[TAHAIC (Nige)]

Originally Posted by mental4astro

Barrel size is the limiting factor to the widest the apparent field of view can be for a given focal length. An example will best demonstrate this: the longest 1.25" EP that will give an AFOV of 68º is 24mm. In the 2" format this is 42mm.

ok - how are you working this out exactly? when you say barrel size are you referring to the diameter of the EP or the scope?

Originally Posted by mental4astro

But, having such a fast scope (f/4), another factor comes into play, coma. It is a phenomenon that affects reflectors where the resulting light cone coming off the primary mirror is too stumpy for an eyepiece to handle, and the resulting aberration is seen as the stars along the edge of the field of view having little 'comet tails' that rediate out from center. This is not a flaw, but something that just is. Longer focal length eyepieces show this much more than shorter ones.

I think I understand this thanks to your excellent explanation - your saying that coma is a result of the geometry i.e. the shorter the scope the larger the angle of curvature towards the outer regions of the primary mirror.

Originally Posted by mental4astro

* 300x magnification is possible, optical quality allowing. Even 400X is not out of the question. But the one all limiting factor is the atmosphere. And it doesn't matter how good your gear is, the atmosphere will determine how high magnification can be used. Typical conditions allow 120X tops. Good conditions allow 250x. Exceptional conditions allow 400X.

I too have Barlow lenses. Both 1.25" & 2". I never used the 1.25", & only once the 2" in the four years I've had it! Most folks you'll find prefer to use the appropriate short focal length EP over a Barlow.

I can appreciate the effect of the atmosphere but I think I should be able to see more than what I can. I don't seem to be able to see much at all, when I use my PL 6.4 EP I can only see the surface of the moon and not quite as good as an old $15 60mm Tasco refractor we bought for the kids - using a 3x barlow and a H12.5 EP.

If most folks don't use a barlow what can they actually see - can you see Jupiter? I can't and I expected to see Jupiter and a couple of it's moons on a good day. I tried looking a Venus also but can't see more than a slightly enlarged star. This is disappointing, is it the quality of my EP? What can I expect to see with say a good quality PL 8mm EP and no barlow? The mag factor just seems too low to me.

Originally Posted by mental4astro

But, all the same, a Barlow is a good idea for your scope (just determine optical quality first before commiting more funds.

I agree completely - I am happy to spend a little money on a better quality EP and a barlow but how do I determine the optical quality of the mirrors in the scope?

Is it better to upgrade to a 2" focuser and EP as opossed to 1.25"? If so I would be happy to upgrade the focuser to 2" providing the scopes mirrors are going to preform. I don't want to spend the money on 1.25" gear and then find out that I should have bought 2" gear.

Should I ditch this F/4 scope and get a 1m 8" so its not such a fast scope - or would this not make much difference? No point spending money on EP's and focuser if the scope is not going to preform - better to change the scope now if I need to.

Thanks for your great advise.

TAHAIC