Telescope Advice: 6" vs 8" Dobsonian

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In summary, the conversation discussed the decision between purchasing a 6 inch or 8 inch dobsonian telescope. The main difference between the two is the light-gathering power, with the 8 inch having almost twice the power of the 6 inch. The price difference for this increase in power is not justified, so it is suggested to shop around for a better deal. Ultimately, the group recommends going with the 8 inch telescope, specifically the one listed, as it is a great deal and a reputable brand. It is also advised to invest in a sky atlas to learn how to navigate the night sky and observe faint objects.
  • #1
Sheneron
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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
 
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  • #2
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 [itex]36\pi[/itex] square inches.
The cross-sectional area of an 8-inch scope is [itex]64\pi[/itex] 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.
 
  • #3
Get the 8" with the full-sized Dob mount. One of the best deals in the market.
 
  • #4
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.
 
  • #5
James R said:
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.
 
  • #6
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.
 
  • #7
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.
 
  • #8
Sheneron said:
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.
 
  • #9
James R said:
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 [itex]36\pi[/itex] square inches.
The cross-sectional area of an 8-inch scope is [itex]64\pi[/itex] 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
 
  • #10
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.
 
  • #11
The ratio russ gave is correct- the square of the diameter. 50/28 is still 1.8, by my math.
 
Last edited:
  • #12
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.
 
  • #13
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.
 
  • #14
B. Elliott said:
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 [itex]A=\pi d^2[/itex] (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.

russ_watters 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.
 
  • #15
ASTRIC said:
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.
 

1. What is the difference between a 6" and 8" Dobsonian telescope?

The main difference between a 6" and 8" Dobsonian telescope is the size of their primary mirror. The 6" has a 6-inch diameter mirror, while the 8" has an 8-inch diameter mirror. This results in the 8" telescope having a larger light-gathering capacity and being able to provide higher magnification and resolution compared to the 6" telescope.

2. Which one is better for viewing planets and deep-sky objects?

The 8" Dobsonian telescope is better for viewing both planets and deep-sky objects. The larger mirror allows for better resolution and light-gathering, making it easier to see faint objects in the night sky. However, the 6" telescope may still be suitable for viewing planets, but may not provide as clear and detailed views as the 8" telescope.

3. Is there a significant price difference between the two?

There is typically a price difference between the 6" and 8" Dobsonian telescopes. The 8" telescope is usually more expensive due to its larger size and capabilities. However, the exact price difference may vary depending on the brand and model of the telescope.

4. Which one is easier to transport and set up?

The 6" Dobsonian telescope is generally easier to transport and set up compared to the 8" telescope. The smaller size and weight of the 6" telescope make it more manageable to carry and assemble, making it a good option for those who need a more portable telescope.

5. Can I upgrade from a 6" to an 8" telescope in the future?

Yes, it is possible to upgrade from a 6" to an 8" Dobsonian telescope in the future. However, it may require purchasing a new telescope altogether or purchasing larger components for the existing telescope, depending on the brand and model. It is always best to research and consult with a professional before attempting to upgrade your telescope.

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