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General Astronomy => Beginners Forum => Topic started by: unmoharib on December 24, 2017, 10:43:31 AM

Title: Beginner dob Recommendation is almost always 8", why?
Post by: unmoharib on December 24, 2017, 10:43:31 AM
Hi guys

All over the net when I see new folks searching for a dob telescope everyone urges the 8".  I know the cost to aperture ratio is decent but what about the usually forgotten 6" dob?

I've owned a 6" and an 8" in precisely the same time.  Had them both cooled and aimed high.  I had the luxury of directly comparing them.  I had 24mm Panoptic X2 so every scope had one.  (I had been holding them for a friend who purchased them used for binos).

I have to say I did not see much difference between the two scopes concerning viewing more in the bigger one.  I'm in the city, so perhaps at a dark site it'd have been apparent?  What I did notice is how clean the opinion was in the 6" dob, filled with richness and contrast.  Not to say the 8" was awful or anything but as hard as I tried to like the 8" longer, I ended up enjoying the 6" more.

The 8" isn't a monster but when it came time to transport them in the 6" was easier, to me that the opinion was better, it looked considerably smaller although it's the same focal length.  And it was cute as a button in my living space.

I feel the 6" needs some love.  It brings a lot to the table in a much better price then the".

Last, there was something magical about the 24mm Panoptic from the 6" F8 dob.
Title: Re: Beginner dob Recommendation is almost always 8", why?
Post by: sennessningwilch on December 24, 2017, 04:06:38 PM
The gap in light collecting between a 6 and 8" is the greatest given a 2" growth at 78%.  The distinction is most evident on globular clusters.  The over 30 percent increase in resolution can be appreciated on near doubles and solar system observing.  The general dimensions and weight gap in the mounted scopes isn't as much a factor given the profits realized.
Title: Re: Beginner dob Recommendation is almost always 8", why?
Post by: Jacob Cota on December 29, 2017, 01:41:30 AM
What was the f-ratio of your 8 inch? If it was f5 or so, then I think the higher contrast from the 6inch at f8 made the 6 inch more enjoyable under an urban sky.

I've seen a similar thing in refractors. When viewing from the city I prefer the view through an f7 instrument rather than an f5 one. The higher f-ratio seems to improve contrast and the perceived quality of view goes up. I think the faster instruments bring inmore light pollution which brightens the background and degrade contrast. How much of this is physics and how much is human perception, I don't know. But as the art critic says... "I know what I like".

Regarding dark skies, yes the difference seems to vanish and the larger instrument will usually outperform the smaller one. Not that the small ones are bad but more aperture means more resolving power which normally translates into more detail at the eyepiece.
Title: Re: Beginner dob Recommendation is almost always 8", why?
Post by: subhymerlo on December 29, 2017, 04:06:02 AM
It was the Orion combo.

8" - F5.9
6" - F8I hear what your saying, and the reasons listed are good reasons to lean towards an 8" dob. And believe me I am like thinking ok, I should enjoy the 8" more right? lol It was a lot of fun being able to compare these two scopes side by side. Really neat experience actually.
Title: Re: Beginner dob Recommendation is almost always 8", why?
Post by: blufdestholreng on December 29, 2017, 04:06:42 AM
Ok, not as much of a difference then but still possibly a contributing factor.

Its also possible that the 8 was maybe not as well collimated or maybe just the 6 has an outstanding mirror. In any event, I know what you mean. Besides the refractors I also have a little 6 inch Newtonian and I really like it. Its small, light and gives really good views for its size and cost. At the moment its my favorite grab-n-go scope.I've also recommended it to at least one friend considering his first scope.
Title: Re: Beginner dob Recommendation is almost always 8", why?
Post by: Duane Berhane on December 30, 2017, 10:07:15 PM
Actually, my default recommendation for beginners is the 6" f/8 for the reasons given above, plus cost. As you noted, I'm in the minority, though. One big difference for me, though, is with 8" of aperture, I get better color on Saturn and Jupiter than with 6" scopes. I'm not a beginner who may or may not like observing, though.
Title: Re: Beginner dob Recommendation is almost always 8", why?
Post by: kondcongrese on December 31, 2017, 01:13:06 PM
Along with factoring in affordability, I usually talk to beginners about their physical capacity (larger scopes are heavier) and their perceived zeal and what objects they are most interested in. Usually after some frank talk, you can recommend a scope that best fits all of these factors.

