Attn Canon Owners.

[acropolite]

Found this thread on a 20D problem. The thread describes a problem with 20D sensors that may be caused by manufacturing defects with the sensor. It looks like dust on the sensor but is not removed by cleaning and is increasingly apparent when the lens is stopped down. I have checked my 20D and found the same problem, but the spots on the sensor aren't apparent in everyday use. As stated they are particularly noticeable if you take an image of an overcast sky and stop the lens down. Here's the link http://forums.photographysites.com/forum9/78.html and it makes interesting reading. This may be a quirk of all digital cameras, I would be interested to know if other members (with canon and other brands of camera) have the same blotches apparent under the test conditions described. I have attached a very compressed image with the offending spots circled to indicate the problem. The dark patches are much more apparent in the original image.

[MiG]

I've noticed it on our 20D. But in that link you posted, people seem to think that it's a matter of cleaning. If it's under the IR filter from the factory, then that sucks. That's more than a maintenance issue.

On another note, I just found out that my focusser on the GS 10" dob is too long for the camera sensor to get to prime focus. Boo.

[EddieT]

Phil,
They look like classic dust motes to me.

I notice in your image there is also an uneven background, i.e. the top right portion is brighter than the left-side.

Perhaps you should read the How-To article on Darks and Flats. There is a technique in there that will allow you to calculate the distance of the specks from the chip.

By eyeballing, I would guess that they aren't on the chip as they are slightly defocused, but judging by their size they are closer to the lens than any internal lens surface. That leaves whatever optical surfaces are in-between the lens and the chip. i.e. the IR filter.

It may not be dust, but could be blemishes/spots in the IR filters coatings. Either way taking and applying flats to your images would remove the uneven background and the spots from your images.

[acropolite]

The general theory seems to be dust, a couple of the spots responded to a puff with a blower, but the remaining 6 are still there. Cleaning kits are outrageously expensive, around $150 for wipes, fluid and a how to Video. The image was simply my wall with natural light, not meant to have any consistency; in fact I deliverately kept the exposure long and moved the camera to make sure I didn't pick up spots on the wall. Under average daylight conditions the spots are invisible, but they tend to show readily against an overcast sky when the aperture is stopped down to F11 and higher. I have a mate with a cleaning kit who has volunteered one of his precious wipes; I will post the results.

[tornado33]

if it gets worse when lens stopped down it must be something on the glass above the sensor like dust, but perhaps its badly stuck there and wont come off with a blower brush, it may need wet cleaning with one of those sensor cleaning kits like your mates.
Scott

[iceman]

That's not uncommon for newts, MiG. You'll need to move the mirror forward in the OTA to be able to reach focus.