Astro-Tech 6″ Ritchey-Chrétien Imaging Review

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You may have seen my first impressions and unboxing of this telescope. After having a few good nights of imaging to test it out, I’m happy to say it has lived up to expectations.

Why choose a Ritchey-Chrétien Astrograph?

Astro-Tech makes a range of Ritchey-Chrétien (RC) telescopes that are all designed specifically for imaging. (The Ritchey-Chrétien design is what’s used by the Hubble and most other large aperture professional instruments.) The AT6RC is the baby of the bunch, and with an entry price of only $399, putting it right next to similar sized Schmidt-Cassegrain telescopes from Celestron and Meade. The RC is heavier than an SCT of the same size, which also made me opt for the 6″ version instead of the larger 8″ (13 vs. 19 lbs). Given I still use a little Celestron CG-5 EQ mount, I felt going over 25 lbs when adding the cameras and guide scope would be pushing the mount a bit much.

The big reason to choose an RC over an SCT is the flatter-field, no-coma imaging. While there are coma corrector & field flatteners available for SCTs, the example images I’ve seen still show a lot of elongation at the edges of the field. The AT6RC does have a little of this as well, fortunately Astro-Tech also makes a field flattener for $150 that works exceptionally well with these scopes. Note that there are coma-free SCTs available at larger apertures. Pricing tends to be similar to comparable sized RCs.

The other question would be why choose this over an APO refractor. This really comes down to focal length and price. APOs are popular imaging instruments. Unfortunately, they’re also expensive (and heavy) for large apertures. A 100mm APO refractor will go for upwards of $1500. They have their advantages and disadvantages, but for me it came down to getting a larger aperture for a lower cost with a long focal length (1370mm for the AT6RC) for imaging the smaller deep space objects.

I’ll add that I just ordered an Astro-Tech 65 EDQ, which is an APO with only a 65mm aperture and a 420mm focal length. I bought this specifically for doing wider-field imaging for objects like the Andromeda Galaxy, Pleiades cluster, and bright comets, and I’m impatiently waiting for it to ship!

What you Get

There are no eye pieces or star diagonal – this is an imaging telescope designed for use with DSLRs, CCDs, and WebCams. I already talked about build quality in the initial article, so I won’t repeat that here, other than to say the only negative I’ve found is the front dust cap fits either too tightly in warm weather or too loosely in very cold weather.

• 150 mm f/9 (1350 mm focal length) hyperbolic quartz-coated mirror Ritchey-Chrétien astrograph
• Front Dust cap (plastic)
• 2″ Crayford focuser with 34mm of travel on a rotating ring with a 10:1 fine focus nob and focus lock screws.
• 1 1/4″ adaptor
• 1 x 2″ extension ring
• 2 x 1″ extension rings
• Vixen dovetail
• Finder shoe (no included finder scope)

As far as I can tell, the telescope was perfectly collimated out of the box, and I haven’t had a reason to touch the collimation screws yet.

The AT6RC doesn’t include a case, and the packing is molded styrofoam, which isn’t a good long-term padding choice since little pieces will break off that could wind up inside the instrument. I wound up buying a large rubbermaid type bin at a home depot, and cutting a chunk of extra memory foam I had to line it. This works extremely well for transporting the telescope, and cost less than $20.

Focuser

The scope includes a 2-inch crayford focuser with fine focus adjustment (10:1), a tension screw, and a lock screw. It also has a rotatable collar that you can loosen to re-orient the camera. This all works perfectly well with the Canon T3i and AstroTech Field Flattener, which makes for a very lightweight imaging chain (less than 2 lbs). I found for my camera with the field flattener, I needed just the 2″ extension ring and only extend the focuser a few mm to achieve focus.

I have noticed the focus shift ever so slightly over a period of about 2 hours on a night that got down to about 16° F. I believe this was due to the temperature drop over that period and the effect that change had on the steel tube. The only way to even really notice was by using a Bahtinov mask.

Images

This is the heart of what you buy this telescope for. I’m taking all of my images with a Canon T3i DSLR, just like I used with the C6 SCT. First, while this IS NOT a fair comparison since I never bought a field flattener or coma corrector for the SCT, I want to show a pair of corner images from the C6 SCT and from the AT6RC. I’ll also add that the slight elongation in the AT6RC images is due to a tracking error, not the optics of the telescope. These corner images are completely unprocessed; the redder background is due to more sky-glow in the SCT image since they were taken different nights at different locations.

I’ve found no reason to complain about the flatness of the field with this telescope and the ATFF. This flat field and the focal length made it just possible to capture both M81 and 82 in the same field, as shown below.

Another great example is the Orion Nebula. Here you’ll really see the diffraction spikes on the brighter stars. This is arguably the one drawback of the RC design. The diffraction spikes are caused by the spider veins that hold the secondary mirror in place. Personally, I find they add interest and a bit of depth to the images, as they only appear on the brightest stars in long exposures.

This last image of M74 is more heavily cropped but presented me with a surprise – a supernova, which at the time of this picture was down to 16th magnitude. While I still need a great deal more experience processing, and a better EQ mount, I’m quite impressed this little 6″ telescope can reveal such a faint object with a series of 5 minute exposures.

I’ll also add that there is no vignetting with the APS-C size sensor in the T3i (22.3mm x 14.9mm). I took a series of flat frames that showed only a 5% light fall off from the center to the corners. A full frame camera would obviously give different results.

Summary

Bottom line, I think the AT6RC is a great astrograph at a great price, even adding in $150 for the field flattener, which based on other sources, seems to work well on a wide range of telescopes. The only aspect that leaves me wanting is the aperture, but I feel the need to upgrade my CG-5 mount to something more robust before moving to a larger RC. The quality of the optics appears to be very good. Anecdotally, I think I got slightly sharper stars in the C6 SCT at the center of the field, but I’d need to do some side by side testing to quantify that. The focuser works smoothly and stays put, and the telescope delivers an excellent flat field with very little light fall off on the APS-C sensor of the Canon T3i.

Highly recommended for any budget astrophotographer that wants a long focal length telescope.

Where to Buy

While you can sometimes find Astro-Tech scopes at other resellers, Astronomics actually owns the brand. View their list of RC Scopes for 6″ – 20″. The 6″ and 8″ models spend more time on back-order than available, but there are alternatives.

Orion Telescopes has recently begun offering Ritchey-Chrétien scopes in 3 sizes: 6″ RC, 8″ RC, and 10″ RC. These appear to be identical to those offered by Astro-Tech, which are also branded elsewhere as GSO. Each retailer seems to offer a slightly different package, but the same scope and focuser. The Orion scopes quite handily include dual-finder scope shoes, so you can use one for an auto guider and the other for a visual finder. Regardless of the brand label, they are very good imaging scopes!