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
(http://gallery.orpington-astronomy.org.uk/albums/userpics/10046/veil_290608.jpg)
Did you do flat frames, Mark? It's a nice image other than that. It does look reflective though.
Hmmmm forbidden doughnut.............. :)
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?
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 :o
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.
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.
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...
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
I'd guess additional images are going to be required...
Quote from: MarkS on Jul 01, 2008, 22:50:34
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 (http://www.rc-astro.com/resources/reducer.html) 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?
Knowing how little I know, I think the elements in the focal reducer may be the culprit :oops:
It seems similar to lens flare, but with cake :)
not beer then Mick? You're slipping mate.
Got some Mag ribbon too now :twisted:
Back home now :oops:
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 :-?
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
thing is, a dust bunny is a different thing to an out of focus star image. You didn't answer the question though. Is there a piece of planar glass 2.625mm from the CCD (give or take)? :)
how thick was the glass that you replaced the IR filter with, if thats 2.5ish mm then that would give you your 5mm ish distance,
the relection being from the top of the ccd, relfecting back off of the internal top side of of the replacement glasscover.
Top of glass--------------------------------------------------------------
/\ |
/ \ |
/ \ thickness of glass ~2.5mm
/ \ |
/ \ |
top of CCD-----------------------------------------------------------
total distance ~5mm
Mac
Ian, Mac,
Sorry - I didn't answer the question. The glass is approx 3mm thick and it may be about 2.625mm from the CCD (give or take).
But how does that create an out of focus star image?
Mark
Actually I got my sum wrong. The image would be 9mm away from the focus to be that size, or 4.5mm assuming it's a single reflection. If I hadn't got my sums wrong I'd have said Mac hit the nail on the head. Thinking about it, there the possibility that we're looking for two planar surfaces 4.5mm apart, one of may or may not be the sensor. But since the sensor cover glass is unlikely to be coated that'd be a prime candidate, together with the fact that the image is very nearly at focus and very bright (comparatively) as it reaches the sensor cover glass making the reflection more visible.
It's an out of focus star image because it's formed by light reflecting off on a surface and then reflecting back onto the ccd. because of the additional distance the light has travelled, by the time it finally gets to the ccd it's no longer in focus.
A dust bunny is a diffraction pattern caused by a mote of dust and it's size is related to the distance from the imaging plane and the size and shape of the mote itself, the fact it looks a bit like a doughnut or out of focus star image is coincidence. Refractors also show doughnut dust bunnies, but an internal reflection wouldn't look like a doughnut.
Some lenses are made up of sandwiches of different pices of glass. Could it be that?
it could be, but I wouldn't look there at the moment for two reasons.
To keep things sensible in the thinking department, I would keep to planar surfaces as reflections from non-planar ones are much harder to calculate (and impossible for us for all intents and purposes as we would need to know the radius of curvature).
In a lens there are rarely planar surfaces, let alone two parallel ones, and secondly in good lens construction the designer would have gone to some lengths to avoid internal reflections using cements, placement and coatings.
Ian,
I still disagree with your 9mm I think it should be 15mm. F6.3 means that the cone of light will be a particular shape: in this case the height is 6.3 times the width. The doughnut is 2.4 mm wide so the "height" is 6.3 times this i.e. 15mm.
So 15mm is the extra path length if the cause is reflections off planar surfaces. But I can't see anything that could account for 15mm.
My best guess is the following: light is being reflected off my new glass "filter" back up to the inside glass face of the focal reducer (2-3 inches away) and then back to the CCD. The glass face of the focal reducer is non-planar so it could act to refocus the light for its retun journey to the CCD. Now the focal reducer is anti-reflective coated but maybe, for instance, it is not optimised for infrared. Under this hypothesis, infrared is coming through the scope and focal reducer; reflected off the planar glass back to the reducer; reflected off the non-planar reducer and back to the CCD where it is not completely in focus. As discussed elsewhere, the RGB pixels are all sensitive to infrared so the doughnut appears whitish.
This hypothesis is easily tested by putting an IR filter between the scope and the reducer.
If the test is positive, the solution is either
1) Put an IR filter in the optical train
or
2) Re-mod my camera with something specifically designed for the purpose i.e. not cheap, uncoated glass!!
Mark
Mark, the optical system is not f6.3. It's f10 for about 2.5m and then gets significantly shortened after the FR to give an apparent f6.3. The geometry of the light cone is not that straightforward.
