William Optics GTF-81 - Very Initial Observations

[tferrari]

I have owned the William Optics GTF-81 scope for about 4 months. I have not performed any field tests with it other than a few star tests (winter here is VERY cold and I'm not too brave)

I have no regrets about the purchase. That's a reasonably summary considering the little amount of information I've gathered.

Some initial observations regarding the GTF-81:

Based on my star tests -- due to integral flattener (3 element + 2 element flattener), the stars are perfectly round to the edge of the field (the primary reason I bought the scope). No concerns with field curvature.

Collimation was absolutely bang on.

Focuser is rack & pinion style (very tight, nice feeling focuser)

Focuser has digital display to indicate backfocus.. quite accurate. The measurement given on digital readout is the exact same as what is seen on the physical ruler.

Focuser has lock control which does change the focus slightly when tightened but is measurable as you can see the focus on digital display change sightly.

Focuser is very solid.

Focuser assembly can rotate 360 degrees (very snug, no concerns).

Focuser has two thumbscrews used to tighten eyepiece etc into focuser. The bottom of focuser has a plasic grub screw which can be removed and an additional thumbscrew added for greater stability (3 thumbscrews total).

I will give a full review on it once I get it out of my house, but that will honestly be a couple of months (I work out of town as well).

If anyone wants to know any specific information or has questions regarding the scope, let me know.

I know there is very little information about this scope on the net and I'll do my best to give whatever information I can at this stage.

Tyler

[michael.h.f.wilkinson]

Sounds good, enjoy the scope!

[Donkeiller]

Good Grief!!!

A WO scope with a focuser that (apparently) actually works!!

What is the World coming too?

[ItIsOnlyMe]

It looks to be a beautiful thing, would you consider it to be value for money? Would love to own one but nearly £900 seems a lot for such a small scope.

[michael.h.f.wilkinson]

Good Grief!!!

A WO scope with a focuser that (apparently) actually works!!

What is the World coming too?

Olly visited them with a big stick and a nail in it. He made the entire board write a thousand lines each "I shall not use a Crayford focuser!"

[tferrari]

It looks to be a beautiful thing, would you consider it to be value for money? Would love to own one but nearly £900 seems a lot for such a small scope.

I'd first like to state that my GTF-81 will only be used for AP. I have zero use for refractors of this size for visual work. I'd rather buy a huge dob to look at objects if visual observation became an interest of mine.

I assume your question is not, "Do you think a small APO refractor has greater value than a larger diameter reflector?". This topic has certainly been covered many times on these boards and some other professionals/gurus can offer great advice on this question.

If your question is, "Do you think the price for this small APO refractor is justified relative to other similarly-sized refractors?" - then, the answer is yes (in my opinion).

A bit of a background - It was a long-time struggle to get perfectly flat field with my WO Megrez 80mm LOMO Triple APO (I still have this scope, yes) when taking photos with my DSLR. I was using a TRF-2008 flattener in conjunction with a DSLR sensor. Using a smaller sensor, my field flatness/apparent star elongation issues MAY have disappeared, but I wasn't about to tolerate less than acceptable results with my DSLR when I knew that complete field flatness with large sensors was achievable with other scopes. This issue, by the way, had nothing to do with collimation, flattener type, sensor orthoganality, focuser orthoganality or orthoganality of any of components in the optical train. I spent months diagnosing the issue with no change in results

The opportunity to achieve a flat field by purchasing a single scope in a tight package was enough of a reason to take a gamble and purchase a scope with an integral flattener and leave my 80mm LOMO triplet and TRF-2008 for the resale bin.

If you compare the price of a GTF-81 to a FSQ85, you'll see quite a difference in price. The Takahashi is a marvelous scope, but I like to gamble on cheaper options (GTF-81) and support new innovative designs from the lower-cost producers. Considering the price (and the apparent build quality) I felt the potential for value greatly outweighed the risk of purchasing this. I am aware that the VERY early models may have had some issues with collimation, but that issue has most-likely since disappeared.

I think the GTF-81 is very competitvely priced considering there is absolutely no concern with purchasing a field flattener or matching it to a scope which it was not necessarily designed for (compromise to begin with). A Televue TRF-2008 flattener for AP might run you around $275USD - so if you factor the need to purchase one of those with any other scope, suddently the GTF-81 begins to make sense from a dollars value. However, if a person has reservations regarding the use of a refractor for AP (a small diameter one at that), then the above point is relatively moot.

