Haloes...

[RobertM]

Some of you may have noticed the haloes around the bright stars in Orions belt in my recent widefield image on the otherhand I may be wrong and you didn't notice.

For those that did (or didn't)...

I was curious as to what caused them since they hadn't really bothered me before but were really obvious in that image.  Anyway I thought I'd share my findings as to what the cause is and the solution...

Haloes are images of a reflection from one window that appear on another.  There may in fact be many concentric (or at least symmetrical) haloes which go unnoticed dependent on the reflectivity of each glass surface.  There was only one in this case so I first needed to calculate which glass surface the haloes were being imaged from. 

1) Calculate how large the halos were in mm:

Size = Diameter of halo in pixels * pixel size = 62 * 0.0054 mm = 0.33mm

2) Calculate the distance from the CCD surface:

Distance = Diameter of halo in mm * focal ratio = 0.33 * 3.2 = 1.05mm

This puts the reflective surface at ~1mm from the CCD surface i.e. on the CCD optical window, which I've subsequently confirmed with Finger Lakes Instrumentation.

I now have a dilemma ! do I send the out of warranty camera back for the newer double coated MAR window at a cost of $200 + shipping (and that's the killer) or make do with haloes for perpetuity.  For that sort of money I could buy a second hand 40D and get Mick to mod it for me with Tony's pink toolkit 

Decisions, decisions.

PS. I'm sure someone posted the calculations in a thread a while back, alollogies if they did, but I couldn't find it!

Robert

[mickw]

Get the 40D - You know you want to  
Or get FL to send over the new window and Tony and me can fit it - I've already done the same thing to a 350D and consider myself an expert 


Tony, polish up the big pinky  

[Ian]

I believe tony maintains his various pink accoutriments in a high state of polish.

Some are getting worn to a point apparently.

[Tony G]

Talking of halos     



Tony G

[Fay]

I could help Tony & Mick as well Robert...........

[mickw]

Now I'm scared 

[Rocket Pooch]

Hi,

Send the thing back and get it upgraded, you know you want too.

Chris

[Carole]

the big pinky

I believe tony maintains his various pink accoutriments in a high state of polish

I could help Tony & Mick as well Robert...........

the mind boggles!!!

[MarkS]

Robert,

The distance you have calculated, 1.05mm, is not the distance of the culprit window from the CCD but it is the extra distance travelled by the reflected ray compared with a direct ray.
This extra distance is 1.05mm in air or 1.5 times that (i.e. 1.575 mm) in glass.  I think you have a double reflection in a glass pane 1.575/2=0.7875mm thick.  This is either the CCD cover glass or the Baader filter.  

Mark

[mickw]

Does that mean we don't get to fix it 

[RobertM]

Mark,

It's just a reflection between surfaces but you're right about the extra distance.  I shall check results unfiltered and see whether there is any difference.

Cheers
Robert

[MarkS]

I think you have a double reflection in a glass pane 1.575/2=0.7875mm thick.  This is either the CCD cover glass or the Baader filter.  

According to http://www.ccd.com/pdf/ccd_8300.pdf (page 38) thickness of the cover glass is  0.76mm +/- 0.05mm
It matches my prediction quite well.
However there is also a 0.57mm air gap between the sensor surface and the cover slip - a reflection there would add 1.14mm to the ray path - this is not a bad match for your 1.05mm calculation.

In either case the cover slip is implicated in one way or another.

Looks like you'll be sending it back for the upgrade!

Mark

[RobertM]

I think at that f-ratio the halo would be quite large if it was a reflection from any surface of the filter.  I believe the reflection produces a cone of light that could only be from the cover slip.

Definitely bad news but thanks for your input anyway.

Robert

[MarkS]

Robert,

You have already calculated the extra ray path at that F-ratio:  1.05mm in air or 1.575mm in glass
The Baader filter is too thick to generate such a small increase in ray path.  The only mechanism by which the Baader filter could cause this is by reflecting the light back up to the lens where it is subsequently, somehow refocused and sent back to the CCD.  This was what was happening with my CCD replacement glass here:
http://forum.orpington-astronomy.org.uk/index.php?topic=3535.msg16742#msg16742

Anyway, reusing a famous "Mac" diagram from that discussion, this is what I think is happening in your case:

 \  Direct          \ Reflected               \ Reflected
  \ ray               \ ray 1                     \ ray 2
 --\--------------\---------------------\---------------------------- Top surface of cover slip glass
     \                    \        /\                   \                                      
       \                   \      /  \                   \                                    
        \                   \    /    \                   \                        thickness of glass ~0.76mm
         \                   \  /      \                   \                                  
           \                  \/        \                   \                                  
 --------\--------------------\--------------\----------------------  Bottom surface of cover slip glass
             \                            \                    \        /\                      
              \                            \                    \      /  \                      
               \                            \                    \    /    \         thickness of air gap ~0.57mm
                \                            \                    \  /      \                    
                 \                            \                    \/        \                  
 --------------------------------------------------------------------  Top surface of CCD


A total internal reflection within the glass (reflected ray 1) would cause the right size halo because of the increase in path length within the glass.
A reflection between the CCD surface and the bottom of the cover slip glass (reflected ray 2) would cause approx the right size halo because of the increase in path length in the air gap.

Mark

[RobertM]

Thanks Mark, for confirming the bad news 

I think it's going back ! While it's there hopefully they'll be able to fit the newer upgraded teflon shutter; fortunately that's free.

Robert