William Optics Space Cat Review

November 24, 2019  •  25 Comments

William Optics Space Cat Review

For the past two years I’ve done most of my deep space astrophotography with an iOptron SkyGuider Pro, Nikon D750, and a Tamron 150-600mm lens.  This combo works surprisingly well, and I like the versatility it offers.  However, I have been looking to upgrade to a real telescope for some time now.  The clear choice for me was William Optics.  They seem to make telescopes for DSLR users like myself, who want something relatively lightweight and portable. 

Earlier this year, I remember being intrigued by William Optics’ latest telescope – The RedCat!  It looked a lot like a 70-200mm lens, which I’m very familiar with, but with some nice upgrades – a built-in bahtinov mask, lightweight and compact design, versatile arca-swiss / dovetail plate, smooth focusing ring, and more!

In early 2019 William Optics released the first version of their RedCat 51 telescope.  It went on to be a very popular telescope, especially among DSLR users.    

In September 2019, William Optics introduced a new version of the RedCat with a few small improvements including – a longer dovetail plate, a better-marked focusing ring, and the tilt-adjuster built into the telescope.  William Optics also released the Space Cat and the Black Cat in September 2019.  These variants share all the same features and upgrades as the new RedCat, but with a different color scheme.  I will be reviewing the Space Cat in this article, but the RedCat will perform exactly the same!  It’s up to you to choose your preferred color.

In early October, AgenaAstro reached out and offered to send me a Space Cat telescope to review.  When the Space Cat arrived, I was really impressed by the quality of the telescope!  To start, the Space Cat comes in a beautiful padded carrying case.  I spend six months each year traveling across the country and living out of my car.  Therefore, I keep all of my camera gear in two hardcases for protection.  However, I didn’t have any room left for a telescope.  I’m glad the Space Cat comes with a practical case to keep it well protected on my adventures.


Bahtinov Mask

The Space Cat itself is beautifully designed, and every aspect has been carefully engineered.  Take for example the lens hood.  Not only is it completely reversible, reducing the packed size, it also includes a built-in bahtinov mask!

For those who are new to astrophotography, a bahtinov mask is basically a piece of plastic with a bunch of lines in it.  This creates a unique pattern when photographing the stars and helps with focusing.  Once the 3 spikes are aligned perfectly, you have a sharp image!  In fact, if you look closely at the Space Cat logo, you’ll notice the cat’s whiskers are diffraction spikes!  This careful attention to detail is present throughout the Space Cat.

For many people, focusing on the stars can be quite difficult!  Essentially, you’re trying to determine if the star is as small as possible.  For people with bad eyes, this can be incredibly hard.  The diffraction spikes make it much simpler, just verify that the middle line is actually in the middle of the other two lines, and you’re all set!

Unfortunately, my Nikon D750’s Live View is starting to show its age.  If I’m lucky, I can see one or two bright stars in Live View.  This makes it hard to see the diffractions spikes in Live View.  Once I take a short test photo though, I can clearly see the diffraction spikes.  If you have a newer Mirrorless camera, you should have a much easier experience!  The new mirrorless cameras are much more sensitive at night when using Live View.  In fact, you should be able to see the Milky Way galaxy in real time!  With this increased sensitivity, you should have no trouble focusing with the Bahtinov Mask! 


Dovetail Plate

Another great feature of the Space Cat is the dovetail plate.  Since the dovetail plate is so long, you can easily balance your camera rig by adjusting the screw locations.  This can be especially helpful if you have a very large DSLR!  Although, a battery grip may prevent the dovetail plate from sliding back.

William Optics has kept photographers in mind with the Space Cat by including an arca-swiss plate built into the dovetail!  If you’d like to attach your Space Cat to a ballhead, just unscrew the dovetail plate from the Space Cat, flip it around, and reattach it.  The arca-swiss plate will now be ready to use.  I’ve never seen another telescope even consider this feature!  Kudos to WO!


Focusing Ring

When I first saw the RedCat advertised online, I noticed that the design was very similar to a 70-200mm lens.  Not only are the size, weight and focal length similar, the focusing ring is very reminiscent of a telephoto lens.  The official term for this is a “helical focuser”, but I’ll be referring to it as the focusing ring.

When you look at the Space Cat you’ll notice there are two rings, the large ring controls the focus while the small ring controls the tension.  The focusing ring is clearly marked, with very precise markings near infinity.  These smaller markings will help you to find the correct focus for your particular telescope.  For example, the image below shows the exact location where the stars are sharp for me.  Once you learn this position, you can quickly put your focus there, then do some small tweaks to get it perfect.

