William Optics SpaceCat 61mm Review
Over the past 5 years I've been using the SpaceCat 51mm telescope for most of my deep space imagery. The wide field of view allows me to capture large regions of space, and showcase the phenomenal dust and nebulosity throughout the cosmos. Earlier this year, William Optics reached out to me and offered to send a SpaceCat 61mm telescope for review. Since I am a big fan of the original SpaceCat 51mm, I agreed to test it out and write a review.
With clear skies being a rare treat in the Pacific Northwest, I drove down to Kanab to test out the SpaceCat 61mm. While the weather wasn't great, I was still able to capture enough data for this review. The main focus of this trip was to create my first deep space mosaic using the SpaceCat 61mm. This was a massive undertaking! To learn more about the mosaic process, from the initial setup to the post-processing, watch my full-length YouTube tutorial.
This Mosaic is composed of 9 panels. Each panel is just 30 minutes, but even with this short integration time, the SpaceCat was able to resolve plenty of detail with minimal grain. Click here to view the full-res photo.
A Quick History of the Cat Series
William Optics has released many popular telescopes over the past few years, from the Zenithstars to the Pleiades. However, my favorite scope has been the RedCat 51mm. I love how compact, portable, and beginner-friendly this telescope is. The 250mm focal length provides a wide field of view, which makes it easy to find a good composition. (Although it's often too wide for Full Frame cameras.) Another benefit of the wide FOV is that it is more forgiving of tracking / guiding / polar alignment errors, which is helpful for beginners.
In early 2022, WO released the RedCat 71mm, which has a focal length of 350mm. This telescope is fairly large and heavy, and has the same overall design as the Cat 51mm. In 2023 William Optics released the RedCat 61mm, which features an internal focuser, 300mm focal length, and f/4.9 aperture. The RedCat version retails for $1,500, while the limited edition SpaceCat retails for $1,700.
The main benefit of the "Cats" is their Petzval design. This creates a flat field from corner to corner, with no star distortion. This eliminates the need for a flattener, which can add more complications to the setup and shooting process. The Petzval design also results in smaller and shorter telescopes. This compact size will help to prevent the telescope from banging into a tripod leg, or accidentally ripping out cables in weird orientations. If you're looking to purchase your first telescope, a Petzval is the way to go!
The SpaceCat 61mm pairs perfectly with the ASI 2600MC Duo and AM5 mount. The Petzval design keeps the system compact, without requiring any additional spacers or flatteners. Check out the Zenithstar 61 to see just how bad things can get when you need a flattener.
Internal Focuser
The star of the Cat 61mm is the new internal focuser, abbreviated WIFD. This focuser is incredibly smooth and precise, a huge improvement over the focus ring seen in the original Cat 51 telescope. With the old focus ring, I would often spend 5+ minutes making the smallest adjustments possible, only to have the blurry stars. The new focuser has both a rough and precise adjustment knob, but the rough adjustment is accurate enough to get perfectly sharp stars in seconds! The WIFD makes things so much easier, and I'm seriously considering buying the updated RedCat 51mm WIFD as well.
This brings us to another important feature of the Cat series - the built-in Bahtinov Mask. A Bahtinov Mask creates a unique diffraction spike pattern over all bright stars. This spike pattern allows you to quickly see if the stars are sharp or not. Every Cat telescope has this Bahtinov Mask included inside the front lens cap. When you combine the Bahtinov Mask with the WIFD, focusing becomes the easiest part of the night.
The focus system is compatible with EAFs, if you want to go that route. The Cat61mm ships with the necessary allen keys, so you can take the telescope apart. However, I find electronic focusers to be slow, inaccurate, and cumbersome. (At least with the ASIAir software) I would rather focus the telescope manually. Thanks to the improved WIFD focuser, this only takes a few seconds.
The black knob is the rough focus, and the silver knob is the fine focus. I didn't even need to use the fine-focus knob, since the rough focus is so precise and easy to adjust. Once you determine the proper focus point, shown in the window to the left, you can leave the focus there indefinitely. It should only take a few seconds to re-focus the telescope each night, with the help of the Bahtinov Mask.
