William Optics GT-81 Review Part 2 and First Light

[m37]

Finally I have had a chance to use my new scope. Report is as follows:

[if you want the less long-winded version, this scope is totally awesome and I absolutely love it]

Long-winded nothing to do because it's raining version:

Just to recap, the GT-81 is a triplet air spaced APO with Ohara FPL-53 low dispersion glass costing £725 from FLO. Focal ratio is f/5.9 native or f/4.72 with the recommended 'William Optics Flattener Reducer 6' at 0.8x (£139 from FLO). Aperture is 81mm and focal length is 478mm.

Total price for the pair from FLO is £864 plus postage which is as good a price as you will find.

Already covered the unboxing and a tour of the main features here http://stargazerslounge.com/topic/202558-william-optics-gt-81-unboxing-and-review-part-1/ so I will just go over some of the performance and use after a couple of weeks.

Finish and durability

As mentioned before the finish is superb and looks and feels very nice. It seems like it would be very durable although I don't intend on testing this out !!!

The metalwork is all anodized and coloured gold but the mounting bracket is quite soft in its finish. This means after mounting and adjusting on my HEQ5 a series of small dents was left in the finish. I have absolutely no problem with this as I intend to be buried with this scope! but it could affect resale value perhaps a little.

Stability and Construction

Having used this scope a few times I can report that without excessive tightening everything fits together and lines up well e.g. the finder scope brackets are nice and solid and when mounted on the handle they line up really well. The bolts used are of a very high quality and this was one of the things I really noticed - they don't have that cheap finish where you can see marks left by use of allen wrenches.

Focuser

Very impressed with this, super solid Rack and Pinion focuser which feels very, very sturdy out of the box. I increased the tension a little to make things extra secure but when in position and locked down with the focus lock bolt it is going absolutely nowhere. Conducted a small test where I set perfect focus on a target of small dots and then locked the focuser with my DSLR and a small weight attached. After nearly an hour I could detect no shift of focus at all. Very, very smooth and the micro focuser allowed such minute changes I am not going to bother with a motor focuser any more. Absolutely no perceptible image shifting on focusing, even with the larger focuser control. Brilliant. I can't see how it could be any better than this although I have never used a moonlite focuser so I couldn't say!

Rotatable Focuser and Balance.

It is not possible to balance this scope with a DSLR fitted and racked out to 61mm on the draw tube (required to get focus) with the recommended Reducer/Flattener. It becomes back heavy (on the camera end) even with a guide scope mounted forwards and the only way I could do this and still have both mount bolts in contact with the bracket was to reverse the mounting bracket. This balances nicely but means you can no longer rotate the focuser as the bracket is in the way. Not yet tried it with the R/F removed. I have the tube rings so this should present no long term problem but I found it strange. Unless I am doing something silly, I suppose my DMK21 guide cam is a bit on the heavy side. I think it weighs about the same as my OAG though. More testing is required here I think.

Also if you remove the handle to make room for tube rings then there is nothing stopping small flecks of the finish dropping inside the tube. Removed safely with a hoover but still, bit annoying.

Oh, and the thermometer in the focuser seems to be accurate How ever did I manage without one of these before?

Dew Shield

This is retractable leaving you with a very small tube length. Very high quality, nice flat black finish on the inside with no discernible reflections. Also has a very smooth action. The fully extended position is such that it is very easy to get a dew strip in the right place without anything being in the way.

I used to get shocking dew on my cheap SW achro refractor and I have always used a dew strip. Whether it is to do with the atmosphere or weather, or the dew shield or mounting of the lens cell I don't know but I tried this scope without any dew heating and there was not a trace of dewing on the objective. The guide scope dewed up but the GT-81 did not.

The lens cap is very nicely finished and fits on and off very smoothly and securely. Flocking on the inside of the dew cap is a nice touch for taking darks.

Fixtures and fittings

All supplied fixtures, eyepiece holders/reducers have a very nice feel and weight to them. The brass compression rings perform well, all adjustable bits have very strong, positive and well sized nobs on them.

I have the WO red dot finder too and this mounts very nicely in the provided space on the focuser.

