Why do my stars look like this?

[gooseholla]

Not sure if this is the best place but my stars look strange. Last night the effect wasn't there but it is most nights. It happens in all eyepieces whether lower power or high power. Telescope is 18" F/4 reflector.

Let me help with some pictures. The star below has normal diffraction spikes (red). But then it has a shaft of light either side stretching across a large part of the view brightly. Sometimes it is like a line other times like a cone.

It was lovely viewing last night without it but now the effect is back. Everything is saying the scope is collimated - star test and Cheshire. Any ideas what it is? Really annoying!!
Thanks

John
 

[malc-c]

Not sure if it's the same, but I had issues with an addition diffraction spike on images which was due to flats on the secondary mirror's minor axis, in that where the secondary was held in the coating process ment the minor axis wasn't fully coated.  I'll see if I can find the thread, which has some serious therory and documentation explaining why the spike occured.  You might find moving the primary and secondary mirrors closer to each other as part of the collimation process will remove the spike, so that the secondary diverts all of the light cone out of the focuser

[malc-c]

Found the thread - quite lengthy, but might help

http://stargazerslounge.com/topic/159748-200p-colimation-or-poor-optics/

[gooseholla]

Thanks reading thread with interest but the effect is gone now! Most annoying and strange.

[Owmuchonomy]

Just a thought.... Dew?

[gooseholla]

You know I really am starting to believe that my secondary and eyepieces dew up something rotten with the slightest bit of moisture. The night got a bit warmer and the Telrad wasn't dewed up anymore and the picture improved no end.

John

[John]

I have seen that effect when I've been viewing and the object has been near a telephone wire or similar. The wire does not have to be across the object but just somewhere in the general line of sight of the scope.

[alan potts]

I get spikes on bright stars on my Dob which is also 18 inch and i don't really think there is anything wrong with it as everything else is as sharp as a razor, i rather like the effect. I just think it is the spider veins.

However Like John i too have seen additional spikes when near to power lines or other objects. I was looking at Antares a while back and the effect was crazy then I realised I was looking through part of the safety net of my sons trampet

Alan

[gooseholla]

I think that may account for Vega - it was partway behind a tree. But it baffles me as to why Deneb, clear line of sight was doing that. I am starting to suspect dew, but will also be reading malc-c's thread very carefully, so I am equipped if it isn't dew!

Thanks

John

 

[malc-c]

Some of the imaged DaveD posted have sinced expired, but he is sending me a pdf with them in if you need them.  If you can't repeat the error then it would seem to be down to dew or a defraction caused by an obstruction (as been suggested).  I would be suprised if the secondary optics used in your 18" scope were not perfect eliptical in profile

[niallk]

You know I really am starting to believe that my secondary and eyepieces dew up something rotten with the slightest bit of moisture. The night got a bit warmer and the Telrad wasn't dewed up anymore and the picture improved no end.

John

That sounds promising. I got really perplexed one night when I just couldn't focus my 15" properly - until I shone a light on my secondary from below and saw the amount of dew. I was amazed I was even seeing anything at the EP. I have an Astrosystems dew heater behind the secondary, but I guess the humidity and dew point just overwhelmed it... scope was dripping wet.

Hope this is all it is,

Niall