Dob purchasing issues, and what might I see?

[fragchamp]

Hi all.
I've been reading as much as I can around here and everywhere, as I am interested in buying a first scope. I currently live in a dark sky area north west of Bendigo. It's strange that it's taken me this long to realize that I should take advantage of the dark skies by getting a scope (duh), but at least they seem to be pretty cheap now. I would like to test some out, but I'd probably have to travel at least 200km to get to any of your various club meetings, so unfortunately I don't think that will happen.

I am currently considering dob reflectors in the 8" to 12" range. My thoughts were, if I'm going to get a dob, then since I don't need to transport it, I should go for 12" since that's the largest "cheap size." But then I spoke to a club member (who sells them) and he said the mirrors used are not precise enough to get any more detail above 8" size. A little brighter perhaps but not more detail. I would assume that all the cheap dobs would use the same mirrors (GSO?), but it's possible that some brands have their own testing and better quality control.
Some people around here were happy:
http://www.iceinspace.com.au/index.p...41,410,0,0,1,0
Some were not:
http://www.iceinspace.com.au/forum/s...&highlight=dob
I am not going to bother with this or any other product if it has poor quality control. If something is advertised to be 1/12th wave then it should be, end of story. If it isn't, then the dealer should replace it with their apologies. Would anyone who's looked through a few of these things care to comment on the 8" vs 12" difference, or on quality control?

The last thing I want to know is whether I will see anything coloured through it (other than coloured stars, which I can already see). I find reports that most things are white to the eye, but tracking cameras can bring out colour and then of course filters can be applied etc. I am not interested in photography at the moment, so I just want to know what I might see. It seems to me that there ARE some coloured nebulae out there, but I've yet to find out whether these are obviously coloured, faintly "tinged" with colour, or not coloured at all to the eye (through a dob). It's possible to photograph anything in a neutral way, so that the picture "looks like what you see." Unfortunately this seems to be rare in astrophotography so I need help from viewers. Any comments?

[Rodstar (Rod)]

Welcome fragchamp!

There is a huge difference between an 8 inch and a 12 inch scope in terms of what you will see at the eyepiece. A 12 inch scope gathers 2.25 x as much light for a start, so the objects will appear more than twice as bright, with far greater clarity in the images.

Of course a scope of any aperture will suffer if the conditions are poor, but on a good still night, the 12 inch scope will totally blitz the views through an 8 inch scope.

Colour perception is a very individual thing. Some people never see any colour in any nebulae. Others see quite a bit, but only in the brighter nebulae and some planetaries. I see a distinct green tinge in M42 with a 12 inch scope, but I have to get up to about 18 inches to begin to see pinks.

If I were you, I would jump straight at the 12 inch, particularly if you are not going to be lugging it around too much.

[Blue Skies (Jacquie)]

Originally Posted by fragchamp

Some people around here were happy:
http://www.iceinspace.com.au/index.p...41,410,0,0,1,0
Some were not:
http://www.iceinspace.com.au/forum/s...&highlight=dob
I am not going to bother with this or any other product if it has poor quality control.

I wouldn't get too worried about the complaints in the second thread. From the comments that were made it seems to me that the purchaser of the so-called faulty scope had unrealistic expectations of what it could do. Or he didn't take the full-aperture cap off, just the smaller one that can be used for solar or full moon viewing (hey, it happens, and to intelligent people too!). And I've seen enough GSO mirrors now to know that you'd have to be really unlucky to get a 'dog' mirror (eg a totally bad bad one) that couldn't outdo an 8". Yes, there is probably better quality around for more $$, but as a beginner I doubt you'll be able to tell the difference. Takes a while and looking through a lot of different scope to educate you eye as to what is good and what is not. And some people are just ultra fussy...

Originally Posted by fragchamp

The last thing I want to know is whether I will see anything coloured through it (other than coloured stars, which I can already see). comments?

Ditto to what Rodstar says about colour. Added to that is that you will likely see a blue-green colour in bright planetary nebulae. I've seen a dark red around the fringes of M42 in a 16", worms of pink in the Trifid Nebula at high power in an immaculately kept 17.5" but generally colour is not seen in nebs. The planets of our solar system will definitely be in colour.

[madtuna (Steve)]

Hi Fragchamp!

I've only had experience with the 10" GSO dob I convinced a mate to buy, I was so impressed I'm buying the 12".

If portability was an issue I'd have no hesitation going with a 10" as the 12" is a beast. But I've done my measurements and a 12" it is.

That bad experiance withe the GSO dob you cited is honestly the only bad review I can recall ever seeing, but I've see too many positive reviews to count.

[Tannehill]

The GSO and Synta Dobs are a great bargain for the aperture. If you can afford a 12" Dob, go for it.