The tension ring can be adjusted separately at any time to change the speed of the focusing ring.  If you plan to use this telescope to photograph birds, you can loosen the tension ring, which will allow you to focus faster on a quick moving subject.  I prefer to keep my tension ring fairly tight, so that all of my adjustments are a bit more precise.


Aperture and Star Sharpness

The Space Cat has a 51mm aperture with a focal ratio of f/4.9.  This allows a good amount of light into the telescope.  For the photographers out there, you may already have a 70-200mm f/2.8 lens.  If you were to use that lens at f/2.8, you could capture almost 4 times as much light as the Space Cat.  Granted, a 70-200mm lens also costs anywhere from $500 to $2,000 more than the Space Cat!

I have a Tamron 70-200mm f/2.8 lens, which I used quite a bit before the Space Cat.  I did enjoy using this lens for my astrophotography, and I want to show some comparison images between the Space Cat and the Tamron 70-200mm lens.

Here's a close crop of the upper-right corner from my William Optics Space Cat.  You'll notice a very slight amount of coma distortion.  If you were using a crop-sensor, or dedicated astro camera, you wouldn't see this part of the image circle.

This is a close crop of the center from the Space Cat.  The stars are nice and spherical, with no noticeable distortion.  The multi-colored dots are hot pixels, caused by my DSLR sensor overheating.

This image was taken with the Tamron 70-200mm G2 lens, at ~175mm.  The image has been cropped to the upper-right corner.  There's a small amount of coma distortion on the brighter stars.


Focal Length

The Space Cat is a 250mm focal length telescope.  This is considered very wide for deep space astrophotography.  However, there are some benefits to using a wider focal length at night.  One of the biggest problems that beginners have is finding objects in the night sky, especially with a tracker like the SkyGuider Pro or Star Adventurer.  These portable star trackers have no Go-To functionality, which means you have to manually move the telescope and find the objects yourself.  Good luck doing that with a 600mm+ telescope / lens!  The 250mm focal length of the Space Cat makes finding objects in the night sky quite a bit easier.

The image circle produced by the Space Cat is large enough to cover a full frame camera sensor.  I personally use a Nikon D750, which is a full-frame camera.  Generally, full frame cameras have larger pixels, which translates to better low-light performance.  However, there are some benefits to using a smaller sensor. 

If you have a crop-sensor camera, like a Nikon D5600 or Canon Rebel T7i, your sensor is physically smaller.  This will effectively magnify the image by 1.5 times (or 1.6 on Canon).  Therefore, the 250mm focal length of the Space Cat will look more like 375mm on a Nikon and 400mm on a Canon.

Now that I’ve photographed most of the large objects in the night sky, I do prefer a focal length of 400mm or longer.  This allows me to fill the frame with the object, which means more resolution and detail!  Looking ahead, I may get a dedicated astrophotography camera like the ZWO ASI 1600 or an APS-C camera to use specifically with the Space Cat.

If you already have a dedicated astrophotography camera, like the ZWO ASI 1600, this might be the perfect camera for the Space Cat!  The ASI 1600MM Pro has a micro four-thirds sensor (4/3” CMOS).  This small sensor will give a Field Of View similar to 500mm on a full frame camera.  (250mm x 2 = 500mm).  A 500mm focal length is ideal for many of the best objects, including: The Horsehead Nebula, Rosette Nebula, Pleiades, Triangulum, Orion, and more!

Picture saved with settings embedded.


T Mount Adapter

Like every other telescope, you will need a T-Mount adapter to connect the Space Cat to your DSLR or mirrorless camera.  The Space Cat uses a 48mm T-Mount adapter.  If this is your first telescope, be sure to pick the correct 48mm T-Mount for your camera body (Canon, Sony, Nikon, etc…)  You will not be able to use the Space Cat until you have this adapter for your camera.

The T-Mount adapter will screw onto the rear-end of the Space Cat.  Once it has been screwed on, you can attach the telescope to your DSLR like a normal lens.  Unfortunately, I had a lot of problems with the William Optics 48mm T-Mount adapter.

After securely attaching the T-Mount adapter to the Space Cat, I connected the telescope to my Nikon D750 like any other lens.  Upon doing so, I noticed that the T-Mount adapter never “clicked” into place. 

