Taming a GSO Ritchey-Chrétien Telescope (iOptron Photron RC8)
Updated: Jan 29
They look sexy - a cheap price tag for the aperture, light-weight for the focal length, cool rainbow-colored star spikes, awesome resolution, and not too terribly slow optics at f/8 (can also be reduced for speed). What's not to like? Shoot, iOptron claims "coma free, chromatic aberration-free, spherical aberration-free optics". Sounds like a no-brainer for those wanting to move into the world of long focal length astrophotography.
Buuuuuuuuuut, hold on a second. Put your credit card back into your wallet. Please, let me tell you my experience and the many challenges of this telescope so that you can make an educated purchase. You need to know what you're getting into - and how to fix some of the inherent design flaws of this budget RC scope if you dare try.
I've owned this telescope for about a year now, and finally feel satisfied with star shapes and the flatness of the field. I've spent hours scouring astronomy forums and experimenting to solve many of the issues of the telescope. The main challenges are:
Focuser and primary mirror cell flexure (they are unfortunately attached to each other!)
Unwanted off-axis bright star diffraction caused by primary mirror edge imperfections
Baffle extension for older models
So, let's just go down the list.
If you've done any research on the GSO RC8, then you know the hot topic of debate is achieving collimation and which procedure to utilize. People struggle with it. They spend a lot of money on lasers and tools designed to help collimate. Aligning the two hyperboloidal mirrors on an RC requires precision to produce nice stars and images. Failure to do so is a frustrating thing - as none of us like to waste quality imaging/clear sky time.
I have a feeling one of the main reasons people struggle with collimation of these GSO RC's is due to the next topic of conversation regarding the focuser and the primary mirror being attached to each other. I'll dig in deeper there.
THE FIX: I've developed a way to consistently and easily bench collimate my RC8 using only a Cheshire eyepiece. I know, some RC owners will say that's impossible - but I'll share it with you all in a separate post anyway (very soon) - and you can see if it works for your set up.
Focuser and Primary Mirror Cell
This is the biggest design flaw of the telescope, and is responsible for the difficulties of collimation. The focuser attaches right to the back of the primary mirror cell. What this means, is that any adjustment you make to the primary mirror during collimation inherently tilts the focuser. A tilted focuser is never a good thing. Second, and probably more importantly, since they are attached (and backfocus is about 10 inches!), the weight of the imaging train will pull on the primary mirror cell causing collimation shifts when slewing to different parts of the sky. Let me repeat that in all bold, yellow caps: THE WEIGHT OF THE IMAGING TRAIN WILL PULL ON THE PRIMARY MIRROR CELL CAUSING COLLIMATION SHIFTS WHEN SLEWING TO DIFFERENT PARTS OF THE SKY. Why is this important? Because people are collimating their RC's on the bench or on actual stars only to see mis-collimation as soon as they change altitude - therefore assuming their collimation adjustments were incorrect. Here's an example:
Near zenith - nice round stars, well-collimated (center crop):
Same FOV and imaging session. Target was at 50° altitude. Notice the poor star shape - a clear indication of mis-collimation/tilt.
It's important to point out that the weight of your imaging train makes a difference. When I've used a OSC camera without a filter wheel, the issue was minimal but still present. Using a mono camera with a filter wheel adds substantial weight and therefore causes more flexure. I found that the weight of my ASI2600MM, EFW, and ZWO OAG-L caused collimation shift in many parts of the sky below 70° altitude. This is substantial, and truly limits imaging.
You may be thinking that some of this sag may be due to the cheap stock focuser. You are probably partially correct, but I replaced my stock focuser with a quality Baader Diamond Steeltrack only to find that the issue remained (if not worsened due to the Baader being heavier).
FIX #1 (the ultimate fix): Have a custom collimation adapter backplate installed that "de-couples" the focuser from the primary mirror cell. I found a Cloudy Nights user that custom builds them. Here's a link to the thread. You can also see the results of my testing of the backplate in post #384 of the thread. This fix is the best solution, though it comes at a cost ($). It also solves the problem of being able to separately and properly align your focuser with the secondary mirror.
FIX #2: Shoot with OSC camera only, and use a light-weight OAG. Essentially, lighten the load of your imaging train as much as possible. I wouldn't recommend guiding with a guide scope, though, as an OAG is almost a necessity at this focal length (~1600mm).
In both cases, a focuser upgrade is nice. The stock focuser doesn't take an electronic focuser very well, and will slip with a heavier load. I don't doubt that it has some degree of sag as well with heavier payloads.
