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Scope upgrade advice sought


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Hi all,

I have had a Skywatcher 120ST since September, and have managed to accumulate enough experience to know that this scope just ain't up to it any more. It has a lot of CA and is not easy to persuade to stay in focus. Another issue is that it is a little too short on focal length for working with a DSLR on DSOs and the moon. However, it was a great learning tool. Now I'm looking for an upgrade path.

Originally, I had thought about an ED80, as everyone raves about them, but that would give me even less focal length, less diameter, and be a large financial outlay. (I had thought about a 150 apo, looked at the prices, and then soiled myself!) I then thought about going Newtonian with a 200PDS. This seems to fit the bill apart from being very large. I have an EQ5 mount, which I have seen many people get good results from with a 200PDS, but probably not such a good match at this point in my learning curve. 

My next thought was going down in size to a 150PDS, which also has good reviews. This would probably be more of a better introduction to the world of reflectors. At the moment, this is my favourite, as it is a reasonable price, good performance, an increase in light gathering power and a good match for the mount.

I would appreciate thoughts on these choices and any other alternatives that people can throw into the pot.

Kind regards

RIchard

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When I bought my first scope a few months ago, I was intending to go with n 8" Newtonian for my ZEQ25GT mount. I probably would have been ok, but decided to go with a 6" f/5 newt from GSO. I believe it was the right choice.

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The 130PDS seems to be another popular choice with the experienced imagers. I seriously considered the 150PDS but read several people say its focal length was a bit of a poor compromise between the wide view of the 130PDS and the extra magnification of the 200PDS.

That's not to say you shouldn't get the 150PDS, its a good scope.

If you do get one of these newtonians don't forget to allow for the cost of a coma corrector.

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Longer answer.  Everyone has their own priorities, in order to make an informed decision there are several factors. The scope combines with your chosen camera to determine pixel scale and FOV.

P = (FR * S) / (FL)

Where:

P is the image scale per pixel in arc seconds
S is the size of the pixel in microns
FL is the focal length in inches
FR is the focal ratio of the OTA

This determines the resolution of the rig. 1.5" to 2.5" seems to be a comfortable range

FOV (in arc minutes) = 135.3xD/ FL  where D is chip width in mm and FL is focal length in inches

Knowing the FOV, one can compare to some common targets to see if they will fit. A program like Stellarium has a calculator so you can enter different combinations, select a target, and get a preview of what an actual image framing would look like.

The same scope with a cam with a tiny chip will produce a zoomed in look, while a very large chip like in a full frame DSLR will do the opposite.

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