Every lens will “click” once it has been properly connected to a DSLR, and it cannot be removed until the Lens Release Button has been pressed.  However, the William Optics T-Mount adapter never fully locks into place.  This can be very dangerous.  In fact, I had placed my camera and Space Cat on the back seat, and forgotten they were there.  Once I started driving, I heard something fall.  I turned around to see the D750 had come off of the Space Cat entirely!  This would never happen with a proper connection.

After messing around with the adapter for a few weeks, I decided to contact William Optics.  After waiting a week with no reply, I tried their Facebook page.  I saw that WO had read my Facebook message, but they never replied.  I’m not sure what’s going on with their customer service, but I am not impressed.

Thankfully, AgenaAstro.com has fantastic customer service!  They are the dealer who sent me the telescope and T-Mount adapter, so I also contacted them.  I receive an email reply within a few hours, and the owner sent me a replacement T-Mount Adapter the following day.  Apparently the original T-Mount had been discontinued.  I’m happy to report that the new T-Mount adapter works fine, and securely attaches to my DSLR now. 

If you find yourself having similar problems with your T-Mount adapter, I would try contacting your dealer, rather than William Optics.  They should be able to help you out and get you a replacement.


Lens Hood

The lens hood on the William Optics Space Cat can be taken off completely, or installed in reverse to save space.  This helps to cut down on the size of your Space Cat, if you need to pack it for a trip.  I’m a big fan of this versatility!  For example, let’s say it’s a bit windy outside.  That large lens hood will cause the entire setup to shake.  Once you take off the lens hood, the wind shake should be noticeably less.  Alternatively, if there’s a lot of dew, the lens hood will help prevent condensation from settling on the glass.


Filter Slot

For those who already have a telescope or dedicated astrophotography camera, the Space Cat includes a built-in thread for 2” astronomy filters which have an M48x0.75 thread.  This allows you to easily install filters to the back of the telescope.  I personally use a clip-in filter for my Nikon D750 when I’m shooting in a light polluted area.  However, I’m not a huge fan of clip-in filters, for a variety of reasons.  Now that I have the Space Cat though, I may invest in some 2” astronomy filters.  If you plan to use a monochrome astrophotography camera, this might be a great fit for you!


Field Rotator

The Space Cat also features a field rotator.  According to the official documentation, this is designed for creating mosaics.  However, I use it to change the composition and the angle of my camera.  For example, if my D750 is at a weird angle, due to the position of the object in the night sky, I can quickly adjust the Field Rotator screw and rotate my camera to a horizontal position.  If you’ve ever used a telephoto lens for deep space astrophotography before, it’s essentially the same thing as loosening the lens collar a bit and rotating the camera around.

For more advanced users, you can use the Field Rotator to create impressive mosaics.  The entire ring is clearly marked from 0 to 350 degrees to help make precise adjustments.


Shooting Experience

My first night out with the Space Cat was a lot of fun!  As I’ve mentioned earlier, I normally use a SkyGuider Pro, Nikon D750, and Tamron 70-200mm or Tamron 150-600mm lens.  I’m happy to report that using the Space Cat was very easy!  The only real problem I encountered at first was the T-Mount adapter.  As I was connecting the Space Cat to my D750, the T-Mount adapter started to unscrew itself from the telescope!  This give me a scare, and I was worried something would fall.  I was very careful to tighten everything down again, and slide the dovetail plate onto my SkyGuider Pro.

Once I had attached my Space Cat, I needed to double check my polar alignment.  If you have a SkyGuider Pro, you’ve probably gotten very tired of dealing with the flimsy plastic base.  The screws can be hard to turn, and the adjustments are never that precise.  Thankfully, AgenaAstro.com sent me out a William Optics High Latitude base to review.  I was really impressed by the quality of this mount, and it made the polar alignment process noticeably easier.  If you’d like to read more about this base, be sure to check out my full review here.

After I did a precise polar alignment, with help from the William Optics High Latitude Base, I needed to get the Space Cat focused.  I unscrewed the front lens cap from the Bahtinov Mask and began focusing.  It was nice to have the diffraction spikes as a visual aid!  Within about 30 seconds I had beautiful, sharp stars and I was ready to find my object.

The 250mm focal length is great for beginners!  One of the most common problems my students face is too much zoom.  With the 250mm FOV, most objects can be found quite easily.  I ended up choosing the Pleiades as my first object.

After taking my first series of photos, I immediately noticed a problem.  There was a fairly large dust spot near the center of the frame.  As a telephoto lens user, this is something I’ve never really had to deal with before.  Usually dust spots are only visible when using f/8+, which we never use at night.  This meant I had to do some more work and create Flat Frames.

