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William Optics FLT120 owners thread


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I wanted to create a thread to contain all information, tips, guides etc. related to the William Optics FLT120 telescope in one place. An index has been included which includes links to the direct post for each topic in question.

This telescope was released in the UK circa September 2021 so is pretty new to the market, therefore I will be adding my learnings with this telescope along the way and will invite others to do the same! Here are some product listings to William Optics website which include the telescope and the two recommended flatteners, followed by a list of the critical product specifications:

https://williamoptics.com/products/telescope/fluorostar/fluorostar-120

https://williamoptics.com/products/accessories/barlows-flatteners/flat7a

https://williamoptics.com/new-william-optics-flattener-68iii

  • Focal Length: 780mm (native) | 780mm (Flat 68III) | 624mm (Flat 7A)
  • Diameter: 120mm
  • Aperture: f/6.5 (native) | f/6.5 (Flat 78III) | f/5.2 (Flat 7A)
  • Image circle: >full frame
  • Recommended flatteners: Flat 7A and Flat 68III
  • Focuser: 3.3" Rack and Pinion
  • Tube weight: 7.2kg
  • Optics: FPL-53 triplet APO
  • Length (full retracted/fully extended): 680mm/850mm
  • Accessories included: Soft carry bag; anodised scope rings, carry handle w/integrated vixen shoe mount and losmandy plate; dust cover with integrated bahtinov mask

Index (with hyperlinks to comment)

 

FLT120_1.thumb.jpg.554992b5efd2c0741151541864bb9399.jpg

Edited by Richard_
Updated index
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First up is the focuser. The telescope features an impressive 3.3" Rack and Pinion (R&P) focuser which offers a wide range of focus and can handle heavy imaging equipment. When standing behind the camera/eye piece end, the left side of the focuser features the coarse focus knob with integrated temperature dial, and the right side features a coarse knob with a 1:10 fine focus knob.

Out of the box, the coarse knobs felt stiff for half a rotation and smooth for the other half of the rotation, which was repeated across the entire length of adjustment (approximately 90mm as indicated by the white markings on the draw tube). Whilst the coarse adjustment is useable for visual etc, the fine focus knob will not move the draw tueb in when the scope is pointed towards the zenith (it will just spin around), especially when loaded with a flattener and a camera. I am a bit disappointed with this for a scope of this price. I reached out to the vendor (FLO) who provided some instructions on how to adjust tension etc.

Attached is the PDF which FLO provided to me. I've taken a picture of my actual focuser and have annotated per below image. I loosened the "locking screw" and backed out the "adjust tension" screw to no avail. I then loosened the "gear meshing screws" out 1/8 to 1/4 turn each and voila, problem solved, the focus knobs are now smooth for a full rotation across the entire length of focusing. Whilst these screws hold the focuser to the body, it's possible that the gears are meshed too tightly causing higher friction and no reaction from the fine focus knob if too much weight is loaded. With my Flat 68III, ASI533 and Mini-EFW installed, I can now point the telescope to zenith and move focus inwards using the fine adjustment knob. Awesome!

I'll follow up with another post on the installation of the ZWO EAF to the coarse knob of the FLT120.

1968191225_FLT120annotatedfocuser.thumb.png.632ffc5426ef2c8d6c6515f1140d3c06.png

FLT 120.pdf

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25 minutes ago, scotty38 said:

Why would you do such a thing, you're no better than those folk stood on street corners plying their wares on the weak! 🤣🤣🤣

Haha, please accept my apologies scotty! Just think, when you eventually find a buyer for one of your kidneys and have bought this telescope (if it's on your 'to buy list' that is), then you'll have all the information in this thread to get you up and running :D

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Installation of the ZWO EAF

I actually completed the installation of the ZWO EAF back in October 2021 but I never wrote it up. At the time, there were no online guides showing how to remove the focus knob and mount the EAF specifically for this telescope/focuser so I spent a couple of hours figuring it out. Once you understand what you need to do, this entire install can be done in less than 15 minutes. Since October, I've had a number of sessions with this autofocuser and I've had no issues. Refer to Fig.15 and Fig.16 at the end if you wish to see the finished install.

