Tak Epsilon, Square Stars and Spider Vanes

[MarkS]

There's been a lot of discussion about the square stars (or even rectangular when the guiding is bad) in my Tak Epsilon images.  As Chris mentioned, this is a result of the thick spider vanes that hold the secondary.  To achieve F2.8 the secondary is large and heavy and needs thick vanes to support it.  Square stars are also often mentioned in connection with the Vixen VC200L - again due to the thick vanes.

To be honest, I didn't notice it in any Tak Epsilon images I looked at before buying the scope but now I know they're there, I can see them clearly in the same images!  And now it bugs me a bit ...

I'm still not clear on the exact mechanism that causes this "square" effect but it must be related to the bright diffraction spikes.  It's only obvious on stars of a certain size.  My suspicion is that it is to do with seeing - the same effect that makes brighter stars appear bigger when imaging terrrestrially.  If the scope were used in orbit (like Hubble) I bet this effect would disappear.

If anyone knows more about the precise cause, I'd love to know.

Mark

[mickw]

You might get some info here.............

http://www.takahashi-europe.com/en/index.php

or

http://www.takahashiamerica.com/
The Americans are usually pretty generous with their advice.

Must admit, I still can't get my head round how an obstruction in the light path produces a bright artefact 

[RobertM]

Do a google search for 'VC200L square stars'.  There are quite a few articles that discuss it in depth as well as solutions like grinding the spider vanes down 

I'm sure accurate focus is critical to minimising star shapes and therefore the spider vane effects.  I must admit I don't find them objectionable.

Robert

[Tony G]

Mark,

If the square star phenomenon is bugging you, then I think the only thing to do is get rid of it, but not for nothing, as i have a pair of Meade bino's which I got for Aldi a few years back, but they are nearly as good as new, excepted for a few scratches on the lens and the support strap missing. If you are interested in a straight swap then PM me. 

Tony G

[MarkS]

Take a look at the hi-res version of the Cat's Paw here (click on the image on the web page):
http://apod.nasa.gov/apod/ap100421.html

It has square stars!

So if the European Southern Observatory is also generating square stars, I'm in good company.   Sorry Tony,  it means I won't be swapping the Tak for a pair of battered binoculars!

Mark

[Tony G]

I suppose I could ask the European Southern Observatory if they are interested in swapping.

Tony G

[RobertM]

That's quite a noisy Cats Paw image.  Tony maybe you could swop your webcam for their camera