TS 203/800 ONTC assembly

After 4 years of astrophotography, I finally bought my dream setup for many more years to come! A lot of thought and planning went into this: I wanted a fast astrograph with longer focal length, excellent color correction and a flat field.

Newtonian telescopes usually offer the best price to performance ratio for a given aperture, since you only need to figure two optical surfaces (the primary and secondary mirror). Much less than the 6 surfaces in a typical triplet apochromatic refractor. However, good optics are only part of the story. Excellent mechanical properties of the tube, focuser and mirror supports are essential to obtain the best optical performance. That’s why I decided to order a custom 8″ f/4 ONTC system from TS-Optics in Germany. 

It also helps if you’re a bit mechanically inclined, because almost every element of a Newtonian can be modified, replaced or upgraded if you’re not satisfied with the performance.

My order was placed on 11 January 2020 and I quickly received e-mails from the owner of Teleskop-Express to finalize the details of the build. There was some delay from the carbon fiber tube supplier, but the telescope was eventually delivered on March 4th.

The tube came in an oversized box and was packed very well. This shipping box also contained smaller boxes with the primary mirror and accessories, packed next to the tube.

Two white tube rings are supplied by default, it’s a 235 mm set from Orion. These are made from cast aluminium and each end is tapped with 1/4″ – 20 UNC thread. The provided washers were too small for the holes in my TS L-UP13 Losmandy plate (330 mm length), so I added larger M6 washers to secure the rings to the plate.

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My telescope came with the secondary mirror already installed. It was protected by a layer of cotton wool. The collimation push screws are secured with a rubber band.

I purchased the standard ONTC stainless steel spider and secondary mirror holder with pre-designed offset. The edges of the secondary mirror were blackened to reduce glare and the diameter (74 mm) is optimized for illumination of APS-C sized camera sensors.

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The tube is made from thick (5 mm) layered carbon fiber. This offers very high stiffness (low tube flex), low thermal expansion (low focus drift) and low weight (good for my EQ6-R).

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TS paints the inside of the tube with Berger anti-reflex paint and also applied a black flocking material (velours) on the tube wall opposite the focuser to improve contrast.

Average optical quality of commercial Chinese  mirrors (e.g. Synta or GSO) is quite good these days, but there’s no guarantee you’ll receive a good one. Selected (Taiwanese?) optics are used by TS for every ONTC they produce.

The primary mirror is 203 mm f/4 (800 mm focal length) and comes secured in a German-made mirror cell. TS uses tape to mark the orientation of the cell in relation to the tube. I did not pay extra for an optical test report for the mirror.

Primary mirror focus position(s) are fully customizable. Mine has holes drilled for 115 mm (visual), 145 mm (standard coma correctors) and 170 mm (ASA reducer or GPU corrector) above the tube wall.

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I purchased a Moonlite CRL 2.5 inch large format focuser with 1″ drawtube travel. This high-end Crayford focuser has many features including collimation screws for both the base and C-axis (drawtube), which should allow perfect collimation of the entire optical system.

Collimation is checked at TS using an artificial star, before the telescope is disassembled and shipped to the customer. Transport of the telescope seemed to have a remarkably low effect on collimation of my system, only a small adjustment of the primary was needed after installation of all components.

I read a lot about proper collimation and bought a TS 2″ barlowed laser collimator instead of the more common (non-barlow) laser collimators. However, I still need to do a bit more research about the tools needed for secondary mirror collimation.

A coma corrector is needed for astrophotography with fast Newtonian telescopes. The GPU 4-element aplanatic corrector is designed by Hungarian optician Gyulai Pal for modern high-resolution sensors up to APS-C size and uses FPL-51 low dispersion glass. Click here (use google translate) and here or here to see comparisons of popular coma correctors. This corrector does not affect focal length of the system and is more tolerant to tilt. Focuser and focus distance were optimized for this popular 2″ coma corrector.

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GPU corrector installed with 2″ parfocalizing ring and slip-security straps 🙂

Since the corrector body is quite long (100 mm), it can protrude into the tube of the telescope. The optics are multicoated but this is not enough to prevent all unwanted artifacts/reflections. Care must be taken to hide the front element from stray light.

I saddled everything on my EQ6-R during the day. Handeling “the beast” for the first time was thrilling! It’s actually not that heavy at all, given its size.

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Weather forecast does not show any clear skies for the coming nights, so I used a distant tree to test the optics and focus point.

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ONTC first light

Next were some quick flat frames. I covered the opening of the telescope with a white t-shirt and pointed the tube to the overcast sky. My luminance filter shows symmetric illumination of the sensor with 18% vignetting of the corners.

Next steps will be installation of a top rail with guide scope and a proper test under the stars.

Overall really happy and great service from TS.

 

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