8 & 10 inch Skywatcher collapsible v solid tube dobsonians

Mountain_Wanderer · #1 16-09-2010, 07:39 AM

OK, after much research I find myself stuck with the decision on what dobsonian I should buy.

I've narrowed it down to 4 choices:

- Skywatcher 8 inch black diamond collapsible

- Skywatcher 8 inch solid tube

- Skywatcher 10 inch black diamond collapsible

- Skywatcher 10 inch solid tube

I'm a completely new beginner and so this will be my first scope. I require ease of transport, which means I favour the collapsible version which shrinks the length a little bit to make it easier to cart around in the car. It would be interesting to know how much the collapsible versions shrink by if someone has one.

I also am leaning towards observational astronomy and not astrophotography. A solid tube version would allow a potential upgrade onto an equatorial mount in the future if I ever get into astrophotography, but I think that for now observational astronomy will be my interest.

Both the 8 and 10 inch versions have 1200mm focal lengths. In terms of aperature, a 10 inch gathers twice as much light than an 8 inch. However, if I'm only to use it for observational astronomy and not astrophotography, would a 10 inch actually deliver much more benefit? Do deep sky objects in both scopes just appear as white whisps and no colour? I know that 8 inch dobs can deliver some great views so perhaps an 8 inch is more appropriate, particularly also because it is lighter in weight and less bulky for ease of transport.

Just comparing the 10 inch collapsible to the 10 inch solid tube, I notice that the collapsible has a focal ratio of F/4.7 compared to the solid tube version of F/5. I've heard anything below 5 can be a problem for the occurance of coma. Does anyone have a 10 inch collapsible version and is the focal ratio a problem? I also notice the highest practical power is 508x in the 10 inch collapsible compared to 600x in the solid tube version. Why the drop? Can this be overcome by the choice of eyepiece?

Now looking at the 8 inch, I notice that the focal ratio of F/6 is the same in the collapsible and the solid tube versions. But, the highest practical power is 406x in the collapsible and 480x in the solid tube version. Is this a concern? Can the issue be fixed with an appropriate choice of eyepiece?

Looking at the 8 and 10 inch collapsible versions, it seems to me that they're not quite up to the same quality as the equivalent solid tube versions. Yes, a 10 inch collapsible would be great for light collection, but does the focal ratio (F/4.7) screw up the optics? Therefore would an 8 inch collapsible with a better forcal ratio (F/6) be more appropriate?

Hope someone can help me here?

Cheers
Jowel

astro744 · #2 16-09-2010, 10:17 AM

Just a couple of points to clarify.

A 10" delivers 100/64 = 1.56x the light gathering of a 8" NOT 2x.

It is for visual that you will notice and benefit from the larger aperture more than for photography since the camera records light whereas your eye does not.

Objects will appear as white wisps in boths 'scopes only brighter in the 10" at the same power. The 10" will also show faint galaxies that may be invisible in the 8" but these galaxies will be barely visible in the 10".

The highest practical power will be 300x on both 'scopes. The sky will not allow better due to seeing. The image will be brighter in the 10" at the same power.

I would go with a solid tube 8" for portability and stability. No need for shroud and no need to assemble. This telescope will be for life but you can complement it with a 12" later for brighter views again 144/64 = 2.25x brighter. Being f6 the 8" will be kinder on your eyepieces unless you also want to invest in Tele Vue or others highly corrected to f4.

bmitchell82 (Brendan) · #3 16-09-2010, 10:52 AM

Just to compliment what has already been said, the 10" is the same length as a solid tube 8" 1.2m Mine used to fit into the back of a Mazda MX6 coupe. The weight and size is comparable to the 8". The 10" will show you a lot more as you can see it without having to strain to see it.

E.G. I can see the flame nebula JUST in perfect dark skies. Running man on M42 is visable, Cent A is clearly visible as is pinwheel m83. in a 8" scope flame nebula no, running man you can semi make it out, cent a kind of, pinwheel kind of.

If you can get the 10 great, if you cant the 8 will be a good scope too.

The difference between the F ratios will be collimation accuracy. The lower the F# the more critical it is to have your collimation spot on. Don't see this as a deal breaker though, some decent collimation equipment will make light and accurate work of the job. Just don't skimp and get a 40 dollar laser this will almost guarantee the same kind of return in views Further more you will get brighter images with the faster F# (well that is my experience)...

Good luck

mswhin63 (Malcolm) · #4 16-09-2010, 11:04 AM

Considering my situation I went for collapsable for portability but thinking back I could have gone for a solid tube I think. Alway hard to decide for a 12", but for an 8 or 10" the choice would have been a bit easier for a solid tube (for me at least) as my cars can easily fit a solid 8 or 10".