In speaking with WO, it is clear that the integral flattener was computer modeled to deliver a flat field specifically for that scope; I guess that comes as no surprise. Who would build an integral flattener for a scope from the ground up to only let the flattener fail because it wasn't designed for the scope? Logically, it makes sense that the integral flattener should perform well. Whether or not it does, that comes down to some kind of quantitative analysis (or a quick qualitative assessment like I've done). At this point, I have zero concerns (especially compared to what I was dealing with before with my Megrez 80mm triple APO)

Overall, I purchased the GTF-81 because I wanted a reasonably-priced solution to the headache I dealt with for months. There come a time where you just need to say, "fu*k it", and just move on when the pleasure you get from trying to solve problems is outweighed by the headache of not being able to enjoy the hobby (complete personal experience-type thing).

My post would be much more comprehensive and would carry more weight if I had some actual data to show to help others base their opinion on. However, I just simply haven't had the time.

Thanks for the question. Hope my explanation helps you somehow.

[tferrari]

More observations regarding WO GTF-81 focuser:

The focuser digital display runs on two button-cell batteries which can be changed with a small phillips screwdriver in a minute or two.

When you turn the focuser digital display on, you rack the focuser in all the way, press 'ZERO' on the digital display and that becomes your reference zero point (0.00mm). If you rack your focus out to a set point (i.e. 23.55 mm), turn the digital display off/on again you do NOT need to rezero it. It will still display 23.55mm. If you move the focuser while the display is not powered on, it will still display the correct measurement when it is turned on. This shows that the distance being measured is not simply based on some relative measurement solely based on your zero. It is something intrinsic to the relationship between the physical position of the focuser tube and whatever mechanics are being used to measure it and display it.

If you rack the focuser in and out (even quite violently) from full back focus and back in, and return it to its ZERO position (fully racked in) the digital display will still read 0.00mm. This is great. There are cases (one time out of ten) where very forceful racking may cause the zero to slip by 0.01mm, but you really have to be trying to cause it to go out of its initial calibration/zero-ing (couldn't see it happen with normal focuser use in a session). There is great stability with this.

This accurate correlation between the actual focus position and precise measurable readout may prove extremely useful for people who want to return back to some kind of focus (or at least get 99% of the way there without even having to look through the scope). The final 1% can be dialed by your preferred method (eye, bahtinov mask, FWHM measurement etc.)

The focuser has two large screws which can be used to adjust the tightness of the focuser (amount of force required to turn knobs which rack the focuser tube out). They are engaged by use of a large flathead screwdriver and are countersunk into the focuser (to decrease likelyhood of accidental loosening I assume).

If the focuser tightness is relatively high (as I like it to be for AP), when the focus lock thumbscrew on the bottom of the focuser is engaged, it only has a very minor effect on the focus (even when the focus lock thumbscrew is engaged quite tightly). I think this is an important component to this quality of this focuser. There's nothing worse than purposely putting your scope out of focus so that when you tighten the focus lock screw, the slight shift in focuser tube assembly will bring you into focus. This is a way to compensate for poor mechanical build quality. The end result is the same (you get your proper focus), but it's just unnecessary to have slop like that. In the case of the GTF-81, this is not a requirement; however, I will note that the tighter your focuser is to begin with, the less the focus lock knob has an effect on your focus when it is engaged.

There is a measureable amount of backlash in the focuser but it is extremely consistent and isn't much of a nuisance. I would say that on the lowest speed focus knob, there is 1/24 of a rotation of backlash. This is completely manageable.

There is absolutely zero mechanical play noticed in the focuser. You can not move the knobs laterally or off axis in any way (definitely expected for a focuser with any type of quality).

As the focuser tube is racked out, the rack can be seen. It is some kind of copper alloy and is VERY nicely machined. Face width of each tooth is machined to perfection. They didn't skimp out here. As for the pinion, I haven't taken the focuser apart and don't intend to - it works perfectly.

The whole focuser assembly can rotate 360 degrees and can be locked down with a single thumb screw. The focuser assembly is machined to fit very snugly onto the telescope body and its tightness is controlled by 5 plasic grub screws which use friction to grip the body of the telescope and fix the entire focuser assembly to it. The tighter the grub screws are engaged, the more force is required to turn it. This is a very typical William Optics design for these components. I wouldn't use the focuser assembly rotation for AP, however, you probably could given the tight tolerances where the focuser assembly joins to scope body. This has been typical with WO scopes for the last 15 years.

Overall, the focuser assembly is very high quality. I can't see how anyone would complain about the quality.

