A New Ritchey-Chrétien Telescope for Astrophotography
I’ve been given the unique opportunity to review a new Ritchey-Chrétien Telescope, the iOptron Photron RC6. This telescope has a longer focal length than any of my refractors, which I plan on putting to good use. The impressive 1370mm focal length means that this stocky red Ritchey-Chrétien is a perfect choice for astrophotography during galaxy season. I’m looking forward to some large, high-resolution images of galaxies such as M101, the Pinwheel Galaxy.
With this added “reach” also comes an increased demand for focus and tracking accuracy. Added challenges include a strong need for an effective plate-solving solution, as locating deep sky objects at this magnification can be very time-consuming. These issues are not nearly as apparent when dealing with wide-field refractors, and one of the countless reasons I recommend them to beginners.
Although the iOptron RC6 telescope presents a new learning curve, it also means that I’ll be photographing a deeper view of space than ever before.
The iOptron Photron RC6 Telescope
Until now, I’ve only photographed galaxies through wide-field imaging telescopes such as refractors and Newtonian reflectors. A Ritchey-Chrétien telescope is a much better fit for smaller deep-sky targets, including many of the galaxies up for grabs at this time of year. The increased magnification means that smaller galaxies will appear in higher resolution, and take up more real estate in the frame.
Deep Sky Astrophotography with the iOptron Photron RC6 (Video)
The Ritchey-Chrétien Telescope Design
The iOptron Photron RC6 is a Ritchey-Chrétien telescope, the first of its kind to ever grace my backyard. By design, an “RC” telescope uses hyperbolic primary and secondary mirrors. Compared to a traditional Newtonian reflector design, this RC6 is said to offer a coma and chromatic aberration free results. Quite an impressive feat for a modest 6-inch diameter telescope with a conveniently stocky design. Mounting the tube to my equatorial mount is quick and easy, as the stubby design of the RC6 is compact and easy to manage.
What is an RC Telescope? Discussion on Quora
The Ritchey-Chrétien design is known for its great imaging performance, which is why you’ll find this design used in large professional telescopes including the Hubble Space Telescope. The RCT sitting in my backyard is just a few meters smaller than the HST, but the little iOptron Photron has the heart of a champion.
The Photron includes a 2″ dual-speed Crayford focuser. This is a nice touch for a telescope that will need to execute precision adjustments when in use for deep sky astrophotography. The design of the fixed primary mirror is said to eliminate the image shift that other telescope designs can suffer from. I have not experienced this phenomenon myself (or I didn’t notice), but perhaps any readers that have used a Newtonian reflector can weigh-in on this subject in the comments.
iOptron Photron RC6 Specifications:
- Aperture: 150mm
- Focal Length: 1370mm
- Focal Ratio: F/9
- Focuser: 2″ Dual-Speed Crayford
- Dovetail Bar: Vixen-Style
- Tube Weight: 18 lbs
The iOptron Photron RC6 was built for astrophotography. Its steel tube includes internal knife-edge baffles that reduce the stray light entering the objective of the scope. In a long exposure photo, the smallest amount of stray light can become quite bright, and it reduces the overall contrast of your image. Light baffles are present in most of the astrophotography telescopes I have used, but its a nice feature nonetheless. I’ll take all the help I can get when it comes to reducing the negative effects of light pollution from home.
The Knife Edge Light Baffles on the Photron RC6
Similar variations of this particular Ritchey-Chrétien model exist, each with its own particular flair. Previously I have seen a lot of fantastic images taken using the Astro Tech RC6, or ATRC6. Browsing equipment photos of this version of the telescope being used for deep-sky astrophotography was helpful when trying to determine the correct back focus for my camera.
Compared to a Newtonian Reflector
I’ve spent a number of astrophotography imaging sessions using an Orion 8″ F/4 Newtonian reflector telescope. Unlike this RC, the reflector required constant collimation adjustments, and a coma corrector to produce a flat imaging field. A coma corrector is an essential added expense for anyone looking to take pictures with a “Newt”. In the case of my Orion 8″ astrograph, the Baader MPCC II (coma corrector) cost nearly as much as the OTA itself.
Compared to Refractor
A small apochromatic refractor telescope such as the William Optics Z61 is much easier to use than an RCT. The compact size and forgiving wide field of view make the entire deep sky astrophotography experience less labor intensive. The drawbacks of a small apochromatic refractor telescope are their high price tag, and short focal lengths. As convenient and versatile as they may be, a Ritchey-Chrétien telescope is a better choice for capturing small targets such as galaxies.