To be clear, this isn’t a problem with the telescope itself.  The dust spot was on my camera’s sensor.  However, since the rear element of the telescope is much further from the sensor than a normal lens, the dust spot appears quite large in the frame.  A telephoto lens’ rear element would be closer to the camera’s sensor, which would normally blur out most dust spots.   

Now that I know dust will be a problem with the Space Cat, I will make sure my sensor is clean before any serious shoots.  If you would like to verify your sensor is clean, it’s a fairly easy process.  First, find a blank, white wall in your house, or a clear blue sky.  Next, set the camera to Aperture Priority mode, with f/22 and a low ISO.  Now take a photo.  Any dust on your sensor should be visible if you zoom in. 

It’s actually very easy to clean a camera sensor, although it is scary the first time you do it.  I recommend using VSGO sensor swabs with a drop of the included cleaning solution.  Now lock your camera’s mirror up, this setting is usually found in the “Settings” menu.  Once the mirror is locked up, you can quickly swab the sensor a few times.  When you think you’ve gotten all the dust, turn the camera off.  Now re-attach a lens and take another test photo.  You may need to repeat this process two or three times to get a perfectly clean sensor.

Speaking of Flat Frames, the Space Cat has a small amount of vignette.  If you’re not familiar with that term, it just means how dark the corners of the photo are.  The amount of vignette is mainly determined by the lens / telescope design.  The image below has been heavily stretched to give you a clear look at the vignette. 

Keep in mind, if you are using a smaller sensor, like an APS-C or Micro 4/3rds, the Vignette will be completely absent!  That’s because you will only be capturing the center of the image area.  A Full Frame camera captures much more area, and thus the Vignette is visible.

Next, I wanted to show you how the Vignette will look with a crop-sensor camera, so I borrowed a Nikon D5600 for a night.  Once I had two photos, one from my D750 and one from the D5600, I used Photoshop to align them properly.  As you can see, the crop-sensor on the D5600 is only able to capture the center of the image circle.  In other words, the vignette is automatically cropped out!  This is another great reason to use a crop sensor with the Space Cat. 

Now that we've covered the vignette, let’s take a closer look at the stars.  This is arguably the most important aspect of a telescope after all!  The Space Cat exhibits a small amount of chromatic aberration around the stars.  Meanwhile, the coma (star distortion) is very well controlled.  If you look very closely at the edge of the frame, on a full frame camera, you’ll see that the stars aren’t completely spherical.  All things considered, the stars look great on the Space Cat, especially if you will be using a crop-sensor or dedicated astro camera!  In that case, you'll only capture the center of the image circle, which is the sharpest area.




Pairing a Camera with the Space Cat

Undersampling or oversampling is something you must always consider when purchasing a new telescope or camera.  This will depend on the focal length of your lens / telescope and the pixel size of your camera.  Thankfully, there’s an easy equation we can use:

(Pixel size (um) / focal length (mm))  x 206 = arc-seconds per pixel

My Nikon D750 has a pixel size of 5.95 um.  The focal length of the Space Cat is 250mm.  Therefore, my equation is:

(5.95um / 250mm) x 206 = 4.9 arc-seconds per pixel

Okay, so I have about 5 arc-seconds per pixel, but what does that actually mean?  Very simply, the higher the number, the worse the results.  Ideally, the arc-seconds per pixel would be between 1 - 2.  If you’re a photographer like me, this would make more sense in terms of focal length.  The more zoom we have, the larger the object will appear in the frame.  This will increase the resolution and detail that we can capture.  This is why I tend to recommend 400mm+ when shooting deep space astrophotography with a full frame DSLR.

Generally, we want to have a value of 1 – 2 arc-seconds per pixel for optimal results.  This will give clear, detailed photos with good resolution.  For example, my Tamron 150-600mm lens, when zoomed into 600mm, gives me a value of 2.  It’s no coincidence that I love using that focal length for many objects at night, especially the Orion Nebula!

With all that in mind, I may consider getting a new camera specifically for use with the Space Cat.  Since my D750 has very large pixels I tend to undersample with any given lens / telescope. 

At this point, I’ve been considering two options.  First is the Nikon D5600, which has a pixel size of 3.89um.  There’s also the ZWO ASI 1600, which is a dedicated astrophotography camera.  The 1600 has a pixel size of 3.8 um.  Let’s try the equation again and see how this will perform.