Focal Length
The Cat 61 has a focal length of 300mm. You'll want to use telescopius.com to determine if this is a good fit for your camera sensor. Start by finding the object(s) you want to photograph, then scroll down to the telescope simulator. Enter the 300mm focal length along with your camera's sensor size. I'm currently using the ZWO ASI 2600MC Duo, which has an APS-C sensor (23.5mm x 15.7mm) This provides the perfect field of view for targets like Andromeda, Orion, Pleiades, North America Nebula, California Nebula, and more!
Since the Cat telescopes do not use flatteners, reducers, or extenders, you'll need to attach a camera with a different sensor size to change the field of view. For example, the ASI 533MC has a sensor size of 11.31mm x 11.31mm. This would significantly shrink the field of view, making any nebula or galaxy appear larger in the frame. A full frame camera, with a sensor size of 36mm x 24mm, would create a wide FOV at 300mm, making most nebulae and galaxies appear small. (Try using these various configurations on telescopius.com to see the difference!)
If you already have the RedCat 51mm or 71mm, then I don't really see the point of getting the 61mm. The focal length is only 50mm +/- of the other Cats. You'd be better off investing your money into a larger telescope with more zoom. This opens up a rabbit hole that I don't want to get into today though. I'd' recommend watching Nico's video to see the problems associated with larger telescopes.
The Lagoon Nebula is a fun summer target that's perfect for the SpaceCat 61mm. Since I captured this data from a Bortle 2, no filters were required!
Aperture
Every Cat telescope (51, 61, 71) has the same aperture - f/4.9. This lets in a considerable amount of light, especially when compared to the typical f/8 or f/10 telescopes. The f/stop is determined by the focal length and aperture diameter. You can use this calculator to get an idea of how things work. An f/4.9 telescope will capture more than twice as much light as an f/8 telescope, and over 4 times as much light as an f/11 aperture! That means you can get a cleaner photo in a fraction of the time.
I can get clean images with the f/4.9 aperture in as little as 2 hours. That's assuming I'm shooting from a dark sky (Bortle 2), no filters, and the target is fairly bright. Of course, this will change if I'm shooting with a narrowband filter from a light polluted area. Still, the f/4.9 aperture does a fine job in most cases.
The "Blue Horsehead" is located just above Rho Ophiuchi. It is a challenging target to find and photograph, and is best seen from a Bortle 1 or 2. I found that the 300mm focal length was a bit too tight for this composition, and the SpaceCat 51's 250mm focal length would have been a better fit.
Size & Weight
The RedCat 61mm weighs 7.47lbs, which makes it fairly lightweight. For comparison, the RedCat 51 weighs 5.25lbs and the RedCat 71 weighs 9.46lbs. (I will say that the 61mm is noticeably heavier than the 51mm, even though it's only ~2lbs more.) The Cat 61 is 365mm long (14.37 inches) and it does not have a reversible lens hood. This makes it substantially larger than the Cat 51. The included carrying case, while nice, is also quite large. When I travel to dark skies, I like to throw the Cat 51 in a hardcase, along with the camera, filters, cables, and accessories. If I must fly, then I know my gear will be safe in this carry-on sized case. I could even fit the Cat 51 inside my backpack and take it on a hike through the mountains. However, the same cannot be said for the Cat 61. It's simply too large and heavy to fit in the backpack, and the guide scope bracket would need to be removed before it would fit inside the hardcase.
I realize these concerns won't mean much to a backyard astrophotographer, but if you travel to dark skies frequently, it is something to be aware of. If I was flying to South America for an astro trip, I'd rather bring the RedCat 51mm WIFD.
Image Quality
Of course, the most important consideration when purchasing a new telescope is the image quality. Are the stars reasonably sharp with minimal distortion? How bad is the vignette?
Assuming you can achieve perfect focus, which is easy to do, then the stars are quite sharp. I'd say they're on par with the Cat 51 and Cat 71 (which are known for producing sharp stars with minimal distortion). The stars are a bit softer in the corners though, and this will be exacerbated if you don't focus properly. Feel free to download some sample photos to get an idea of how sharp the telescope actually is.
