diameter. 50/28 is still 1.8, by my math.

 

[ASTRIC]

Has anyone used the below telescope, is it any good. This will be my first telescope and i want to get a decent one.

SkyWatcher Skyhawk 1145PM Newtonian Reflector Telescope

It was recommended by Sir patrick moore for amatures.

 

[Chronos]

The price differential [250 vs 300] is illogical, IMO. Some bells and whistles must be missing from the 8" tube. A basic 8" for $300 is more cost effective.

 

[James R]

Just wondering, but how did you get those numbers for the cross sectional area and why couldn't you just calculate for the available area?

6" diameter: Area = 28.26 in2
8" diameter: Area = 50.24 in2

My mistake. I used for the area of a circle $A=\pi d^2$ (i.e. squared the diameter instead of the radius). That means I was out by a factor of 4 on both the areas. But that doesn't affect the ratio of the areas, which is still 1.8, as I said.

Yeah, that post had a few things wrong with it. Something else in there is that there is a central obstruction from 10-15% and usually the fraction is bigger for a smaller aperture.

True.

That said, the focal ratios on the two scopes are different - the 8" scope has a higher focal ratio, so things will actually appear dimmer than in the smaller scope. But the magnification is much higher.

Magnification isn't determined by the focal ratio. The magnification you get is the focal length of the scope divided by the focal length of the eyepiece. Essentially, magnification is not the biggest issue for a telescope, anyway. Often the best views are at lower magnifications (depending on what you're looking at, of course). But changing eyepieces can give you whatever magnification you want, up to a practical limit determined by the scope aperture.

I'm not sure why you think things will be dimmer in a larger scope. Given the same magnification, things will always look brighter in the larger scope.

 

[James R]

Has anyone used the below telescope, is it any good. This will be my first telescope and i want to get a decent one.

SkyWatcher Skyhawk 1145PM Newtonian Reflector Telescope

It was recommended by Sir patrick moore for amatures.

That's only a 4.5 inch scope. For a reflector it is probably too small; I'd recommend something larger.