SCT or Dob ?

[Ally8446]

Good evening peeps. Clear skies clear skies, my kingdom for clear skies. On a more serious note, Would a 6" Sct give decent views of DSOs. I have seriously been considering an 8" dob but am now concerned about portability. I have only recently got into this amazing hobby so my eye isn't as trained as it it could be. Any advice would be great as Im planning on making a purchase in the new year. Thanks folks

[Capricorn]

What are you planing on looking at and doing with the scope ?

If pure visual and you can locate things then the Dobsonian will win out, they come in 3 parts so are reasonably easy to carry round and 8" is not that big, my handspan is 8" and I do not have big hands.

Is it just an SCT or is the SCT part of a goto system, as about the only 6" SCT I can think of is the Nexstar 6SE.

If OTA then what mount is the SCT to go on.

You are looking at an EQ5 for something like a 6" SCT OTA.

6SE's are portable but still not a lightweight item and an EQ5 is a fair chunk as well.

If the idea is in time to put a webcam on and image planets then really a dobsonian is not the instrument.

[Ally8446]

Thanks for the reply Capricorn. I'm really only interested in DSOs and certainly won't be even thinking about AP for a long time (years, if ever). As far as Goto is concerned I would rather learn manually first then maybe add Goto at a later date. So really it would just be the SCT. Might stretch to an 8" second hand SCT or 10" Dob. Won't be making any snap decisions as its quite a financial layout for me so I want to get it right.

[JamesF]

For visual only use on DSOs I'd go for the 8" dob every time. SCTs aren't particularly well-suited to DSO viewing because of the narrow field of view, and there's no substitute for aperture. If budget is an issue then I'd probably avoid the Skywatcher f/4.7 10" dob to be honest. The views are fantastic, but the focal ratio makes it quite unkind to many eyepieces.

James

[Eastridge]

Yes, for what you want the Dob is the instrument and a 8" will be more portable than the 10" and kinder on your pocket for EP's. ( Don't I know it!!!)

[Dunkster]

For visual only use on DSOs I'd go for the 8" dob every time. SCTs aren't particularly well-suited to DSO viewing because of the narrow field of view, and there's no substitute for aperture. If budget is an issue then I'd probably avoid the Skywatcher f/4.7 10" dob to be honest. The views are fantastic, but the focal ratio makes it quite unkind to many eyepieces.

And here's the rub... many Dob/newt users save cash on the tube itself but end up splashing out just as much on expensive eyepieces, coma correctors, etc... not so with an SCT (you just need a dew shield and probably heater!)

SCTs are just as well suited to DSO viewing so long as the object fits in the FOV. For an 8" SCT the max is about 1.3 degrees without resorting to a focal reducer. That doesn't exclude an awful lot of objects that wouldn't be better viewed with either a pair of bins or a short tube refractor anyhow

For visual, 8" aperture is 8" aperture whichever way you cut it, and if you can pick up a nice one at a good price used, at less than 6kg they're a great compact and portable scope.

[Ally8446]

Thanks guys, I've got some thinking to do then. Will let you know which way it goes asap.

Clear Skies

[Ajohn]

That is sound advice from Dunkster. I started with a 10in dobsonian. F6.2 out of interest for a good compromise on eyepieces, field of view etc etc. I then bought a C8 and later sold that too and went for a C10. The C8 size are fairly portable and easy to manage. The C10 takes more effort and takes longer to settle down. The field of view of these scopes is just about ideal for most things really that's why they have been made in more or less the same form for so long and why so many people buy them what ever the size. For photography they are rather slow but reducers are available. In fact all sorts of things are available for them. They are best viewed as a jack of all trades.

A more interesting question really is fork or german equatorially mounted. The fork mount has a lot going for it. It's much easier to make a rigid fork mount with a lot less material than the german type, no counterweight for one. The downside is that more types of telescope can be directly mounted on the german type. On the other hand people mount small refractors directly onto the telescope when they are fork mounted. Personally for some one with one telescope I would go for one with a full fork mount. If they ever find themselves buying a 100mm or larger APO refractor they will need another mount to go with it. With a C8 they may also find that mounting even a small apo is problematic. Personally in this case which might also apply to a C10 I would prefer 2 stands. I would stay away from the 1/2 fork types. It tends to show up deficiencies in the bearings. Just bought a skywatcher 127 synscan mac. Things are reasonably solid really but let down by play and flex in one of the bearings. Why - well I just fancy trying something small and have an interest in fitting a 110mm apo to the stand.

