Confused by star shapes through WO Prism

[Chris]

Ok, not quite an eyepiece I know, but it does involve first light with the new on the scene Vixen HR 3.4mm!

Setup the Tak fs60, WO prism diagonal, and Vixen HR 3.4 last night in order to observe Jupiter before it disappeared behind the neighbours garage. Try as a might couldn't really get a good focus, so assumed the seeing was really bad so decided to check this with a bright star by doing a star test.

Now here comes the weird bit, I couldn't achieve anything resembling a round refractor like star, I can only describe it as a short vertical white/yellow line with a red dot either side, kind of like a colourful division sign turned by 90 degrees!? 

Curious, I popped the star either side of focus to check the Fresnel ring pattern to discover not one but two central dots! huh? and a grey line left of centre going vertically through the rings? This line switched sides the other side of focus. 

 Suspecting the diagonal I grabbed the cheap mirror diagonal from my CPC800 and switched them. This solved the problem immediately thankfully!

I've emailed FLO where I purchased the WO prism from and great as always they have agreed to take it back so I can exchange it for another diagonal. I'm thinking either the Tak prism which I've had before and have been pleased with, or the WO Dura Bright mirror diagonal?

Before I decide I'm just wondering if prism diagonals struggle with fast scopes? and if that was at least partly responsible for the aberrations I saw. The star test was at 105x in a f/5.9 scope. If this is the case It will be safer to go mirror the mirror route. Also has anyone here seen stars how I described through a scope and prism diagonal before? because I haven't!  

Not quite the first light with Vixen HR I was expecting, but sometimes surprising things happen.  

 

  

[Ben the Ignorant]

Your William prism was obviously badly defective. But to be sure whether your scope needs a prism or a mirror, email the maker. Only they know if their objective was designed to be complemented by a prism or not, and if the size has to be 1.25" or 2", since the optical path is not the same. Glass type matters, too.

[johninderby]

I had a WO prism diagonal and promptly got rid of it. Very disappointing performance. Replaced it with an Omegon 1,25'' Amici Prism 90° which has been excellent. Star shapes are normal in my f4.5 frac. Bought it originally for daytime use but has been an excellent performer for regular nighttime observing.

https://www.omegon.eu/amici-prisms-90d/omegon-1-25-amici-prism-90d/p,14172

[Chris]

13 minutes ago, johninderby said:

I had a WO prism diagonal and promptly got rid of it. Very disappointing performance. Replaced it with an Omegon 1,25'' Amici Prism 90° which has been excellent. Star shapes are normal in my f4.5 frac. Bought it originally for daytime use but has been an excellent performer for regular nighttime observing.

https://www.omegon.eu/amici-prisms-90d/omegon-1-25-amici-prism-90d/p,14172

I did think mine was the only negative review of this prism so it's interesting to hear this. It's also good to hear that some prisms work well with fast scopes, thanks.