Spider prevents me from getting diffraction spikes?

[EzyStyles]

Hi,

I was asking someone about diffraction spikes when imaging. He told me that my spider is too thick that is why my stars has 6 spikes instead of 4? how true is this?

thanks

[ving]

wouldnt have thought so... buthow thick are ya spiders anyhow?

[EzyStyles]

not the thin ones thats for sure. see pic.
maybe i have only 3 spider legs instead of 4?

[Hitchhiker]

Hi Ezy, I only see 3 vanes on that spider? Am I missing something here?

That Sirius image looks fine to me for a newt with a 3 vane diagonal - ie 6 spikes. The number of spikes has nothing to do with the thickness of the vanes - the thickness of the vanes affects the intensity of the spike, not the number of spikes.

[vespine]

Yeah each vane makes 2 spikes 180 degrees apart. The reason 4 vanes make only 4 spikes is because each spike you see is actually 2 spikes on top of each other, as each vane has another vane 180 degrees to it.

[EzyStyles]

arh that explains why i have 6 spikes instead of 4. So if i change the spider to a 4 vain one, i'll have those pretty 4 diff spikes? worth the money? :)

Thanks for clarifying.

[Striker]

Well I learnt something from this thread...thanks.

[Vermin]

Prostar have this to say about 3 vs 4 vaned spiders:

Should I use a 3-vane or 4-vane spider?
The choice between a 3-vane or 4-vane spider is largely an aesthetic preference, and the consequence of your decision will be the pattern of the diffraction spikes that emanate from bright objects. A 4-vane spider produces four brighter spikes. A 3-vane spider produces six, dimmer, spikes. A 3-vane spider is usually a good choice for telescope apertures up to about 15.0 inches.
3-vane spiders are a particularly good choice for planetary and lunar observers. The brightness of the spikes is inversely proportional to the viewing magnification. Thus, at the high powers typical of planetary observation, the spikes almost disappear, and the view is more "refractor like." (Conversely, if you mostly observe at low magnifications, you might prefer a 4-vane spider.)

I think all you need to do is get some thinner vanes for that mount, they look excessively thick to me. My 16" has 4 vanes less than 1mm thick to support a 2.6" secondary.

[ving]

ditto, tony... thanks for the clarificatio vespine :)

[vespine]

Yeah I guess you'll get 4, but I reckon six is pretty, isn't it? :lol:

or just modify those 3 legs into a T shape, that will give you 4 spikes too, not sure if you can get it as stable as one with 4 legs.

[Astroman]

or you could curve them and have no spikes :)

[netwolf]

How easy/dificult would it be to convert a GSO dob to a curved spider. Is there a drop in solution that works without hacking or drilling the tube.

Regards

[rmcpb]

Wolfie,

Have a look at http://www.1800destiny.com/ for curved spider ideas.

Cheers

[ving]

hmm... interesting

[rmcpb]

Gunna have a go Dave??????

[ving]

I think it would only be useful for imaging(?)

in truth, there are too many things i want and that one, while good would be down the list somewhat :)

[rmcpb]

Could make one cheaply using the GS gizzards of your secondary. Think about it, we'll get you into the ATM fold yet :)

[ving]

i wonder ho much they need to curve? :confuse3:

[rmcpb]

Not much I think. Have a look a the "How much vane is in the light path" page in the above site. It appears that the best way is to make the vane with an even curve through 180^o ie. half circle to fit into your tube. With a 50mm secondary and a 230mm ID on your tube that would be about a 90mm diameter.

[Satchmo]

Ving

The spider design that produces no diffraction spikes and minimum diffracted light haze around a bright star is a 3 vane spider with each vane bent so that it is essentially a 1/6 segement ( 60 degree angle ) of larger circle from the ponit touching the edge of the secondary hub to th edge of the mirror. This design has been verified in a number of independent studies and always comes up a winner.

Mark

[ving]

my head hurts! :(