Everything you need to complete the installation is provided in the ZWO EAF box. Here's a list of the exact components:

  • ZWO mounting bracket
  • ZWO EAF
  • 4 x M4x8mm cap head screws (3mm hex key fitting)
  • 4 x washers
  • 4 x M4x6mm grub screws (2mm hex key fitting)
  • Bushing with innder diameter ~6mm (second from right inside ZWO EAF box)
  • 3mm hex key
  • 2mm hex key

All items are laid out as shown in Fig.1 below. The fancy coloured hex keys are my own, the ones provided by ZWO are silver 🙂

Fig.1

813959398_1.Allequipment.png.c3502420176c37ce9a5fef1613c66ba6.png

 

The EAF will fit onto the coarse adjustment knob which features the embedded thermometer. Unlike the coarse/fine adjustment knobs on the other side, there is more than one screw which needs to be removed and these are hidden behind an access screw as indicated in Fig.2. Use the 2mm hex key to fully remove this access screw.  Important: this access screw will not be screwed back in after EAF installation, so I'd recommend keeping this safe. I ended up wrapping this screw with clear sticky tape which I stuck to the inside of the coarse knob after it was removed! If you do happen to lose it, it is a standard "cup-point M4x4mm" grub screw which can be easily found online.

Fig.2

1241345765_2.Removeaccessscrew.png.de61a94e236b4bfd383d37a9f9b9d997.png

 

Before we remove the two screws holding the knob onto the main shaft, it's easier if I show you the picture after removal so you know what to feel for inside the access hole. Fig.3 shows two screws which are about 90 degrees from each other which hold the knob directly onto the focuser main shaft. Referring back to Fig.2, we will be loosening these two screws by inserting the 2mm hex key into the access hole which we have just cleared. It can be a little tricky to see the screws through the access hole (black screw on black body), so I'd recommend using the light from a camera phone or perhaps a headtorch to keep both hands free. One of the screws will be clamped onto a flat portion of the shaft whilst the other will be on the curved surface, so just back them out 1/2 to 1 full turn just to loosen them enough to wiggle the knob free (you do not need to fully remove the screws).

Fig.3

789128050_3.Coarseknobscrews.png.37860ca920ac5d0e0f8e4a61652fb7a8.png

 

After freeing the coarse knob from the focuser, you will see the main shaft which the ZWO EAF will be attached to (Fig.4). Notice the flat portion of the shaft: this is where one grub screw from the original focuser was tightened against, we will do the same with the EAF. Measuring the outside diameter of this shaft at the curved edges yields a measurement of approximately 6mm (Fig.5), which is great for us because ZWO provide a bushing with an inner diameter of ~6.2mm (Fig.6) which will easily slip over the main shaft.

Fig.4

1628353771_4.Mainshaft.png.46f5e32200099c4ab75dd902a0397123.png

 

Fig.5

1743663584_5.Mainshaftmeasurement.png.efbd5eae1289f7ad2f445e279db0c214.png

 

Fig.6

124608045_6.EAFBushing.png.f085e4289dd5113cc72531d09b103bce.png

 

Before attaching the bushing to the main shaft, it's a good idea to screw all four of the M4x6mm grub screws into the bushing using the 2mm hex key just enough so that they are sub-flush on the inside and the outside of the bushing surface (Fig.7). This will make it less fiddly when we go to tighten them up!

Fig.7

1761487815_7.EAFBushingwithscrews.png.227550846663776ff4994914c6be7937.png

Rotate the opposing coarse knob until the flat portion of the main shaft is facing upwards. Whilst holding the coarse knob in place to prevent rotation, slip one end of the bushing over the main shaft and line up one of the screws through the access hole (Fig.8). Luckily, not only are the black grub screws easier to see against the silver bushing, they are also in alignment with the grub screws on the outside so you can use these to line yourself up through the access hole. Ensure that the first grub screw is directly over the flat portion of the main shaft and tighten using the 2mm hex key. This is a very important step as you don't want to introduce any slippage between the bushing and the grub screws. The bushing may slip at this point so support the bushing loosely in one hand and turn the coarse knob with the other hand until you see the next screw through the access hole. Proceed to tighten the second screw with the 2mm hex key and go back and forth between the two screws, nipping them up until they feel hand tight. Test the coarse knob and make sure that there is no slippage between the bushing and the main shaft.

Fig.8

57864388_8.EAFBushingfittedtomainshaft.png.e34f686c8632fffed53382b2d706bec2.png

 

Now the fun begins! Pick up your EAF and take a look at the shaft (Fig.9). Notice again that there is a flat portion on the EAF shaft just like the telescope focuser main shaft. We will be connecting the EAF to the bushing in a similar manner to how we attached the bushing to the telescope. Rotate the focuser and bushing so that one of the screws is facing upwards through the access hole. Carefully slide the EAF shaft into the bushing as far as it will go, ensuring that the flat portion is also facing up (Fig.10) as this will align the flat portion with the grub screw. Tighten the grub screw onto the flat portion of the EAF shaft using the 2mm hex key followed by the second screw. Go back and forth between the two screws and nip them up ensuring there is no slop or slippage present between the EAF and the bushing and that they are hand tight.

