How an AZ4 really copes with a big frac

[parallaxerr]

I have, for a long time, restricted myself to the use of a Sky-Watcher AZ4 mount and for good reason. There’s no two ways about it, I like refractors. I’ve had quite a few over the last few years and the pick of the bunch was a lovely Sky-watcher Equinox 120ED. I picked the Equinox up at a bargain price and very much regret letting go of it, but it wasn’t getting the use it deserved. The mounting requirements were significant and to enable the scope to perform at its best I mounted it on a Giro Ercole atop a 16” steel pier extension and a CG5 2” stainless steel tripod. Having spent a lot of money arriving at this setup, I stopped using it. Why? Size & weight.

The whole mount assembly was very heavy. Combined with the required journey through the house to get to the garden (resulting in many door frame dings), followed by the many steps to set it up on the top level of our tiered garden, it just became too cumbersome and I stopped bothering.

Shortly after selling that setup I discovered the AZ4 and it was a bit of a revelation for me. With a bit of tuning I found I have a great Alt-Az mount with smooth motion that handled a 4” scope with ease. One of the features I particularly like is the fact that the scope rotates around the Az axis, whereas with Giro style mounts the scope is mounted out on an arm and the throw and motion of the scope at the eyepiece end felt a little un-natural to me. For a while I used a 102mm ED with the AZ4 which was a perfect match, but that scope didn’t really float my boat. Then I picked up an 80mm F11 Vixen achro which is a great performer, but of course all the while I’ve been suffering aperture fever knowing what a 120mm scope can pull in from my location.

So, I started researching how the AZ4 would handle a 120mm refractor, I read many posts and spoke directly via PM with several other owners. There were mixed opinions that ranged from “unusable” to “rock-solid” but of course there are variables such as the tripod used, the weight and focal length of the scope etc. I am confident that my AZ4, again mounted on top of a steel pier but this time on a 1.75” steel legged tripod is as sturdy as an AZ4 can get. This mount weighs considerably less than the Giro and 2” legged version and is far easier to handle. When I found a batch of new Celestron Omni XLT120 F8.3 achros for sale, I pulled the trigger.

First impressions of the scope on the mount were that it looked perfectly acceptable and everything balanced up nicely. The Axes remained smooth in operation giving me some confidence that it would all be OK. First light came at the end of last week. The forecast was not great with gusty winds and scattered cloud, but I setup anyway. The moon presented itself in the perfect pre-dusk position, nice and high just past the meridian and at approximately 50% phase.

The lighting was near perfect for observing one of my favourite lunar features – the Apennine Mountains and the Apollo 15 landing site at Mons Hadley. I decided I’d have a bash at Hadley Rille and set about testing the scopes performance at a range of magnifications right down to my shortest 5mm EP, yielding a mag of X200. The scope held up VERY well, far better than I had imagined it would. I noted that the view was particularly cool, compared to me previous ED scopes. CA was present of course, but only really became distracting at the higher mags. I tried the Baader semi-apo filter but didn’t really get on with it this time, instead opting for a #8 light yellow filter which suppressed the CA more than satisfactorily and only gave a very light cast to the image. The shadows of the Apennine Mountains were a deep dark black with fantastic resolution. The 5mm LER eyepiece presented a very sharp image which I was pleased about considering the image starting breaking down in my 80mm scope with this EP, obviously a function of the scopes aperture. In fact, so sharp was the image I pushed further using the 9mm LER barlowed to approximately 4.5mm yielding X222 and Hadley Rille was there in all its glory. Clearly defined at its Southern end with the Northern end a little harder to define, but it came and went with the seeing. The resolution of the image with this 120mm glass was notably superior to the 80mm.

When reasonable darkness set in I slewed the scope around to the East, away from bright moon and inserted the 32mm plossl, this time un-filtered. I scanned around waiting for my eye to adjust after the bright moon, taking in the very sharp pinpoint stars displaying no discernible CA at this low mag. I decided to carry out a star test and with my limited horizons, Arcturus presented itself well. The star test was all that I had hoped for. Very nice concentric rings inside of focus and slightly softer outside of focus suggesting slight under-correction. I am aware though that the spread of colour due to being an achro can cause the softness so would need a green filter to test properly. Most importantly though, the lens cell is collimated well so I know at least it’s performing at its best and I am more than happy with the result.

So far so good then. Very happy with the scope but how did the AZ4 perform?

I would say that the 120mm F8 frac has the AZ4 at its limit for high mag work. There was some wobble when the wind picked up, however I would not normally observe in such winds. Pre-empting vibration during focusing, I had already fitted the scope with a Sky-Watcher dual speed Crayford focuser which worked very well. The light touch required to focus with the fine knob did not impart excessive vibration, it was a level I can certainly live with. The limitation comes when manually tracking while observing. The 55° FoV of my short FL EP’s results in lots of manual correction at high mag with the associated vibration. A wider apparent FoV EP would be better suited here, or a tracking mount. Placing the target in the upper right of the FoV and allowing it to drift was the best option, allowing enough dwell time to make out fine detail, though at X222 the target moves fast!

