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Weird circles in Meade Lx200 system – help please..


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Derbyshire Dave

I have a Meade Lx200, and a Celestron f6.3 reducer, connected to my ZWO ASI183 camera via some spacers and an off axis guider.

 

I get weird circular marks on the images, which appear to be some sort of reflections. There are several of these ‘circles’ of varying intensities in the image below. Tackling different targets results in these circles being seen at different points and intensities in the image.

 

M51 with bad circles

 

So the equipment is:

Meade Lx200, with two stage focuser attached,

Celestron f6.3 reducer

Some spacers

ZWO OAG2 off axis guider

ZWO EFW filter wheel

ZWO ASI 183mm mono camera.

Guide camera is a ZWO 174

 

With the F6.3 reducer I believe that the back focal distance is 105mmm. I have tried various arrangements with distances from about 115 down to the latest, below, which has a back focus of about 95mm.

LX200 Setup 95mm back focus

 

 

I have tried so many iterations that I am beginning to confuse myself, but I think I tried without the OAG and had no circles. I have tried removing the prism from the OAG, and taping up the hole to stop light entry, but in that case I still get the circles.

 

I am unsure how close the OAG should be to the filter wheel, but realistically it can’t go any closer because the holder for the guide camera is very nearly touching the filter wheel already.

I was wondering if any of you has seen anything like this, and can offer any advice on how I might try and track down this problem..

 

 

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Dave try pulling the OAG prism back out a little. You might be in a touch too far and getting a refraction off the bottom corner of the prism

 

No scrub that I just read the full item

Edited by Graham
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Derbyshire Dave

Hi Graham,

 I did leave the OAG in, but removed the prism, and taped over the hole. I still got the circles. I'm pretty sure that when I removed the OAG all together, the circles went. Of course removing the OAG shortened the back focal length too. And of course I need to use the OAG.

 I posted this on SGL as well, and there is some though there that I have 'too much hanging out the back'..

https://stargazerslounge.com/topic/375589-weird-circles-in-meade-lx200-system-–-help-please/

 

But I can't see that I can maintain the back focal length if I reduce the connectors in some way. I'm not sure how critical that back focal lenth is..

 

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Heres a thought how about we remove the extra focuser and when you use the stock focuser always do the final tweak anticlockwise using the "peg" upgrade.

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Derbyshire Dave
10 minutes ago, Ibbo said:

Heres a thought how about we remove the extra focuser and when you use the stock focuser always do the final tweak anticlockwise using the "peg" upgrade.

Thanks Steve,

 

I have just removed the extra focuser, and screwed the reducer directly onto the back of the LX200, same thread so that works. I'll be honest, I found it almost as easy to use the peg anyway. I wasn't aware that there is a difference in the action depending on which way you turn the peg .. does it work in some kind of backlash type way then?

 

Actually imaging using the GT81 tonight, but can try out the basix LX200 focussing in the daytime tomorrow.

 

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5 hours ago, Derbyshire Dave said:

Hi Graham,

 I did leave the OAG in, but removed the prism, and taped over the hole. I still got the circles. I'm pretty sure that when I removed the OAG all together, the circles went. Of course removing the OAG shortened the back focal length too. And of course I need to use the OAG.

 I posted this on SGL as well, and there is some though there that I have 'too much hanging out the back'..

https://stargazerslounge.com/topic/375589-weird-circles-in-meade-lx200-system-–-help-please/

 

But I can't see that I can maintain the back focal length if I reduce the connectors in some way. I'm not sure how critical that back focal lenth is..

 

 

Hi Dave

The distance between the fr and the camera chip is critical.

If the distance is wrong you will introduce all sorts of problems with your images.

I have always made my own extensions which are a lot heavier built than the commercial ones to stop the sag.

Edited by Graham
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