Well, if light gathering is dependant on area of the objective, then the difference is significant
6 inch = 28 sq inch surface area
8 inch = 50 sq inch surface area.
(however, the respective size of the central obstruction (secondary mirror) would also affect this, slightly.
My first scope was a 6" Newt and my second scope was an 8" Newt.
Practically, the 8" produced a slightly brighter image of DSO's but you had to look close to see any greater detail. (which there was but not hugely apparent from casual observation).
What was noticeable was that the 8" was much more awkward to handle and move about. So if this is a factor, it should be weighed into the decision.
But the two general rules of thumb are
1. Go for the biggest aperture you can handle (and afford), and
2. The best telescope for you is the one you are going to get out with and set up, and use more often.
A further disclaimer: For observing a Dobsonian mount is easier to set up and use than an Equatorial mount. But Equatorial is virtually essential if you are going into astrophotography. Other than that they are both Newtonian scopes, except that on a DOB mount you can have a collapsible scope, that makes handling much easier.
Sorry if this is too much information, but it all is relevant to a choice
Mirror area ratio is approximately 16/9 = 77% more light for the 8" (based on pi*r2 - I say approximately because the secondary mirror % obstruction can vary a little.) So fainter DSOs will definitely look significantly brighter on the 8", but you won't notice much difference for the brighter ones, or planets etc.
Allan, your detailed analysis of the 2 scopes are invaluable information. Do you use your 8" tube for astrophotography as well? if so, what mount that you use. I'm planning to get HEQ5 Pro, am I pushing the limit if I get 8"? (total weight of tube, DSLR and autoguiding scope/camera would be around 10kg).
Many thanks again.
Is there a significant difference when observing DSO? would the 8" give much clearer and detailed view of galaxies and Nebula than the 6"?
Thanks.
Astroboyz, galaxies are pretty faint in any telescope and nebula are not much better, unless photographed. That's why we call them faint fuzzies.
I sense you are trying to get a feel for what each scope can "see", but that's not really something we can convey via a forum. If you can find an astronomy club near you that are having a viewing night, and can see through various scopes for real, that would be better than any 1000 words we can type.
For visual there is a saying that says the best 6" f8 is an 8" f6. You get noticeably more light gathering power and better resolution.
The size of these 'scopes is the same except for a larger diameter tube for the 8".
If you are going to mount onto an EQ are you still talking f8 or f6 or perhaps f5 or f4?
A 6" f5 will be shorter and lighter than an 8" f5 and giver wider fields of view. An 8" f4 would be the fastest photographically and give almost as wide a field of view as the 6" f5. However you would need some sort of off axis correction at f4 (and perhaps even at f5). I'm not into imaging much so perhaps others can comment here.
Except for its effect on TFOV and magnification, for a given FL eyepiece, and optical aberration creep. Makes no difference to brightness at a fixed magnification, which I'm sure is what you meant
The brightness of DSOs which are extended objects (nebulas) is dependent on f ratio/exit pupil.
But the brightness of DSOs which are made up of point sources (open clusters) is dependent on aperture.
So when you look at say the Eta Carina nebula or Lagoon nebula, the nebulas themselves may look the same brightness wise when viewed with the same exit pupil (though one image will be smaller than the other, because of different magnifications to achieve the same exit pupil), but the image in the 8" will always look livelier than in the 6" because the stars around and in the nebula are brighter. Similarly with globular clusters - the tiny point source stars are brighter in the 8".
And galaxies plainly stand out better in an 8".
The downside of an 8", apart from being heavier, is that it is touchier to atmospheric turbulence. So while the stars are brighter, they may be blobbier or less crisp on some nights compared to a six inch or smaller telescope on the same night (this was certainly the case in suburbia, I remember at times getting frustrated with the 8", and using my 4.5" reflector instead)
Regards,
Renato
Except for its effect on TFOV and magnification, for a given FL eyepiece, and optical aberration creep. Makes no difference to brightness at a fixed magnification, which I'm sure is what you meant
Yes. I meant an 8" telescope is an 8" telescope regardless of f ratio and is capable of a given brightness factor increase over the unaided eye.
A 6" telescope is a 6" telescope regardles of f ratio and is also capable of a given brightness factor increase over that of the unaided eye.
An 8" brightness factor increase over a 6" is equal to 64/36=1.78. (Or if you like 16/9=1.78) which is considerable and noticeable.
An 8" brightness factor increase over a 6" is equal to 64/36=1.78. (Or if you like 16/9=1.78) which is considerable and noticeable.
Interesting, as per my original post, 50sq inches compared to 28sq inches = 1.78 increase.
Astroboyz, I had my 8" Newt on an EQ5. A HEQ5 should cope more than adequately.
But as I did not do much serious astrophotography, I soon found comfort of observing was more important and now I have a 12" Dobsonian.
I used to (like every newbie seems to) think Astrophotography was a natural progression, and hence I ventured down the EQ path. But now I strongly suggest to new comers, unless you definitely intend to spend the time and money required to practice astrophotography, settle for observing (at least initially), and go DOB. That would have saved me heaps of money and frustration. But like most I didn't listen.
I have an 8" f/5 Newt on my HEQ5, along with an electric focuser, a
9x50mm finder, and my DSLR, and sometimes my SW80 in place of
the finderscope. It handles the load just fine.
raymo
Hi Allan,
is your DOB GOTO or manual? If manual, I'm just wondering if it's difficult to learn locating DSO ? Any particular brand that you would recommend. You were right, like many beginners, I thought why not choosing something that could be used for AP as well .... but not quite sure I have commitment in such hobby :-)
Hi Raymo,
do you use any autoguider for you AP? how long were those exposures? Many thanks.
Hi Astroboyz,
My current telescope is a 12" Skywatcher collapsible GoTo DOB.
Just had it a short time.
Before that I had a 10" non goto Skywatcher collapsible DOB for 3 years.
Sure finding faint stuff was a bit harder without goto, but I used setting circles and smart phone programs to help me find some pretty impressive faint stuff with it.
It (the 10") was definitely a lot easier to move around and setup, and it probably got more use because of that.
My recommendation would be either the skywatcher goto 10" or 12" collapsible (Flex tube) DOB
or
The Orion DOB.
There was a post today I think that pointed out a really great price on an orion goto dob (at Bintel I believe). http://www.iceinspace.com.au/forum/s...d.php?t=120105
They are my two brands of choice. (Just my humble personal opinion) having looked through quite a few different scopes.
Hi Astroboyz, I don't know which specific exposures you were referring to,
so I can't tell you how long they were. Anyway, I don't guide at all; I just
take and stack lots of short exposures of between 15 and 120secs.
depending upon how good my polar alignment is on the night. I only use
my 1100D. CCD cameras and autoguiders are beyond my budget and
my inclination.
raymo
do you mind show us those photos taken with unguided mount, and short exposure? Many thanks in advance.
By the way, do you use any light pollution filter for DSO, if so any suggestion on the brand ?
A UHC/LPR filter can be good for some types of nebulae, and an OIII filter is terrific for planetary nebulae and some other DSOs (e.g. I've had wonderful views with it of Rosette, Keyhole, etc.). No good for galaxies, alas - they are basically just lots of stars, and so all you get is dimming!
Hi again Astroboyz, these images were taken with subs of 15 to 45secs.
In camera noise reduction enabled, no separate darks, flats, etc:
Stacked in DSS. I don't use an LP filter.
NGC 253, M83, NGC5128, NGC4945 [cigar galaxy].
raymo
