bright stars appear as a fuzy ball

[dizzy2005 (Mike Diskett)]

Is it bad seeing that causes bright stars to appear as a fuzzy ball, I'm using an 8 inch dob focal length 1200mm and a 10 mm eyepiece.

Dim stars are ok, and If I put the dustcap on but open the 2 inch hole that reduces the fuzzyness (but I guess its just making bright stars dimmer)

It seems like stars are more pinpont like in my crappy 60mm refractor.

I think I might have read somewhere that bigger apperture shows up the problems of bad seeing more is that right?

Am I just trying to view too early in the evening, 8 pm? Should I be staying up past midnight. (I get up for work at 6:00 am )


Mike

[Robh (Rob)]

Mike,

I'm assuming your 8" dob is well collimated.
There are a number of factors coming in to play here.
If you are viewing early evening, are you giving your primary mirror time to cool down to the outside temperature? Convection currents off a cooling mirror will give stars a hot fuzzy look. Also, early evening, heat from the ground will cause convection currents in the lower atmosphere and this make seeing worse the lower the star is to the horizon. Things should settle down more a few hours after sunset. If sunset is about 5pm, by 7pm things should be settling down.
The 8" dob will gather more light. Therefore, stars will appear brighter. Brighter stars will produce more glare which is accentuated by poor seeing and poor transparency.

Regards, Rob

[ngcles]

Hi Mike, Rob & All,

Yep, I've got little doubt the big contributor to what you are seeing here is "seeing". The effect is more noticeable on brighter stars than faint ones. The advice offered by Rob is spot-on, particularly about letting the 'scope cool before expecting it to really perform

I'd expect that it will be about 45 mins before the mirror settles down to ambient temperature outside -- so don't expect it to perform well for at least a 1/2 hr, particularly after taking it outside from a warm house to a cold backyard in winter.

If it is well cooled and well collimated it will show nice sharp stars assuming the seeing is good. So called "fast seeing" usually caused by high-altitude, high velocity winds will make bright stars look like tennis balls -- similar to what you are seeing. "Slow seeing" on the other hand will make a relatively good image ripple and wobble about and the cause is likely to be more local or even inside the telescope itself as Rob pointed out. Even a 2 deg C difference between the mirror and the air immediately in front of it will create a rising column of air within the telescope tube which will disturb the image. The only solution is to wait until it stabilises or use a fan to actively cool your primary mirror -- this usually cuts the cooling process in half.

Yes, larger apertures are more "seeing sensitive" than small ones. They look through a larger diameter column of air for a start and because a larger 'scope has inherently better native resolution, it will show up imperfections small 'scopes won't notice.


Best,

Les D

[dizzy2005 (Mike Diskett)]

The scope had cooled (a couple of hours) but I guess if the outside tempreature is still dropping then the mirror might lag behind constantly cooling, I hadnt collimated it prior to viewing. (no fan assist its a 8 inch collapsible SW)

I did star test it and it looked pretty good to me, but my pretty good is probably quite poor to the experts.

Still getting to grips with collimation, I dont have any fancy tools just a hand made site tube and a lot of patience, first go at collimation took me 2 hours, next go an hour and 3rd go 30 mins, so theres a healthy pattern there, allthough I've only collimated in the day. Not sure how to even start in the dark, tried using the full moon the other day but i cant see the eyepiece exit hole reflected and the dot on the mirror. I guess I will need a laser for easy nighttime collimation.

Last nights viewing was still fairly fuzzy, even stayed up till 10:00

It was better than the previous night though. And I did notice a marginal improvement as the night went on.

I also noticed if I went totally out of focus so the rings almost filled the view (well 50% or so) I could see turbulance in the air by movement in the concentric rings (not that I could make out individual rings more a large disk). And I could quite noticeably see convection currents in the scope when I held the collapsable struts with my hand. The heat from my hand sent endless convections currents through the diffraction rings.

Mike.

[Lumen Miner (Mitchell)]

Alert noob Warning!!

Is it just me or when someone says "bright stars just appear as a fuzy ball" does it not suggest over-magnification?

IMO a distant star should appear as a minimal size to be inherant of its original composition. Try enlarging a distant star and you get blob, focus and retract, you get a pin point accurate resembalance.


In all though, that is most probably irrelevant to the problem at hand. It seems to me you issue is more technical, in which I can not help.