Veil + Bright Doughnut

[MarkS]

I've just done some more experiments - pointing the scope at Vega.  An unmodified Canon EOS 300D shows exactly the same bright doughnut though perhaps not quite so bright.  So it's not entirely the fault of modding the camera.  Putting an IR filter in the optical train makes no difference whatsoever.  So IR is not a contributory factor either.

But if I take the reducer/flattener out then the problem disappears.

Putting the reducer/flattener back I used an eyepiece but could not see the doughnut visually because it was too dim.  So I put the camera on the eyepiece (eyepiece projection) and the doughnut appeared in the photo.  So this is conclusive proof that the doughnut is created by multiple element lens design of the reducer/flattener. 

Mick - you were right all along - well done mate!

But it's only a problem when there's a super bright star in frame.  This doesn't happen all that often ...

[Ian]

you know that means it's got to be taken apart so we can work out exactly where the reflection is coming from, don't you...

[Rocket Pooch]

Its the secondary, I had the same problem on the VC200L if you take a flat with the reducer off you will see the dog nut but it will be spread out over a large area, with an F6.7 it was in the centre third and with my old F3.3 it was in the middle.

SCT's are not flat so when you put a reducer on the cone is reduce and therefore the effect of the dognut is.

I'm positive this was what the problem was.

[Carole]

OK this thread went way over my head a long time ago

Me too!!!

Carole

[Mike]

Is this a Dog Nut... ?

http://farm1.static.flickr.com/140/317362825_d83694c5a7.jpg?v=0


To John:

From Ian

[Fay]

Mike you are funny!!!!!!

[JohnP]

Mike you are funny!!!!!!

I'd say perverted.....

[Ian]

Mike you are funny!!!!!!

I'd say perverted.....

I'd say Mike needs at least a back, sack and crack.

[Rocket Pooch]

what i think is odd is that mike has a dog suit

[MarkS]

Its the secondary, I had the same problem on the VC200L if you take a flat with the reducer off you will see the dog nut but it will be spread out over a large area

You're absolutely right, Chris.  With the focal reducer off, this is what I got - a huge doughnut filling most of the frame.  It's centre is left of the main star and slightly down a bit.  I haven't applied a flat to this image so there is also some unrelated vignetting.

But technically, what is going on?  Is it the secondary creating an image of, say, the corrector plate?

In any case, the fact that the star and the doughnut are ont concentric must surely indicate that my scope is out of collimation?

[Rocket Pooch]

I was talking to John the other day about this, the problem the S/N is not good you also reduce a lot you kinda double the effect, the only thing which will get rid of it is a Flat, the secondary is effectively a big bit of dust, and you can't move it.

Also it might not be the collimation, sometimes its where the camera and telescope is not aligned properly, again I had this issue with the Vixen and I made real sure it was set proper, even on my ED80 I get about a 11% field flatness variation and also some mine flexture between the focuser and camera, the old one that is.  The NEW SCT RC is a lot better, as a side point the C11 is not really designed as an astro graph either this is not going to help. 

Couple of options, get a better reducer, this will help but don't push it to F3.3, if you want to get a hyperstar

[MarkS]

I think I may have finally diagnosed the problem.

An object placed 110mm from the back of the focal reducer produces a neat image 130mm away (see photo).
110mm is the distance of the CCD from the back of the focal reducer.

So a bright star image at focus on the CCD can be reflected back by the uncoated glass in front of the CCD and form at out of focus image (the doughnut) back at the CCD after being reflected by the focal reducer.  So I definitely need coated glass in front of the CCD to prevent this.  Either that, or a better designed focal reducer.  I'm currently using the Celestron one.

[RobertM]

That's what I was trying to explain on Friday.  The trouble is you have three surfaces that could contribute - front/back surfaces of your replacement glass and the sensor itself.  I can't imagine it would be the sensor as that would result in a serious loss of sensitivity (if it is your in trouble) so that leaves the optical glass.  Maybe it's optically flat and polished but not coated ?

[MarkS]

Robert,

Yes, it's very flat, maybe polished but definitely uncoated.

[JohnP]

Well done on finding the issue - so I guess that means you'll be replacing the glass once it's available from supplier....? Out of interest I wonder if Fay's modded camera has the same problem? (I expect not)..

John


PS - Any chance you could remove that image of the dog Mike... put's me off my meals everytime I look at this thread...