# Likely location of dust bunny ?

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Cleaned filters, cleaned barlow, cleaned ccd, cleaned outside SCT...Possibly inside SCT ?  There are fewer since all that cleaning, still a few stubborn ones.

Thanks !

ML

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Turn the camera and see if it moves, if so then it's on the camera and you can eliminate the telescope and other kit as the culprit...

if it does not move, then move turn the filter then Barlow one at a time to see if the dust bunny moves, on the images, eventually you will find where the problem is....

hope that helps

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If you don't mind a little maths you can calculate the distance of the dust particle from the CCD surface using the information contained in the image and this will help you locate it.

The formula is very simple and you just need to know the pixel size of the camera in mm, the number of pixels that the dust donut covers in the image and the focal ratio of the telescope. The result of the calculation tells you how far the dust is from the surface of the sensor.

The formula is D = Pdf

D = the distance of the dust from the CCD surface in mm.

P = the width of the dust donut in pixels.

d = the width of a single pixel in mm.

f = the focal ratio of the telescope.

You don't give any information about the camera used in your post so I can't complete the calculation for you but assuming the scope was the C11 with a f ratio of 10, the image shows the dust donut is approx 26 pixels wide so you just need to plug in the pixel width in mm to complete the calculation.

Distance of dust from the CCD (D) = 26 (pixels width) x ( ?? width of single pixel in mm) x 10 (focal ratio)

As an example, supposing the camera you used was the old Philips Toucam Pro this had pixels 5.6um wide in a 640x480 array so in the formula you convert 5.6um to mm which is 0.0056mm and the calculated distance to the dust would be 26 x 0.0056 x 10 = 1.456mm which would put it right on top of the sensor glass widow.

Be careful to measure the number of pixels width in the raw image file before any downscaling or resizing and if using the video function of a DSLR remember they downscale internally in video mode so you need to recalculate the number of pixels width accordingly.

HTH.

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If you turn the camera with respect to the ota and the bunny doesn't move, then its on the camera- the advice above is wrong.

Thanks !

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