William Optics 73mm APO OTA - 1st impressions

[fwm891]

This scope arrived this afternoon so this is a first impression of the scope.

William Optics ZenithStar 73mm APO – Purchased from First Light Optics, Exeter, UK. March 2019.

Why a William Optics 73mm ZenithStar APO

There are loads of this sized telescope around, rebadged, trinkets added, paint jobs. So Why a WO ZS 73 APO ?

Originally, I started looking at the WO ZS 61 APO to go on my Sky Watcher, Sky Adventurer Pro pack mount. I’d been using the mount with various camera lenses coupled to either of my Nikon cameras (D5100 or D800E). I especially liked the image fov's from lenses in the 400 – 600mm focal lengths. The mount handled them well and unguided subs of 100 – 300 seconds using an intervalometer show very little signs of star trails even with the occasional 600 second sub (due to faulty cable connection).

The WO ZS 61 APO’s focal length at 360mm was just that bit shorter than my 400 -600 ideal range so here enter a WO ZS 73 APO. With a 430mm focal length it crept into my ideal range. At f5.9 it was fast enough (photographically), it came with a variety of goodies and it was not too heavy for the rated loading of the Star Adventurer (less than one of my camera lenses). I looked at longer focal lengths too but their weight didn’t suit the mount when camera, finder etc were all taken into account.

So, I placed my order with FLO and it arrived very promptly (along with the clouds).

It arrived double boxed with semi ridged black foam inserts protecting the contents extremely well.

In the box and assembly:

The WO ZS 73 APO OTA with two finder scope rings (attached – but removable)

A vixen profile dovetail bar anodised gold (matching the finder rings and the scope’s trim)

A 2 inch to 1¼ inch reducer.

End caps, the front-end cap hides a clear Bahtinov mask feature.

The 2018 version has a thermometer fitted in the end of a 1:1 focuser knob.

The focuser has a helical rack-n-pinion adjustable with 1:1 and 10:1 motion control’s

Putting it together meant nothing more than adding the finder scope adjustment screws to the finder scope rings, respacing the OTA rings along the dovetail (if required) – job done.

Visual impressions:

The body has that clean medical exactness about appearance. It’s there to do a job and it intends to do it. The coloured trim adds colour in a not too subtle way. Overall clean, minimalistic and functional.

Other impressions:

I’ll start with the vixen style dovetail bar. What I like about it is the asymmetric hole layout along it’s length, allowing it to be switched 180° to better achieve a balance point* and still retain adjustment on the tube ring positions. It’s 2 inch helical rack and pinion focuser is smooth in operation and I can feel no play even when extended fully with an APS-C DSLR mounted. Focus knobs are large enough for gloved hands when used on colder nights. *The focus lock screw on the underside of the focuser to me is poorly positioned as it hinders the positioning of the dovetail bar if you put a heavy camera or eyepiece assembly in the focuser. The dovetail bar can’t extend backwards far enough for balance unless this screw is removed.

There are three knurled screws holding accessories in the focuser, these press onto a compression ring to secure diagonals, extension tubes etc. I’ve put a 2 inch diagonal and 24mm ES 68° eyepiece in and looked at a tree coming into leaf and I have to say very impressed with the contrast and detail. I was a little disappointed to find that to put my DSLR on I needed a 35mm extension tube to get the scope to focus. Considering how prominent astro imaging is spoken about in the blurb a suitable extension tube should I feel have been included.

Looking down the interior of the OTA from the focuser there are numerous baffle rings along the focuser’ draw tube.

 Tube rings have comfortably sized knobs with nylon washers which either secure or release the tube easily. It’s lens shade/hood slides easily but with enough resistance to prevent it sliding back if viewing near the zenith. There is a small knurled screw supplied to fit into the lens hood but on mine there’s swarf or a bad thread form and my screw doesn’t tighten and secure the hood.

The supplied finder rings are very sturdy and will do the job of securing and adjusting the finder scope well. What I can’t understand is the size and proportion of the adjustment screws for the finder scope. They’re way over sized – especially if you look at the size of knurled screws used to secure equipment in the focuser.

Looking forward to using this in anger under the stars

Once I have some results to share I’ll post them to this review.

Francis

 

 

 

 

[fwm891]

Miraculously it cleared late evening here, not brilliant but good enough to give the scope a quick test. I now know that I will need to be more careful with balance and to that end I will need another small weight on the counterweight bar and also shift the dovetail bar further under the focuser as I didn't feel the dovetail bar was fully supported across the Sky Adventurer's Dec plate.

The built in Bahtinov mask worked brilliantly making focusing a simple task.