If you research it, many people on the internet will tell you that one of the benefits of the RC design is that they don't have dew issues like an SCT or refractor (that use glass). Many swear they don't have to do anything to prevent dew from forming on their RC mirrors. I'm here to tell you that I've experienced both dew and frost on BOTH mirrors. I live in the mid-Atlantic region of the U.S., which is prone to dew because of temperature fluctuations at night. If you live in an area that is not prone to dew, you may be fine. But, don't purchase an RC thinking you absolutely won't need some sort of dew mitigation system.
THE FIX: A 12" dew shield can slow the dewing process a bit, but won't eliminate it on exceptionally "dewy" nights. I've been using an 8" dew strap around the OTA about 1/3rd of the way up from the primary mirror at about 50% power. It seems to help a lot. At some point, I will purchase and install a dew heater for the secondary mirror, but at the moment I don't want to invest any more money into this telescope.
Unwanted Off-Axis Bright Star Diffraction
This issue is best described in a picture:
You'll notice the off-axis star at the top left has some extra diffraction pointing toward the center of the FOV. Here's a tight crop (30s exposure):
While this may not seem too bad, here's what it will look like in a long integration:
Too sloppy for my taste. The cause of the excess diffraction is imperfections around the edge of the primary mirror. The primary mirror doesn't have clips holding it in place; rather, it is held in place by the baffle. But, on the outer edge of the mirror, you can see where clips were used to hold it in place when the mirror coating was applied at the factory. This left small areas of no reflectivity along the outer edge:
This appears minimal when looking at the mirror, but the lack of reflectivity in these small spots makes a huge difference on bright stars. There are four of these spots on my primary, three of which you can visually see partially obstructing the mirror. Maybe I was just unlucky with mine. Hopefully, yours won't be so intrusive.
THE FIX: An "aperture mask" solved this for me. This certainly is not the most elegant of solutions, but it works quite well. Here's what the mask looks like:
The outer dimension is 215mm and the inner dimension is 185mm. I had it custom made online for about $15. Here's what it looks like installed:
Again, not elegant at all. I use two-sided tape to hold it to the OTA. Using a mask is a trade-off. It essentially reduces the telescope to f/8.7 at the expense of better-looking off-axis bright stars. Here's the result:
If I'm imaging an object with bright stars, I usually install the mask - otherwise I leave it out. I haven't noticed any other side effects of using it.
If you are buying or own a used RC8 manufactured before 2020 or so, you'll need to get a baffle extension. Essentially, the baffle tubes weren't quite long enough in the earlier GSO models. This allowed stray light to hit the sensor, creating strange gradients and making it impossible to produce flats capable of properly calibrating lights. You can watch a video explaining it in more detail here. You can buy an extension here, if you need one.
My iOptron RC8 already has a baffle extension from the factory. One of the things I learned, though, is that if your focuser is not aligned with the tube and secondary (and therefore the baffle), you'll still see some strange reflections from the baffle when imaging LRGB. It appears that having the focuser aligned has fixed this issue for me.
To sum it up, this is not a low-maintenance telescope. But, if you put some effort into getting it collimated and fixing its flaws, it will produce some nice images. Now that I've finally got everything set up, I think it will rival my Edge HD 8. In fact, I'm able to get a nice flat field over an APS-C size sensor when using a Hotech SCA flattener. That's not achievable with my Edge 8 at f/7. Here's a quick 21 minutes (7 x 60s each of RGB) test stack on M37 with a ZWO ASI2600MM and the Hotech SCA Flattener (un-cropped for viewing the edges and FOV). This image includes all of the fixes I've done to the telescope, and is the best collimation I've achieved to date:
M37, uncropped (test shot)
I have yet to complete a project with all of the fixes in place - still waiting on some clear skies and good seeing. But if you're curious as to what this telescope can produce with some reasonable integration time, here's an image I shot last year with it:
If you look closely, you can see the star shape is imperfect (bean-shaped). This is due to the collimation shift caused by the imaging train pulling on the primary mirror cell, since the target never reached above 70° altitude when I imaged it. Nonetheless, some nice details came out. At this point, I had installed the aperture mask so that off-axis stars were more balanced. The stock focuser was still installed, and the de-coupling plate wasn't available yet.
Would I recommend this telescope? Sure, if you like tinkering. Is it ready to go, out of the box? Not if you want to produce excellent images. You'll have to invest some more money to make it a truly great imaging scope. My hope is that this blog post will help those interested in buying an RC8 know what to expect, and provide solutions for those who are struggling with it. All in all, it's been a great learning experience for me.
Stay tuned for a blog post about collimating my RC8 using only a Cheshire!