PREVIEW The William Optics GT81 Triplet Refractor

[RobH]

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.

Steve

I know.....simply making the point that the number of elements in the objective makes no difference....I had one of those awful AT65 quadruplets with the rear flattener if you remember

Rob.

[Deneb]

Some more teaser pics with the guider attachment...

The guidescope compromises of a 60mm binocular lens with pvc piping providing focus distance for the QHY5, the main body of the guidescope is PVC piping & a few toilet cardboard rolls bonded together with black tape...

My recent submission of M11 was guided with this guidescope.

Nadeem.

[buzz]

I have a GT81 - I have shot images in daylight with a D60 and can easily see the field curvature. There is an array of pylons on the horizon, several miles away. The centre ones are sharp but those on the edge of the frame are very poor. A seagull was in the frame, about 200 yds away right in front of the pylon on the edge of field. It was tack sharp.

I have heard that there is a new flattener coming out. I already own the Mk IV WO version and checked the website. If you plot the recommended settings for the M90, FLt 98,110 and 132, you get a reasonable relationship between focal length and flattener distance. By my reckoning, the Flattener IV should be about 78-79mm. I have only fixed extension tubes but I will experiment tonight with 77.6 and 85.1mm and image widefield with a H18 sensor. It is not the size of 35mm, but certainly will give an indication.

I'll download the CCD analysis program and will give it a go for a more numerical analysis. This will be first digital light for me, after several weeks of getting acquainted with setup, balance and settings of an EQ6.

[buzz]

I did the analysis and posted on the other GT81 thread. In case you are wondering, the flat IV does not work with the GT81. The focus tube is too long. I used CCD Inspector and confirmed Deneb's findings. It needs a flattener and it is a bit of a shame that WO didn't plan their products more coherently.

[Photosbykev]

The positioning of the handle is very similar to the high end Canon prime lens like the 400mm f2.8L and 600mm f4L. The Canon handles have two tapped threads to take a 1/4" and 3/8" fitting to allow the lens to mounted on a pan/tilt head like the Kirk Cobra or Manfrotto long lens support head or on a home made dovetail. It's a very solid way of mounting the lens, but not as good as ring clamps, and makes carrying the lens very comfortable.