The other main reason is apeture which was the final decision to go for collapsible 12". I must admit I dont have a problem with colimation of secondary but am starting to experience shifting of primary collimation a lot now. Although colimation is now quite easy.

If you can go for a solid tube (fitting in your car then go for it).

I am also curious now whether apeture in relation to vision is a linear path. I know for my experince in hearing/electrical it is a logarithmic path so 2 x power or 3db does not provide a 2 x actual listening volume. I am wondering if vision has the same pattern.
For CCD the vision is near linear but not sure for the eyes. (This is more a biology query)

Liz · #5 17-09-2010, 02:07 PM

Hi Jowell

If you can afford the 10" collapsible dob, go for it!!
My 10" can see a lot more than my previous 8", so you mose well get the 10" now, instead of getting the 8" and wishing you had the 10.

noswonky (Peter) · #6 17-09-2010, 04:01 PM

Regarding how much the tubes collapse, the Skywatcher site specifies two numbers for the tube length on the specs page under 'tube dimensions'.
For example, the 10" is given as
380x(800-1120) mm
Which means 1120mm extended and 800mm collapsed.

See:
http://www.skywatchertelescope.net/s...1=1&class2=106

Mountain_Wanderer · #7 18-09-2010, 11:32 AM

Thanks for the advice everyone.

The question I think I've reached is whether the 10 inch can deliver a significantly greater benefit than the 8 inch for observational astronomy. And therefore whether this benefit is worth the additional weight, additional bulk, additional cost and the additional attention needed for collimation.

I want to get a scope that won't dissapoint me with its limitations. As I see it, an 8 inch should do a decent job and therefore I'm favouring that. I need to be convinced that a 10 inch can deliver much greater observational experiences and is therefore worth the extra pain which comes with it.

Screwdriverone (Chris) · #8 18-09-2010, 04:15 PM

Jowel,

I got the 12" collapsible after the age old question of " will I want the bigger one in a month from now?" considering there was a $700 difference between the 10" and the 12", I would have to say the 12" isnt worth the extra $700...

Having said that, the other day, I had my 12" collapsible DOB and my 8" on the HEQ5 pro side by side with 13mm eyepieces in each.

Looking at NGC 253 (Silver coin Galaxy) in Sculptor with these two side by side showed a SLIGHTLY brighter galaxy on the 12" over the 8"....TRUE STORY! A few more details on the 12" but not so much to make much difference.

I know everyone will now throw radius and light gathering equations at you etc etc but realistic comparison on both my scopes didnt make much of a difference.

The 8 and the 10 have the same length and focal length so I would suggest that unless you have an extra $200 for the 10",, get the 8" for $499.

AND, my serious suggestion is to consider the GOTO version, convenience of computer control and finding is something I am Hooked on now, with my HEQ5, star hopping with my 12" is OK, but getting very very old now I have goto.

Chris

bmitchell82 (Brendan) · #9 19-09-2010, 11:50 PM

Ill throw in a my piece on that. the difference between the 8, 10, 12 scopes is significant. But to the untrained eye very little difference can be seen. Unfortunately people have a pre conceived idea that they will see all this different detail and become disappointed when its not there.

I gave up showing people Nebula and the like in my 10" because they just couldn't see it. I could see it there as if it was a neon sign flashing away. so in short once you can train your eyes to work in low light situations then the 12" will show phenomenal details way more than the 8" will ever see.

Also the 10 and 8 might have the same focal length but they are different scopes. they have different F ratios and operate differently and collect light differently!

The goto suggestion is great idea but once you get good at it, star hopping is very easy because you get the idea of what your field of views.

though ill say that my goto i couldn't live without it!

keep it up youll get there in the end

Mountain_Wanderer · #10 22-09-2010, 07:55 AM

Thanks for your info fellas.

It seems things are brighter in a 10 inch and may let you see a few additional things than an 8 inch. I've got a nagging feeling that if I go with an 8 inch I'll soon be regretting not getting a 10 inch. Oh, my brain is hurting! Who actually has a 10 inch collapsible anyway?

I've been thinking a lot about how I could overcome the weight issue with a 10 inch. I could attach wheels to the base, I could make up a small cart to move eveything from a car to an abservation point etc etc etc. It's possible to do that.

My concern still remains over the focal ratio of 4.7. From what I gather, you need a good collimator to correctly align the mirrors before observing. However some reviews I've read say that coma still exists and that many people with scopes with 4.5 to 5 usually replace their eyepieces. I guess better eyepieces would correct coma? How bad is the coma on these things and can it be easily overcome or is it a nagging problem?

Cheers
Jowel