There was some trailing to images but the fall-off in image quality from centre to edge of my DX (APS-C) size sensor is quite marked. The trailing I'm putting down to the balance issues and the dovetail bar position making the mounts axles non orthogonal. Smaller sensors using the centre of the fov will be fine. I've added an overlay to one of last nights images showing the sensor sizes I have.

For me as this was bought for imaging - it looks like I'm going to have to get the dedicated field flattener at some stage. Whilst not unexpected I was expecting better. 

On a more positive note before packing everything away last night I put a star diagonal (2 inch) and my ES 68° and 82° eyepieces on the scope and had a quick look around. As with looking at the tree yesterday when unpacking the scope, stars were crisp with good contrast, focusing was very precise - a nice ending to a new scopes first day.

So next I'll move the dovetail bar further back (focus lock screw will have to go to do this), make a small additional weight for the Star Adventurer to better control the overall balance...

Image is 5x90 sec 1600 iso aligned and integrated in PI with a histogram stretch.

TBC

 

[fwm891]

A few photos to show my concerns over the balance point:

Images 1 - 4 show the balance point as it was last night - very near the end of the dovetail bar. Image 4 shows the locking screw's position blocking further movement of the dovetail bar. Image 5 shows the dovetail bar shifted back after removing the locking screw.

 

[fwm891]

Update - the 73A flattener arrived today and as it was cloudy, raining, cold I thought I'd adjust the flattener to WO's specs (11.4 on the barrel scale), put a full frame DSLR (Nikon 800E) focus on an effectively infinite chimney, point said scope at the cloud base and take a look at the results.

There are two images attached. One is as-is camera set to auto. The second is the same frame but given a mild stretch in Lightroom.

Mottling in the frame area is I think from variations in the 'greys' of the cloud base. What I was really interested in was the corner vignettes. I'm pleased as that amount of 'corner' would likely be lost in stacking alignments or taken out with flats.

[fwm891]

First image is without the field flattener with Nikon DX format sensor (APS-C) Next is the DX sensor with flattener, final 3 are with flattener and FX Full frame sensor.  All exposures are around 60 sec, there is some trailing due to imbalance especially with the FX  800E body.

 

 

 

 

 

Edited April 3, 2019 by fwm891
Images annotated

[fwm891]

Quote

I've annotated the images for clarity

 

[eshy76]

Thanks for this comprehensive review - I've had the same scope for a year and I love it! The only fault I found was an inconveniently placed focus lock screw which means I need to turn the scope upside down if I need the dovetail to extend beyond the focuser for balance. Very minor issue.

Also would have been nice if the rotator was marked for degrees...available as a £200 extra!

[RolandKol]

Thanks for a nice review.

As per balance and the locking screw,  - how stiff is the drawtube without locking? And won't you lose camera at some point?

Why not to turn OTA in the rings 180 degrees?

 

Edited April 26, 2019 by RolandKol

[fwm891]

I turned mine through 90° putting the fine focus knob on top where it's easy to get to.

[RolandKol]

And did I understand it correctly,

35mm extension tube is needed for DSLR even if FF is used?

[Han Solo]

I’ve got this scope and I came across the same balancing issue as you did. I had a local machine shop make me a couple of aluminium stand-offs for use with a couple of longer M6 stainless steel machine screws. I can now push the dovetail plate as far back as I need to whilst providing clearance on the focus lock screw. Works a treat. Did cost me £30 though.

 

[fwm891]

On ‎26‎/‎04‎/‎2019 at 11:29, RolandKol said:

And did I understand it correctly,

35mm extension tube is needed for DSLR even if FF is used? 

No, the Flat 73A is set to 11.4 (on the WO scale - probably mm but not sure?) then the 48mm T camera adapter. Total. If your using a cooled camera (CCD or CMOS) then some adapter will be needed to make up the difference between a dslr and short back focal plane cameras.

My pic's above were without the flattener in place - Han Solo has a flattener.

Mine with guide scope. Flat 73A and D5100 on Star Adventurer

Edited April 27, 2019 by fwm891
text added

[Han Solo]

On ‎27‎/‎04‎/‎2019 at 19:54, fwm891 said:

No, the Flat 73A is set to 11.4 (on the WO scale - probably mm but not sure?) then the 48mm T camera adapter. Total. If your using a cooled camera (CCD or CMOS) then some adapter will be needed to make up the difference between a dslr and short back focal plane cameras.

My pic's above were without the flattener in place - Han Solo has a flattener.

