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William Optics Zenithstar 73 III APO Refractor OTA

William Optics Zenithstar 73 III APO Refractor OTA
Price:  £629.00(Inc. 20% VAT)

Model:  A-Z73IIGD
Brand:  William Optics

2 Year Warranty On All New Items
(Excluding Batteries & Globes)

  (2 Reviews)

Now complete with soft case as standard

Special Offer:  A free Zygo Interferometer Test Report & Tune is available when you order 1 item or more of selected products.

2019 All New Zenithstar 73 APO

Zenithstar 73 is a greatly versatile and portable refractor telescope for beginners and veterans alike. Designed for both astrophotography and wildlife photography, high power visual observing through the Zenithstar 73 is also hugely satisfying. The image is crisp, sharp and colour aberration free thanks to the use of top quality glass — FPL53, closely matching the legendary fluorite material. The 43mm image circle when Flat73 is used covers full frame size sensor nicely.

ZenithStar 73 f=430mm
F/5.9 FPL53 doublet APO

Retracted Length:390mm
Weight :2.83kg
2.5” R&P Focuser

Package includes:

‧ Z73 APO

‧ Innovative Bahtinov Mask

‧ Mounting rings

‧ Vixen style Dovetail

‧ Soft Carry Case

Optional accessories:

‧ Guiding rings

Average Rating (2 Reviews):  
Write a Review and share your opinions!

09 November 2022  | 

superb quality scope , kindly delivered to NI by the RVO team. Not their normal policy.Better price than main rivals too

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Awesome Telescope
21 February 2021  | 

I bought the Z73 purely on the basis of Trevor Jonesís gushing reviews and I purchased it purely for Astrophotography purposes.

Having found a new passion for Astronomy only 2 years ago after a 20 year hiatus and discovering that you could actually image deep space objects from your own back yard was an opportunity I couldnít allow to pass me by.

I bought the scope as a package from RVO which included the Z73, 50mm guide scope, mounting rings, dove tail bar and integral bathinov mask. I also bought the Skywatcher HEQ5 Pro mount to get me off and running.

The telescope itself looks awesome (Iíve got the Red trim model) Itís rock solid, sleek, finished beautifully and is really well made.

Now onto the important bits -

- As a wide field telescope for astrophotography this scope is excellent. In my opinion, there isnít another telescope on the market that compares to the super sharp optics, quality of build and usability of this scope at what is a very reasonable price for what you get.

You will however 100% need the William Optics dedicated field flattener if you plan to use this scope for astrophotography.

I use the Z73 with the field flattener and a Canon 200d Astro modded DSLR camera which has a C-Size sensor mounted on the HEQ5 Pro mount and the results are simply astounding.

This telescope is a wide field scope and is absolutely perfect for imaging open star clusters (Pleiades) vast nebulae (Orion, North America, California) globular clusters and also certain galaxies (Andromeda, Triangulum, Pinwheel) to name just a few.

If you want to take images of distant galaxies then this isnít the scope for you. If you however want to image vast nebulae, star forming regions and their surrounding nebulosity then this is the scope for you.

I cannot say enough about this scope and even seasoned pros hold this telescope in high regard.

I would pigeon hole this telescope as an upper mid range piece of equipment. There are a lot more expensive scopes out there and there are a lot less expensive but as far as quality of optics and affordability is concerned, you will not find a better 430mm focal length - 73mm apochromatic refracting telescope on the market to come anywhere close.

Even the more expensive apochromatic triplets and quads that are available of similar focal length and aperture will not come close to the quality of this scope.

5 stars all the way. 10 out of 10. Simply put the best scope on the market in this focal length / aperture range.

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13 of 13 people found this review helpful.

Some items that are showing as in stock are in stock at our holding warehouses, not at the shop. To double check the stock levels at the shop, please contact us on 01909774369 or

The delivery times for most items is 1 - 2 days but again, please double check stock levels on important orders before placing an order via the website.

The delivery costs for courier next day are for mainland UK only. If you live in the Scottish Highlands/Islands, Channel Islands or Northern Ireland, then a shipping surchage will apply. please contact us for prices. Smaller items can still go 1st class via Royal Mail.

