Some newbie questions: EP, Jupiter and "tails"

[freespace]

Finally the clouds departed, and I got to try out my bintel 2x barlow, my bintel 12mm plossl, and UO 5mm.

I found Jupiter, then tried the new EPs with and with out the barlow, and found I can never see any detail on Jupiter. Its always a soft glowy circle, and I have to admit I am a little disappointed (its moons were easy to see though).

I have a 102mm f/5 skywatcher, should I be able to see /some/ details on Jupiter? Was it just bad seeing that caused the blurriness?

I also find that even when focus, the stars don't come down a point - it can kinda see like diffraction spikes, to one side. So it looks like this: >.

The direction of the > is also dependent on my eye position. I don't notice this much on the 12mm bintel, only with the 25mm super wide that came with the scope - is this a function of the EP?

And, there seems to be a vertical line straight down the field of view - this is noticable only when I am focusing, so it maybe I am focusing on defects in the primary or diagonal - should I be worried?

Sorry for the barrage of questions, but I dont know so many things!

Cheers,
Steve

[Dennis]

Hi Steve

Is the 'scope in this CN article the same as yours?

Cheers

Dennis

[freespace]

Originally Posted by Dennis

Hi Steve

Is the 'scope in this CN article the same as yours?

Cheers

Dennis

Yeah it is Dennis. It is exactly the same, except with a red dot finder and a altz3 mount.

Does it make some difference?

Cheers,
Steve

[rmcpb (Rob)]

When I first started reading this I immediately thought "collimation" then found its a refractor and the "diffraction spikes" change with eye position. If these spikes move when you rotate an eyepiece then suspect the eyepiece, if they move as you rotate your head around the optical axis then suspect your eyes. It may be that you have a mild form of astigmatism in your eyes?? Wouldn't hurt to have it checked out with an optometrist.

[Dennis]

Hi Steve

I just wanted to clarify what model ‘scope you had before I thought about the problems you are having. These 120mm aperture F5 ‘scopes are classified as wide field ‘scopes, which are designed for providing you with low magnification, wide views of star fields, nebulae, etc. So, they are not really optimised for planets.

Unless you spend $1000’s, a mass produced 120mm F5 objective lens will suffer from chromatic aberration and another CN Report at http://www.cloudynights.com/item.php?item_id=1602 opens with this line:

“Being an F5, 600mm scope, the 120st is known primarily as a "rich field" scope. This scope has been reported to show chromatic aberration (CA) at higher powers and is not meant to be used as a planetary scope”.

The author also mentions the use of a Minus Violet Filter to reduce the effects of chromatic aberration. However, the CN report author does indicate that by using his Meade 5000 Plossl eyepieces, he was able to obtain some pleasing views of Saturn and the Moon, so it seems that good quality eyepieces and Barlow’s can help with seeing detail on the planets. He quotes using magnifications of x135 and x220, which indicate eyepiece/Barlow combinations of 4.5mm and 2.7mm respectively, which are very short focal length combinations.

I see you have a UO 5mm and I would have expected this to perform quite well on Jupiter? Did you view Jupiter when the planet was above, say 60 degrees altitude (above the horizon)? When low down in the skies, the Moon and planets tend to produce views such as you described due to atmospheric distortion.

Traditionally, a refractor "should" produce point like images with concentric diffraction rings when the star is slightly inside or outside of focus. The description of seeing little tails > may indicate some problems, as does the vertical line?

With low cost wide field eyepieces, you can expect coma or distorted stars when your eye moves to the edge of the field of view. This effect may start to appear approx halfway out towards the edge. However, I would expect to see round stars at the centre of the field of view.

Cheers

Dennis

[freespace]

Originally Posted by rmcpb

When I first started reading this I immediately thought "collimation" then found its a refractor and the "diffraction spikes" change with eye position. If these spikes move when you rotate an eyepiece then suspect the eyepiece, if they move as you rotate your head around the optical axis then suspect your eyes. It may be that you have a mild form of astigmatism in your eyes?? Wouldn't hurt to have it checked out with an optometrist.

I might. When I got radiotherapy as conditioning for my transplant they did warn me something about my eyes... I will get it checked, thanks.

Originally Posted by Dennis

I see you have a UO 5mm and I would have expected this to perform quite well on Jupiter? Did you view Jupiter when the planet was above, say 60 degrees altitude (above the horizon)? When low down in the skies, the Moon and planets tend to produce views such as you described due to atmospheric distortion.

Thats what I thought as well. However Jupiter was fairly flow in the sky, and sydney is apparently suffering from a jetstream at the moment.

Traditionally, a refractor "should" produce point like images with concentric diffraction rings when the star is slightly inside or outside of focus. The description of seeing little tails > may indicate some problems, as does the vertical line?

Perhaps, It might be just something on the diagonal too, I will test it tonight. The bintel plossl just in and just out of focus gives these "spikes" too, but they are more symmetric and they also change as I move my eyes.

It could just be I have doggy eyes I am extremely short sighted, and have fairly thick glasses. I normally take my glasses off when I look through the EP, as I can get closer, but it might also be having certain optical effects I am no familar with. This does not seem to be a problem with the 5mm UO when I focused on the Jewel Box nebula, however stars had sort of a fuzzy halo around them.

Thank you very much for your help Dennis, it was very informative :-)

Cheers,
Steve