PREVIEW The William Optics GT81 Triplet Refractor

[andrew63]

But the above data from the vixen was without a flattner - Perhaps it's just an indication of the quality of the Japan optics, which should not come as as surprise.

andrew

[Tim]

I am a reflector fan as you know Nadeem but I was under the impression that the extra glass sorts out the colours, not the coma.

[Deneb]

Im under that impression as well Tim, but I was shocked to see that much coma & being very truthful I can't even remember my ED80 exhibiting that much coma even without a reducer...

[Psychobilly]

With a Matching Flattner I am sure the GT-81 will be a beast... It's shorter focal length and a lot faster...

Besides which you also have something to carry a flask and sarnies in

[Deneb]

Not to sound too hasty the next test will be with a TRF2008 Reducer, if that yields the results I want, then it will be a keeper otherwise it's going on a Journey !

Nadeem.

[Deneb]

With a Matching Flattner I am sure the GT-81 will be a beast... It's shorter focal lenght and a lot faster...

Besides which you also have somethign to carry a flask and sarnies in

I suppose that backpack is good for something

[FLO]

The focus was not that good on your test image which could make things look worse. All fracs of this F ratio will need a flattner for imaging.

I am a reflector fan as you know Nadeem but I was under the impression that the extra glass sorts out the colours, not the coma.

Granted the focus was not entirely there, but for a triplet I would expect that curvature to be a lot better even without a reducer...

They are right Nadeem, the third lens is for enhanced colour correction. Triplet refractors typically exhibit more field curvature than a doublet, not less. For imaging it is normal practise to use a 'field flattener' or combined reducer/flattener. This is something we mentioned briefly HERE.

You have what is almost certainly the first GT-81 sold in the UK so we are as keen to see your results as anybody else. Please take your time, there is no deadline, you can return it at any point And do keep in mind that you are assessing a budget triplet, it isn't an FLT-98.

Hope that helps,

Steve

[Coco]

I'm wondering if the handle would fit inside the standard syntra/vixen dovetail between the parallel swage's..

Would be a simple drill and tap M8 holes in the scope handle and turn it upside down, if your careful it would look tidy.. can the handle be pointed backward, handy if the scope centre of gravity is backwards with a camera and filter wheel.

[Deneb]

Your thoughts are appreciated, Im not in any doubt they were wrong Steve, far from it actually, also I would expect the triplet to have better colour correction too, is that not why people buy them.. or do they have to spend out for a FLT98 to get the premium optics as the GT81 is just a budget scope from the WO Stable... I have to admit & apologise for any false expectations I have created about the scope. The Field Curvature was bit of a shock & had not realised triplets suffered this more then doublets. As stated the next field will be done with a TRF2008 Reducer/Flattener. I would like to think not all is lost, time will tell...

Nadeem.

[Deneb]

Would be a simple drill and tap M8 holes in the scope handle and turn it upside down, if your careful it would look tidy..

I let you Mod yours when you get it in Pink !

Nadeem.

[FLO]

When you get a chance Nadeem please post another image with the focus well and truly nailed so we can see a more accurate field curvature chart. As Kevin says, if the focus is off it affects the results

[steppenwolf]

I think it is critical that the focus is correct for a review of field curvature to have any validity - I'd also like to see more stars available for the CCDInspector report as 58 is on the low side. Also, make sure that you have set the Image Scale to the correct pixels/arcsec. figure in CCDInspector.

[Deneb]

Thank you - It's duly noted...

Nadeem

[gbealnz]

I'll echo Steves comments, and in my case pull my head in until I get a decent night.

In the euphoria of having a new toy I shot haphazardly at the first few breaks in the weather, and just got the gear in after the M8 shot, before it pelted down.

In an effort to keep objective about this, at this stage I cannot see any reason to criticize the GT-81, it is a fantastic scope. It will require, and it is not alone here, a decent flattener, even the FLT98 does, so don't poke the poo at it for this. In my case I am hinging my bets on the TRF2008 as a reducer, and the AT2FF as a flattener. In is my belief that W/O will produce a flattener in the next month or so.

In the fullness of time, I will trial this scope and post the results, good bad, and otherwise. Until then I reckon this is what both of us should be doing.

