nPAE Theia90 Star Diagonal
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| nPAE Precision Astro Engineering (https://www.npae.net) is a relative newcomer to the astronomy equipment scene. Their take on common accessories, such as the ubiquitous star diagonal, is innovative and fresh. I received their Theia90 Star Diagonal directly from the company, specifically for this review. |
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Included
in the box:
- Theia90 star diagonal body with 2-inch eyepiece holder and 2-inch front nosepiece
- Three filter sleds
- Zygo report of mirror test
- Instructions (download here)
- Cerberus case
- Hex wrench
Construction and Optical Performance
The Theia90 Star Diagonal sports a precision polished and dielectric-coated
1/10th wave mirror. Dielectric coatings are very durable, allowing for
stress-free cleaning when necessary, and provide nearly 100% reflectivity.
The 1/10th wavefront error is assessed after coating, though their website
currently only mentions before coating. nPAE is so confident in the
optical quality of their mirrors that they provide a Zygo wavefront report of
the actual mirror installed in the diagonal.
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The diagonal body is a single solid piece of aluminum, precision cut and expertly coated in subdued black. The entire diagonal weighs in at 540g (19 ounces). The 2-inch eyepiece holder is solidly connected to the body. Large thumbscrews which drive a brass compression ring to hold your eyepieces firmly without introducing marks on the barrel. As with most eyepiece holders with a compression ring, you will usually find it necessary to significantly loosen both thumbscrews to remove eyepieces with a "safety" groove. The 2-inch nose can be unscrewed so you can attached the body directly to any M48 x 0.75 male threaded focuser or exit port. There is currently no Schmidt Cassegrain telescope (SCT) version of the diagonal (replacing the nosepiece with an SCT threaded nose), so a 2-inch visual back will be required for SCTs. |
As
you can see from the photos, the Theia90 body has a relatively long front
section to allow easier access to the the filter sled (discussed below), but the
overall light path isn't much longer than other typical 2-inch diagonals - 14cm
(5.5 in). By comparison, the Baader MaxBright reviewed elsewhere on this
website is approximately 13cm. Still, be sure your scope can accommodate
the 14cm of focal length the diagonal adds (mainly an issue for refractors, as
SCTs and Maks can almost always accommodate extra focal length). The
physical length from the front of the body to the back of the body is 11.4cm
(4.5 in), for fork-mounted SCTs, the length of the diagonal coupled with a 2-inch visual back could result in clearance issues when pointing towards the
zenith (alt-az mount) or polar region (wedge mount).
Views in the eyepiece are bright with no loss of contrast. There are no noticeable aberrations with images looking virtually identical to views straight through the scope, confirming what the Zygo report indicated about the optical quality of the mirror.
Filter Exchange System
The Theia90 star diagonal's main innovation is its filter exchange system.
The included filter sleds (three in the box and available separately on nPAE's
website) hold standard 2-inch filters with a thickness of 10mm or less (almost
any filter on the market). The sled is held in place via high power
permanent magnets in the body of the diagonal and the filter sled itself. The
magnetic force is
strong
enough to firmly hold the sled without worry of your expensive filters falling
to the ground while still being easy to remove with even gloved hands. As
mentioned earlier, the longer than normal front section of the diagonal body
provides additional clearance to make room for removing and inserting the sled.
For added flexibility, you can add two additional filters (thus three total in
the light path simultaneously) at the front of the nosepiece and between the
nosepiece and the body.
nPAE's filter exchange system encourages filter changes, eliminating the
usual complication of pulling out the eyepiece, unthreading the current filter,
and threading in the replacement filter. As such, this diagonal is the
natural choice for visual astronomy. It is also an excellent choice for
astrophotography in a setup where a diagonal is useful for clearance reasons.
If you intend to leave your expensive filters mounted in the sleds, you will
want to source your own plastic containers to keep your filters safe from
scratches and dust when you place them back into your gear case as none are
included. nPAE tells me they may start including such cases in the future.
The Zygo Report
The included Zygo report is a nice touch (click
here to download a PDF of the
report included with this diagonal). Each mirror is tested with a Zygo
interferometer. Interpreting a specific report is quite technical, but the
fact that nPAE includes a Zygo report with each mirror indicates their
confidence in the quality of their products and their ability to consistently
produce 1/10th wave mirrors. For those interested, I found the following
discussion to be useful:
https://www.orionoptics.co.uk/OPTICS/zygo.html
For general information about Zygo testing concepts and equipment, visit the
actual Zygo website:
https://www.zygo.com/products/metrology-systems/laser-interferometers
Collimation
From the factory, the mirror comes aligned at a precise 45 degree angle in
relationship to the center line of the front of the body, but the back of the
diagonal offers four screws for user collimation. This is an advanced
feature that allows you to tilt the mirror to compensate for misalignment of the
telescope's optics or focuser. A small hex wrench included for the
collimation screws. The included instructions provide a brief section on
the collimation process when using a laser collimator. While this feature
is a nice touch, most owners would be best served by not adjusting the
mirror, unless they have experience in collimating the focuser end of their
telescope.
Conclusion
The Theia90 Star Diagonal offers impeccable mechanical and optical quality in a
2-inch dielectric mirror format. The innovative filter exchange system
makes it an excellent choice for visual observers and for many modern imaging
scenarios. The price is quite reasonable for a high-quality diagonal which
is built to last a lifetime. Highly recommended.
You will find ordering information and additional reviews at nPAE's website - https://www.npae.net
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