(3.8um / 250mm) x 206 = 3.1 arc seconds per pixel

This is just one more reason to use a crop-sensor or dedicated astrophotography camera with the Space Cat.  The smaller pixels essentially translate into more zoom and resolution.  Remember, the Field Of View with the Space Cat and ASI 1600 will be similar to 500mm on my full frame Nikon D750.


Final Thoughts

The Space Cat is a very well-designed telescope for folks with a DSLR and portable star tracker who want to get into astrophotography.  The Space Cat is lightweight, portable, and works perfectly with a SkyGuider Pro or Star Adventurer!  If you don't currently have a good lens for astrophotography, the Space Cat may be the perfect addition to your setup.  If you are using a crop sensor camera, the ~400mm equivalent focal length will work nicely for many of the brightest objects in the night sky.  However, if you have a full-frame camera, you may find yourself wishing you had a bit more zoom.  If you're a serious astrophotography, and you've already got a set of 2" filters and a dedicated astrophotography camera like the ZWO ASI 1600, then you'll love the Space Cat!



  • Compact and lightweight
  • Padded carrying case
  • Built-in Bahtinov Mask
  • Wide FOV makes finding objects easy
  • Perfect fit for a SkyGuider Pro / Star Adventurer
  • Minimal star distortion
  • Minimal vignette, especially if using a crop sensor camera
  • Great dovetail bar!
  • Can be paired with Crop-Sensor or dedicated astro camera for optimal results
  • Built-in 2" filter slot



  • Received a defective T-Mount adapter (which was later rectified by AgenaAstro, who sent me a free replacement)
  • William Optics Customer Service never responded via email or Facebook
  • 250mm is a bit wide if you are using a full frame camera
  • A telephoto lens would be more versatile 


Photo Gallery

Heart Nebula

  • 3 Minute Exposures x 40 photos (H-Alpha)
  • 3 Minute Exposures x 20 photos (Blue)
  • ASI 1600MM Pro
  • SkyGuider Pro
  • ZWO ASIAir + ZWO 120MM Mini
  • Photos taken in Kanab, Utah

Picture saved with settings embedded.



  • 4 Minute Exposures x 20 photos (Total 80 minutes)
  • ISO 800
  • Nikon D750
  • SkyGuider Pro
  • ZWO ASIAir + ZWO 120mm Mini
  • Photos taken near Lake Placid, New York (Bortle 3)


North American Nebula

  • 4 Minute Exposures x 37 photos (Total 148 minutes)
  • ISO 800
  • Nikon D750
  • SkyGuider Pro
  • ZWO ASIAir + ZWO 120mm Mini
  • Photos taken near Lisbon, Ohio (Bortle 4)


Andromeda Galaxy

  • 4 Minute Exposures x 38 photos (Total 152 minutes)
  • ISO 1600
  • Nikon D750
  • Optolong L-Pro Filter
  • SkyGuider Pro
  • ZWO ASIAir + ZWO 120mm Mini
  • Photos taken near Lisbon, Ohio (Bortle 4)
  • Image has been heavily cropped


Orion + Horsehead Nebula

  • 2 minute exposures x 61 exposures (Total 122 minutes)
  • ISO 200
  • Nikon D750
  • SkyGuider Pro
  • ZWO ASIAir + ZWO 120mm Mini Auto-guider
  • Photos taken in Kanab Utah (Bortle 2)


Rosette Nebula

  • 2 minute exposures x 51 exposures (Total 102 minutes)
  • ISO 800
  • Nikon D750
  • SkyGuider Pro
  • ZWO ASIAir + ZWO 120mm Mini Auto-guider
  • Photos taken in Kanab Utah (Bortle 2)


Rosette Nebula

  • 3 minute exposures x 10 exposures (H-Alpha)
  • 3 minute exposures x 8 exposures (Oxygen)
  • ASI 1600MM Pro
  • SkyGuider Pro
  • ZWO ASIAir + ZWO 120mm Mini Auto-guider
  • Photos taken in Kanab Utah (Bortle 2)

Picture saved with settings embedded.


Veil Nebulae

  • 1 Hour of H-Alpha and 1 Hour of Oxygen (ZWO Narrowband Filters)
  • ZWO ASI 1600MM Pro
  • SkyGuider Pro
  • ZWO ASIAir + ZWO 120mm Mini
  • Photos taken in Kanab, Utah (Bortle 2)

Picture saved with settings embedded.


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