A Work Commenced June 21 2025

Back in 2015,  I parted company with refractors as my main astronomical viewing instruments having ‘seen the light’ when I acquired a brilliantly simple 200mm F/5.9 Dobsonian from Skywatcher. Once properly collimated and acclimated under good seeing conditions, it significantly out-performed a top-rated 5 inch F/12 refractor on pretty much everything.  The experience radically changed my perception of the true value of refractors, particularly when scaled up beyond 5 inches. 

A year ago, I was delighted to see an astro imager had performed a shootout between a Skywatcher Explorer P 200mm F/5 Newtonian and a high-end 5 inch F/7.8 ED refractor on a high resolution lunar target, showing clearly the former’s superiority despite a nearly five-fold price differential. 

Over the last decade, I’ve made a series of modifications to this 200mm Newtonian, called Octavius, owing to its 8 inches of aperture. The primary mirror from Skywatcher was good but I decided to replace it with a true 8-inch (203mm) F/6 Ultra Grade mirror with an accuracy exceeding 1/10th wave PV from Orion Optics UK, which also came with state of the art reflective coatings – HiLux enhanced aluminium – increasing the mirror’s reflectivity from 85 per cent to over 97 percent across the visible spectrum. 

I then upgraded the secondary mirror by ordering up a smaller(1.52 inches) and more optically flat Antares unit with a 1/15th wave PV surface accuracy, whilst also reducing the central obstruction to less than 20 per cent by aperture.

Like the primary, the small secondary mirror came with enhanced aluminium coatings as standard, to maximise image brightness. Because it’s such a small and lightweight mirror, I could attach it to the stalk using double-sided tape.

My own personal experience with enhanced aluminium coatings is that they have much greater longevity compared with standard coatings. Indeed I have older mirrors treated with HiLux that are 10 years old and show no signs of deterioration despite regular use in my damp Scottish climate. 
The somewhat flimsy secondary housing on the original SkyWatcher 200P was removed and replaced by a much more rigid and solidly-built structure manufactured by the German company, BackYard Universe.

The secondary holder has a diameter of 40mm – slightly larger, in fact, than the 38.6mm elliptical flat mirror – giving a minimum central obstruction of 19.7 per cent. 
Of course, If I so desired, I could have installed a low-profile focuser and decrease the secondary obstruction to just 10 per cent using a even smaller secondary, not to mention the need to redesign the spider structure. But in the end I decided against it, as I wanted to create a more all-purpose visual instrument rather than a specialised lunar and planet killer. And in any case, optical theory informs me that once the central obstruction falls below 20 per cent or so, the effects are almost indiscernible from a non obstructed optic of the same aperture.

I did however replace the original single-speed rack and pinion focuser on the SkyWatcher Explorer P after ten years of heavy use, with SkyWatcher’s dual-speed Crayford  focuser from SkyWatcher, which allows me to achieve smooth and precise micro-focusing on high resolution targets.  

I also attached an inexpensive USB type cooling fan to the rear of the primary mirror which helps remove the boundary layer of warm air accumulating above the surface of the mirror, to help keep the images as crisp and detailed as possible. 

The interior of the tube is lined with cork and overlaid by matt black flocking material to further enhance its thermal properties, as well as maximising image contrast.

The telescope sits in the original lazy susan altazimuth mount with the stalks under its base fitting into some pre-cut holes  I made in an inexpensive plastic water butt. This raises the height of the telescope further above the ground providing a supremely comfortable standing height – I’m a six footer – for all my astronomical observations. And that brings me to the second reason for abandoning refractors- I hate sitting down, huddling over an eyepiece, straining my back while trying to study a target located high in the sky. Those kinds of observations are much more comfortably executed while standing up. Indeed, I even use my travel scope- a high quality 130mm F/5 Newtonian placed on an altazimuth mount- in exactly the same way i.e. standing up. Once I got used to standing with my Newtonians, there was no going back to refractors. 

Just as any fine musical instrument benefits from occasional tuning up, so too does Octavius. At F/6 collimation is very easy, so easy in fact that it only requires a simple collimation cap to get you 90 per cent of the way there, and quite often better. The final tweaking is done manually by performing a star test at high power and making those final adjustments using the collimation screws on the primary mirror. It’s quick and easy, taking only a couple of minutes to get perfect alignment of the optics.

The more rigid secondary structure has greatly helped to maintain precise collimation from night to night, so much so that the instrument only needs occasional tweaking when moved from its indoor storage space to the outside air. That’s how it should be!

So how good are the optics? In a word- excellent! But to elaborate: star testing at 200x under good conditions shows identical diffraction patterns inside and outside focus. Stars focus down to tiny, round Airy disks. By far the biggest benefit has come from the primary mirror. On the night the new primary arrived back in January, I was immediately able to discern the improvement on Jupiter. The new mirror snapped to focus better, showed more contrast and routinely revealed subtle details on the Giant planet that I had only vaguely discerned using the original mirror. Magnifying the first quarter Moon up to well beyond 400x showed no image breakdown whatsoever. The image below is a single shot image of the first quarter Moon taken with the Svbony SC001 2 Megapixel imaging camera, on the evening of May 3 2025, showing some very fine details.

To be Continued……

De Fideli.