The vignette is minimal, especially when using an APS-C sensor or smaller. If you're using a full frame camera, then you'll likely see some vignette in the corners. Thankfully this can be fixed with a proper set of Flat Frames, DBE / Gradient Correction, or a quick crop.
There really isn't much more to say about the image quality. The telescope does a nice job! Even if you get lazy and don't refocus throughout the night, there's always BlurXterminator. This handy PixInsight process will take even the most deformed looking stars and turn them into perfectly spherical pinpoints of light.
Just keep an eye out for "pinched optics", which is a fairly common problem with refractor telescopes. Do a quick Google search to see some examples. This can be fixed by adjusting a few screws, but be careful! If you accidentally loosen or tighten the screws too much, you could damage the telescope. I have not seen any pinched optics on my Cat telescopes, but I have had some students experience this problem.
With all this talk of cats, I figured I'd try to photograph the Cat's Paw Nebula for the first time. I was surprised just how well it turned out, considering this is only 60 minutes of data! The f/4.9 aperture was a lifesaver on this particular trip, since time was limited due to wind and clouds.
Final Thoughts
If I had to condense this review down into one word, it would be - Simplicity. The Cat 61 makes things so much easier at night! There's no fiddling around with reducers, extenders, or flatteners. The built-in Bahtinov Mask combined with the internal focusing system makes focusing a breeze. The image quality is quite good, with minimal star distortion and vignette. The telescope is also fairly compact, which makes it easy to bring to a dark sky away from home.
My main gripe with the SpaceCat 61mm is the size and weight. That might seem odd, especially if you compare it to any other telescope out there. It's really not that large or heavy. Still, I miss the reversible lens hood that was featured on the SpaceCat 51mm; and I wish it would fit in my hardcase without needing disassembled. The included soft-case has enough padding that I'm not worried about taking it on roadtrips, but I wouldn't want to fly with it.
Another issue I have with the Cat 61mm is the price. The RedCat 51mm WIFD retails for just $900. The RedCat 61mm WIFD costs $1,500, and the RedCat 71mm WIFD costs $1,800. Each scope is f/4.9, but the focal length goes from 250mm, 300mm, and 350mm respectively. Based on these facts, I think the RedCat 61mm should be priced at $1,350, as that would be an even $450 price increase for focal length / aperture diameter. To make matters worse, if you order directly from William Optics you will be charged an import fee by UPS, which could cost upwards of $150. For this reason I recommend buying your telescopes through a third party astro retailer, like AgenaAstro.
Throughout the review I've mainly focused on the RedCat models, but there are also "SpaceCats". The SpaceCat is functionally identical to the RedCat, but it has a gray color scheme. You will pay a premium for this monochrome finish - a SpaceCat will cost between $100 - $200 extra. The SpaceCats are also exclusive to William Optics website, you won't find them anywhere else. If you use my coupon code: PETERZELINKA during checkout, William Optics will cover the import fee, which should save you at least $100. This code is only valid when ordering directly from William Optics, and you must purchase the SpaceCat model (either the 51mm or 61mm).
Remember, the main consideration when buying any telescope is the focal length. If the focal length is too long, then you won't be able to see the entire nebula or galaxy. If the focal length is too short, the object will look boring and small. You'll want to use Telescopius.com and look at a variety of targets to determine if the 300mm focal length will work for you. If 300mm isn't working, try 250mm or 350mm. These correspond to the RedCat 51mm and 71mm, both of which I'd also recommend.
So, if you're looking for an easy-to-use telescope around 300mm, then the RedCat 61mm WIFD is a great choice! This is a quality telescope that will last for years to come. However, if you plan on traveling around the world to dark skies, I think you'd be better off with the RedCat 51mm WIFD instead. For backyard astrophotographers looking for a bit more zoom, consider the RedCat 71mm WIFD.
The Dark Shark Nebula is a difficult target to capture, especially with only 3 hours of data. The 300mm focal length was a bit too short, and I wasn't able to get the composition I wanted. In hindsight, I would've been better off photographing a different target.