There is no reliable objective evidence to support the claim that the 8" mirrors are higher quality than the 12" mirrors from these mass produced vendors. A shorter f ratio mirror is a bit more work to make well than an f6, so yes the median 12 f5 could arguably be less well figured than the median 8 f6. Nevertheless, you will find hundreds of GSO and Synta users here and on Cloudy Nights who will testify to the great enjoyment quality from these scopes. I have an Obsession and a TV refractor, and the 12" mirror may have been a tad less quality than those units, but to be honest, you have to work to notice that. Furthermore, the quality of these mirrors has improved mightily now that there is competition in the market AND because the internet (that's us, with our giant implacable information network) keeps them honest. Put out consistently poor product - not just an isolated flaw, everyone has bad days - and word gets out quick.

The 12" f5 will be a bit more touchy about collimation than an 8" f6. You will need some collimation gear either way, but for a given degree of mis-collimation the 12" will be affected more than the 8". That said, it is an easy thing to learn.

The biggest issue is the mass. A 12" is considerably more work to haul and set up, even tho 4" sounds like a small difference. The best scope is the one you use the most.

Scott

[peter_4059 (Peter)]

Fragchamp,

It's all been said already. The only thing I have to offer in addition is I got a 10" GSO dob that had the primary mirror clamps done up too tight. It took me a while to realise things weren't quite right but after I had someone who knew what the problem was make some adjustments it was just fine.

I'm agree with the others... the 10" is great and the 12" is probably better if you don't have to move it around.

[dugnsuz (Doug)]

Get a laser collimator for 10 or 12" scopes - necessary!
Every time you move a scope of this size you have an opportunity for the mirrors to become slightly mis-aligned.

True the mirrors are mass produced in China, but the bang for buck is immense! Compare GSO price with hand figured mirror in 12" size.
Outside the beginners budget!?

Colour - ethereal green every time, but it's gonna be a hell of a brighter green in a 12" Dob!
All the best
Doug

[GrahamL]

I had a 10" gso dob for a couple of years and found it great ...I bought a 12" gso mirror only for my truss project and it to has far surpassed my expectations for the small amount of cash it cost.

I have looked through the 8" dobs as well ,which are also fine scopes
I find theres an fairly obvious increase in detail AND brightness with each step up .. maybe the seller has a lot of 8 " scopes
Any of three of the above will give you years of enjoyment
If home is were you'll use it the most I'd grab the 12" in a second

cheers

[CoombellKid]

Having owned an 8" for several years and also had the opportunity of
looking through several 10" and 12" I dont think the difference is huge.
More subtle than anything else or I would of upgraded a long time ago.
However if I had the choice again I would go the 12" every bit of extra
aperture helps!!

regards,CS

[ngcles]

Hi Fragchmp & All,

I agree with most of those who contributed above -- I'd go for the 12.

I had a chance nearly 5 years ago to do a test report for a magazine on the 12" GSO dob and found the optics to be excellent. All round construction of the 'scope was very good considering the price. Fortunately I own an excellent 'scope to compare it with -- my own 31cm f/5.3 on a GEM which has a mirror by Mark Suchting.

I evaluated the optics using my own eyepieces (they were Televue plossls and Naglers) of known quality and also the GSO supplied ones. The star test on the GSO I evaluated was excellent. I detected none of the common faults frequently found in mass-produced optics. There was no detectable astigmatisim, it seemed to be well parabolised, there were no apparent zones or turned edges on the mirror. A Ronchi eyepiece was applied at focus and it similarly showed no detectable defects -- the lines were ruler straight (as they should be in an _eyepiece_ ronchi), evenly spaced and evenly illuminated.

Mars was at its BIG opposition (2003) and it showed excellent detail -- hardly less than my Suchting mirror. In summary, in the 'scope I tested, the optics appeared to be of very good if not excellent quality.

I have since looked through several other 10 and 12" GSOs and they too seemed to have pretty good mirrors. There have been some others who have had (and expressed) unsatisfactory views but they seem to be in a very small minority.

My only complaint with the optics was a "design" issue not quality. The secondary mirror in the ''scope I tested was a 70mm minor axis job which I felt for a visual 'scope is unnecessarily large -- it is almost "photographic" in size. The secondary in my own 'scope (which has a slightly longer fl at f/5.3 as opposed to f/5) is only 54mm. The GSO could easily get away with something about 10% smaller -- say 62mm. This would pull the secondary obstruction by diameter down below 20% -- a desirable thing to obtain best contrast.

But, all in all, the optics were difficult to fault and the overall execution of the telescope was good -- it presents as excellent value for money.

As said above by many, 12" -v- 8" is a big increase in light-gathering power and it will make a substantial difference on deep-sky stuff. Nebulae will be much brighter and more detailed. Globular clusters will regularly exhibit a well-resolved appearance, and to the experienced observer who knows exactly what to look for, some spiral galaxies will start to show evidence of spiral arms.

The view of the planets will be much brighter and colours more saturated. However, 19/20 nights, the level of detail visible in say Mars or Jupiter will be the same in both 'scopes. On that 20th night, the 12" will show a better view.