Altair Astro 60mm mini guider mounted nicely in the included mounting rings with a bit to spare although a bigger guide scope would struggle to fit and leave any adjustment room. Adjuster bolts in the guidescope rings are tipped with that plastic stuff to protect your guide scope.

Imaging

Earlier in the week I got my first decent chance at imaging anything with this scope. Aligning guide scope and RDF was very easy.

Setup is GT-81 with WO flattener reducer 6 0.8x into the provided T-Thread Canon adaptor on an unmodified Canon EOS1000D. Images captured in the totally awesome Backyard EOS.

Guiding was handled using PHD, a DMK21au04.as and an Altair Mini Guide Scope/Finder which incidentally I would highly recommend. F3.75, light weight, well made, excellent focuser mechanism, diagonal with helical focuser for eyepieces, mounting rings all for £149. Ace.

Gives me a whopping choice of guide stars, even at just 0.5sec exposure with about 50% gain in quite poor seeing I regularly get this many:

After some failed attempts due to not having balanced the setup properly (d'oh) PHD whipped through calibration in about 15 steps each way and guided really nicely. For me anyway, I'm very new to guiding

First Imaging Run (M35) (finally!)

Decidedly average seeing conditions, a bit misty and damp air, fairly light polluted, 5 x five-minute exposures *guided), ISO 400, 5 dark frames, 30 bias, 20 flats.

1. Stacked in DSS with recommended settings (reduced size PNGs)

2. Histogram of the above picture

3. with a very quick and dirty RGB level balance

4. Few zooms of the centre, and the four corners

Centre

Top Left

Top Right

Bottom Left

Bottom Right

Summary

I absolutely love this scope. As a fairly big step up from my ST102 I am blown away by the quality of the scope, the performance of the optics and how it handled colour correction and field curvature. There is perhaps a slight stretch at the very edges if you zoom right in but I'll have a look at the other images I took and see how they are.

The colour looked really good. Admittedly I'm a total noob to this lark with under a year's imaging experience but I couldn't see any colour problems even at a pretty fast f/4.72.

A few minor niggles then, but these can be easily overcome and I could probably find little things I didn't like about most scopes. For the money I am super happy that this scope will last me an age. I would recommend this without reservation at only £200 more than the Skywatcher Equinox ED80.

cheers

Christoph

[Uranium235]

Excellent review, thanks for posing this. Three of the corner shots are acceptable, with the only exception being the bottom right - which as you said is still a bit elongated. Im just wondering whether this will give me a perfect field with the 383, which chip size isnt far behind the APS sensors you get in a DSLR.

[m37]

Yeah, I noticed that in the bottom right and wasn't sure how this could be when it's coming through a round hole!

I was thinking maybe an issue with the flattener/reducer, or my DSLR chip is not perfectly aligned, whether it's out of line in the camera body or maybe the focuser could be a little off.

I have one of the Skywatcher flatteners which does not reduce, I might run something through this and see how it looks.

cheers

[m37]

Addendum: I just looked at another image and there is possibly a bit of elongation in all four corners. I reckon it's the spacing of the flattener being a little off.

I was wondering if different T-Adaptors might have slightly different thicknesses or be not quite orthogonal to the required plane.

I have a few things to try out anyway. I shall report back!

[Ewan]

Nice review Christoph, great when you get a new bit of kit but even more so when it's a new scope

Sure the elongation is not down to spacing as it's showing in each corner of the image ?

Just need some better weather & soon to enjoy / test it some more.

[m37]

Nice review Christoph, great when you get a new bit of kit but even more so when it's a new scope

Sure the elongation is not down to spacing as it's showing in each corner of the image ?

Just need some better weather & soon to enjoy / test it some more.

Hey dude. Yeah, reckon it is spacing. Apparently at this sort of f ratio it needs to be millimeter perfect. Got some of those delrin spacers to try, as you say, if it ever stops raining or being cloudy!

[Alien 13]

Hey dude. Yeah, reckon it is spacing. Apparently at this sort of f ratio it needs to be millimeter perfect. Got some of those delrin spacers to try, as you say, if it ever stops raining or being cloudy!

 

I have the ZS71 and if anything the spacing has to be sub millimeter perfect.

Alan

[sloz1664]

I find ccd spacing is the most frustrating aspect of imaging, especially like me having scope, lens & ccd camera permutations.