Fig.9

770080223_9_EAF.png.d95cc74374f38867767958e1cc70247e.png

 

Fig.10

1504942325_10.EAFonbushing.png.dc009073bf0809bff68c70528a9f0c01.png

 

The next step is to mount the bracket onto the focuser, rotate the EAF into position and ensure the bracket is square with the EAF. The long horizontal part of the mounting bracket will be affixed to the flat portion of the focuser indicated in Fig.11 whilst the short vertical part will be affixed to the EAF. Grab the four M4x8mm screws and place a washer over each one. Fig.12 shows where we need to get to, but let's break it down into a couple of smaller steps:

  1. Place the long side of the bracket onto the flat portion of the focuser with the shorter part of the bracket facing "up". Align the long centre slot of the bracket with the screw holes indicated in Fig.11. Insert and finger tighten two of the M4x8mm screws with washers into the aforementioned screw holes on the focuser, ensuring that you can slide the bracket a little bit which will help us with aligning the EAF for mounting.
  2. Whilst supporting the EAF with one hand, rotate the coarse knob with your off hand until the screw holes on the front face of the EAF are lined up with the slots in the vertical part of the bracket
  3. Slide the bracket flush against the face of the EAF

Fig.11

636891726_11.Mountingbracketscrewholes.png.2706d24c7172726e6b5505a77d2231fc.png

 

Fig.12

1951144126_12.Bracketscrewsandalignment.png.680c84a602ac6a97c089a7d9c9dd50a7.png

 

After that has been completed, insert the other two M4x8mm screws and washer to connect bracket and EAF (see Fig.13). Loosely finger tighten these screws for now. Ensure everything is square then proceed to first tighten the screws attached to the focuser body using the 3mm hex key. You may need to loosen the screws attached to the EAF first to ensure we haven't applied any torque, but you can tighten these straight back up. The final step is to screw the thumb-screw back into the original position per Fig.14. This screw will contact and tighten against the bracket before it starts tensioning the focuser.

And with that, the installation is now complete! Refer to Fig.15 and Fig.16 for the finished look.

Fig.13

925546524_13.ScrewsonEAF.png.ca265e990631e29b9d9cf8f43dc7a8c8.png

 

Fig.14

1490795883_14.Thumbscrew.png.69b719b0c21c4e7a974181bbdfad59ee.png

 

Fig.15

1606958778_15.Finalassembly.png.6fbc22368f057357fc12eb0497db98d9.png

 

Fig.16

805582662_16.Finalassembly.png.7ecdebe0479c80ec4d6b4753dec73b1c.png

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Installation of the PrimaLuceLab Sesto Senso 2

In order to get imaging with the FLT120 quickly I “borrowed” the ZWO EAF from my other telescope instead of buying another one. It was time for the EAF to go back to the other telescope so I decided to buy a new electronic focuser. The PrimaLuceLab Sesto Senso 2 (quite the mouthful) seemed quite good so I went for that.

The Sesto Senso (Italian translation being “extrasensory perception”) is a little bit different to the EAF as it doesn’t require a base plate to attach to the focuser. For installation to this focuser you will also need an additional 33mm adapter as the most common shaft size is 25mm (which the Sesto Senso is designed for) so you will need to purchase this to attach the focuser to the 33mm WO shaft. Here's a list of the exact components to complete the install:

Equipment

  • 3mm hex key
  • 2mm hex key
  • 1.5mm hex key

PLL Sesto Senso Box

  • PLL Sesto Senso 2
  • 2 x screws (3mm hex key)
  • 3 x screws (1.5mm hex key)

33mm Adapter

  • Adapter
  • Black shaft adapter
  • 4 x screws (1.5mm hex key)

!Important note! Before we proceed with the install it’s important to note that this focuser will be attached to the fine focus knob and not the coarse focus knob. When my telescope was delivered I found that the fine focus knob would not move the focuser inwards when the telescope was loaded with camera and pointing towards the zenith. This was resolved per my instructions posted above so I would highly recommend you check movement of your fine focus knob and resolve if required before commencing with the rest of this install.

