Williams Optics Redcat 51 Petzval APO Review

The Redcat 51 first appeared on the global market in 2019 and as expected Williams Optics produced a high quality, well-engineered telescope.  GLAgencies in South Africa are now the local distributers for this (and other) quality Williams Optics products.   

I was privileged to review a Redcat 51 over the course of July/August 2021.  I have always been interested in this telescope because of its striking design and the many excellent reviews it has received from other Astro-photographers around the world.  So, what has all the fuss been about?  Is this little APO everything it is claimed to be? 

The answer is a resounding YES! Read more to find out why… 

RedCat 51 Technical Specs 

In summary the Redcat 51 is a highly portable apochromatic refractor with a flat field from edge to edge therefore removing the need for any field flatteners.  As an additional bonus, the scope is designed to accommodate a full frame sensor and doubles up as an excellent wildlife / portrait / landscape lens when used with a DSLR.  In summary, here are the basic specifications: 

  • Scope Design: Petzval Apochromatic Refractor (4 element prime lens) 
  • Diameter: 51mm 
  • Focal Length: 250mm 
  • F-Ratio: F/4.9 
  • Focuser: Calibrated Helical 
  • OTA Length:  Retracted – 210mm 
  • Mounting Style: Vixen/Arca-Swiss 
  • Weight: 1.45kgs 
  • Image Circle Size: 44mm 


The unboxing process was a delight – everything beautifully (and logically) laid out.  One can tell that a lot of effort was applied to the design of the Redcat 51 and the packaging was made to compliment the quality one expects from Williams Optics telescopes. 

I particularly liked the quirky cat design on the soft carrying case and the accompanying stickers added a nice touch too. 

The first impression when you lift the scope out of its case is its tiny size.  This truly is a portable and versatile telescope. Made of a strong aluminium alloy that is anodized a striking red colour, the Redcat 51 has an accompanying dew shield (which inverts to protect the lens when the scope is not in use and for ease of transport).  

The box contains the following: 

  • Williams Optics 51mm Petzval APO Refractor Telescope (Redcat 51) 
  • Focuser with focusing index 
  • Aluminium dust caps (front and rear) 
  • Bahtinov mask 
  • CAT series mounting ring and Plate 
  • Field Rotator 
  • Soft Carrying Case 

Great design features 

The Redcat 51 telescope has a 48mm thread at the rear making it compatible with a wide range of DSLR cameras.  Note: the M48mm T-Ring (to attach a DSLR to the scope) is sold separately. A wide range of other Williams Optics accessories are also available via GLAgencies and select suppliers in South Africa.  Likewise, a diagonal (and relevant eyepiece) can be acquired locally and attached for superb observational astronomy / wildlife spotting. 

The built-in field rotator with clearly marked degree graduations is a fantastic design inclusion for accurately framing celestial objects without affecting your focus and the filter slot just after the rotator accepts 48mm filters. 

Focusing the Redcat 51 is simple with the helical focuser – the tension ring allows you to lock the focus or adjust the tension/drag to meet whatever your requirements might be.  The focusing index is also a great practical addition to the design.   

Attaching a CMOS camera and achieving correct back focus for my ZWO294MC Pro was easy.  Likewise attaching a DSLR camera was just as straightforward. 

The cleverly designed ring mount has a flat section on top with well positioned holes for attaching a finderscope, guide-scope or another relevant accessory. 

The included Bahtinov mask is a useful addition when aiming for pinpoint focusing accuracy with astrophotography – in this regard a really nice touch to the threaded telescope cap is that the illustration of the Redcat “cats whiskers” on the front of the cap illustrates the diffraction spike pattern you should strive for with the Bahtinov mask to achieve perfect focus. 

What about the optics? 

The telescope is fitted with Ohara FPL53 and FPL51 synthetic fluorite glass optics that produce high contrast, superbly sharp results in both photographic and visual observational contexts.  Additionally, being an apochromatic refractor means almost zero chromatic aberration which make this scope ideal for high quality astrophotography endeavors. 

The Petzval design which consists of 4 elements in 3 groups renders a full frame flat field with an accompanying image circle of 44mm accommodating full frame sensors and smaller. 

How does the scope perform under night skies? 

Setting everything up for the Redcat to operate was quick and within minutes the camera was attached and everything correctly balanced.  I opted to primarily use my ZWO 294mc Pro to do a few test runs on various celestial objects but also attached my Canon 60Da DSLR for a quick moon shot. 

From an Astrophotography perspective the Redcat 51 is a fast-little telescope and provides for wide field images of expansive nebulae. I did a series of imaging sessions under less than ideal seeing conditions (Bortle 7/8 skies) but wanted to see how the scope would perform with an Optolong-L-extreme filter inserted and my ZWO camera attached.  

