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Frac vs dob on star clusters


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Hi all I've posted a similar thread elsewhere and thought it would be better here, I'm torn between a 5" frac and an 8" dob I'm aware that both will give me good views of the planets with the frac giving a crisper image. The thing is that there is a street light being built right outside my garden so I'm limited to star clusters and planetary, on star clusters how much more will the dob show compared to the frac. If the difference under heavy light pollution is negligible then my tendency would be to go for the frac, although if the dob would show many many more stars then I would probably go for that instead. My intention here is to go for a good planetary/stellar scope so if anyone has a better suggestion I would greatly appreciate it :)

here are the two I'm currently thinking of I have a budget of around £400 (not sure until I sell the guitar)

Evostar - Skywatcher Evostar 120 (EQ3-2)

Dobsonians - Skywatcher Skyliner 200P Dobsonian

Again thank you for any replies people :):):hello2:

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Just thinking of that refractor set up - that EQ3 will probably need its legs pretty much fully extended and it could be a bit wobbly up there on the aluminium tripod. I think the Dob may be a much more use friendly scope. I love my refractor but if I had to get rid of one or t'other I'd keep my dob

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The dob is likely to give you much better views of globular clusters with better resolution of the core stars. Open clusters - the dimmer ones will benfit from the dob but bright targets such as the double cluster could be a close run thing.

As for planetary, in good seeing and with a well cooled scope the dob should show more features and will allow you to push the magnification more. However, in moderate seeing, although the dob vies will be brighter the resolution gain from the aperture will be lost and the refractor is likely to give you steadier views (dobs are more affected by thermal currents)

The mount issue is very important. My 12" Skyliner has given me the best views of Saturn and Jupiter I've ever had but having to keep moving the scope hinders prolonged "immersive" viewing. when using my ED120 on a tracking mount if I just keep peering at jupiter by brain starts to ignore the fuzzy periods and justs builds a much more detailed picture from the steady moments, a bit like registax really. It is amazing how much you can see if you persevere like this. Much easier with a tracking mount.

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I have a 12" dob and the evostar 120 on eq3-2, both are fab however i would choose the dob if i had the choice... really just for the fact the bigger light grasp of the reflector. The eq3-2 is fairly sturdy enoough to give steady views providing theres good seeing and no wind again the dob wins. You can get a motor drive for the 120 and an autofocussor to steady things a bit more. I use the 120 for double star work and some planetary (for the motor drive) and for photos. For everything else i use the dob. The only down side of the dob is you cant really take long exposure photos with it. Hope this helps?

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I'm a big fan of refractors but I'd go for an 8" F/6 dob in this case.

Clusters, especially globular clusters, really benefit from the extra aperture - an 8" shows many more resolved stars in M13 than a 120mm refractor does.

Of course, if you can go to 10" or even 12" then it gets even better ..... :D

Edited by John
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Second hand sounds like a good idea actually as I'm not to fused about it being used, even if there's a bit of cosmetic damage. This might be one of those situations where it would be a good idea to hang back with your money for a bit till a good opportunity pops up :D

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hi Matt

I think you hit the nail on the head with your last post. wait for an ideal scope (for you) to come along and then pounce when it does. in the meantime do lots of reading and searching and if you can see a few scopes before then even better.

I have a 12" dob and a 120mm refractor (soon also to collect a 6" dob - but f11). I have to say that I have hardly used my refractor since I got the dob and even when I do use it I regret not getting the dob out.

the reason I am buying the 6" is that I feel this will be slightly better for planets and double stars than the 12" given it will be a slower scope. it will certainly (I expect) be better than the refractor. I suspect I will sell the refractor but not sure yet.

as you are in Bolton (I am in Stockport), you'd be welcome to have a look at my two scopes some time and you could then see just how big a 12" dob is (they are big but manageable) and compare the refractor views with it. with my two scopes anyway, there is seriously no comparison on almost all objects with the dob being the winner in nearly all cases.

to answer your specific question, I'd say of the two you mention (surprise surprise given the above) that the dob would be the better choice unless you wish to concentrate on planets and (importantly) also wish to photograph them.

cheers

Shane

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