Question about star spikes with Vixen ED81s

[konstantinos75]

Hi
In the last years the telescope I use the most for astrophotography is Vixen ED81s with the focal reducer f/5.2.
The camera to capture images is sbig st2000xm

The problem with Vixen ED81s is that it makes star spikes on the brightest stars. These spikes are typical of this telescope and are caused by the three metallic holders underneath the main mirror (they are pointed by the red arrows on the image).

[konstantinos75]

I would like to ask if someone has solved this problem e.g. by using an aperture mask...

[steppenwolf]

An aperture mask would resolve this issue but it doesn't have to be as restrictive as the one shown in your second post.

[ollypenrice]

Exactly. You can make a minimally restrictive one using tools from a graphics shop;

Olly

[konstantinos75]

Hi

I made a aperture mask and put it on the Vixen ED81s.

I took some test shots of Venus and a dim star.

As you can see the mask creates more spikes than expected which I don't like especially on bright objects like Venus.

I am trying to explain why I get these spikes when the mask is on. One explanation could be that the inner part of mask is not cut as a perfect circle. Other explanation could be that the mask must be increased in size so that more light entering the telescope is obstructed.

But this will definetly lead to more vignetting which is another problem.

Any ideas how to eliminate the star spikes would be welcomed.

Thanks

[lensman57]

Hi

I made a aperture mask and put it on the Vixen ED81s.

I took some test shots of Venus and a dim star.

As you can see the mask creates more spikes than expected which I don't like especially on bright objects like Venus.

I am trying to explain why I get these spikes when the mask is on. One explanation could be that the inner part of mask is not cut as a perfect circle. Other explanation could be that the mask must be increased in size so that more light entering the telescope is obstructed.

But this will definetly lead to more vignetting which is another problem.

Any ideas how to eliminate the star spikes would be welcomed.

Thanks

The mask can help but it needs to be perfectly round, yours is not it has many sharp edges. It also needs to be placed as close as possible to the front element and be true to the optical axis. I use a proper metal step down ring on my Canon 200 L to reduce the CA and it works without causing unnecessary spikes. It looks like the one in the link. If you could find one suitable for your scope it is worth having a go with it. I also have to either find one for my WO Star 71 or make one as mine spikes like your ED. The other point is depending on the size of the mask aperture the effective F ratio will alter as the scope lets less light in my 200L works @ F3.5  . http://www.ebay.co.uk/itm/like/261450244174?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=108&chn=ps&device=c&rlsatarget=&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0-L&ff12=67&ff13=80&ff14=108&ff19=0

A.G

[steppenwolf]

I agree with Lensman - the aperture mask MUST be perfectly circular otherwise you will simply be emulating the leaves of the camera lens iris.