Whichever way you go, best of luck with your purchase.

Best,

Les D
Contributing Editor
AS&T

[fragchamp]

Thanks to you all for your comments. It certainly helps to have a forum in these cases, and this seems like a good one I'm pretty solid on the 12" now, which was my original choice anyway. The size and weight is not a big deal as I'm strong and live in an aircraft hangar. Seriously though I will likely only use it at home and I'll just make a roller base for it if it's too heavy. After reading the collimation threads my choice is most likely going to be the Orion pair. Thanks again!

[erick (Eric)]

Last night I set out to have a shoot-out between my 8" which I have used for some 10 months and my new 12", which I have only looked through twice before then.

However, clouds intervened and I'll have to come back to this.

On M42 and Comet 8P/Tuttle, the 12" was markedly brighter. I also was sure I was picking up fringing pink in M42, but I need to spend more time on that.

[Paddy (Patrick)]

Sounds to me like you've already decided on the 12" GSO. I good choice I reckon. A few of us in the Bendigo District Astronomical Society have them, but I'm not sure when the next dark sky gathering would be happening for you to have a look. Buy one anyway! I find mine to be fantastic and have got to really enjoy finding galaxies with it and with increased experience see more structure all the time. Cruising around the Fornax galaxy cluster has been a lot of fun on recent nights - but it has taken a while to get my eye in. Nebulae and clusters are great through it, but I couldn't say that I see colour in them. Even with M42 I'm not sure if its colour or imagination that I experience. I really just want to suggest that you don't expect too much by way of colour etc to start with and you will get to discover a lot with practice.

A trolley is a great accessory! For collimation, I have found the bintel own brand laser collimator, Bobs Knobs and some stronger springs for the primary to be of great benefit.

[fragchamp]

I guess the colour thing is just hit and miss. Eye sensitivity and/or image brightness could be responsible for not seeing it. To me it makes sense to buy the biggie and give myself the best chance, since popular consensus says it will be better than the 8". Cheers Paddy for the heads up on the BDAS, I'll look out for it.

[Paddy (Patrick)]

The web address for the BDAS is

bdas.fastmail.fm

A very nice bunch of people indeed.

[Sammas]

Just a note to support the people intending to buy an 8" and starting to get concerned that they should be buying a 12"...

If you're reading this thread and considering the 8" but starting to worry that maybe it's going to be too small (you want something great from the start, of course!) then I'd not be too worried about sticking with the 8".

An 8" is a great scope. They're physically smaller and, for the inexperienced, they give you truly amazing views of the night sky. Although I have not had the opportunity to look through a 12", my understanding is that will not be missing out on anything with an 8" - everything you can see with the 8" will be visible in a 12" as well - it'll just be more subtle in the 12".

To the inexperienced observer the 8" and 12" give similar views - there's just a lot more subtlety in the 12" - which you'll appreciate more once you've been observing for a while. Also, the 12" is much bigger - harder to move and transport, which means you may use it less.

For me, I will probably stick with my 8". It will take me a long time to exhaust the detail I can see through it. For me the next upgrade will not be more aperture, but some way to track the sky so I can take pictures - either a tracking mount (HEQ5) which I can mount my 8" on, or an equatorial platform. Without tracking your photo taking is really limited to the moon - everything else needs long exposure or high magnification (which moves through the scope too fast to snap easily with a dob, though it can be done

[tbentley]

I'd just like to throw in something to consider in favour of the 8" as well. If money is any consideration the smaller scope may be better value. As has been stated there is very little that you will be able to see in a 12 that you won't also see in an 8, although perhaps not as well defined.
For a beginner, as I still consider myself to be after a couple of years, the thrill is more in seeing something rather than seeing it in detail. The money you save should be put towards eyepeices, books and accessories to improve the experience further.
I guess what I mean is that aperture isn't everything (blaspheme I know) and what I can see now in my little 8" dob is far better than I could see in club members' 12" scopes when I started. If anything, a smaller scope has helped hone my skills in order to see better.
If you don't ever plan to upgrade then I would say:
a) Who do you think you're kidding?
b) Go for the 12".
Otherwise buy the 8", get some good quality eyepieces and accessories then learn as much as you can. I'd recommend reading and asking as many questions as you can here and a book called "Astronomy Hacks" by Robert and Barbara Thompson, especially good for dob owners.
Hope this helps.
Travis

[Starkler (Geoff)]

My 2c worth.

If money is tight or you are not sure if astronomy will be an ongoing passion, get the 8.

If you prefer a scope that is easy to move and handle, get the 8.

If you want the best views and bang for the buck, get the 12 and you wont look back.

Whilst an 8" is in no way a toy and that size suits many people as trade off between aperture and portability, the difference 12" of aperture will make should keep you happy for a long time. As for mirror quality, its a bit of a lucky dip as to what you get. Mirror quality ranges between fairly good and excellent.