With so little precious time for imaging, it is as Alan states " spacing has to be sub millimetre perfect."

Steve

[Alien 13]

I find ccd spacing is the most frustrating aspect of imaging, especially like me having scope, lens & ccd camera permutations.

With so little precious time for imaging, it is as Alan states " spacing has to be sub millimetre perfect."

Steve

Hi Steve,

I can only imagine the problems you have.

Recently i have been running without my FF/FR and am much happier to me the images are a little sharper and yes i have to crop a third of the image away but thats not so much of a problem with my scope camera setup.

Alan

[Roy Challen]

Hi Alan,

I am considering buying the WO ZS 71, so I found it interesting that you are happier without the reducer on it. I guess the images producer would be sharper due to the stopping down effect? If you had the chance to buy the scope again would have it with or without FF/FR?

I also have the same mount and similar camera/lenses combo, is the spacing more forgiving?

Roy

[Alien 13]

Hi Alan,

 

I am considering buying the WO ZS 71, so I found it interesting that you are happier without the reducer on it. I guess the images producer would be sharper due to the stopping down effect? If you had the chance to buy the scope again would have it with or without FF/FR?

 

I also have the same mount and similar camera/lenses combo, is the spacing more forgiving?

 

Roy

 

I would still get it with the FF great for realy wide field but using it without the FF helps on the smaller objects.

Alan

[aPolleti]

Hello, I'm from Brazil and just get my GT-81, this is the beautiest scope ever made, and It seems like a tank, very impressive.

I have a doubt, there is a screw in the top of the scope, behind the guidescope support, seems the description in the manual is "360º Rotatable Focuser Lock Screw", but since the real lock screw is beneath the scope, what is this 360º Rotatable Focuser Lock Screw?

Thanks in advance

[Uranium235]

aPolleti, on 31 Oct 2014 - 9:52 PM, said:

Hello, I'm from Brazil and just get my GT-81, this is the beautiest scope ever made, and It seems like a tank, very impressive.

I have a doubt, there is a screw in the top of the scope, behind the guidescope support, seems the description in the manual is "360º Rotatable Focuser Lock Screw", but since the real lock screw is beneath the scope, what is this 360º Rotatable Focuser Lock Screw?

Thanks in advance

Its exactly what it says it is   You can rotate the focuser by 360, and use that screw to lock it in to position.

However, I would not use it because it may run the risk of upsetting your field flatness if the rotator is not a mechanically perfect fit. Nothing worse than spending time getting your corners correct - only to lose them again if you make a change that wasnt needed (so I tend to mosaic, rather than rotate).

[Ewan]

Its exactly what it says it is   You can rotate the focuser by 360, and use that screw to lock it in to position.

However, I would not use it because it may run the risk of upsetting your field flatness if the rotator is not a mechanically perfect fit. Nothing worse than spending time getting your corners correct - only to lose them again if you make a change that wasnt needed (so I tend to mosaic, rather than rotate).

That's what i found a little annoying to be fair as well.

My 80EDT has two rotators, if these are not adjusted bang on you will get elongated stars in the corner(s), too tight it will bind, too loose there will be slop, kind of defeats the object of building ota's with rotatable focussers if i am honest.

A better bet if you do need to re-frame a target is maybe loosen the main rings & rotate the scope instead.

[ollypenrice]

Good review and so nice to find a proper R and P focuser from WO.

I'd say the right hand side was slightly out on both corners but not severe. Chip spacing may well sort it.

Imaging with a nice little apo takes some beating.

Olly

[Owmuchonomy]

Great review. Just picking up on one observation I concur with and that is the AA finder guider. It comes with all the correct accessories, helical focusser, illuminated reticle eyepiece, erecting prism etc. I am very pleased with mine too.

[gary777]

You seem to be very happy. how well does it do on naked eye viewing of the moon and planets.  Are you able to see the Cassini Division easily? I am looking for a 80mm f6 scope for observing, but it has to be short and light weight.

 

Thanks

[ultranova]

A nice review on the scope, I think the GT81 is turning

into a classic, like the SW ed 80, it works out the box

once you have worked out the f/r distance to your chip

you will be very happy, with the results

hope you get some clear skies soon

Paul