[tferrari]

One last focuser comment:

I believe the focuser has 4 screws which can be used to ensure focuser othoganality with the optical train. Basically, this allows adjustments to be made to ensure that the focuser draw tube is aligned wth optical axis.

[ollypenrice]

Hi Tyler, nice to hear from you here. Sorry the Lomo proved difficult. Also great that WO have finally moved beyond frictionless Crayford! They really did ship out some abominable rubbish before going R and P and their competitors are still doing so.

As you say, the Tak FSQ85 defines the term 'expensive' when it comes to dollar per millimetre, but it also sets the standard. (It ruddy well should, too!!) If you get a flat field with the WO then the acid test will be blue stars. Will they bloat? This is a problem for all refractors, including multi element apos, and generally sorts out the sheep from the goats. However, post processing can be your friend here.

Really looking forward to some images. Is this scope still available?

Olly

[mytola]

In my GTF81, I've found that locking the focus doesn't affect the focus distance if I turn the focuser in the outwards direction before locking it. In the opposite direction it will slightly shift the focus. I've also notices that there's a slight image shift when changing focus direction. Might be that some adjustments can be done to remedy this, but I haven't managed to do that.

Overall I agree that the focuser is sturdy, I imaged near the zenit this evening and focusing was very smooth.

[ItIsOnlyMe]

If your question is, "Do you think the price for this small APO refractor is justified relative to other similarly-sized refractors?" - then, the answer is yes (in my opinion).

Overall, I purchased the GTF-81 because I wanted a reasonably-priced solution to the headache I dealt with for months. There come a time where you just need to say, "fu*k it", and just move on when the pleasure you get from trying to solve problems is outweighed by the headache of not being able to enjoy the hobby (complete personal experience-type thing).

I suppose I was thinking that many people get excellent results with the likes of the Skywatcher Equinox 80 @circa £520, though as you point out this is plus flattener which brings it to more like £700. Then you start mucking about with the focuser e.t.c e.t.c.

Having recently pushed the button on a 400mm L series Canon prime lens I totally understand the "spend the money and stop trying to fix the inadequate equipment" argument. Nice to have something that just works and you can get on with enjoying it, spent 3 hours birding with this lens today and had a great time and didn't even consider the money. Did notice several times the fantastic optics and supremely quick focus, most of the time you don't even notice it focus it is just somehow already tack sharp. Unlike the 300mm zoom it displaces which spent all day hunting for focus on anything that wasn't sat still.

Odd how we see these different items in different ways. 400M f5.6 lens with circa 77mm aperture didn't feel expensive, but somehow an 80mm scope at the same £1100 would do.

Sure you will have a fantastic time with your new scope and very much look forward to seeing the images.

[tferrari]

...the acid test will be blue stars. Will they bloat? This is a problem for all refractors, including multi element apos, and generally sorts out the sheep from the goats.

Hi Olly - no surprise seeing you here! Ha. I love it when you chime in - thanks.

I would certainly like to do a more appropriate star test for this scope. Do you have any specific suggestions for choosing a particular star? Are there guidelines that people typically use when trying to compare how scopes handle blue bloating?

Some more detailed info on this would be great.

Is this scope still available?

You ask if the scope still available - If you mean the lomo triplet, yes.

In my GTF81, I've found that locking the focus doesn't affect the focus distance if I turn the focuser in the outwards direction before locking it. In the opposite direction it will slightly shift the focus. I've also notices that there's a slight image shift when changing focus direction.

Good info, thanks

[ollypenrice]

Any field with bright hot stars will give you a good idea. If you go for an ultra-crazy star like Rigel then you'll probably struggle because everyone does, whatever their scope. I think you just need to get some images in and see how the stars look. Blue bloat is easy to see. It just gives a broad blue glow around the star. Noel's Actions have a routine for reducing it or you can do it yourself in Curves by putting a wide, feathered circular selection around the halo, pinning the background sky level on the curve and then lowering it just above the pinned point.

Olly

[FLO]

Blue bloat around bright stars is more commonly associated with doublets, not triplets. The GTF81 is a triplet with an inbuilt flattener. I would be disappointed if it showed significant blue bloat.

Regarding whether the GTF81 is available, we stopped offering them following some collimation issues but that was the first batch. If they have overcome that we will restock them because it is a promising design.

HTH

[ItIsOnlyMe]

Widescreen centre have it listed as in stock at £899, though they list a gt81 rather than gtf81

http://www.widescree...s_GT81-DDG.html

[ollypenrice]

Blue bloat around bright stars is more commonly associated with doublets, not triplets. The GTF81 is a triplet with an inbuilt flattener. I would be disappointed if it showed significant blue bloat.