My desire for more aperture though comes from improving DSO observations, and for this, at lower magnifications this setup will be AOK. With a 25mm EP giving me close to my known preferred exit pupil, vibration is not an issue and I’m really looking forward to seeing what can be pulled in come the Autumn when the skies darken and more interesting objects come in to my view. I am particularly looking forward to viewing the Veil Nebula considering the success I had with the 4” ED using an Astronomk OIII filter.

All in all, a positive first test of the scope and how the mount handles it. Hopefully I’ve put a bit more meat on the bones on the subject of how the AZ4 handles a 120mm refractor and this will help someone considering a similar setup to make an informed decision. If you give the AZ4 a solid foundation it can work well, depending on the type of observation you want to do. My setup is very sturdy and any residual  vibration comes from the mount head itself as opposed to the tripod, I think I’d be inclined to agree that this wouldn’t work on the aluminium legged version!

 Cheers,

Jon

Edited May 4, 2020 by parallaxerr

[John]

Good report and I'm glad you are pleased with this setup Jon

I tried my ED120 on my AZ-4 a few times but overall I found that combination did not work for me other than at low to medium powers.

I reckon your pillar adds a bit of stability though, and the strap from the central tray to the ground helps as well. These things might make all the difference.

I do like the rugged simplicity of the AZ-4 mount. 

 

 

[parallaxerr]

1 hour ago, John said:

I tried my ED120 on my AZ-4 a few times but overall I found that combination did not work for me other than at low to medium powers.

I should have mentioned the weight. The Omni with Crayford and finder weighs in at 6kg. Do you know what your ED120 weighs John?

[parallaxerr]

1 hour ago, John said:

I reckon your pillar adds a bit of stability though, and the strap from the central tray to the ground helps as well. These things might make all the difference.

Agreed. I found if I tap the pillar there was next to no vibration, but tapping the scope there was. This suggests the flex is in the mount head itself and the pillar and tripod are sturdy enough.

Edited May 4, 2020 by parallaxerr

[dweller25]

Strap is a very good idea - I’m guessing you have a big metal screw in the ground ?

[parallaxerr]

10 minutes ago, dweller25 said:

Strap is a very good idea - I’m guessing you have a big metal screw in the ground ?

Yes, that's right. It's a ground anchor, cant remember what the original use for it was but it's about 12" long. It's been there for about 18 months now, never budges.

[John]

55 minutes ago, parallaxerr said:

I should have mentioned the weight. The Omni with Crayford and finder weighs in at 6kg. Do you know what your ED120 weighs John?

My ED120 with the Moonlite focusser, 2 inch diagonal and 9x50 RACI finder is 6.5kg.

 

 

[jock1958]

Hi Jon great write up, your Celestron looks the bees knees atop your AZ4 combination.

Like you I wanted a lightweight set up with the ability to move it out of the house quickly when observing opportunities present themselves without taking anything apart.

I've added and subtracted bits and bobs over the last few years and settled with the AZ4 steel leg tripod /  Skytee 2 combo with a 5" ally extension tube for my Tak FC100 DL. 

Question whats that little silver bracket on the bottom of your AZ4 mount for?

[parallaxerr]

7 minutes ago, jock1958 said:

Question whats that little silver bracket on the bottom of your AZ4 mount for?

TADA.. they say a picture paints a thousand words! It's a mobile phone holder so I can use stellarium to pull up alt-az cords and find the fuzzies using the az ring and a wixey angle finder

 

[jock1958]

1 hour ago, parallaxerr said:

TADA.. they say a picture paints a thousand words! It's a mobile phone holder so I can use stellarium to pull up alt-az cords and find the fuzzies using the az ring and a wixey angle finder

 

Aha I thought it was a phone holder but couldn’t see it properly. I’ve got a similar set up though not as neat or lightweight as yours!

Edited May 4, 2020 by jock1958

[Timebandit]

 

 

For many years now I have used my SW Equinox 120Ed with an AZ4 without any real problems. It is on a 2" leg tripod , and a slight vibration at times on very high powers is noticeable for a few seconds(even though with the tripod sitting on the grass the vibration seem dampened considerably). But overall nothing that offends me. Seems to handle the weight of the 120ED fully loaded with 2" TV diagonal and some large 2" eyepiece's without a problem. I have heard that the AZ4 supposedly is not up to a 120 size refractor scope. But from my experience all is good and a very sensible priced and. effective mount it is too.