Refractor 120/600

Hello,

I like know if a refractor of 120mm with F/5 (Orion,Synta..)can be a good performer for open cluster and others DSO...

Thanks,
Paulo

[QUOTE]Paulo martin said:

Hello,

I like know if a refractor of 120mm with F/5 (Orion,Synta..)can be a good performer for open cluster and others DSO...

Thanks,
Paulo[/QUOTE

Yes, Paulo. In fact that is what they are for. They are also good for hunting comets.

Not so good for planetary, lunar, or imaging (unless it's a three element APO, and pretty expensive).

It depends on how fussy you are. I was going to get an inexpensive 5 inch refractor, I would get it at f/8, because the optical correction is easier to make at a longer focal length, so the inexpensive part won't compromise your quality as much. It would also have less chromatic aberration on the planets and moon if you were going to use it for that.

To my mind, when you get a refractor for dsos, it is because you like open clusters and big wide views. I think this is an acquired taste; people new to astronomy usually like bigger and brighter (meaning a larger scope).

But I think you would do better to get an f/5 dob in the eight or ten inch category. The extra aperture would do well for dsos and you would still get nice wide fields. You wouldn't have to worry about chromatic abberation,the mount is part of the package and more stable than typical inexpensive refractor mounts.

I'm not saying don't get a 5 inch refractor. I have one myself. But I think you'd be better off with an 8 inch dob. At 25x it should give you a maximum field of view of around 3 degrees, not as much as a five inch refractor, but very good, and brighter. My other choice, if you want a mount with tracking, would be an 8 inch SCT. This will give you a lot more light than a refractor and be better on dsos but you won't have a nice wide field view. The SCT, being shorter, is intrinsically more stable than a five inch refractor. The SCT also won't have color issues.

regards
Greg N