Stars are blurry despite collimation

[Ledge1962]

Hi all.  I have a 14"Dob (f4.7) that is well collimated, and used to have really sharp views, but not so much any more. Nothing has been changed, the mirrors are clean, the secondary is heated, it has 3x 80mm fans on the primary. It is stored in an insulated garage. 

I use the Sky-watcher 82 degree EPs and the scope cools for an hour at least before use.  The stars look like they are suffering from atmospheric turbulence (on fire) according to astrobaby's guide, but I have a 10in dob (250p) that shows clear stars on the same night at the same time.  I am at a loss as to why it is like this?  I have tried collimating using a Cheshire, a farpoint 2" laser and a barlowed laser.  The results are similar. 

When looking at the Orion Nebula, the nebula is fine but the stars look like they have little spikes coming off them. When defocused, the star test does have the concentric rings but the star still looks like it has small spikes.

Has anyone had any issues like this before? 

I would appreciate any suggestions or further tests to help find a solution.

Thanks

 

[Ruud]

I think it may be the seeing that limits your observations. Check the link for Palmerston North. The seeing there  is somewhere between 1 and 2 arc seconds as I post this. Larger telescopes are hindered more by less than perfect seeing. That is because in the wide column of air you're looking through there's more traffic of warm bubbles rising and cold ones sinking.

https://www.meteoblue.com/en/weather/outdoorsports/seeing/palmerston-north_new-zealand_2185018

I just took a town in the region, you'll have to fill in your own for an improved forecast.

If you want to observe planets or the Moon in better detail, try using an off centre aperture mask. That narrows the column of air you're looking through and takes the secondary mirror out of the equation.

 

 

Edited May 7, 2020 by Ruud

[Ledge1962]

Thanks Ruud, I will give that a try. It is a concern when the scope cost $5000 NZD. I was worried the mirror may have warped somehow.

 

[CraigT82]

Is the 14" a flex tube? Just thinking it could be your body heat entering the tube. 

[Ledge1962]

It is a truss  (Orion xx14g) , but does have a shroud attached. Being a 14" mirror, I am wondering if it isn't cooling down enough. It has a heater on the secondary, but it didn't change the look of the stars both on and off.