Lines coming from star.

[Joecobbs]

Hi there, I was viewing venus earlier with my 12 inch dob, which I've just upgraded from an 8 inch. I noticed that there were four bright lines coming out of venus vertically and horizontally, like a cross, is this normal because venus is so bright, or is it a collimation/seeing issue? 

[joe aguiar]

Sounds like maybe your spider vanes maybe in the 12 it's so bright your seeing it

Joejaguar 

[RT65CB-SWL]

Hi @Joecobbs and welcome to SGL. 

As per @joe aguiar has said. They are 'defraction spikes'. I don't mind seeing them on star clusters, i.e. Pleiades [M45] in Taurus [Tau], but for planets it's a "No!" from me... and I do not have a Newtonian/Dobsonian/other reflector; that has the tube open to the elements on one end. 

[Joecobbs]

I thought as much, but didn't know the name of them, thanks. I presume as I've got a newtonian, there's nothing I can do about it? I don't mind them and have noticed them on brighter stars, but they were just a lot more noticeable on venus. 

[Aramcheck]

I haven't tried it, but have read here on the SGL that the small cap on the front of the scope cap can be removed leaving the large front cap in place on the scope. That will cut down the effective aperture of the scope, but it can be rotated so that you avoid creating diffraction spikes. It will of course reduce the achievable resolution too.

Cheers
Ivor

[Carbon Brush]

Ref Ivor.

Yes using the front mask is a good way to remove spikes. On a really bright target like Venus, it will avoiding you being dazzled!

HTH, David.

[ollypenrice]

8 hours ago, Aramcheck said:

I haven't tried it, but have read here on the SGL that the small cap on the front of the scope cap can be removed leaving the large front cap in place on the scope. That will cut down the effective aperture of the scope, but it can be rotated so that you avoid creating diffraction spikes. It will of course reduce the achievable resolution too.

Cheers
Ivor

It should indeed improve the view. Resolution may not be adversely affected because less use will be made of the outer part of the mirror.

8 hours ago, Joecobbs said:

I thought as much, but didn't know the name of them, thanks. I presume as I've got a newtonian, there's nothing I can do about it? 

There is a solution. You can use a curved spider which will smear out the spikes, spreading the diffraction effects around the source so that they are no longer visible. Such spiders are quite widely used, notably on Obsession Dobs. Quite honestly you have more than enough light for the planets, though, when using an aperture mask.

Olly

[CraigT82]

You could also ramp up the magnification in order to dim the spikes.  In my scope viewing Jupiter at 175x the spikes are all but invisible (venus might require a bit more mag as it's so bright

[ScouseSpaceCadet]

5 hours ago, CraigT82 said:

You could also ramp up the magnification in order to dim the spikes.  In my scope viewing Jupiter at 175x the spikes are all but invisible (venus might require a bit more mag as it's so bright

Viewing Venus for the first time through my 8" dob, I had the same issue. Increasing the mag to 300x removed the diffraction spikes, clearly showing the planet and phase.