Taking Flats with a DSLR

[PhilB]

Carole, sorry wasn't sure what image I was looking at. As regards defraction spikes, you have a real problem if you're getting them in a refractor 'cos they shouldn't be there!

[MarkS]

As regards defraction spikes, you have a real problem if you're getting them in a refractor 'cos they shouldn't be there!

I agree with Phil. 

Diffraction spikes are typically caused by the spider that holds a Newtonian's secondary in place. 

If your refractor is producing  them then it indicates that something, somewhere is obstructing the light path.  Maybe something has come loose?   Have a careful look at the objective lens and inside the optical tube to see if anything is amiss.  Check for loose bits in the focal reducer and inside the camera.  Give it a gentle shake and listen for any rattles.

[Carole]

Diffraction spikes are typically caused by the spider that holds a Newtonian's secondary in place. 

Silly moi, I should have realised that.

I have just had a thought I wonder whether my "clip in" CLS filter has come loose, will have to check.

Carole

[Rick]

If your refractor is producing  them then it indicates that something, somewhere is obstructing the light path.

Even very small obstructions affecting the circularity of the aperture can cause quite noticable effects.

[Carole]

I have just checked the CLS clip filter and it was in place.  However I notice the filter is not circular, two opposite sides are squared off a bit.  Ive placed them top and bottom.  Everything seems to be in place, so have taken some more flats to see what I get this time.

Carole

[The Thing]

Carole, the filter is rectagular(ish). The ccd is rectangular... They need to line up.

Duncan

[MarkS]

Cycling home tonight I was thinking through the maths of why using a lightbox close to the scope produces an incorrect flat.  I now know I can definitely prove it mathematically but its a bit complex and few people will understand it.  However, I believe can demonstrate it with a simple practical experiment and almost everyone will understand it.

I'll carry out the experiment and post the results.  Watch this space!

[Mac]

but its a bit complex and few people will understand it.

Post the maths as well as the results

[PhilB]

Mathematician fuel on standby, I think this may be good.

[MarkS]

I think I need to qualify my earlier assertion.  It may well be the case that for a naked refractor, Newtonian or SCT then a lightbox immediately in front of the scope may give an acceptable flat. However, as soon as you add a focal reducer (which most people do) then a very close lightbox does not give a correct flat.

My argument proceeds by experimental demonstration followed by a thought experiment.
A focal reducer is actually a convex lens and it will produce an image of whatever you put immediately in front of the scope.  So I put a computer monitor showing a stripy pattern directly in front of my Celestron C11 SCT.  Hold a piece of paper well behind the focal reducer and you can see the image - note the sharp disc (the shape of the scope's aperture) the stripy pattern and the scope's central obstruction.  There is also an outer ring - probably an internal reflection - but this forms no part of my argument.  Note that a lightbox would not normally use a stripy pattern - I've just done this to make clearer what is going on.



Now as I move this piece of paper closer to the focal reducer the original image appears out of focus.  So that this is an out of focus image of the part of the lightbox seen through the scope's aperture.



Continuing to move the piece of paper closer to the focal reducer, we reach the plane where the CCD would sit - at this point we have a very out of focus image of the lightbox.  This is the image you are taking when you shoot the flat - the camera CCD sits within the central blurry bright disc.



Now for the thought experiment.  
Remember we've just demonstrated that a flat is a very out-of-focus image of the lightbox as seen through the telescope aperture.
We now move the lightbox 10 or 20 feet in front of the telescope (and assume it is large enough to fill the scope's field of view).  As the lightbox moves then so does the the focal plane of it's image behind the focal reducer - in fact the focal plane moves nearer to the scope.  As before, we can place a piece of paper in this plane and see the image of the lightbox.  As we move the piece of paper towards the scope, the image becomes out-of-focus as before.  By the time we reach the plane where the CCD would sit the image is very out of focus, but not quite as out of focus as in the original experiment.  So the flat that we shoot with the lightbox 10-20 feet in front of the scope is DIFFERENT to the flat that we would shoot with the lightbox immediately in front of the scope.  This is the whole crux of the argument - the position of the lightbox DOES make a difference to the flat.  An infinitely large lightbox at an infinite distance away would give yet another different blur in the plane of the CCD.  This is the flat that we really want and it is best approximated by taking an image of a uniformly illuminated sky (usually near dusk or dawn).

To drive the point home further, here is a 2008 Horsehead using a lightbox flat:


And here is a 2009 Horsehead using a flat created from the sky:


It took me a long time to realise why my flats didn't work properly!

Mark

[PhilB]

Sorry, but I'm not sure that I agree. When you use a light-box to produce flats you're not trying to form an image of the light-box itself. Surely the purpose of the light-box or, for that matter, any type of illumination used for flat production, is to provide a uniform source of light with which to illuminate the telescopes internal components.This then enables things such as errors, marks, dust and other debris within the optical system to be photographed so that they may be subtracted from the final image. It seems to me that it's actually quite important that you should not be able to image the light-box when making flats. If you do then you've introduced an artefact into the flat image which isn't there in the light frames but the software will still attempt to remove it during processing.

[The Thing]

I rotate my diffuser or now my electroluminescent panel for each flat and then stack the flats as median or average to avoid pattern and non-uniformity issues.

[MarkS]

Phil, yes, I (almost) totally agree.  A light box is an physical object that you are putting in front of an optical train.  So an image of it will be created somewhere (as demonstrated by my experiment).  The flat you create by using a light box is simply a very out-of-focus image of that light-box.

You have fallen straight into the trap I carefully prepared!  My whole experiment was designed to prove that you should not create flats by sticking illuminated or diffuse objects of any kind, in front the scope!  

I agree that when use use a light-box you are not trying to create an image of the light-box itself.  But, unfortunately, like it or not, that is precisely what you are doing!  A flat created in this manner is simply the wrong thing for correcting vignetting etc in your astro-images.

[RobertM]

That's very interesting and I agree with you Mark.

Another angle on this is that when you are imaging a nebula or other very remote object the photons are travelling parallel to the optical tube by the time they reach it.  Because of that they do not illuminate the inside of the tube, edges of optics etc, that may be in or near the light path.  An artificial light source in front of the tube is very diffuse and illuminates everything at varying incident angles.  It couldn't give you a perfect flat but for most optics and imaging rigs it's obviously good enough.

Hope that makes sense!

Robert

[PhilB]

Mark, I think I might be starting to see the light. However, it's the first image in your experiment, where there are in-focus stripes, that worries me. When taking a flat the telescope would be focused on infinity and nothing placed at close range would be in-focus. This being so, I not sure that moving the image plane through what is a relatively short distance would have any noticeable effect on the out of focus image.

Robert, yup, the thought that the light beam is not parallel had occurred to me also. I suppose that you could get round it by collimating the light-box beam but that would require some awfully big optics and an associated price tag.

I obviously need to go away and make a few more mistakes, whilst keeping what's been said in mind, at which point all will become clear!