I call sct c this and c that who ever makes them. My C8 was Celestron, the C10 is Meade. If you buy a used one make sure it does come with it's equatorial wedge. Fork stands can have a bit of a problem in this area when they are polar aligned. A german stand should have fine adjustments to align north and to align for latitude. The fork mount may have neither, one of them or both. For visual use I have never found that this is a problem. I have just drift aligned and adjusted the scope accordingly by swinging the head around to align north while viewing a star.

The SCT do have one drawback but it also often applies to any form of newtonian or any scope with a central obstruction, mac's etc included. The obstruction reduces contrast and the bigger it is the worse the effect is. 1/5 the diameter of the main mirror is regarded a the maximum for "acceptable" results but 10% is even better and can even approach the performance of a refractor. Spec's often quote this as area rather than diameter % but that only relates to loss of light gathering power. I don't think any commercial scopes of any type have central obstructions at or below 1/5 and compound scopes tend to have bigger obstructions than newtonians. Where this really matter though is for high resolution work in relationship to planets in particular. For that sort of thing high precision optics are needed too really even better than Oldham Opticals 1/10 wave wave front error mirrors along with seeing conditions that allow their full potential to be used. That gets harder and harder as the size goes up. As an aside I think it's rather naughty of Orion to quote p/v errors. In short I wouldn't worry to much about this aspect. I probably do because I know about it. Many old amateur astronomy books mention the contrast aspect. They suggest 4in F15 refractors and 6in newtonians for equal levels of performance. I'm inclined to think that is true but these days the refractor needs to be a high end apo to keep the size resonable. A 5ft long telescope needs a hell of a stand to keep it stable.

John

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[LukeSkywatcher]

My scope is an 8" SCT. its brilliant for me because it is portable. I'm in a wheelchair. I know an 8" Dob would gather more light because there is no central obstruction, but it wouldn't be as portable for me. How much light the SCT loses....................i have no idea but i am not complaining with the views.............even of DSO's.

[Ajohn]

There would still an obstruction in the DOB Paul a fair chance that it will be smaller than a SCT but on the other hand it may well not be. A Dob is just a type of a Newtonian.

John

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[swamp thing]

1355656386[/url'>' post='1743136']

And here's the rub... many Dob/newt users save cash on the tube itself but end up splashing out just as much on expensive eyepieces, coma correctors, etc... not so with an SCT (you just need a dew shield and probably heater!)

This should be true, but you also find a lot of SCT owners splashing out on Naglers, Ethoi etc as well.

Reckon we're all as daft as one another on this one.

[michael.h.f.wilkinson]

For visual only use on DSOs I'd go for the 8" dob every time. SCTs aren't particularly well-suited to DSO viewing because of the narrow field of view, and there's no substitute for aperture. If budget is an issue then I'd probably avoid the Skywatcher f/4.7 10" dob to be honest. The views are fantastic, but the focal ratio makes it quite unkind to many eyepieces.

James

Wrong, I am afraid. For visual aperture is what counts, not speed. For a given magnification, the brightness of the image to the eye depends on the aperture of the scope, not the speed. For a given aperture, an F/10 scope with a 40mm 68deg EP gives exactly the same view as an F/5 scope with a 20mm 68deg EP. The same amount of light spread out over the size area of your retina.

SCTs are not the first choice for DSO imaging due to their slow speed, but the "narrow" (1.34 deg for my C8) FOV is only a problem for half a dozen really wide-field objects (and for those a short frac like my 80mm F/6, or a good pair of bins beats a dob). All, all globulars, all planetaries I know, and all galaxies bar 3 fit into the FOV (the exceptions are M31, the LMC (which does not quite fit into the FOV of my 15x70 bins), and the SMC). I have spotted over 600 DSOs with my C8, so it is no slouch. The C8 OTA is lighter than most (or even all commercial ?) 6" Newtonian OTAs. It is so portable I can take it on holidays with all the camping equipment, the wife's shoes, the kids' toys, etc. I even took it to France to spot the eclipse in 1999 (in a Peugeot 106, sans children but with the missus (and shoes )).

When portability is an issue, nothing beats SCTs.

[michael.h.f.wilkinson]

This should be true, but you also find a lot of SCT owners splashing out on Naglers, Ethoi etc as well.

Reckon we're all as daft as one another on this one.

Yep, I have Naglers and XWs, but the views with very modest EPs are very good (like the kids' 20mm Plossl, a steal at 15 euro), just not quite as nice (and not nearly as wide) as with the premium ones. I also have an F/6 scope, so at least I have an excuse to buy expensive stuff . On the other hand, having had the scope for over 17 years, I have not spent much on the scope itself, one a yearly basis. If the missus complains about me buying EPs, I just start counting her shoes

[Sarah]

For me aperture wins, which is why we tend to mainly use dobs for visual. But you can't get away from the fact that the SCTs are much more compact than the dobs. The main problem with the SCT is that it is a dew magnet, so you really need a dew shield and a dew heater.