Setting up for a night of astrophotography with the RC6
First Impressions of the iOptron Photron RC6
I received an early demo model of the RC6 in late 2017, and am only just now getting chance to use it. The vibrant red finish of this telescope is hard to ignore. This was evident with the audience at NEAF 2018, where iOptron displayed several sizes of the Photron RC in matching red on the show floor. This is my first iOptron telescope, and I have high expectations based on the tremendous experiences I’ve had with their reliable astrophotography mounts.
Related Post: iOptron CEM60 First Impressions
The included dovetail bar comes in matching anodized red paint, which I promptly marked up after securing it into the base of my mount. Speaking of mounting the telescope, I had a bit of tough time properly balancing the load, particularly with a guide scope attached. Luckily, the RC6 includes not only a finder scope base on the left-hand side but drilled holes to easily add another finder/guide scope base on the right side.
The RC6 with a guide scope and finder scope mounted
This option proved to be the perfect solution to my balancing issues. I mounted an old finder scope to the newly installed base to for better balance, and a useful way to align the RC6. I’ve accumulated a number of telescope accessories from previous ‘scopes over the years, which is handy in situations like this. The finder scope I’m using was an Orion 9 x 50 Achromat I ordered years ago. Having a few extra accessories including finder scope brackets and bases can be a lifesaver in a crunch.
I’ll be adjusting the focus of the telescope manually during my astrophotography sessions on the RC6. iOptron does offer an electronic focuser built exclusively for their Ritchey-Chrétien telescopes, which will set you back about $200 USD. This is a nice available upgrade, which would certainly come in handy when fine-tuning the focus on a distant galaxy.
If you’re looking to install an aftermarket focus motor to your telescope, be sure to check out my experiences using this model from Pegasus Astro.
Guiding the RC6 on the iOptron CEM60
The iOptron CEM60 mount is more than capable of giving the RC6 a smooth ride. The telescope and all imaging accessories fall well below the maximum payload capacity of the mount, meaning any minor balance issues are unlikely to affect autoguiding. The auto guiding telescope I am using is a new StarField 50mm F/3.4 model, from Ontario Telescope and Accessories. This has replaced my old Altair guide scope but continues to house the reliable and no-nonsense Altair GPCAM2 AR0130 mono guide camera.
I’ll have much more information about the Starfield line of products including the reducer/flattener in the near future.
As I touched on earlier, a deeper view into space puts a stronger demand on tracking accuracy. The 1370mm focal length of the RC6 will not forgive a rough ride through a 3-4 minute long exposure, so I’ll be paying close attention to the graph in PHD2 guiding. As always, a precise polar alignment of the mount early on will save headaches later. Those of you that own a PoleMaster already understand the importance of this step. (I’ll get one this year, I promise!)
Finding Focus with the iOptron RC6
I was happy to see that iOptron made the effort to include the necessary accessories needed to reach focus for astrophotography. The RC6 includes three focuser extension rings for a variety of back focus applications. For my Canon DSLR camera, I found focus using the 1″ and 2″ extension tubes as seen in the image below.
To reach focus on the iOptron Photrom RC6 with a DSLR, use the 1″ + 2″ extension tubes.
If you’re having trouble reaching focus with RC6 and your particular camera, try setting up during the day (that’s what I did). This year, our Canadian winter lasted well into the spring. The sky was cloudy for weeks on end, meaning I didn’t have a single star to focus on. Instead, I used a distant street light to find focus. At a focal ratio of F/9, this telescope produces a dim view through the eyepiece. You’ll need to use a bright object if your focusing at night.
Once you have found focus on your test subject, you can mark the location on the focuser, take a photo of it, or simply keep it locked in. The idea is to have a benchmark to use when you’re ready to start imaging with the RC6 on a clear night. You’ll have to tweak the focus on a real star of course, but you should be close. Without knowing the correct back focus of a new telescope with your camera, it can take hours to find the right combination of extension tubes. The iOptron RC6 has a long focal length and a slow focal ratio (F9), making this process more difficult than it is with a fast little refractor.
Which Camera to use with a Ritchey-Chrétien Telescope?
When it comes to choosing the “right” camera to use for a particular telescope, understanding pixel scale is essential. This involves the correlation between the size of the imaging chip (camera sensor), and the telescopes focal length. Most planetariums and imaging software programs will calculate the most common camera chip sizes for you, or you can do the math for yourself. Richard Wright shared an excellent article on pixel scale for astrophotography in this Sky and Telescope article.