Gary

[Deneb]

No one is criticising the scope & your input would be appreciated in this Gary...

Nadeem.

[gbealnz]

No, but deep down I got the feeling we were both going off half-cocked with posts that showed a slant on the scope that wasn't needed. In my case I rushed through two shots, between foul weather, that ordinarily should have taken longer. This if anything didn't do the scope justice, and the casual onlooker could think that the fault lay with the scope, where it lay in my lap instead. In your case, a picture or two with soft focus and no corrector again showed the scope in a bad light, something that wasn't intended of course, but nevertheless it did.

Like Steve said, give it some time, get a decent night, and produce some stunners, I am sure waiting for this to happen at this end.

Gary

[ollypenrice]

No, it is impossible to create a flat field without a rear lens element. The faster the optics the more this will be so, if I'm not mistaken. So to be fair, the Vixen is a slower scope - though clearly a good 'un!

This WO will need a flattener but as we saw with the Lagoon it can be done. The next thing to find out is how well it controls the colour. The test I'd suggest is simply to shoot a bright star, ideally a hot blue one, on axis and then in a corner. Box and paste the corner crop next to the on axis star for comparison.

Our TEC140 is as good an apo as it's possible to buy but it still needs a flattener beyond about a 12mm square chip.

According to FLO's site a flattener is proposed for your scope. It will still be cheaper than an FSQ85 by a factor of about five!

Olly

[Deneb]

Dear All

I believe I have started this review on a wrong note about this Scope, it was not my intention to criticise or degrade the scope. I do believe the GT81 will perform the way it is expecting to. Just like a kid with a new toy, I was just too excited to take the scope out & have a play, but not realising my debunked results would create some false pretenses for overs to think the scope is a troubled performer.

To make sure there is less confusion regarding the ongoing testing of the scope I am going to request the Admin of SGL to cut the thread back down only to the scope images & have the Title changed to 'Preview' instead of 'Review'.

I would like to think that would be acceptable...

Nadeem.

[Psychobilly]

The causes and likely solutions have been discussed?

But I guess what will be will be...

[ollypenrice]

Is that necessary? I think you've reported honestly as you saw things and provided first class clarification. To my mind that Lagoon image really does look promising with the flattener.

Olly

[Deneb]

I believe that maybe if a new thread is created as a Review, that Myself, Gary & anyone else could contribute to the thread in a constructive way, not to say you have not done already... I think for clarification it would be better...

Nadeem.

[Psychobilly]

I dont see anything wrong with the thead as it evolved... The only CCD inspector image I had were the one's Steve (Steppenwolf) had generated from a full frame image from my Vixen.. but that is a different animal...

Garys images from the GT-81 and TRF-2008 show the potential and the fact that WO are designing, manufacturing and releasing a matched flattner can only be a good thing...

Peter...

[RobH]

Doublet, triplet, quadruplet or quintuplet....it makes no difference to field flatness.

Short F-Ratio refractors need field flatteners.

I tried out my TMB 80/480 F6 triplet last night with the relatively small chipped Atik 16HR, and there is field curvature showing, whereas with my F6.9 WO ZS80 doublet and the same chip there isn't any, and that's just a difference of F0.9.

Rob

EDIT.......missed some of the intervening posts.....I never realised that triplets were more prone to field curvature than doublets.....you learn something every day :-)

[Ant]

I'm going to be culling this thread later at Nadeems request.

Cheers

Ant

[FLO]

Doublet, triplet, quadruplet or quintuplet....it makes no difference to field flatness.

That isn't the case

Generally a third element is used for colour correction, a fourth element is most often used for field flatness and the quintuplet design is usually employed as an alternative to exotic low dispersion glass. I am generalising, there are exceptions.

Short F-Ratio refractors need field flatteners.

Yes, and large apertures combined with long focal lengths usually exhibit less field curvature.

I tried out my TMB 80/480 F6 triplet last night with the relatively small chipped Atik 16HR, and there is field curvature showing, whereas with my F6.9 WO ZS80 doublet and the same chip there isn't any, and that's just a difference of F0.9

Precisely

HTH,

Steve