When we first started we had an 8" Newtonian on a CG5 GoTo mount and shortly after purchased a 6" SCT on a manual CG5 mount (and added motors). We mainly bought the SCT as the package was on a good price and we really wanted a second mount. We were surprised how great the views were through the SCT, so much so that we preferred the views from the 6" SCT over those of the 8" Newtonian. We took the SCT to our first few star parties and considering it's 6" of aperture, it was a great little scope.

It probably comes down to the physical size of the different set ups and budget.

[Mr Fiery Jack]

Its like anything else, you can spend as much or as little as you like on this hobby, I buy almost exclusively second hand...all comes to he who waits....Astro buy and sell will provide

[E621Keith]

8" dob vs 6" SCT on DSO, the 8" dob wins, but 8" dob vs 8" SCT is much harder. Dob has a lower purchase price and a simpler mount. SCT is normally used with a more complex mounting system which adds additional cost. However, as it has been pointed out already, a fast Newtonian needs higher quality eyepiece to achieve the same quality of view as a SCT with cheaper eyepiece. The differences between my Meade S5000 UWA, Vixen LVW and Nikon NAV-SW are hardly distinguishable in my f10 SCT, but the difference starts to show in my f6 scopes.

Having said that, a 200p dob cost £300 while a C8 OTA cost £900. Assuming you buy a used EQ5 to mount it, it will cost you £1050. The difference is £750, which is the price difference between 5 Meade 5000 UWAs and 5 TV Naglers, so the dob still works out cheaper for your wallet despite the more demanding eyepiece requirement.

[RikM]

And here's the rub... many Dob/newt users save cash on the tube itself but end up splashing out just as much on expensive eyepieces, coma correctors, etc... not so with an SCT (you just need a dew shield and probably heater!)

SCTs are just as well suited to DSO viewing so long as the object fits in the FOV. For an 8" SCT the max is about 1.3 degrees without resorting to a focal reducer. That doesn't exclude an awful lot of objects that wouldn't be better viewed with either a pair of bins or a short tube refractor anyhow

For visual, 8" aperture is 8" aperture whichever way you cut it, and if you can pick up a nice one at a good price used, at less than 6kg they're a great compact and portable scope.

I totally agree with you regarding aperture for visual use. 8" is 8" is 8" but the great thing about a Dob, when you are on a budget is that it all be bought in small manageable bits. £300 for the scope, and then an eyepiece at a time until you build up a nice collection. With an SCT you have shell out a big chunk of change up front. I've spent about the same on a 10" dob and Pentax XW EP's as a C8 on a GoTo mount, but i did it in three sets of £500 as that is all I can really spend in one go. Then you have to consider EP's for the SCT on top.

Not an easy decision to make and there are no 'right' answers, you just have to make the best compromise for your situation. I like big faint DSO's so I bought the largest aperture, widest field instrumnet I could afford, lift and store.

Edit: beaten to it by Keith

[Sarah]

1355736050[/url'>' post='1744088']

8" dob vs 6" SCT on DSO, the 8" dob wins, but 8" dob vs 8" SCT is much harder. Dob has a lower purchase price and a simpler mount. SCT is normally used with a more complex mounting system which adds additional cost. However, as it has been pointed out already, a fast Newtonian needs higher quality eyepiece to achieve the same quality of view as a SCT with cheaper eyepiece. The differences between my Meade S5000 UWA, Vixen LVW and Nikon NAV-SW are hardly distinguishable in my f10 SCT, but the difference starts to show in my f6 scopes.

Having said that, a 200p dob cost £300 while a C8 OTA cost £900. Assuming you buy a used EQ5 to mount it, it will cost you £1050. The difference is £750, which is the price difference between 5 Meade 5000 UWAs and 5 TV Naglers, so the dob still works out cheaper for your wallet despite the more demanding eyepiece requirement.

Yes 8" is better than 6" of aperture. As Keith says an 8" dob v an 8" SCT would be a tough call. But usually the dob wins because of the much cheaper cost.

What I was trying to say in my previous post was that the SCTs can be underrated a bit, as they do make great visual scopes. The views we were comparing through the 8" Newtonian and the 6" SCT were brighter objects. And no doubt the 8" would have performed better under darker skies, but the compactness of the SCT meant that was the one we took away with us.