Did you take a look at that link I posted? The subtended angle of the light cone after the focal reducer is tan-1(1/(2NR)) where N is the natural focal ratio of the scope (f10) and R is the reducers focal multiplier (0.63).
Substituting: angle=tan-1(1.2/x) where 1.2 is the radius of the out of focus star image and x is the distance from the focal point gives an increased light path of 9mm and as it'd folded back on itself (coz you can see it) the light is traversing 4.5mm twice before finally reaching the sensor.
I got the sum wrong the first time because I used diameter, not radius in the second expression.
My point exactly! :D
Mark, I agree with you. To me it looks like a magnification of an out of focus star image which could only occur from a curved optical surface. The doghnut moving non linearly with the star image backs up that hypothesis. The only curved optical surface is the Reducer Corrector and if that was the case then the size of the reflection would depend on the accuracy of focus rather than being something that could easily be calculated i.e. dust bunnie. This could easily be proven by changing the focus slightly between images.
Actually Robert that's a good point. We can work out if the reflection is off of a planar surface by looking at the way it moves across the image plane in relation to the star image. It would be superimposed if the star is on the optical axis and then as the star moves away the reflection will be 2x as far from the axis but in the same direction. If the reflection moves in a non-linear fashion, it'll be off of a curved surface. Then all bets are off regarding distance from the imaging plane.
Obviously bear in mind the optical centre of the telescope may not correspond with the centre of the sensor, but you could work that out too ;)
Ian,
I still don't understand where 9mm comes from.
Let me work from your own equations:
(1) angle = arctan(1/(2NR)) = arctan(1/(2*6.3))
(2) angle = arctan(1.2/x)
setting (1) equal to (2):
arctan(1.2/x) = arctan(1/(2*6.3))
Hence,
1.2/x = 1/(2*6.3)
So,
x = 1.2*2*6.3 = 2.4*6.3 = approx 15mm !!!
So your complicated method gives the same answer as my simple method.
So, yes, the geometry of light cones really are that straightforward. :D
yep, you're right, I did the substitution wrong. :oops:
We've successfully proved that the addition of a focal reducer changes the geometry of the light cone to be effectively the same as an equivalent scope of the modified focal length.
And that's why I don't earn my living doing sums. I'd be skint. :roll:
However, we've still got the question of what is adding 15mm to the light path and that would be two surfaces 7.5mm (check my maths on that too) apart. What do you think about my postulation regarding the curvature or not of the surfaces?
OK this thread went way over my head a long time ago :o
QuoteSorry - I didn't answer the question. The glass is approx 3mm thick and it may be about 2.625mm from the CCD (give or take).
amended ray diagram
Top of glass--------------------------------------------------------------
\ /\ |
\ / \ |
\ / \ thickness of glass ~3mm
\ / \ |
\ / \ |
btm of Glass-----------------------------------------------------------
\ / \ \ |
\ / \ \ |
\ / \ \ thickness of air gap ~3mm
\ / \ \ |
\/ \ \ |
Top of CCD-----------------------------------------------------------
depending on the light path the reflection image will be either 6mm or 12mm
red incoming
green first reflection
blue second reflection.
if your glass is .5mm thicker and the distance between ccd and glass is .5mm difference that would allow
you the calculated 14mm
as each light path reflection is extended by 7mm.
The front of the CCD / glass is probably in the region of about 25mm from the back of your focal reducer, which means the reflected light path
would be in the order of 50mm, so you could probably rule this out as the source of your reflection.
Mac.
Mac,
I love those diagrams - thanks for taking the time and trouble!
I see one problem - because of refractive index, the raypath through 3mm of glass is equivalent to only 1mm of air so your multiple raypath suggestion is looking less likely as an explanation.
The refractive index of the glass will only alter the angle of internal reflection, the light path will still be ~3mm
Snell's Law. http://en.wikipedia.org/wiki/Snells_law (http://en.wikipedia.org/wiki/Snells_law)
The distance is still ~3mm for the air gap and ~3mm for the thickness of glass.
The difference in refractive index for the glass and air, will change the angles of reflections at the boundrys, The ray paths will still be the same. give or take the refractive indexes.
Sorry, I got my figures the wrong way round - I think 3mm of glass is equivalent to either 4mm or 4.5mm of air.
The way it works is like this. If you focus your camera and then put a sheet of plane glass between the lens and the CCD then the rays of light will no longer focus on the CCD - they will focus behind it. The reason is that glass has extended the effective raypath. It's because the speed of light in glass is only 2/3 of that in air so the waves "bunch up" closer together in glass.
