Short vs. Long Tube Refractors

[Mr. Subatomic]

Hi Everyone,

I'm considering buying an 80 mm refractor, but I noticed that they come in both short and long tubes. What are the advantages and disadvantages of having more or less focal length, specifically for astronomical observing?

Thanks for your help,

Mischa. :)

[bmitchell82]

difference is FoV

go grab yourself a 3m long tube. you will notice that theres less light and a greater effect of zooming in then go and grab your 1 meter long pipe and youll notice theres a bit more light and a wider view.

The other thing is that a ST is alot more portable than a LT, then theres the question of what are you using it for and what are you future intentions. If you are going for observational usage why are you going a 80mm refractor?

Post up a bit more detail and we can help you from there.

[Smirnoff]

In achromatic models, shorter focal lengths produce more chromatic aberration than a longer focal length in any given aperture. Longer focal lengths may also produce sharper images due to reduced chromatic aberration; and because long focal length optics are easier to make, there's a reduced likelyhood of imperfections. For this reason Long focal lengths are better for lunar and planetary targets.

Short F/L achromatic refractors are more designed towards wide-field deep sky viewing and are not particularly suited to planets.

[Mr. Subatomic]

The reason why I ask is that I think a refractor will be better suited for my needs. It will be more portable so I can take it away and I'll be able to use it on the balcony or roof. Generally, it will get me outside more, which is something I find that I don't get to do enough.

So I'm actually downgrading from a 10" Dob (shocking, isn't it!) :jawdrop: