Amateur Astronomy and Imaging.

Reviews > Telescopes > Orion Optics CT8


The Orion Optics CT8 OTA

After 7 years of faithful service I decided it was time to replace the Intes Micro Alter M603 with something more suitable for deep sky imaging. When I bought the Maksutov Cassegrain my main interest was observing the planets visually. Since then I have become more interested in deep sky objects, almost exclusively imaging rather than visual observations. The long focal length (1500mm) and F/10 focal ratio meant I had to use a focal reducer with the Maksutov Cassegrain and even then the usable field of view was very limited. I spent almost 2 years reviewing and comparing my options. Initially my heart was set on a fast apochromatic refractor, and those designed by Stellarvue (SV105), Takahashi (TSA-102), Televue (NP101is) and Vixen (ED103) were high on the list.

The models I was most interested in from Takahashi and Televue were prohibitively expensive, and I had my doubts about importing a Stellarvue scope from the US in case there were any problems and I needed to return it. I would have been able to get hold of the Vixen quite easily, but I wasn't convinced it would have met my objectives.

Rear of the CT8

My attention eventually turned to the realm of the fast reflectors. While the price tags were far more appealing, my primary concern was keeping the weight within the tolerances of the Losmandy GM8. If I were to opt for a reflector I had decided to choose an 8" model between f/4 and f/5 with a focal length of between 800mm and 900mm. These days it almost seems that reflectors are not quite as fashionable as refractors and this was reflected (no pun intended!) in the available options. Although there were plenty of 8" reflectors to choose from, many were much heavier than I could accommodate on my mount. I did consider a Vixen R200SS but the thick spider vanes tend to cause square shaped stars. As a company I had heard many good things about Orion Optics UK customer service and they have a reputation for their high quality mirrors. My attention was drawn to their 8" carbon fibre CT8 reflector. At F/4.5 with a focal length of 900mm it met my criteria and the carbon fibre OTA kept the weight within my limits. After contacting several other owners I was reassured by their opinions of the scope, though I was advised to consider replacing the stock focuser.

Moonlite focuser

Eventually I placed an order for a CT8 along with a Moonlite focuser and the Catseye collimation tools. I sent the Catseye hotspot to Orion Optics for them to spot the primary mirror as part of the build process. They were more than happy to oblige. The scope arrived 2 weeks later, well packaged with no evidence of any mistreatment during delivery. I carefully removed all the protective polystyrene and layers of bubble wrap until I was acquainted with the carbon fibre OTA for the first time. I was equally impressed with the quality of the OTA and the Moonlite focuser. I decided not to fit the finderscope since I had no use for it.

As I've never owned a reflector before I was somewhat hesitant about collimating the scope. I decided to give this a trial run in the comfort of the house before I progressed to doing it outside in the observatory. With the scope on the floor and a mirror carefully positioned to reflect light into the OTA I began the process of collimating the scope with the Catseye collimation tools. Positioning the secondary mirror beneath the focuser was relatively straight forward, but I found aligning the secondary mirror an extremely difficult task. After spending 4 or more hours tweaking the collimation I realised it was every bit as difficult as I expected. I wanted to familiarise myself with the process, so the following few evenings I repeated the process. I was far from confident I had properly aligned the scope, but felt I had it as close as I could get to being properly aligned.

Inside the CT8 with Catseye reflection spot visible.

The following weekend I installed the scope in the observatory and rechecked the collimation. As expected the short trip from the house to the observatory was enough to knock it out of collimation, so I dialled it in using just the primary mirror alignment knobs. Fortunately I only had to wait a few days before my first opportunity to carry out some tests arrived. I imaged the moon using my Canon 350D at prime focus using the Baader MPCC focal reducer to get a feel for the field of view. The moon easily fitted within the field of view, and ascertained the field of view to be about 1.5 degree by 1 degree. This was about what I had expected, and I was also happy to discover that the calculations I had carried out to determine the focus travel I would need had paid off. I also took a test image of M45 to determine whether there was a significant degree of coma at the edge of the field. This is most easily determined by using a bright star field as it keeps exposures short and increases the probability that there will be stars across the whole field. I was very happy to discover a very minimal amount of coma at the edge of the field. In fact, it was apparent that the scope was not perfectly collimated since there was no coma at all on one corner of the frame and a small degree of coma at the opposite corner. The initial results were very pleasing indeed.

First test shot of the moon to determine FOV.

It was several more days before I was able to carry out any proper long exposure images, but my first targets were M81 and M45. Even watching the subs download to the laptop I noticed the improvements this new scope was delivering. For the first time I was able to make use of the full field offered by the Canon 350D. I also discovered that exposures of 20 minutes (my standard exposure length with the Maksutov Cassegrain) offered no improvement over 10 minute exposures. This was another satisfying discovery since shorter exposures mean I can obtain satisfactory images in less time.

Unfortunately I did discover a slight problem with the camera. When I processed my first images I discovered dust motes and what looked like 3 dark parallel lines. Closer inspection of the camera sensor revealed these to be slight scratches on the filter. I can only assume I had caused these inadvertently when I modified the camera. They hadn't been noticeable until now due to the fact that I had such a limited field of view using the Maksutov Cassegrain. Fortunately flat fields effectively remove these marks from my images.

Test shot of M45 to assess coma.

One of my first surprises came the morning after an imaging session. The primary mirror had dewed up due to the high humidity, but I also noticed what appeared to be brown stains near the mirror clips. I demisted the mirror using a hair drier on its lowest setting and after the brown stain completely disappeared. It seems it may have been an optical illusion. When I mentioned this to a friend with considerable experience using reflectors he informed me he has often noticed this before too. I've also discovered that collimating the scope is far from easy even after acquainting myself with the tools and the procedure. Much of the difficulty arises from the fact that I need to use an extension tube to reach focus if I use the Catseye autocollimator (else I can't see the fourth reflection). The tools are so sensitive that even the very small amount of play between the extension tube and the focuser changes the results considerably. Ideally I would prefer a method of collimating the scope with the camera attached since this would surely offer the most accurate results. So far I have not found an adequate way of doing this.

I've now owned the CT8 for about 5 months and so far I'm very happy with it and I don't regret veering away from the 4" apochromatic market at all. I've not had any need to contact Orion Optics customer services since I received the scope, but both John and Barry were very accommodating and helpful in answering my many questions prior to making the purchase. I would highly recommend both Orion Optics UK and the CT8.