Veil + Bright Doughnut

[MarkS]

This is the first light with my DIY modded Canon EOS 350D.
Celestron C11 with F6.3 focal reducer. 
22 x 5min exposure. 
This is the full frame but scaled down by a factor of 4.

It's in my "Works in Progress" folder because of one big problem: what on earth is that bright doughnut?  Maybe the bright star is causing flare somewhere in the optical train.  Maybe part of the cause is the glass I used to replace the original IR filter - it doesn't have anti-relective coatings.  Maybe the cause is completely different?

On the original image the doughnut is 310 pixels in diameter i.e. approx 2.4mm on the CCD

[Fay]

Did you do flat frames, Mark? It's a nice image other than that. It does look reflective though.

[mickw]

Hmmmm  forbidden doughnut..............           

[Rick]

I guess it could be internal reflection in the replacement glass (from the image of the brightest star off the lower, then off the upper and down through the lower onto the sensor). The shape is consistent with an out-of-focus star through an annular aperture. Catch will be to see whether it appears on other images. Presumably it didn't appear in any images pre-modding?

[mickw]

If it's an internal reflection caused by the plain glass "filter" shouldn't there be other doughnuts, to a lesser degree, corresponding to other stars ?
You did the same mod to the 300 with no ill effects ?
Wouldn't flats reveal a likely cause ?

The results you are getting your modded cameras still amaze me 

[RobertM]

Excellent image but yes that bright ring ... very strange.  Dust produces shadows and won't be very pronounced so it is definitely reflection.  There is one particularly bright star which is likely related but because the reflection is off centre I would imagine there could also be some alignment issue i.e. not all the optical surfaces are parallel.  As has already been mentioned there should be other lesser donuts which might become visible if you stretch the image. 

What's different to the 300d ? or is it just that this problem hasn't been noticed before - it is a fairly bright (mag 4.2) star to image for so long after all.

[MarkS]

Fay, Mick, yes I take flats.  The flats got rid of a couple of dust bunnies but there's nothing in the flats in the same position as the bright doughnut.

Rick, you asked if it appeared pre-modding.  Dunno.  I bought the camera, modded it and now this is the first picture I've taken with it.

Mick, yes, if it's caused by reflection then in theory there ought to be other doughnuts for other stars.  But this star is much much brighter than the rest.  I've done the same mod to the 300D but this is the first image I've taken with a very bright star in the frame.

[Ian]

I think the safe bet is internal reflection. You could work out how far away from the image plane it is by it's apparent diameter (focal ratio determines the geometry of the light cone etc, hey you're the mathematician you do the sums ).

It's be quicker to look for planar surfaces first though, as I doubt you'd get such a clear out of focus star image from a convex (or concave for that matter) surface. There aren't many to choose from...

[MarkS]

Ian,

If it comes from a point source then that source would have to be 2.4mm * F6.3 = 16mm in front of the CCD.  But the only thing 16mm in front of the CCD is thin air!! 

I guess I can test the theory by taking a series of shots of a bright star in different positions in the frame and see if the doughnut moves with it.

Doughnuts, I'm beginning to feel hungry ...

Mark

[Rick]

I'd guess additional images are going to be required...

[Ian]

If it comes from a point source then that source would have to be 2.4mm * F6.3 = 16mm in front of the CCD.  But the only thing 16mm in front of the CCD is thin air!! 

Hmm, interesting. I've checked your working and agree. I have now put my socks back on. However, I wonder if it's as simple as that. What shape is the light cone when there's a focal reducer in the way?

I've found a website and have a revised location for you. I think the star image is 5.25mm from the CCD, or as it's a reflection, is there a surface 2.625mm from the CCD?

[mickw]

Knowing how little I know, I think the elements in the focal reducer may be the culprit  

It seems similar to lens flare, but with cake  

[Ian]

not beer then Mick? You're slipping mate.

Got some Mag ribbon too now 

[mickw]

Back home now 

Now attacking Z from fridge 

Oh - mag ribbon - It's not fair - I wanted to burn something down first.......................

Was I thinking that or did I just type it out loud 

[MarkS]

Ian,

Dust on the surface of the glass "filter" is quite a few mm from the CCD but it creates creates a dust bunny much, much smaller then this doughnut.  So I disagree with your 5mm.

I've checked one or two of the subs and I've found that as the star moves across the frame to the right, the doughnut chases it and begins to catch it up.

So it's definitely an internal reflection.  But from what?

Mark