I targeted a few well known and some lesser-known Nebulae and also created a composite moon shot with the DSLR for this review.  Here are the results… 

Cropped image of the Fighting Dragons of Ara (NGC 6188) – 5 hours total integration captured with a ZWO ASI294mc Pro camera through the Redcat 51 (calibrated and stacked in APP and finalized in Light Room/CC2019/Astropanel) 

The Cats Paw (NGC 6334) and Lobster Nebula (NGC 6357) – 2.5 hours total integration captured with a ZWO ASI294mc Pro camera through the Redcat 51 (calibrated and stacked in APP and finalized in Light Room/CC2019/Astropanel) 

First Quarter “Mineral Moon” Composite image – single shot with a Canon 60da through the Redcat 51 and then cropped/combined with a starfield to produce a composite image (Note: the Optolong L-Extreme filter was removed for this capture). 

Wide field image of the Eta Carina nebula (NGC 3372) and Gabriela Mistral Nebula (NGC 3324) – 2 hours total integration captured with a ZWO ASI294mc Pro camera through the Redcat 51 (calibrated and stacked in APP and finalized in Light Room/CC2019/Astropanel) 

Lagoon Nebula (NGC 6523) and Trifid Nebula (NGC 6514) – 2 hours total integration captured with a ZWO ASI294mc Pro camera through the Redcat 51 (calibrated and stacked in APP and finalized in Light Room/CC2019/Astropanel) 

Running Chicken Nebula (IC 2944) – 2 hours total integration captured with a ZWO ASI294mc Pro camera through the Redcat 51 (calibrated and stacked in APP and finalized in Light Room/CC2019/Astropanel). 

The Verdict for Astrophotography? 

A truly superb astrophotography telescope that produces pinpoint, aberration free stars and captures faint Nebulae with ease.  Note that I captured these objects under extremely challenging conditions – Bortle 7/8 skies caused by grassland fires which resulted in moisture laden, fine haze.  In addition, several of the images were captured through bright moon light to add to the difficulty.  Despite this, the scope performed beautifully and combined with the L-Extreme filter to cut through a lot of the light pollution, I was able to produce pleasing images. 

Focusing was easy, the field of view impressive and optics mind blowing. I cannot wait to use this impressive telescope under dark skies. 

What about daytime photography? 

The Redcat 51 is promoted as also being a great wildlife lens and spotting scope.  I attached my Canon 5d Mark IV DSLR to it with a M48mm adapter and chose a few subjects to photograph.  I used a tripod for stability and obviously manually focused the lens using the helical focuser.  It was easy to capture great shots of birds, insects and flowers.  Here are a few examples… 

There was no discernable chromatic aberration or image distortion.  There was also no vignetting of the image proving that the Redcat 51 is built to accommodate full frame sensors.   

I also wanted to test how the bokeh looked by comparison to other high-end photographic lenses.  I chose a well illuminated flower with a sparkly background and include the shot below clearly showing the pleasing, creamy background bokeh produced by the amazing optics at F/4.9 

I think this lens will have even wider application as a prime lens for portrait and landscape photography.  In summary The Redcat 51 is basically a high-end photographic lens at a very affordable price. 

I love the bokeh in this image – The Redcat 51 is clearly an extremely versatile lens 

The Verdict for daytime photography? 

The Redcat 51 is a very welcome addition to any photographer’s lens collection.  I was impressed with the clarity of images produced by it as well as the zero distortion and chromatic aberration free photos.  Focusing was easy and results impressive.  I read that with an extension tube, the lens can be converted to a Macro lens.  I shall have to test this on another occasion but have no doubt it will produce excellent results. 

In Conclusion 

This is a “must have” telescope.  It is incredibly versatile, well priced, and wonderfully engineered/constructed.  I love the look of it, the results speak for themselves, and the field of view is impressive.  It can accommodate a wide range of cameras, is brilliant for observational astronomy, daytime general photography and is very portable.  It also makes a great spotting scope. 

An additional bonus is that GLAgencies have imported a wide range of Williams Optics telescopes and Accessories.  The complete list of what is available in South Africa is at the end of this review. 

I was so impressed by the Redcat 51 that I decided to purchase one for myself so if that is anything to go by, it should convince you of just how amazing this little powerhouse APO is! 

In closing, my advice is as follows: 




Gran Turismo 81  

Zenithstar 81 Blue 

Zenithstar 73 III Gold 

Zenithstar 61 Gold 

Zenithstar 61 Red 

Zenithstar 61 Space Grey 

Redcat 51 


Uniguide 50mm guidescope Red  

Uniguide 50mm guidescope Gold 

Uniguide 32mm guidescope Red 

Uniguide 32mm guidescope Gold 

Uniguide 32mm guidescope Space Grey 


Adjustable Reducer for Zenithstar 73 

Adjustable Reducer for Zenithstar 61 

Adjustable Flat 73 for Zenithstar 73 

Adjustable Flat 61 for Zenithstar 61 

Diagonals/erecting prisms 

1.25” Redcat erecting prism silver 

1.25” Dura Bright Diagonal 

48mm T mounts / Mirrorless Camera T Mounts 

48mm T mount for Canon in Black 

48mm T mount for Nikon in Black 

48mm T mount for Fuji in Black 

48mm T mount for Sony in Black 

T Mount for Canon Mirrorless Cameras (EOS R/ RA) 

T Mount for Nikon Mirrorless Cameras (Nikon Z) 

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