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WO GT81 First Test Results


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I have mounted this scope on my 8"RC.

Getting the reducer/flattener to the best position needed several attempts. I suppose this

is only to be expected working at F4.7 - I think I have just about got it right (53/54mm to CCD chip)though it may still need a minor adjustment.(WO say 55 to 57)

Last night ( 19th dec) I managed to get a few images though the GT81. It was murky, cold and a nearly full moon was up. I opted to test the scope on NGC2244.

I took a series of images using my Atik 460EX ( + ha filter - to reduce moonlight)

1x3min , 1X5min , 1X 8min and 1 X 10min

There was no problem guiding using the EQ8 and PHD and EQMOD.

After looking at the images I decided to stack them all. I then did some stretching/sharpening.

No flats,darks or bias frames were used.( image below)

Fairly happy with this first go. The reducer/flattener lens to chip distance looks like a criticalfactor. I do have the TS field flattener so I will have a go at F5.9 next clear night.

John

post-415-0-57120800-1387555670_thumb.jpg

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Hi John,

You've got some good detail there for such a limited time

especially with a near full moon. But that's the beauty of

Ha and an f/l of 4.72. It is the scope I'm looking at for a

future purchase in the New Year.

I get similar results with my Z71 at f/l 4.72 but it suffers

in the blue spectrum of my LRGB subs. I'm hoping this isn't the case with

the GT81 Triplet.

Are going to image (L)RGB with your scope?

If so, any feedback would be appreciated.

cheers

Steve

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Hi Steve,

I do have all the necessary filters!

I still need to do so more testing but with some clear skies I hope to get a fully calibrated/processed image soon.

It may depend how long I "fiddle" with the reducer/chip distance.

John

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  • 2 weeks later...

Hi John have you settled on a distance to chip yet?  Is the reducer you are using the FLAT-6A?  I have a bit elongation in the corners and I wonder if this is distance or possible tilt in the focuser / imaging rig.

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  • 3 weeks later...

Sorry to jump in on an old thread but I too have a little stretching in the corners from this scope and wondered if either of you chaps had come up with an ideal distance? Possible tilt has crossed my mind also.

Cheers

Chris

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Do you have elongation in all corners? Is it equal or worse in some than others?

I just got hold of an artificial star so if it stops raining I will try and do some testing.

I found I was getting a stretch in one corner only.

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Hi Ian/Chris

I have a different scope the WOZS71 but have had similar problems with the WO FF6 flattener i am using so was wondering if it could be a generic thing anyway with these fast scopes.

I found i had to get the spacing to 56.5mm to work and yes 0.5mm does make a difference i used the method someone posted in another thread which said that if the star shapes run away from the center then the flattener to chip spacing was too far if they where parallel to the center (looking like field rotation) the spacing was too close.

I still get a slight twist on one side so still have to play around with getting the focus tube etc in line but its marginal and easily cropped away.

I will be interested though in your findings.

Alan

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  • 4 weeks later...

Quick update on this - been a while due to the weather!

I had another look at the WO diagram and realised the length of the entire unit as shipped (flattener/spacer/EOS camera adaptor) is a bit easier to measure with some calipers than the distance to the focal plane marking on the camera body.

The length of the whole unit is supposed to be 75.58mm but mine came out at 77.0mm (+- 0.5mm). I have checked this several times, odd as this is over the stated distance.

I've rigged up an alternative spacer from a T coupler and the inside part of a T-Adaptor which is very finely adjustable. I've got it down to 75.5mm (+- 0.5mm) and the distance from rear of flattener to focal plane marking looks a lot more like the recommended 64.38mm than something around 67 I had before.

post-31053-0-67225800-1393503790.png

I've mentioned it to FLO to see what they reckon. Good old WO !!! An alternative (if this proves to be the issue) might be a shorter extension and some Delrin spacers.

I'll post something as soon as it's not cloudy/raining/snowing!

cheers

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Quick update on this - been a while due to the weather!

 

I had another look at the WO diagram and realised the length of the entire unit as shipped (flattener/spacer/EOS camera adaptor) is a bit easier to measure with some calipers than the distance to the focal plane marking on the camera body.

 

The length of the whole unit is supposed to be 75.58mm but mine came out at 77.0mm (+- 0.5mm). I have checked this several times, odd as this is over the stated distance.

 

I've rigged up an alternative spacer from a T coupler and the inside part of a T-Adaptor which is very finely adjustable. I've got it down to 75.5mm (+- 0.5mm) and the distance from rear of flattener to focal plane marking looks a lot more like the recommended 64.38mm than something around 67 I had before.

 

attachicon.gifspacer.png

 

I've mentioned it to FLO to see what they reckon. Good old WO !!! An alternative (if this proves to be the issue) might be a shorter extension and some Delrin spacers.

 

I'll post something as soon as it's not cloudy/raining/snowing!

 

cheers

 

Oh the joys of WO ownership i got around my issue (wo zs71) by using a M42 camera lens adapter about 1mm thick screwed into a 10mm T ring spacer and then a thin washer.

There is enough space between the camera flange and the FF you think someone would come up with an adjustable T ring.

Alan

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Ian, glad you got yours sorted mate.

Alan - there does seem a gap in the market there. Mind you WO could make their spacers the right size too :) Found this http://www.scsastro.co.uk/catalogue/orion-variable-12-17mm-t-thread-spacer-ring-5326.htm but the minimum is still too big.

Man, it's so silly. WO are on the verge of true greatness with this scope at this price but for the sake of a dodgy measurement or loose tolerance this kind of frustration could cost them dearly. Some would have returned the scope before now. Not me though, I still love this thing!!!

Wahey, come on clear skies I'm itching to try it out (currently hail).

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I tried some imaging last night with the makeshift spacer and the reduction in distance seems to have nailed it. Totally flat from edge to edge. Beautifully round stars right across the image.

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