William Optics GT81 WIFD Review

The William Optics GT 81 (Gran Turismo) is an apochromatic refractor telescope with an innovative internal focus design (WIFD).

William’s ‘Gran Turismo’ line of refractor telescopes have been around for many years, and are well enjoyed in the amateur astrophotography community. 

This telescope features an impressive full-frame image circle (44.4mm), which is an important specification for those using large sensor astrophotography cameras.

The William Optics GT 81 is available at Agena Astro for $2000 USD.

The William Optics RedCat 61 WIFD shares the internal focus design with the focuser knob placed near the center of the telescope. 

I believe the GT81 WIFD will be a popular choice for astrophotographers looking for a premium mid-range APO in the 500mm focal length range. 

In June 2023, William sent me a demo copy of the new GT81 WIFD. As always, I was not compensated for a positive review.

Lion Nebula

The Lion Nebula in Cepheus (HOO). William Optics GT81 WIFD and ZWO ASI2600MM Pro.

William Optics GT81 WIFD Review

The William Optics GT 81 WIFD offers premium optics using proven FPL-53 glass in an attractive package. For $2,000 (USD) you get an extremely capable, full-frame compatible, apochromatic refractor telescope with a practical focal length and impressive design.

The GT81 performs optimally with the use of the correct adapters to achieve the proper backspacing (see below). Keep in mind that to fully utilize the performance of this telescope the FLAT68 III field flattener is needed. 

Backspacing GT81

Diamgram showing the proper M48 adapter and field flattener with the GT81. 

A focal length of 480mm is very versatile for deep-sky astrophotography projects. This field of view is wide enough to capture nearly all of the brightest nebulae in the night sky and does a commendable job on medium-sized galaxies. 

My first project with the GT81 was the Lion Nebula in Cepheus, which is a rather dim emission nebula that covers a large area of the sky. Most deep-sky projects that I enjoy capturing fall within the 500mm range, such as the Eagle Nebula, Lagoon Nebula, and the Orion Nebula

In the following video, you can see my complete astrophotography setup involving the William Optics GT81.

First Impressions

Because the Gran Turismo line of imaging refractors is so well received in the astrophotography community, the new WIFD version created quite a buzz when it was first announced.

THE WIFD focuser is a nice upgrade, and balancing the rig is indeed easier due to a more centralized weight. However, the large course focuser knob now comes very close to the mounting plate of my ZWO AM5 mount. 

 focus knob GT81

The focuser knob comes very close to running into the mounting saddle of the ZWO AM5. 

I can confirm that I am still able to use the focuser with it slightly touching the mount saddle, but it is something to keep in mind when first setting up the telescope. 

It’s a hefty telescope (10 pounds including the mounting plate and tube rings), so you will need a capable tracking mount to carry the GT81.

As usual, this William Optics refractor includes a handy Bahtinov mask that is built into the lens cap. If you are not using an autofocuser, the focusing mask is a huge help. 

This is a practical design feature that was first featured on the RedCat 51, and I continue to take advantage of this feature. 

William Optics Internal Focus Design (WIFD)

The WIFD design aims to improve on the traditional telescope focuser design to better accommodate today’s heavier image trains.

A modern astrophotography setup may include a rotator, an off-axis guider, a filter wheel, and a camera. The weight of this gear can cause the focuser to slide out of focus over time (especially when pointed near the zenith). 

Sensor tilt is another issue many amateur astrophotographers deal with, and this issue can be extremely difficult and frustrating to solve.

The WIFD design moves the focuser from the rear exterior of the telescope to the inside of the tube. The focuser drawtube stays inside of the telescope, which means it is protected from the elements.

The redistribution of weight can potentially allow for better balance during your imaging session when compared to a traditional ‘back heavy’ system.

The WIFD series of refractor telescopes are still compatible with autofocusers like the popular ZWO EAF. 

William Optics GT81

William Optics WIFD GT81 Specifications


  • Optical Design: APO Triplet Refractor
  • Lens Glass Type: FPL-53
  • Focal Length: 478mm
  • Diameter: 81mm
  • F/ Ratio: f/5.9
  • Focuser: WIFD, Compatible with EAF
  • Total Weight: 10 lbs
  • Camera Format: Full Frame | DSLR | Mirrorless | CMOS | CCD

Included with the Telescope

  • William Optics WIFD GT81 81mm f/5.9 Apo Refractor OTA
  • Soft Embroidered Case
  • Lens Cap with Built-In Bahtinov Mask
  • Zygo Test Report

Recommended Accessories

Solving Back Spacing Issues

The William Optics GT81 gave me some grief when it came to setting up the correct backspacing (often called ‘back focus’) of my camera.

The specifications from the manufacturer were to use 55mm of backspacing, with an additional 7.1mm of spacing when using the FLAT6AIII flattener that I was using. 

The problem was, that I wasn’t using the appropriate adapter between my camera and the adjustable field flattener (see image below).

GT81 adapter for proper backspacing

My early backspacing issues with this telescope fell within the ‘user error’ category (as many things do in the Astro Backyard). However, others can avoid this mistake by using the appropriate adapter. This is the adapter you need

Backspacing Specifications

The following backspacing requirements come directly from William Optics, and the Agena Astro website.

  • Telescope only: Available maximum backfocus is 183.4mm from the base of the M63 male thread.
  • Telescope with P-FLAT6A III:  55mm with flattener adjustment scale set to 7.1mm
  • Telescope with P-FLAT68 III: (sold separately and requires use of an additional adapter): 55mm with flattener adjustment scale set to 10.9mm

Remember, if you are using the adjustable field flattener (FLAT68 III) like I am, you need to use an additional adapter to reach the correct backspacing (see below).

adjustable field flattener


Final Thoughts

I see this telescope fitting well for anyone ready for their second refractor telescope, especially if they are coming from a shorter focal length model like the RedCat 51.

The increased focal length at an f-ratio of F/5.9 means that you can now take on new astrophotography projects that are smaller in size, including medium to large galaxies.

Once you have invested in the dedicated field flattener or reducer for the GT81, you can enjoy flat, full-frame performance with excellent color correction.

It’s a heavy scope for its size (10 lbs), meaning this model is outside of the star tracker category. The ZWO AM5 or Sky-Watcher EQ6-R Pro is a safe choice for the GT81.

If you plan on using filters with this telescope and your DSLR camera, a filter drawer is best. This makes it easy to swap your favorite light pollution filter for a multi-bandpass narrowband filter like the Optolong L-eXtreme.

If you are using a monochrome dedicated astronomy camera, a filter wheel is the best option. The ZWO EFW is an affordable choice and the one that I use with my ASI2600MM Pro camera.

My first ‘real’ astrophotography telescope was an 80mm APO, and I can’t think of a way to enter the hobby and begin exploring more.

Lion Nebula


  • Premium Optics (FPL-53)
  • Robust Focuser
  • Flat Field
  • Great Color Correction


  • Field Flattener Sold Separately
  • Must Use Adapater for Correct Spacing
  • Focuser Knob Position

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