The optional temperature probe, Sesto Senso 2 and adapter packaging is shown in Fig.1. All items are laid out as shown in Fig.2. The fancy coloured hex keys are my own

Official installation instructions are here:

https://www.primalucelab.com/blog/support/installing-sesto-senso-installare-2/

https://www.primalucelab.com/focusers/sesto-senso-2-33mm-adapter.html

 

Fig.1

Fig.1.thumb.png.de6cec2d80c6f6519034936df5891095.png

 

Fig.2

Fig.2.png.3f6b9ec2fad853d4c1bd179ba7f2d629.png

 

Installation

The first step is to remove the protective cover from the fine focus knob per Fig.3. Using the 2mm hex key, loosen but don’t remove the grub screw which holds the fine focus knob onto the shaft. Once loose, you should be able to remove the fine focus knob per Fig.4. You can now remove the coarse focus knob the same way, by loosening the grub screw using the 2mm hex key and pulling the knob away per Fig.5.

Fig.3

Fig.3.png.867dcaf5324863269b0b355d81768def.png

 

Fig.4

Fig.4.png.7934466366d434f94f3c7f95546a00f8.png

 

Fig.5

Fig.5.png.abeedccefc39a8179737ed6c99fabae5.png

 

You should now be presented with the silver metal housing (fixed, does not rotate), the main shaft (brass colour) and fine focus shaft (silver colour spindle) per Fig.6. Notice that the main shaft has a flat section where the grub screw was contacting but the fine focus shaft does not have a flat section. I suspect this is the reason why there are issues with the fine focus knob spinning freely, as the grub screw cannot maintain friction with the shaft. Now is a good opportunity to degrease the fine focus shaft where the grub screws will contact using IPA or similar to ensure that there is no oil which could reduce friction. The Sesto Senso adapter will attach to the fixed silver metal housing whilst the rotating portion of the focuser will attach to the fine focus shaft.

Fig.6

Fig.6.png.4043293c092333a59bafef5556d62ede.png

 

Per Fig.7, the diameter of this metal housing measures around 32.90mm which confirms that the 33mm adapter is the correct size for this focuser. Before we fix the 33mm adapter on, we must first attach the black adapter to the fine focus shaft. One grub screw will already be inserted so go ahead and screw the second 1.5mm hex head screw into the adapter but not all the way as we want to slip the adapter over the main shaft. Once the adapter has attached, proceed to tighten the screws being careful not to strip the head, it may help to nip up each screw bit by bit rather than tightening one screw at a time (refer to Fig.8). Try rotating the coarse knob on the opposite side and visually check the runout of the black adapter. If installed correctly, the black adapter should rotate around its own axis without any wobbling. If you see any wobbling, undo the screws and try again.

Fig.7

 

Fig.7.png.17bb2c57d35cf600f9f0fe18202404ca.png

 

Fig.8

Fig.8.png.0650b060bb3b7e4fd245048277871c6d.png

 

If the main 3mm hex screw in the adapter isn’t already loose, back it out a bit then slip the adapter over the black adapter and the silver metal housing per Fig.9. Proceed with tightening the large screw using the 3mm hex key until the adapter is tight on the housing. Ensure that the adapter sits squarely on the silver housing as it’s easy for it to go lop-sided so perhaps use your third hand to hold the focuser steady! Once this has been attached insert and tighten the three 1.5mm hex head grub screws around the outside of the adapter per Fig.10. We’re now ready to attach the focuser!

Fig.9

Fig.9.png.53e1b36280f7d0baa973e8b6599596e5.png

 

Fig.10

Fig_10.png.85dffb30401cf81312833c1783f194e6.png

 

Place the top of the focuser on the table such that the slot showing the shaft is observed per Fig.11. Using your fingers, rotate this shaft until a screw hole can be seen. Using the 1.5mm hex key, insert one of the screws just enough that it doesn’t go all the way through the shaft. Rotate the shaft around and repeat for the second screw.

Fig.11

Fig_11.png.36139dfab988e6fc8d453444829bac4b.png

 

Slide the Sesto Senso over the black adapter until the clamp sits over the adapter we’ve installed onto the focuser. Refer to Fig.12 and Fig.13 where the dashed yellow box shows the part the clamp should attach to. Tighten the 1.5mm hex head screws which will attach the centre shaft of the Sesto Senso with the black adapter on the fine focus shaft (ensure these screws are tight). Now is the time to decide on the rotation you want the focuser to sit in. I attach a Mini-PC and Pegasus Powerbox Advance on my telescope so I decided to rotate my focuser such that the ports are facing upwards for easy connectivity. Once decided, proceed with tightening the 3mm hex head screw in place and ensure this is tight. Finally, you can insert the three 1.5mm hex head grub screws to the outside of the clamp just like the adapter.

The installation is now complete!

