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WO Zenithstar 81

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#1 AstroME

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Posted 29 November 2021 - 10:59 AM

Hi, I was thinking of getting the Zenithstar 81, and just wanted to know if its a good scope for deep sky imaging (Nebulas, galaxies, etc) and if the only difference between the 81 and the 73 or the 61 is focal length and aperture?

 

I'd also appreciate some clarification about the filter sizes (1.25" vs 31mm vs 36mm). How does it impact the image quality and does it make a big difference?

 

Thanks.

 

 



#2 OldManSky

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Posted 29 November 2021 - 12:15 PM

The WO doublets (of which the ZS81 is one) are pretty well-regarded.  A bit more false color than a triplet, but they're not bad.  I had the ZS61 for about a year and was quite happy with it.

Yes, the only difference is aperture/focal length (and size/weight).  They're all FPL-53 doublets.  They require different flatteners/reducers.

 

What size filter you use will depend on the size of your imaging sensor.  For sensors smaller than APS-C, you can usually use 1.25" filters (but it depends on where you put them in the imaging train!).  For APS-C and above, 31mm or 36mm or 48mm filters will be required -- again, it depends on the size of your imaging sensor and where you put the filters in the optical train.

 

One nice thing about the WO scopes -- very good build quality.  Smooth focusers, powder-coated tubes, nice anodizing.  I have a GT71 and love it.


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#3 AstroME

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Posted 29 November 2021 - 02:09 PM

Wonderful. Thank you for the thoughtful reply OldManSky! Can you please elaborate a bit on what you mean by false colours?

#4 Oort Cloud

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Posted 29 November 2021 - 03:21 PM

I have a Zenithstar 73, with the flat73-r flattener/reducer. With the reducer, there is a noticable blue halo around all white/blue stars. It is not bad, but not perfect either. Using a reducer is probably also part of the problem; with a 1:1 flattener, the blue halos could be negligible, but I cannot confirm since I don't have one test with.

If you can afford a triplet, get one, but if not, the doublets are nice - just don't expect perfect color because you won't get it.
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#5 OldManSky

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Posted 29 November 2021 - 09:35 PM

Wonderful. Thank you for the thoughtful reply OldManSky! Can you please elaborate a bit on what you mean by false colours?

Oort Cloud pretty well covered it above.  You get a bit of blue fringing on bright/white/blue stars.  It's not much, but it is there.  And it's more than you'd get with a well-corrected triplet, which is why well-corrected triplets cost more :)

 

Nothing at all "wrong" with that, it's to be expected from doublets, even good ones.  And the WO ones are good ones.



#6 Oort Cloud

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Posted 29 November 2021 - 09:54 PM

Yes, I should add that optically, I am very happy with mine. The amount of correction is almost unbelievable for a doublet. The focuser seems to be not as great, but with it being my first r&p, I'm not really sure.
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