To be quite honest, I haven’t spent too much time obsessing over exact pixel scale details in the past. I know that my wide field refractor will capture the entire Soul Nebula with my Canon DSLR and that the ED102 frames the Cone Nebula and friends up nicely with the 183M. This is a result of my trial-and-error approach to astrophotography I somehow crave, but I understand why this isn’t for everybody.
Based on the sound advice from my good friend Steve Mallia at Ontario Telescope, I have decided to use my APS-C sensor Canon EOS T3i (600D) with the iOptron RC6. Because a longer focal length shrinks the area of sky you see, the larger imaging chip in this DSLR is a better fit with the focal length of the Photron. Combining this telescope with a camera that houses a small imaging chip such as the Altair Hypercam 183M would make for an extremely restricted field of view.
The iOptron Photron RC6 Includes:
- The Optical Tube Assembly
- A 2″ Dual-speed Crayford focuser
- 2 X 25mm, 1X 50mm focus extension tubes
- 1.25″ accessory adapter
- Vixen dovetail bar
Early Deep Sky Astrophotography Results (First Light)
I decided on the stunning Pinwheel Galaxy (M101) as my first deep-sky target with the RC6. The size and magnitude of M101 make this spiral galaxy a good fit for my full-frame DSLR camera. Historically, April and May are a great time of year to capture some serious light on this deep-sky target. The Pinwheel is one of many fine galaxies up for grabs during “galaxy season“. Unfortunately, my test image was captured under an 82% illuminated moon, on a night that also suffered from poor seeing and transparency.
The following photo was made using 70 x 4-minute exposures. (4 Hours, 40 Minutes). The images were integrated with support frames including dark, bias and flat frames in DeepSkyStacker. The final stacked image was brought into Adobe Photoshop for final image processing, using the techniques outlined in this Photoshop tutorial.
M101 – The Pinwheel Galaxy with the iOptron RC6
Image Quality
I looked closely at my individual light frames produced by the iOptron RC6. What I looked for was a flat field of view, sharp stars, and an increased resolution in my subject. I was thrilled to capture M101 in great detail than ever before, thanks to the increased aperture and focal length of the telescope. The image was captured during a nearly full moon, which made processing the image a challenge.
A single 3-minute exposure @ ISO 1600
Removing some harsh gradients in the final image was the biggest obstacle. I used a Photoshop technique that includes applying a synthetic flat frame and setting the blending mode to subtract. This can be a helpful strategy to use when the Gradient Xterminator plugin does not produce the results you’re looking for. As always, adding more overall exposure time and improving the signal-to-noise ratio would improve the quality of this image immensely.
A Baader Moon and Skyglow filter helped reduce the effects of light pollution in my images and made a big difference in terms of overall contrast. I mounted the 2″ round version of the filter to nosepiece of the camera. The 2″ round mounted version offers the flexibility to shoot using a variety of dedicated astronomy cameras and DSLR’s. Of all the light pollution filters I have used for astrophotography from the backyard, this particular Baader model is one of my favorites.
Baader Moon and Skyglow Filter
Final Thoughts
The iOptron Photron RC6 telescope was a pleasure to use, and provided the deep sky results I was looking for. The optical quality of the Ritchey-Chrétien design produced flat, coma and chromatic free results as advertised. Surprisingly, this F/9 focal ratio did not reduce the amount of light collected in my image subs the way I expected. 3-minute exposures at ISO 1600 were well exposed, likely due to the increased 150mm aperture of the telescope.
Take a look at the dramatic difference in magnification between the following two images of the Pinwheel Galaxy. The version on the left was captured using a crop-sensor DSLR with an 80mm refractor telescope, while the version on the right used the new iOptron Photron RC6. As you can see, an RCT is a much better fit if you want to collect close-ups of smaller DSO’s like the Pinwheel Galaxy.
Those of you that shoot with a DSLR camera should have no issues in terms of pixel scale with the RC6. An APS-C sized sensor is a great match with the focal length of this telescope. My image of the Pinwheel Galaxy should give you a good idea of the field of view you can expect to see through the Photron. At just 18lbs, the iOptron RC6 requires a modest equatorial mount for accurate and smooth tracking. The iOptron CEM60 is overkill for this OTA, and I think an Orion Sirius EQ-G or similar mount would be more than enough to carry a complete imaging payload.
The RC6 fills a noticeable void in my arsenal of optical instruments. Having a no-nonsense telescope that excels in high magnification targets such as small galaxies has given me a more complete set of tools for capturing deep sky objects of all sizes. Next up, I’ll focus on capturing some of the smaller galaxies I have traditionally avoided due to lack of reach.
The iOptron Photron 6″ Ritchey-Chrétien telescope is available at Ontario Telescope and Accessories.
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