Please note that if you are wanting to take advantage of our Saturday and Sunday timed services or next day pre 12 or pre 10:30 services, you MUST check that the stock is here at the shop before placing orders. Also, orders using the Saturday and Sunday services must be placed before 1pm the previous working day. Also, please remember to check that the stock is here at the shop before placing orders. If the stock is not at the shop, we won't be able to ship on any of the timed services. 

Please note International customers will need to contact us for a shipping price prior to ordering via the website. We don't ship all items internationally. It is at our discretion what we ship outside of the UK. This all depends on the size and weight of the items. Please get in touch with us for any postage costs. Please note we do NOT ship telescopes or heavy items outside of the UK. This is due to massive postage costs and problems regarding returns.

If you have any questions regarding stock levels or delivery, please get in touch.

Zygo Interferometer Testing For ED Refractor Telescopes!

We are extremely pleased to announce our Zygo Interferometer is now installed and ready for use! Our experienced staff can test refractor lenses up to 120mm aperture and provide a comprehensive 2 page report showing the optical quality and Strehl ratio, plus much more information. We are using the very latest Zygo MX software to provide a more detailed report compared to other standard reports.


The interferometer is essentially a measuring instrument. Within the interferometer is a calibrated laser that is pointed through the optical test surface, then onto a high quality reference sphere, then back through the test subject then to an internal camera which gives us the measurement on the screen. From this, we can adjust the position of the test optic and the reference sphere to obtain a perfectly calibrated system. The more central the laser to the test optic and then on return, the better and more accurate reading and measurement we can take. This takes a lot of adjustment to get the best results from the optical surface. Once the measurement has been taken, we are provided with a series of results that show the optical quality of the surface plus other information. Continue reading for an explanation of the individual values and you can also see a test report that was done on one of our RVO Horizon 60 OTAs. 

We are offering certain telescopes with a Zygo test at an extra cost. Mainly ED doublets and triplets. All available telescopes with a Zygo test report are listed here. Just select Zygo Test Report from the drop down box at the top of the listing and we will test the optics in house and provide a full 2 page report with your scope. This will show you the optical quality of your telescope optics. We can also offer this service on your refractor telescope. Please get in touch for more details about this. 

At Rother Valley Optics, one thing we pride ourselves on is customer service and quality of our equipment. When we test your telescope, we will ensure that it is of the highest quality otherwise, we will reject the scope under our strict quality control and test another model for your peace of mind. One of the main values to look at when testing a refractor is the Strehl ratio. The higher this number, the higher quality lens you have. At Rother Valley Optics, we will guarantee the highest possible Strehl ratio for your telescope.  We will aim to provide 0.95 Strehl or higher on Takahashi, RVO Horizon, William Optics and many more! Generally, the higher the Strehl ratio, the lower the Astigmatism and Coma meaning a better image quality from your instrument. 

Other measurements such as the PV and RMS are important too. With these measurements, the lower the number the better as it shows less curvature on the lens. The flatter an optical lens, the better it will perform. The PV rating (Peak To Valley) is a measurement from the highest point (Peak) to the lowest point (Valley). From this we get the RMS (Root, Mean, Square). This is a similar figure to the PV and measured exactly the same way. The RMS is basically an average height difference from the PV so again, the lower the better. As you can see form the example below, we measured an RMS of just 0.027 wave which is a very good value.

We also measure the Power with our systems which is essentially the error from the reference sphere to the test optic. The reference sphere we use on our system is a very high quality sphere so the closer to zero the Power, the less error between the 2 elements thus ensuring a higher quality optical surface. 

You will also see some more values such as Astigmatism and Coma. Again, these are measured by the interferometer when we take measurements of the optical surface after adjustment. These are values which ideally would be at 0 but as our test optics are all curved lenses, there will always be astigmatism and coma present. The lower the number, the less of these aberations you will effectively see but with most refractors, you can buy field flatteners which will compensate for the coma anyway. The astigmatisms value is explaining the error between the symmetrical curvature of the lens. Basically, no lens will be perfectly symmetrical, the lower the astigmatism, the more symmetrical the lens surface is. 

Once the Zygo test report has been completed, the optical alignment is thoroughly tested and tuned to ensure they are spot on before they leave us. 

All available telescopes are on the website at the link below and ones we've tested already that are available straight away are also listed.

Here is an example of the test what we provide. This was a test on one of our Horizon 60 ED OTA's.

For any questions about this amazing new service, please either call 01909774369 or email


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