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Old 01-03-2018, 10:12 AM
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PKay (Peter)
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Taking Flats - Problem revealed

This morning i refined my technique for taking flats.

I aimed the scope into the pre dawn sky.
Then (using sharpcap) watched the histogram and the live image.
Varying the exposure time I was able to find the optimum exposure to reveal a lens problem.

Tried cleaning the primary lens and fault is still there.

So I have a field flattener. Maybe the problem is there, or maybe on the camera lens?

Quite interesting.
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Old 01-03-2018, 10:23 AM
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Peter, you can work out where the dust bunnies are by this formula

Distance from CCD chip (mm) = (P x FR x d ) / 1000

where P = pixel size in microns, FR is focal ratio and d is the diameter of the dust bunny in pixels.

This formula also works for halo reflections as well.

Cheers
Bill
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Old 01-03-2018, 10:52 AM
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PKay (Peter)
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Very clever Bill, thanks heaps.

The calculation came out to 9.7 (assume mm).

Took off the camera and there they were, right on the sensor cover.

Tricky to remove.

Tried blower (in a can), no good.

Made up mix of distilled water, ~ 5% alcohol and a touch of detergent.

Dabbed with micro cloth and gone!
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Old 01-03-2018, 02:19 PM
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You can also just leave the dust there and use the flats to do the correction. That's what they're for
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Old 01-03-2018, 05:01 PM
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PKay (Peter)
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Maybe Rick

Back to basics: best to eliminate problems at the source.

As to the formula I really don't understand it.
Does it assume the dust is a certain size?

Anyways the problem happened after using the camera with the reflector.
It has an open exposed path to the sensor.

Remember: Poke hanky into drawtube
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Old 01-03-2018, 07:40 PM
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Quote:
Originally Posted by PKay View Post
Maybe Rick

Back to basics: best to eliminate problems at the source.

As to the formula I really don't understand it.
Does it assume the dust is a certain size?
I was being a little tongue in cheek, Peter. I agree it's better to remove the dust but sometimes you can't. I use remote/unattended systems and flats deal with dust donuts just fine in between infrequent cleaning and maintenance.

I have never thought deeply about the formula but it does seem to work. If you have a bunch of dust specks at the same distance the shadows all look pretty much the same size even though the specks presumably vary in size. They are a shadow of the aperture (solid for a refractor and donuts for scopes with a central obstruction.) Something to ponder when I run out of data to process

Cheers,
Rick.
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Old 01-03-2018, 08:56 PM
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Can you do the math for mine when you do Rick...please (I'll send ya the fits files...)
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Old 01-03-2018, 09:23 PM
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Can you do the math for mine when you do Rick...please (I'll send ya the fits files...)
There's a web calculator, Lewis! http://www.wilmslowastro.com/software/formulae.htm#Dust

But I'm happy to check out a FITS file
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Old 02-03-2018, 12:26 AM
ErwinL (Erwin)
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(P x FR x d ) / 1000 can be rewritten as

(P x d ) / 1000 x FR = D x FR

where D is the image diameter of the dust bunny in mm.
This is an approximation of the geometrical intersection of lines between aperture, dust grain and focus plane.

Here, the dust size is assumed to be zero, a good approximation as long as it is much smaller than it's image. Bigger dust grains will primarily darken the bunny.

Erwin
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