Mine with guide scope. Flat 73A and D5100 on Star Adventurer

Hi Francis,

You have a really cool looking set up. I wonder if you will be able to clarify something for me. I have this scope and the adjustable field flattener. Am I correct in saying that the flattener is adjustable to suit different scopes and not to adjust for required camera sensor back focus? I'm struggling to find what size spacer I need between the back for the flattener and my 183m sensor.

Thanks,

James

Edited April 28, 2019 by Han Solo

[Robny]

I have the GT71 which has the same awkward focus lock screw, I habe put my scope on risers to get balance which helps to clear the focus lock.

Rob

[Stu1smartcookie]

I had this scope before i stupidly sold it !!! ( it will honestly be one of my lifes regrets) , im back with WO though in the shape of a z61 . I thought your review was spot on ..its really good to read actual in depth reviews such as this . I never got to image with my 73, but i cant wait to use the 61 for this purpose . One things for sure , the build quality on WO gear is absolutely fantastic .

[JeffM]

Sorry to resurrect an older thread, but I’m thinking of a Zenithstar 73 in almost the same way, with a DSLR, on my star adventurer with a guiding through my 32mm f4 guidescope/ZWO 224mc. My concern has been weight and how close it pushes the SA to its max. How are you getting in with it now?

[fwm891]

Hi @JeffM , Although the extra weight took the SA to it's limits it also smoothed things down and with a careful PA I've found I can get 3 min subs quite easily. I've now sold the SA and moved to a CEM25P with the WO 73.

The SA and ZS 73 (+flat73A) are a great little combo not just for grab and go stuff.

Edited May 29, 2020 by fwm891

[Coriorda]

On 31/03/2019 at 11:16, fwm891 said:

A few photos to show my concerns over the balance point:

Images 1 - 4 show the balance point as it was last night - very near the end of the dovetail bar. Image 4 shows the locking screw's position blocking further movement of the dovetail bar. Image 5 shows the dovetail bar shifted back after removing the locking screw.

 

What is the weight of this set up- does it cause you any tracking issues with the Star Adventurer ? Thanks 

[AstroM1]

Hi All!

Hi Francis (fwm891). Thank for your very well detail post. I recently bought myself a WO ZS73 and your comment and review helped me to confirm my choice. As you previous mount, I have a Star Adventurer pro. Since I solidified the wedge (added a chim in the alt gear), my setup is more stable. I also image with a Nikon D5100.

But, I am curious to know if you can give me some advices on how to use/optimize my new refractor and my current setup.  Did you had issue with cleaning your lens? Did you do any maintenance on your WO ZS73? Etc.

Thanks in advance.

Cheers, Marc

Edited December 30, 2020 by AstroM1

[callisto]

22 hours ago, AstroM1 said:

I have a Star Adventurer pro.

I think you would be pushing it a bit with ZS73 on the S/A mount....I use a WO ZS61 on my S/G Pro mount an it works a dream

[AstroM1]

56 minutes ago, callisto said:

I think you would be pushing it a bit with ZS73 on the S/A mount....I use a WO ZS61 on my S/G Pro mount an it works a dream

Hi @callisto you are right... I am closed to 4kg. I am planning to replace the mount next summer... I will take shorter exposer

Cheers, Marc

[Stu1smartcookie]

Reviving this thread after a couple of months ,... i have just repurchased a ZS73 ... Can someone  please advise  me if i can mount a finders shoe on the scope ( there are two pairs of Allen headed screws that i can install it ( on the 10 o'clock and 2 o'clock position ) Hope that makes sense 

Stu

[Taman]

4 hours ago, Stu1smartcookie said:

Reviving this thread after a couple of months ,... i have just repurchased a ZS73 ... Can someone  please advise  me if i can mount a finders shoe on the scope ( there are two pairs of Allen headed screws that i can install it ( on the 10 o'clock and 2 o'clock position ) Hope that makes sense 

Stu

Just had a quick look at mine. I don't think those screw points are suitable for a finder shoe. The holes are also at right angles to the shoes that I have.

The latest ZS73 has a finder shoe built in to the handle. 

Tony.

[Stu1smartcookie]

Hi Tony ZZZZ lol ive just bought this and noticed the little handle in the case/bag ... unbelievable . ( message to self ... LOOK ) lol

Thanks for the reply . 

Stu 

[Taman]

Just now, Stu1smartcookie said:

Hi Tony ZZZZ lol ive just bought this and noticed the little handle in the case/bag ... unbelievable . ( message to self ... LOOK ) lol

Thanks for the reply . 

Stu 

No problem Stu, enjoy your new scope, I'm having fun with mine!

They are beautifully engineered.

Tony.