GT81 spacer problem
I recently purchased a GT81 (the one without the digital focuser or built in reducer). I am using the dedicated GT81 0.8x flatener/reducer with a QSI683wsg camera.
I have got the spacing to the distance recommended by WO and at this distance I originally got the attached field from CCDinspector. (image called "correct spacing"). As you can see it isn't perfect so I thought it might have something to do with the orientation in the focuser.
So I took some images rotating the whole focuser itself and also some images where I have rotated the reducer/camera within the focuser. In doing so the field is awful and differs whenever I touch it. I have attached a few 3D plots for you to see.
They all have the same correct spacing and as you can see it originally didn't look too bad. But by rotating the focuser 90 degrees in the first image and then rotating the camera 270 degrees it caused real problems.
So my question is - is there a way of getting the focuser of the GT81 set so it doesn't sag as it is moved around (there are lots of strange screws there). Or do I have a faulty scope here?
- Hi,The focuser rotation issue is pretty easy to work on, and hopefully solve. Most WO focusers are similar in this regard. Here is what I recommend.Remove the thumbscrew that locks the focuser rotation. Loosen (or remove, but don't drop and lose) the 2 to 4 (depends on model) small slotted nylon black set screws using a micro screwdriver, or jewelers screwdriver.They are not metal, be careful not to damage them. Once very lose or removed the entire rear section of the focuser will slide off of the ring that it rotates on.Now (and here is the key), apply a thin layer of high quality grease to the groove which is where the nylon screws ride in when put back in place. Use something like white lithium or Super Lube, something that won't be too sticky, but feel free to experiment. You don't need a lot, just enough to coat the groove surface.Now put it back together, and carefully tighten each of the nylon screws, to get it snug, so that it still rotates but isn't wobbly. The thumbscrew can also be put back in, and tightened. The trick is to check these nylon screws and if they loosen up just carefully tighten them and keep them tight enough, but not too tight. It's really just a touchy feely sort of thing.I found the grease allows you to tighten these screws more than you could without the grease, which I think reduces the wobble.Hope that helps,Timm
- Thanks for that. I'll give it a go.I've just noticed that as well as the two thumb screws that push the brass ring onto the nose-piece in the focuser there is a cross head screw. Is there a particular way of using this so that the nose-piece is held in line with the optical train and so doesn't sag?CheersSteve
- Hi,I haven't really tried adjusting that one, but I would guess you would could adjust it quite easily. Trial and error, I would think would be all that is needed.Timm