I'm not sure of the exact mathematics and the exact ratio though ...
I worked through the maths whilst cycling to work.
Putting 3mm of glass between a (focused) lens and the CCD (or film) pushes the focal point 1mm behind the CCD i.e. it adds 1mm to the effective raypath. So 3mm of glass is equivalent to 4mm of air in terms of "effective raypath".
The same principle is true of astronomical filters. If you focus the telescope onto your camera CCD and then add a filter into the raypath, it will throw the focus out.
which is why type one RGB (or CMY) filter sets should include a plain glass filter for the L exposure.
I think we need to create a new forum sub-section... something along the lines of :
'Highly Technical Question Area - don't bother looking unless you have at least three hours to try & understand the thread......' :-)
'fraid you all lost me hours ago........!!!!! :-)
Mac - have you just been swatting up on reflection theory or something for you Astro degree....?????
John.. (confused) :-(
John I agree ! :o I can feel my synapses popping.
John, Mike,
True - the discussion became a bit technical but it does have some important practical consequences:
1) To avoid the need for re-focusing for every change of filter, filter sets should be made of glass of the same thickness and should include a clear filter (of the same thickness) for the luminance channel.
2) If you mod your camera with a filter of a different thickness you also need to re-position the CCD for autofocus to work properly.
O.K. so I lied about (2) - it's not an common or garden practical consequence!
Mark
Quote1) To avoid the need for re-focusing for every change of filter, filter sets should be made of glass of the same thickness and should include a clear filter (of the same thickness) for the luminance channel.
Mark - Exactly why I bought Astronomik L, RGB type II filters... :-)
Except with Type II filters, the "clear" one is actually an IR block... but, the point remains.
Its the secondary.
Mark, just curious.............
Has this problem not been brought up on any of the forums ?
Or did you use a different thickness glass - Could this be rectified by changing the glass ?
Should only take you 2.5 hrs as you have now got the knack ;)
And stop doing sums on your bike - you'll cause a nasty accident :)
More practical consequences:
(3) Swimming pools always look shallower then they really are
(4) Without this effect, glass lenses would not exist in the first place - light would pass straight through without deviating.
Mick, the reason I have this problem and the reason it's not on the forums (present forum excepted!) is (probably) because I modded my camera using cheap uncoated glass - I gave up waiting for Baader to re-supply the market with a bespoke replacement IR filter. In fact, we're all still waiting ...
Chris, why do you think it's the secondary?
QuoteMac - have you just been swatting up on reflection theory or something for you Astro degree....?????
John.. (confused)
Part of this years course included, light, reflection and refraction boundrys and ray paths throught lens's as well as spectroscopy.
So i was cheating in a sort of way as this info was fresh in my mind.
If you want more detail, explained with loads of pretty diagrams and maths :o have alook here.
http://www.belvedereconcertband.co.uk/Macs/Year3EMU.zip (http://www.belvedereconcertband.co.uk/Macs/Year3EMU.zip)
unzip it and have a look at the physics section 5.
should give you loads of info.
Mac. :lol:
Cheers Mac - That explains a lot - I'll take a look at the notes but I'm not promising to understand any of it... :-)
John.
PS - How's that Pier coming along - Have you got it built yet?
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 ...
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...
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.
QuoteOK this thread went way over my head a long time ago
Me too!!!
Carole
Is this a Dog Nut... ?
http://farm1.static.flickr.com/140/317362825_d83694c5a7.jpg?v=0 (http://farm1.static.flickr.com/140/317362825_d83694c5a7.jpg?v=0)
To John:
(http://www.beyondbakedbeans.com/custom/meat_and_two_veg.jpg)
From Ian
Mike you are funny!!!!!! :lol:
QuoteMike you are funny!!!!!!
I'd say perverted.....
Quote from: JohnP on Jul 04, 2008, 21:17:27
QuoteMike you are funny!!!!!!
I'd say perverted.....
I'd say Mike needs at least a back, sack and crack.
what i think is odd is that mike has a dog suit
Quote from: Space Dog
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?
(http://gallery.orpington-astronomy.org.uk/albums/userpics/10046/doughnut.jpg)
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 :-)
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.
(http://gallery.orpington-astronomy.org.uk/albums/userpics/10046/focalreducer.jpg)
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 ?
Robert,
Yes, it's very flat, maybe polished but definitely uncoated.
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...