For some, 6" is a good trade-off between $$$ and potential abandonment of the hobby.

For others, the cost is less of a factor and they really do seem to be quite serious longer term - and sobigger is better!
Title: Re: Beginner dob Recommendation is almost always 8", why?
Post by: Cory Bass on January 03, 2018, 11:20:17 PM
The biggest consideration that I see here regards at what stage you are in the hobby. Will your choice be as a rank beginner and your purchase represent either your first step up from say a very small refractor, or perhaps be your very first actual telescope? If so, I would definitely go with the 6" f/8. You will find as a beginner that the smaller scope is easier to handle, aim and quite capable to providing nice views of a multitude of celestial object. At the same time such a choice represents a minimal expenditure of funds on a hobby that you may discover you are actually not all that enthusiast about after all. There will always be time later to step up to a larger, bulkier, instrument later if the bug really bites you hard. Too many beginners today get carried away buying gear, which is much less important than learning the hobby really well first.

Title: Re: Beginner dob Recommendation is almost always 8", why?
Post by: Keith Dixon on January 04, 2018, 05:52:13 AM
I think that the 8 inch is popular because this is the point that a telescope is no longer a toy.
Title: Re: Beginner dob Recommendation is almost always 8", why?
Post by: Lauro Mason on January 04, 2018, 07:52:29 AM
In mid to large size city light pollution six inches is the entry point in aperture in a reflector where details in DSO are seen by a beginner. Eight inches is where things start really showing up. Smaller aperture works in dark skies. Eight inches is also a good balance between weight, size, cost, and details in the view.
Title: Re: Beginner dob Recommendation is almost always 8", why?
Post by: Bilal Luck on January 04, 2018, 09:39:37 AM

I have owned several of both apertures through the years and much smaller and much larger and everything in between. To me there is a sweet spot and that sweet spot is the 6 inch. The views are not significantly different than those in the 8 despite what the specs would indicate and the six inch is noticeable easier to move around and transport than the 8, again despite what the specs would indicate.

The only real difference is that most 6 inch scopes have a 1 1/4 inch focuser and that the 8s will usually have a 2 inch. To me the 2 inch is nice but pricey so I've opted for a nice set of 1 1/4 inch Ploslls through the years. They do everything I need done. To me the next aperture where you get more bang for the buck is the 12.5 range. These scopes are getting significantly heavier although in the truss incarnations not that much more difficult to transport. They also don't usually need a ladder or step stool.

I'm a minimalist so please take that into consideration. I own the 6 and the 8 both right now. ( I also own the 4.5 and its as cute as it gets).

It takes me every bit of 5 minutes to set up and break down. The rest of the time is spent taking it all in. ( go for the unit finder - Telrad, Rigel, etc..)

All that said - there is no wrong answer. I like the 6 but you're probably going to end up with the 8 and there's nothing wrong with that.

What ever gets you out under the sky at night.

Title: Re: Beginner dob Recommendation is almost always 8", why?
Post by: junktranasop on January 10, 2018, 02:31:18 AM
I see 6 inch, 8 inch and 10 inch Dobs recommended. I have owned multiple examples of both 6 inch and 8 inch Dobs and have owned a 10 inch GSO for 13 years. Each has its advantages and disadvantages.

In general, I think the 8 inch is a better choice and a better value than the 6 inch. Optically, an 8 inch goes deeper, has better resolution, and shows the planets in greater detail. globular clusters start to come alive in the 8 inch, not so much in a 6 inch.

Typically, a 6 inch comes with a barely adequate 1.25 inch rack and pinion focuser, an 8 inch, a decent 2 inch Crayford and if its a GSO (Zhumell), it will be a very nice 2 invh, 2 speed Crayford. The GSO will also include a 50 mm RACI finder, a reasonable 30 mm, 2 inch wide field. The 2 inch focuser makes star hopping much easier because of the wider field of view possible, a big plus for a beginner.

Usually the 8 inch is about $100 more than the 6 inch but not only do you get a more optically capable scope but a better outfiited scope with better accessories.. Size wise, they're about the same with both having about the same single 20 pound heaviest piece.

That's why I generally recommend the 8 inch over the 6 inch.

Title: Re: Beginner dob Recommendation is almost always 8", why?
Post by: ruesonecrai on January 10, 2018, 08:20:05 AM
the Beginner forum is for beginners to ask questions so... i am locking this thread but will leave it here as the information might be helpful for true beginners to read