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Old 04-02-2019, 10:01 PM
glend (Glen)
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Join Date: Jun 2013
Location: Lake Macquarie, NSW
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Double Star Shoot Out - TS vs iStar

Over two nights (of near identical sky condtions) I conducted a Double Star performance comparison of two telescopes: A TS 115mm f7 Triplet APO, and my iStar 127mm R30 Doublet f12. To make the comparison as equal as possible the TS115 was used with a 2x Televue Barlow to lengthen the focal length from the native 805mm to 1610mm. The iStar native focal length is 1476mm. Eyepiece used was the Superview 15mm, and the same 2" Bintel Dialectric Diagonal was used. . So magification on the barlowed TS was 107,3X and on the iStar it was 98,4. The iStar enjoyed a 12mm apeture advantage.

I was looking at a couple of performance factors: the ability to split well known Double Stars (or Triples) currently in the Eastern sky, the colourisation rendered by each scope, and fringing impacts on splitting.
The target Double stars were:
Beta Monocerotis (actually a triple), Upsilon Carinae (aka Vathorz Prior), and Suhail al Muhlif.
The TS115 is my regular imaging scope, but I rarely use it visually, it did not disappoint. It delivered pin point stars with clear "pop to" focus. No fringing from any target. Colour rendition was good with clear transition boundary to black on the limb of the stars.
The iStar 127, which has the R30 Anastigmatic Doublet (which provides improved colour rendition over standard Achromat Doublet lenses), making the f12 perform as if it is an f15 on the CA charts. No fringe filter was used. While providing very similiar star size and brightness, the iStar suffered from less precise focus point pop (it has a Moonlight Focuser btw). The target limb boundaries to black were also less impressive, with some fringe interference in the gaps. Star colours were not as true as the APO, although not too bad.
In summary, the TS 115 (even barlowed) clearly delivered better visual performance on these targets. and had no trouble providing clear separation and distinct limbs. It's ability to pop to focus, compared to having to hunt for it on the iStar makes it the winner in this test imho.

However, there is a big difference in price point between these two scopes, but some of that is due to the iStar being a DIY build around the iStar R30 objective. For what it is, the iStar would be OK for Double star observation but you need to accept it's limitations and it is less precise in that role. If a fringe filter was used with the iStar it would probably improve its performance in this role, but might still be hampered by its loose Spot diagram (in comparison with the TS Triplet).
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