1355656386[/url'>' post='1743136']

SCTs are just as well suited to DSO viewing so long as the object fits in the FOV. For an 8" SCT the max is about 1.3 degrees without resorting to a focal reducer. That doesn't exclude an awful lot of objects that wouldn't be better viewed with either a pair of bins or a short tube refractor anyhow

I agree completely. And it is no different when you get to the larger aperture dobs, with the increased focal lengths, you have the same issue that there are a few objects that are too large for the field of view and these are, as Dunkster says, best viewed through binoculars or a refractor.

​

If you decide on an SCT then I would personally go for the 8" over the 6", if your budget can stretch to it. Otherwise go for the largest dob you can afford and manage to handle. Either way you should have a great scope for visual use.

[michael.h.f.wilkinson]

BTW, even with a focal reducer, the max FOV is about 1.3 deg for a C8. As the focal reducer has an internal diameter of roughly 48mm, it does not gather image information from a larger section of the image plane than a 2" EP can. I used a focal reducer when I only had 1.25" EPs, and in those circumstances it is great. However, since putting a 2" visual back on the scope, and getting some wide angle 2" EPs, the focal reducer has been gathering dust. I might use it for imaging in the future, but visually using 2" EPs is much nicer.

[Charon]

This should be true, but you also find a lot of SCT owners splashing out on Naglers, Ethoi etc as well.

Reckon we're all as daft as one another on this one.

How right you are!

[Ajohn]

And if you want to take photo's with your dob costs rocket. They aren't too bad on a SCT really. Visually SCT's shouldn't need Naglers. There is another point as well relating to F ratio. Rather than typing it see the bottom of this page http://photo.net/lea...ture/sunmoon��. Also note the comment about sky fog.

When it comes to field of view it's better to have a slower telescope and a large 2ndry. Taking 12in for instance it comes out like this

F5 55' F6 64' F7 73' F8 80' Oddly, not at all really, for an 8in it comes out at F5 80' F6 92' F7 102' F8 110' (nearly 2 degrees)

These allow the star image to elongate to 0.1mm, sort of ok for photography but if more elongation is ok then the fields get bigger but the smaller or slower scope always wins. Taking this idea to the ultimate there was an entry in one of the Amateur Telescope Making books on a richest field telescope. This finished up with a newtoniam telescope of around 4 dia with an F ratio of about 4. I can't remember the exact figures. The equivalent refractor was much smaller. The figures were obtained from star distributions at various magnitudes and field of view. A quick web search showed no signs of the original idea only a couple saying it's all wrong. I'd beg to differ especially in respect to size and F ratio due to field of view limitations that these give and available eyepieces and 2ndry obstruction sizes. Mel's lost the plot. Coma correctors tend to be a bit expensive too. No comment on 100 degree view eyepieces as well. The other problem with larger diameter is minimum magnification. On that score another page shows a view of M31 and can't get it all in! The original reflector version didn't need a mount. Just sit in a chair and scan the sky. If any one is interested I probably have the article some where but I don't think it's possible to buy one. It's a diy project that might make an alternative to a small apo but is still unlikely to cover the same field.

If some one wants to photograph large fields the cheapest option is to mount a camera on the scope.

John

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[JamesF]

Wrong, I am afraid. For visual aperture is what counts, not speed. For a given magnification, the brightness of the image to the eye depends on the aperture of the scope, not the speed. For a given aperture, an F/10 scope with a 40mm 68deg EP gives exactly the same view as an F/5 scope with a 20mm 68deg EP. The same amount of light spread out over the size area of your retina.

Yes, I realise that I didn't explain my point at all well

You're absolutely right and I agree that aperture is what counts. In fact I did say "there's no substitute for aperture".

As regards field of view, in my experience I find that Maks and SCTs are not as good for DSO viewing because I enjoy star-hopping and whereas I find that star-hopping when using a scope with a wide field of view is quite easy, when I try the same with a Mak or SCT it's really quite difficult because the narrow field of view makes it more difficult to see patterns of stars to follow. I'm sure there will be others who don't find it a problem however, and I completely forgot that some people have GOTO

Other than for those objects too large to fit in the field of view, if you're confident of finding your target and tracking it then an SCT should be perfectly fine.

James

[michael.h.f.wilkinson]

I am a dyed-in-the-wool star-hopper, and with a good finder scope (16x70 on mine) an SCT is fine.

[JamesF]

I am a dyed-in-the-wool star-hopper, and with a good finder scope (16x70 on mine) an SCT is fine.

Ah, yes, now you mention it I think I recall you posting images of your monster finder scope Something that size would certainly make life much easier, I agree. Perhaps I should consider doing that.

James

[michael.h.f.wilkinson]

Ah, yes, now you mention it I think I recall you posting images of your monster finder scope Something that size would certainly make life much easier, I agree. Perhaps I should consider doing that.

James

Best afternoon spent tinkering ever!