PETER SHAH reviews Orion Optics UK’s 8” Carbon Fibre Astrograph
Mmmmm . . !
Above: The fully loaded 3” focuser and Right: ready to image!
IN RECENT YEARS astro imaging has become easier and more accessible to the amateur with the introduction of the CCD. As the technology has progressed the CCD has become more sensitive and larger, making your telescope optics more important when capturing that quality data. The basic design of the telescope hasn’t really changed that much. Orion Optics UK have been making Newtonian telescopes for years and have now started developing a series of carbon fibre astrographs. Their astrograph is a Newtonian telescope with a twist, it gives a coma free image across a 50mm field, perfect for those large full format sensors. How do they achieve this? Orion Optics prides themselves on making some of the best telescope mirror sets in the world for the amateur market. Now they have mastered the art of producing fine optics they have taken up the task of producing an almost perfect telescope and making it affordable for the amateur. To make the perfect scope there are a number of problems that you need to over come. The first is to give coma free stars. Unfortunately these scopes usually use extremely hard to make 'hyperbolic' mirrors. The AG
scope use a high quality paraboloid and corrects for the coma inherent in this type of mirror, cutting down the delivery time and making it more affordable. This is done by introducing a lens system that will compensate for this problem. The coma corrector is based on the 3" Wynne design but it uses four lenses instead of three, giving even better optical performance. With this set up the spot model showed a 300mm f/4 paraboloid reduced to f/3.8 would give 2.3µ in the centre and 4.2µ at the edge of the field. The weather here in the UK is a real challenge to astronomers. You may only get a couple of hours on the odd night, and many astro imagers spend many nights scraping together enough data to get a half decent image, I’ve done this many times. This is where the fast f/3.8 optics come into play, you can grab loads more of those precious photons than something with a higher f/ratio. The primary mirror in my AG8 has a strehl of .991 and an RMS of .015,
Bubble Nebula anyone who knows their optics will agree this is no ordinary mirror! The carbon tube isn’t there just to look pretty it has a major roll to play in the design; most people think that it’s used to keep down the weight. This is only partly true, as its real value is its stiffness and thermal qualities. Many conventional imaging set ups suffer from oval stars, this is often caused by poor guiding and polar alignment. One of the most common causes, which are often overlooked, is flexure in the scope. The carbon fibre tube is extremely rigid and has virtually no flex and is also temperature stable. As it goes through daily temperature changes there is very little effect on the scope, this is important when keeping the focus and collimation spot on. All the parts are CNC machined aluminium anodised in black. The three-inch focuser is a monster; it didn’t even work up a sweat when I put on my camera, filter wheel and extension tubes. All together it weighs around 3Kg. First impressions were a sports car of a telescope, very impressive, it oozed
quality, and it didn’t stop there. Earlier that day I had spent about two hours getting it collimated, which isn’t easy with this sort of scope everything needs to be spot on. That night it was clear, which was a surprise because anyone who has
had a new telescope knows its bound to rain or have cloud cover. I opened up the observatory and sent the scope off to Vega to see how it would perform a star test. I could see the out of focus doughnut of Vega dead centre of the frame, it
Detail of the rear cell assembly and the three coolig fans
First impressions were a sports car of a telescope, very impressive, it oozed quality, and it didn’t stop there.
and got it close again. The weather over the summer has been particularly bad this year so I was worried the clear spell wouldn't hold out for long and I was using up precious sky time, I just wanted to get an image done! After focusing and a star alignment I keyed in M31 on my G11, the motors whirred into life. It parked it smack in the centre of the chip. I got it guiding and set off a series of 5 minute exposures, 20 minutes worth in each, red green and blue filters. I could see the sky getting hazy and just managed to grab the last frame before losing the guide star completely. There were several guide errors
Peter Shah is a keen astronomer and an acomplished astrophotographer. He is resident astrophotographer with one of the UK’s leading distributors of digital imaging systems and associated products.
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throughout the session due to the guide star fading in and out behind cloud, so I got one flat and one dark frame and shut up shop. After a very quick and dirty process the result was stunning for such a short exposure, especially with the H16. The QE on the Kodak KAI-4021M sensor is just about passable, but to be fair I have been spoilt, my camera is a Finger Lakes CM8 which boasts 83% QE and I am grateful to my good friend Roger Banks for the loan of his H16. The next clear night came about two weeks later and I set about doing IC1805 the Heat Nebula but it didn’t clear the trees before 10 o'clock and I didn’t want to waist any time and got my teeth stuck into NGC281 while I waited. These were again only 20mins in green and blue and 30 minutes in H-alpha for this shot. After getting the data for The Heart it was time to go to bed, I had work in the morning, but then out of the corner of my eye I saw those ladies of the blue mist, The Pleiades, well it wouldn’t have been polite to ignore them. I think it was about 3am before I hit the sack and I was good for nothing the next day, but what a fantastic night imaging! I’m extremely pleased with these first results and can’t wait for the next clear one, things can only get better as I get used to the scope.
The 10:1 focuser seemed very smooth and the star clicked into focus like a dream
showed the scope to be slightly out of collimation, but I carried on and got it into focus. The 10:1 focuser seemed very smooth and the star clicked into focus like a dream. The diffraction spikes were like a knife-edge and the stars were pin. When I took a closer look the stars down in the lower right of the frame was slightly elongated, this was down to the collimation. When I adjusted it I just made it worse, I have to say my collimation skills are pretty poor, having a scope that stays set up in an observatory means I don’t need to collimate very often. I took the camera off and I got the collimation tools back on
Peter Shah gives "first light" to Orion Optics UK's 8" Carbon Fibre Astrograph