Let's start at the very begining... (well the processing part anyway)

[Carole]

If the dust was on the clip-in filter or on the CCD then when you rotated the camera, the dust bunny would remain in the same place on the image. 

the images had already been done Mark, the rotation took place between the images and the flats.  I'll post up the images and the flats.

Carole

[MarkS]

I'll post up the images and the flats.

I'm looking forward to seeing them.

[mickw]

Round and round we go, where it stops etc................

There seems to be the assumption that Carole's dust is on the optics at the camera end of the scope, which the maths say will not noticeable.

If the camera rotates and the bunny is in a different place, the dust, bird shit or glow-worm is on the scope and will be noticeable.

[Carole]

I agree with Mick, and here's the proof.
I don't really care where the dust particle is, so long as they get subtracted from my image. 

This is a partly processed image with no flats or darks applied to show where the dust is on the light frames. N.B. there is one prominent dust particle near M110 and no dust particle near the galaxy edge on the right.



this is the flat (after DSLR had rotated) as you can see the prominent dust particle is not where it is on the light frame but close to the galaxy edge on the right.



Here is the stacked and partly processed image with flats and darks.
As you can see the dust particle near M110 has not been substracted but a non existent dust particle has been subtracted near the galaxy edge on the right.

[Carole]

There are other dust particles there as well, but not as noticeable.

Carole

[mickw]

I agree with Mick, and here's the proof.

You are both talking about different locations for the dust

[MarkS]

Carole,

Thanks for posting the images.  

From them I deduce the following:
Scaled up to full size (4272 x 2848) the dust bunny is approx 100 pixels across i.e. 0.52mm using the 5.2 micron pixel pitch of the 450D.
Assuming an F-ratio of around F/5 or F/6 this places the dust particle less than 3mm in front of the CCD i.e. the dust is sitting on the CCD cover glass (notice it's the same size as some of the other dust bunnies).
Unfortunately for you, for some reason this dust particle moved between shooting M31 and shooting your flat (I hope you don't have your dust removal vibrator switched on).

Also, I think you'll find that if you rotate your flat (approx 80degrees) , the dust particle doesn't end up sitting in the right place on M31.  So that rules out rotation as the cause in any case.

Mark

[mickw]

Also, I think you'll find that if you rotate your flat (approx 80degrees) , the dust particle doesn't end up sitting in the right place on M31.  So that rules out rotation as the cause in any case.

If the camera wasn't firmly attached and seated squarely to the scope, the combination of rotation and misalignment could possibly be the cause ?

[MarkS]

If the camera wasn't firmly attached and seated squarely to the scope, the combination of rotation and misalignment could possibly be the cause ?

Possibly.  Except for the inconvenient truth that the dust particle in question is sitting on the CCD cover glass.

Ivor - look what you started!!!

[Mac]

Mark is entirely correct at short focal ratios but at longer focal ratios you will still need to take flats to remove any bunnies

#Open frying pan and into the fire#

Question regarding the above. (not the frying pan bit)
My take is that the flats are only for the dust closest to the chip, ie, the top of the chip, and filter wheels,
you would never notice any dust on the front of the lens.
Why,
Mark, do you not have a permanent dust bunny on the front of your C11 in the shape of the secondary mirror as do i on the 10"
as do any newt's ect.
does this mirror shadow show up on a flat?
If so can i see one please.

#close can of worms#



Mac

[mickw]

That's because the secondary in a newt is made from very expensive magic glass 

or

I think I can understand the logic behind that as the CCD is focussed on an object at infinity.  Dust on or near the CCD will be closer to focus than dust on the scope so would be more noticeable.

At a risk of invoking a previous thread about reality
http://forum.orpington-astronomy.org.uk/index.php?topic=7096.msg44801#msg44801

So why can't a star hide behind the secondary if the photons move in a straight line - I will not accept gravity lensing 

[MarkS]

Mark, do you not have a permanent dust bunny on the front of your C11 in the shape of the secondary mirror as do i on the 10"
as do any newt's ect.
does this mirror shadow show up on a flat?

A sensible question!
The primary mirror will certainly create an image of the secondary.  Using the lens formula (1/u + 1/v = 1/f) and remembering to use the correct signs for u,v and f you can calculate where this image plane will be.  In fact it will be nowhere near the CCD and certainly nowhere near enough to produce a dust bunny.  But if you defocus your camera you will see that every defocused star is, in fact, an image of the front opening of the scope (i.e. usually a circle) with a hole for the secondary.

However, there is one other factor to consider.  When you add a focal reducer this changes things.  The reducer is a lens that will create a giant out of focus image of the secondary.  This can sometimes cause havoc like in this image:

http://www.markshelley.co.uk/Astronomy/2008/horsehead2008.html

The central disc is the image of the secondary and the bright ring round the outside is stray light either coming straight into the scope or being difused by the corrector plate of the SCT. In a sense, it is a kind of giant dust bunny but it is more or less impossible to remove using flats.

Mark

[MarkS]

Dust on or near the CCD will be closer to focus than dust on the scope so would be more noticeable.

Precisely!

[mickw]

So having a grot splattered primary, secondary or front element will only degrade the image rather than add bunnies ?

[Ivor]

I can see if you step away from a thread for a while it quickly moves on. It's great to see a practical example of a flat and low level details  you have all started discussing but I'd like to come back a couple of steps (well maybe a couple of miles) to make sure I understand the basics first.

Going back to the logistics of taking flats, I like the idea of Carole's laminated light panel, however I prefer the cost of pointing my telescope at my LCD monitor  Unfortunately my garden isn't secure so I'm reluctant to leave the kit outside until the morning plus I don't a permanent site, so practical I'm looking to create the flats at the start of session.

Flats - my take on flats is the following.
For a refractor and DSLR combination, just point the scope at your LCD monitor using a low ISO (ISO 100).  Adjust the exposure so the pixel values are around half the max possible.

When you do this what do you have as a background a pure white screen?

The other bit I'm still unclear on is the well depth and whether it is either half full or 2/3 full and more to the point the histogram is a bell curve so where does the well come into it?