Regarding whether the GTF81 is available, we stopped offering them following some collimation issues but that was the first batch. If they have overcome that we will restock them because it is a promising design.

HTH

More commonly, yes, I certainly agree. However, I've had it with triplets and even the TEC, which is primarily corrected for visual, does show a slight tendency to blow the blues. One of the worst ever was the short lived WO66 Petzval (quadruplet.) Triplet-dom is not a guarantee of good control in the blue. The smaller the aperture the more it seems to be a problem, though this is just an anecdotal observation. I don't know if you can put it on a sound basis or dismiss it?

Olly

[FLO]

More commonly, yes, I certainly agree. However, I've had it with triplets and even the TEC, which is primarily corrected for visual, does show a slight tendency to blow the blues.

I agree Blue colour fringing is not exclusive to doublets but prominent blue 'bloat' is a characteristic of doublets. Anybody with an ED80 will be familiar with it. Fortunately it is relatively easy to remove or reduce through image processing.

One of the worst ever was the short lived WO66 Petzval (quadruplet.) Triplet-dom is not a guarantee of good control in the blue.

The Petzval design does not have a triplet objective. The design is an old one from the 1800s intended to deliver a flatter field and less false colour than a traditional achromat. It uses a doublet objective and a second reducing/correcting lens group. I agree the WO 66 Petzval was not a good example, probably because it's optical tube was not long enough to place the important fourth element in the optimum position. If you want to take a pop at WO Olly then the 66 Petzval and f6 Megrez 110 doublet are your best bets (not to be confused with the FLT110 triplet, that is a diamond).

The smaller the aperture the more it seems to be a problem, though this is just an anecdotal observation. I don't know if you can put it on a sound basis or dismiss it?

I haven't noticed that. In theory it is the other way around because smaller apertures typically use thinner lenses. Thinner lenses have less opportunity to act as prisms and split light into it's colour components. That is why an optical designer cannot simply scale up a smaller design into a larger one and why FPL51 is often sufficient in smaller aperture telescopes whereas FPL53 (or equivalent) is required for larger apertures. It is also why long f-ratio achromats work so well, the lenses are thinner and less curved. Your anecdotal evidence is interesting, perhaps there is another factor at play

HTH,

Steve

[FLO]

Focuser is rack & pinion style (very tight, nice feeling focuser)

Focuser has digital display to indicate backfocus.. quite accurate. The measurement given on digital readout is the exact same as what is seen on the physical ruler.

Focuser has lock control which does change the focus slightly when tightened but is measurable as you can see the focus on digital display change sightly.

Focuser is very solid.

Focuser assembly can rotate 360 degrees (very snug, no concerns).

Focuser has two thumbscrews used to tighten eyepiece etc into focuser. The bottom of focuser has a plasic grub screw which can be removed and an additional thumbscrew added for greater stability (3 thumbscrews total).

Overall I agree that the focuser is sturdy, I imaged near the zenit this evening and focusing was very smooth.

Overall, the focuser assembly is very high quality. I can't see how anyone would complain about the quality.

That is excellent news

The new R&P focuser does appear to be an improvement over the earlier model.

I will give a full review on it once I get it out of my house, but that will honestly be a couple of months (I work out of town as well).

I'm looking forward to hearing your thoughts.

Steve

[gustavo_sanchez]

I got a blue (limited edition) GT81 just yesterday. I noticed that the difference between my scope and the GTF81 is just the built-in flattener, which is nice. However, my concern is that almost all of the current field reducers I know of are also field flatteners, so the flattening effect of the FR/FF will undo the already flat field of the GTF81. On the other hand, the GT81 will just accept the FR/FF just fine.

What's your opinion on this matter?

[Waylandscape]

This is a scope I have been looking at too.

The fact that it is flat enough to handle my full frame sensor is part of the attraction I must admit.

I'll be watching with interest to see some results when you get the chance.

[tferrari]

I'd like to post that I mentioned the slight image shift and slight backlash to William Optics and they immediately offered to send their "new and improved" focuser assembly to me free of charge (if I returned my existing one to them).

Ever since I've been dealing with them, there has been nothing but perfect customer service - impressive.

All newer GTF-81s (I believe since Nov or Dec 2012) are fitted with the new focuser assemblies (not 100 percent sure on dates). No polishing on draw tube (which helps keep tolerances within spec), better machining of R&P components and overall greater number of threads per inch for R&P assembly.