Fig.12

Fig_12.png.40553074ef5704c4ad3ac0525997c241.png

Fig.13

Fig_13.png.36e3e8738202e384fbadb35f1e7c4c02.png

 

Fig.14

Fig_14.png.69f9899691af7dbdd7eac16a30a13d91.png

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Well tonight has been a bit of a disaster! Tonight I set out to determine the amount of backlash in my Sesto Senso 2 as part of trouble shooting another issue, but I've discovered a potential design flaw with the FLT120 focuser. Whilst checking for backlash I noticed that there was still slippage when I move the focus inwards against gravity despite having corrected this per an earlier comment in this thread. 

During the install of the Sesto Senso 2, I pointed out in Fig6 that there is no flat section on the fine focus shaft where the grub screws of the auto focus adapter would contact agadnst. However, flat sections are present on the main shaft connected to the coarse knobs on either side (Fig6 of Sesto Senso 2 install and Fig4 of ZWO EAF installation). Without a flat section on the fine focus shaft, there is only the friction between the tip of the screws and the shaft itself which will allow it to rotate. If you lose friction, the shaft will not rotate even though the focuser does. This explains why there's no slippage when the focuser moves outward  as gravity is assisting with the downwards movement. 

This also presents a problem when using the original fine focus knob as it attaches in the same way with a grub screw contacting the shaft. I don't really want to take a flat file to the fine focus shaft to create a small, flat section so I'll be in touch with the vendor to find the right solution! 

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  • 2 years later...

I am considering getting an FLT 120 in a few months time. I have heard that the focusers are the weakest point. Any more recent feedback from other FLT 120 users ?  I am also considering the Esprit 120. It s about 2Kg heavier though. for my EQ6-R mount.

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@Richard_ just wanted to say a big thank you for writing all this up for the benefit of others. Not sure why you didn’t get any responses, perhaps there aren’t too many out there, but I do hope it has been useful for some. It is not relevant for me as I don’t own this scope but I still found it an interesting read 👍

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2 hours ago, Stu said:

@Richard_ just wanted to say a big thank you for writing all this up for the benefit of others. Not sure why you didn’t get any responses, perhaps there aren’t too many out there, but I do hope it has been useful for some. It is not relevant for me as I don’t own this scope but I still found it an interesting read 👍

Thanks Stu for your kind words! Compared to other 120mm APOs like the Skywatcher Esprit and the Askar, I think this one flew under the radar.

3 hours ago, Limerick John said:

I am considering getting an FLT 120 in a few months time. I have heard that the focusers are the weakest point. Any more recent feedback from other FLT 120 users ?  I am also considering the Esprit 120. It s about 2Kg heavier though. for my EQ6-R mount.

Are you looking to use yours for visual or astrophotography?

I found that my coarse focus knob wasn't smooth through a full 360 rotation as per my post towards the top of this thread. You can make the rotation a bit smoother which has no negative effects when using an auto focuser for astrophotography. Any focuser attached to the fine focus knob (PrimaLuceLab Sesto Senso or QHY precision focuser) wouldn't work well because of slippage between fine and coarse focus shaft, and there was nothing I could do to improve this. I'm at a stage where I'm happy with my focuser performance for astrophotography (I get perfect V-curves during auto focusing and my actual HFR during imaging is as low as the auto focus routine suggests).

Thinking about visual, whilst you can smooth out the action a bit, you would still feel the difference in resistance through a full rotation. This may be frustrating for some and at this price point, some may not find that to be acceptable.

It could just be my copy where this is an issue. I think WO also sold a variant of this telescope with a Feathertouch so it might be worth looking into that.

 

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Posted (edited)
10 minutes ago, Limerick John said:

The scope is mainly for astrophotography with a ZWO EAF most likely. Are the Esprit focusers any better?

I haven't used the Esprit so I couldn't offer you a comparison. The ZWO EAF will work fine with the FLT120 telescope.

In general, I think all the ZWO EAF's have a little backlash in them, but if you spend 30mins configuring your focuser it won't be a problem. If you're a NINA user, check out the video below where Chad explains how to perform this for a ZWO EAF and Pegasus FocusCube v2. 

I use both focusers explained in the video and both work really well after following Chad's advice 🙂

Edit: the installation for the FocusCube is virtually identical to the ZWO EAF I documented in this thread. My one advice would be to replace the grub screws with a flat point (to offer better contact with the flat portion of the bar) and to put a small blob of blue loctite (non permanent one) on the thread. You shouldn't have to worry about the grub screws backing out over time. 

 

Edited by Richard_
Add info around focuser installation
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