Am I expecting too much?

[ben_e71]

So I've been using my NexStar 4SE both in the daytime to align red-dot finder and at night. But I can't get as clear and large an image as I expect. If I focus on the dome of Herstmonceaux observatory or POlegate Windmill 15 miles away I can see them small and clearly with my 25mm Plossl, slightly less clear but bigger with my 18mm BST and larger but less clear with 8mm BST. In fact with the 8mm the point at which focus is sharpest is miniscule.

At night looking at jupiter I get small but clear (moons ar epin pricks) at 25mm - bigger but hard to focus at 9mm and not tried 18mm yet.

Now I am away it has been windy and a little hazy during the day and at night it has been partially cloudy so am I expecting too much at this stage.... I had read a lot and expected more from the 8mm but maybe I am unrealistic.

I have checked that the lens look clean and clear.

[ronin]

It is an f/13 scope, 1325mm focal length. The 8mm will be pushing your luck at 165x. This 2x diameter is subjective at the end of the day. I would suspect that a 12mm BST would give a good image at 110x, and they do not do a 10mm, which is what I suspect might be the best compromise for magnification and quality with the bias towards magnification. If you were willing to forgo the magnification a bit then the 12mm as already mentioned.

[ben_e71]

Thanks. Did read about one member utilising the 8mm on his 4se to good effect which swayed me. Got the scope out cooling at the moment as clear to the south and east here tonight.

[ronin]

It is always one of those things that is difficult.

It is said that the "best" is when the magnification = diameter in mm, so a 13mm eyepiece.

What then happens is that as magnification goes up the image quality goes down, somewhere along that balance there is a point where you decide that the image is not up to it.

The problem is that only you know when Ok becomes bad.

So someone else may be OK with an 8mm where you are not.

There will be variation both within scopes and eyepieces. I presently have 2 identical scopes - making a small electrical converter for a friend to match mine, and there is a difference in the two scopes with the same set of eyepieces. One is just a little bit sharper then the other, used them on Jupiter and M42.

At the high magnification end I find that even a 1mm difference can make the difference between good and poor. If BST eyepieces came in 6mm and 10mm as well I think that the range would be ideal. Makes me wonder would a 9mm Celestron work where the 8mm BST is just on the wrong side. The catch being it would cost £69 to find out.

It could also be that it was clear to the eye but the scope was picking up disturbances high up that came through. There are several layers of air up there all doing things to upset our viewing. Earlier this year the nights were clear and calm and the seeing absolutely awful - the jet stream was up there directly over us, nice turbulant air refracting the light every possible way.