I have photos which show the difference in R&P thread pitch for new focuser assembly. I'll do a measurement thay others can use to determine which focuser assembly they have.

I will admit that the new focuser assembly does work a measurable amount better than the original onr (which was good to begin with).

More updates to follow.

Any questions, feel free to ask.

[tferrari]

Widescreen centre have it listed as in stock at £899, though they list a gt81 rather than gtf81

http://www.widescree...s_GT81-DDG.html

Bit of a late reply, but I'd like to point out that the GT-81 is different than the GTF-81. The most notable difference being that the GTF is the flat field version (thats what the F means). There are other differences as well but I'll let those who are interested dig up the info on Google.

[tferrari]

Here are some photos of the new focuser that I received. This one is the same as the one I had received with the scope I purchased in September 2012, however the tolerances for fabrication had been improved and a new one was offered to me at no charge (as mentioned in previous post). I disassembled it to show some of the details.

-----------PHOTOS-----------

This is the photo of the bottom of the focuser - the far left and right set screws hold the bearings into the focuser. The large silver focus lock knob presses against an aluminum bar which bites into the focuser shaft. The focus lock is a true shaft lock. The set screw to the right of the focus lock knob applies pressure to an aluminum bar which contains a semi-circle piece of plastic which presses against the pinion shaft and controls the amount of pressure required to turn the shaft in conjunction with the draw tube compression screws shown in photo below. The left hand side of the photo shows the 10:1 dual-speed focuser knobs - the right hand-side is the coarse speed knob.

These screws are use to apply indirect pressure to the focuser draw tube by pressing on plastic spacers that run along the axis of the tube inside of the focuser assembly. The LCD screen on the right hand side indicates draw tube travel to 0.01mm precision.

This is the coarse-speed knob side of the focuser - when removed, the 3.9mm diameter shaft is exposed. Bearing is held in by set screw on bottom of focuser.

The threads on the focuser draw tube are nicely machined - I had to remove some very fine brass shavings from between the rack teeth, nothing major. The new draw tubes on only slighly polished (you can still faintly see original machining marks) because William Optics became aware of some play in the focuser system due to the draw tube tolerances decreasing because of the small amount of material lost during polishing.

The internals don't look pretty but the tolerances are great. The white strips are very well-adhered plastic strips which eat up any of the space between the aluminum wall of focuser assembly and draw tube. The felt also helps tremendously to avoid any wobbles. The IC-circuit-type chip in the unit is the sensor for detecting the movement in the focuser draw tube which is displayed on the LCD micrometer on the exterior of the focuser assembly.

This photo shows the coarse-speed focuser on the left, the plastic semi-circle pinion shaft compression block, the pinion gear (quite nicely machined, I score it 8/10), the shaft lock and the set screw holding the dual-speed knobs to the shaft. Overall there is a very tight fit, however I did take it apart and clean a very small amount of filings out of the pinion gear (negligible amount). Also, disregard the hairs and fuzzies - those were not present when I removed the assembly - they appeared after I played with it for an hour.

Side view of the focuser where the rack on the bottom of the tube would slide through and make contact with pinion gear.

View of focuser bottom with assembly removed - showing draw tube rack.

Well-lubricated dual-speed knob side of focuser - note the flattened part where the set screw will bite.

------------------------------------------------------

If you have any questions, post them here and I will answer them.

[Gasman]

Hi T

I`m interested in how you are finding the focuser with the gtf 81?. I`ve just found out today that the gtf 102 is becoming available for the us in the UK which has got me very interested as I`m in the market for a new refractor and this seems to tick all the boxes but I have an Atik camera and Atik usb filterwheel which is quite a chunk for the focuser to carry and wondered how yours is coping?.

Thanks

Steve

[tferrari]

The upgraded focuser I received (which should come stock on all newer production model GTF81s and all GTF102s ever produced) is very good. Very little image shift when racking in and out, even at high magnifications. The shaft lock has no effect on final focus providing your last focus movement was 'racking out'. The compression ring connector works very well for me and the orthogonality of the focuser to optical train in my particular scope appears to be bang on.

As a star test update - I did some 3 minute exposures with my 450D at iso800 and noticed zero indication of blue/purple fringing of stars on this scope. Although the night I performed the test had particularly bad seeing conditions, I can also report that the stars are very round all the way to the edge of the frame (due to scope's integral flattener). I'd post photos but the noise in my 450D is horrendous and I don't want to give a poor impression of the scope by posting an image taken with a defective camera. I receive my ATIK 383L+ in the next 3 weeks and will post some sample sub-exposures for all to see.

Tyler