Dobsonian 8" with auto tracking/slewing

[Scallywag]

Hello all,

I am new here.Have been delving into the various threads etc regarding dobs. I am about to acquire my first scope and have decided it will be a dob. Initially looking at 8" ( considered 10" but settled back to 8") skywatcher. I noticed that you can buy model with auto track/slew which sounds very appealing albeit at approx £200 premium. My question is it worth the premium or better to buy a basic model with selection of eyepieces? Intend to use scope for both planetary and DSO viewing. No intention at this stage to look at astrophotography.

[DOBBY]

Auto Track has the advantage of holding something 'in view' while you move away & someone else has a look... as for the 'selection of eyepieces' try to find an 'observing' group..pop along with your 'scope, & ask if you can try their eyepieces out..most people are only too happy to help out...reading reviews can help, but...everyones eyes are different... try before you buy

[johninderby]

Scopes'n'Skies has a refurbished Flextube Auto 8" dob in the bargain section for £399.00.

http://www.scopesnskies.com/bargain-zone.html

John

[Scallywag]

Thanks for the head's up re scopennskies, John.

[umadog]

Welcome!

I must say that tracking is very nice for high-power viewing of things like planets. Not having to push does help you see more. I'm neutral on the GoTo: learning the sky isn't hard and I wouldn't miss the GoTo if it wasn't there.

If I were starting out with a limited budget I would probably put tracking quite far down the list of priorities. I'd rather be spending the money on stuff like eyepieces, collimating tools, a Telrad, a fan, dew-heaters, books, or even on a couple of extra inches aperture. Then again, tracking is harder to add after the event whereas those items can be accumulated over time.

[Scallywag]

Thanks Umadog. The more I think about the various angles raised the more I question my original decision and end up back at square one! You raise an interesting point re tracking vs maybe putting that extra premium into a larger aperture. For example for 8" with tracking I could buy 10" or with a little more get within a 12" range! I am thinking that larger aperture may be way to go..though still have niggling concern that the need to constantly manually to keep object centred may become a pain... My primary object to to view DSO but also planetary objects too. Good advice I read in another thread remains valid I guess.. do not rush in..stars will always be there!

[twotter]

Nudging a dob to keep the target in sight isn't everyone's cup of tea. I don't mind it, but then I've also started using EPs with wider field of view

I'm also looking forward to hearing news of the Skywatcher goto dob bases

[JamesM]

Scallywag, I would agree with your last comment in your most recent post regarding the waiting game. I think the tracking element of the kit sounds like a great idea, certainly makes it easier to get that 'extra' bit of focus on a stationary object rather than one that is moving, especially on planets.

However, you have highlighted a dilemma that faces many people, in particular new people starting out and that is that a budget forces them to make negative compromises. Now don't misunderstand me, I know we all exist on a budget which of course varies from one person to another, but for example trading off tracking for aperture or for wide eyepieces, purchasing an 8" when the 10" looks mighty close so why not go for the 12" which will mean cutting back on something else (ignoring arguments regarding mobility, storage etc) are all about balancing the different trade offs, having this advantage instead of that one.

The real solution is to wait, to save up a little more (I understand the impulse to get started!) and to weigh up what is right for you. I spent over a year researching what all those specifications meant to me 'at the eyepiece' by looking through so many scopes and establishing my own datum line against which I could measure my expectations and desires. To that end, I realised that instead of buying and reselling etc I would do better to decide on what I 'needed' from the start and save for it, whilst keeping a look out for used gear that would help cut down on the waiting time. Even if you're not sure that astronomy will be your life's passion, you can always resell what you have bought to exit at minimum cost. Everyone aspires to obtain the bigger kit so there won't be a shortage of buyers.

So I would wait and do some more homework on the differences you can see by looking through some gear.

James

[umadog]

Since you raise the possibility of a 12": given the choice, I would rather have a 12" with no drives than an 8" with tracking. For most DSOs you'll not be using super-high powers so hand-tracking is a non-issue. It's only an issue for powers over about 300x. Everyone has a different cut-off and it depends on how smooth the mount is and how much back-lash it has, etc. I've used hand-tracking for years and years and never had a problem. You can cheaply modify mounts to improve the motions.

The only thing about the 12" is that it's really pretty big. To fit it into a car it will need a collapsible tube of some sort. You need to make sure you have somewhere to store it and can move it about. You need to make sure that the size isn't so large as to put you off observing. Looking around the forums and my own experience, I'd say 12" is the largest size which people feel happy using as their "regular" scope. People with instruments over 12" tend to also have something in the 8" to 12" size as a fall-back option for when the big guns are too much hassle. Chances are, then, that 12" will be OK, but go see one first.

You also need to consider the following hidden issues: Firstly, the cool-down time for the mirror becomes appreciable for larger scopes with thicker mirrors. Cool-down time is proportional to the square of the mirror's thickness. A 1/4 or 1/2 inch thickness change does make a difference. Secondly, most 8" scopes are f/6 which is very forgiving of collimation errors and cheaper eyepieces. Most commercial 10" or 12" scopes are f/4.8 to f/5. Your collimation needs to be more accurate here so you might want more expensive tools. Also, cheaper eyepieces can exhibit astigmatism at these focal lengths which isn't evident at f/6. Generally, however, any reasonably good Plossl will work fine at f/5. It's usually some of the wider-field designs that have problems.

I'm not trying to put you off the larger scopes--they're worth the small extra bother--but you do need to know that it's not just a matter of having a bigger mirror.

[Scallywag]

Thanks for the really valuable advice and taking the time to write back with very insightful comments. I really do appreciate you guys taking the time to do so. I am taking everything I read very seriously and belive it or not beginning to enjoy , maybe peversely, the quest for my first scope.