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Wanting a dobsonian with potential to add motorised tracking

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This is my first post so hello to all and compliments on a fantastic and informative resource!

I currently have a Meade ETX125 that I selected on the basis of a good compromise of aperture, performance and portability and am using that for my experiments with DSLR and webcam (SC2) imaging.

I would also like a large aperture that will remain in the garden - something in the region of 12" (as we all would!) but i'm not looking to spent a fortune. I have decided that a dobsonian would probably best fit the bill and I am ideally looking for a second hand one. I realise there are lots of Dobsonians around and the skywatcher range seem popular. I would really like to add a drive mechanism at a later date, for example, I have seen in the past that someone has added a drive to a skywatcher and had a form of GOTO using startrak on a psion. I have also seen kits for drives for the meade lightbridge at around $500.

I would be really interested in peoples views or experience of adding motors and gearing to any particular makes of Dobsonian as this could affect my decision of what scope to go for.

If this question has been asked before I'd be grateful of anyone could point me in the right direction of the thread/article etc.

Thanks very much


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Welcome to SGL Paul.

You need to be clear on what it is you are trying to achieve. Are you primarily observing or imaging? There are two issues here: aperture and the type of mount used.

Dobsonian mounts are a form of Alt/Az mount - you can spin them round in azimuth and point them up and down in altitude, so manually pointing your scope is intuitive. Equatorial mounts have a fixed axis that you must align with the axis of the earth's rotation - it must be angled up (angle equal to your latitude) and pointed (more or less) at Polaris. To point your equatorially mounted scope at something is not intuitive - I won't try to describe it here.

So why make equatorial mounts so complicated? The answer is so to make it simple for them to follow an object in the sky. The stars appear to move but it is really just the Earth rotating, so to track them a motor can turn the mount very slowly around the axis you so carefully align with the Earth's.

Can a Dobsonian do the same? Yes it can but it must be moved in a more complex way since the apparant speed of the star depends on how far it is away from the line of the Earth's axis. That's why the Dobsonian upgrades are so expensive, it is because they need some form of computer control.

Motorised mounts are nice to have for observing but essential for imaging if longer exposure times are required. A motorised equatorial mount is almost certainly a better bet than a motorised Dobsonian if you are considering imaging.

As for aperture, the simplicity of a Dobsonian mount means you can put a fairly big scope on it at a pretty cheap price. Big heavy scopes on equatorial mounts require big heavy expensive mounts. This is not the bad news it seems. If you intend to image, doing so with a small aperture, short focal length scope is much easier. There is a happy medium where you could use the same scope for imaging and observing but increasingly members who do both tend to have small refractors on an equatorial mount for imaging and a big Dobsonian light bucket for observing.

I hope there is something in the paragraphs above that help you with your research.


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Hi Mike

thanks for your input. I have an ETX125 with GOTO that I am starting to experiment with imaging having modified a philips webcab to SC2 level. I suppose I really wanted a Dobsonian for observing due to the light collecting power at a relatively inexpensive price.

I suppose I would like to have a go at imaging with one that is why I would want to put a drive on it. I am a bit of a tinkerer although I expect that if I find I can invest the time and I do obtain good results with the ETX would probably eventually look at upgrading to a larger scope on an equatorial mount.

I suppose in reality taking an image of a daylight object like a planet could be done with relatively short exposures on a dobsonian and so there may not be any need for tracking anyhow.

Some good points made and I suspect I'll jusdt buy the Dobs for the wowfactor of a large aperture. I would also like it to be able to show non-enthusiasts when they come round just how amazing it is to see something with the naked eye - has a bigger impact on them than a series of pictures taken at long exposures!Cheers


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