Weird circles in Meade Lx200 system – help please..

[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.

 

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.

 

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..

 

 

 

 

 

[ollypenrice]

They are internal reflections but from where, I wouldn't like to say. To be honest, the sheer length of 'stuff' after the end of the drawtube really is a fundamental problem in this setup. As well as the internal reflections you have extreme and asymmetrical vignetting (top left in the image posted). When the leverage against the focuser is as long as this you have little hope of keeping the chip square on to the light path. I'm sure it isn't what you want to hear but I see this setup as fundamentally flawed. If I'm wrong, and others have made such a system work, I'll stand corrected.

Olly

 

[CraigT82]

44 minutes ago, Derbyshire Dave said:

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.

You can get it closer, you just need to use T2 thread spacers to fine tune the stopping point for the OAG as it screws to the filter wheel. You need to OAG turret to stop tight at the 'thin edge' of the filter wheel (as per pic).

Is there any shiny surfaces or edges on the inside of any of the kit in the imaging train? 

[gorann]

Yes, I keep it short. That long thing will sag and have all kinds of internal surfaces for reflection. I would start by taking out the reducer (do you need it?) or find one that can sink into the focuser.

This is what my 14" Meade looks like.

[Derbyshire Dave]

43 minutes ago, ollypenrice said:

They are internal reflections but from where, I wouldn't like to say. To be honest, the sheer length of 'stuff' after the end of the drawtube really is a fundamental problem in this setup. As well as the internal reflections you have extreme and asymmetrical vignetting (top left in the image posted). When the leverage against the focuser is as long as this you have little hope of keeping the chip square on to the light path. I'm sure it isn't what you want to hear but I see this setup as fundamentally flawed. If I'm wrong, and others have made such a system work, I'll stand corrected.

Olly

 

Thanks Olly. I've never done this sort of thing before. My usual setup is a GT81 with a parallel guide scope.

I can believe what you say, if I look down the back of the reducer, with my eye slightly off center, I can circular surface akin to what might be appearing in the picture.

I have a fundamental problem, but if I shorten things a lot, will I not have an incorrect back focal length

[Derbyshire Dave]

29 minutes ago, CraigT82 said:

You can get it closer, you just need to use T2 thread spacers to fine tune the stopping point for the OAG as it screws to the filter wheel. You need to OAG turret to stop tight at the 'thin edge' of the filter wheel (as per pic).

Is there any shiny surfaces or edges on the inside of any of the kit in the imaging train? 

if I look down the back of the reducer, with my eye slightly off center, I can circular surface akin to what might be appearing in the picture, so yes there are..

[MartinB]

I used to image with this reducer along with an SX AO unit and a qsi 532 camera.  A longer and heavier train than yours Dave.  Ccd inspector confirmed a small amount of tilt but I never had internal reflections.  Lots of people have used a similar set up successfully and I certainly think it better to use an oag at this focal length.  With your small pixels, even binned, you are better off with the reducer if you can make it work!  

Now you've spotted a likely reflection source you can hopefully sort it.

[Derbyshire Dave]

2 hours ago, MartinB said:

I used to image with this reducer along with an SX AO unit and a qsi 532 camera.  A longer and heavier train than yours Dave.  Ccd inspector confirmed a small amount of tilt but I never had internal reflections.  Lots of people have used a similar set up successfully and I certainly think it better to use an oag at this focal length.  With your small pixels, even binned, you are better off with the reducer if you can make it work!  

Now you've spotted a likely reflection source you can hopefully sort it.

Thanks Martin. I hadn't heard of CCD inspector.

I'm not sure I have spotted a reflection source, just that if you are off centre in anyway, you can see that sort of circle. Can't see that I can do anything about them, will try and see if I can reduce the droop, if that's what it is.. Probably going to do a daytime test tomorrow, removing the focuser and see if I can adjust and get focus with the 'peg' mirror focuser. Same thread on both, so that will work. Just a little bit of extra infornation to the jigsaw.

 

[bottletopburly]

Dave Astap has a ccd inspector in its tools menu if you already have that .

[Derbyshire Dave]

15 hours ago, bottletopburly said:

Dave Astap has a ccd inspector in its tools menu if you already have that .

Thanks, I haven't got it, but I could download it. Thanks forthe tip Martin