Okay splotch! Where are you!?

[UlteriorModem]

Though maybe we could brainstorm this little issue.

I have noticed a splotch appearing in recent images. At first I thought it was internal reflections, however it has showed up in the same place on different targets. (Examples below).

Also note that flats do not seem to deal with this. I am guessing it is so large scale it is being treated as valid signal.

As I understand it the closer a dust bunny or what ever is to the imaging chip surface the 'tighter' or 'smaller' it would be. So it stands to reason this splotch is relatively far out in the imaging train. Or is is spread out glob. I think I saw somewhere in the past where you could deduce the distance from the imaging plane with some math but cant remember nor seem to be able to find that info.

The 'surfaces' in my image train are like this... Imaging surface, Protective anti reflective glass (part of camera), IR/UV cut filter, Field flattener, Objective lens. Sorry I dont have distances between these surfaces right now. But estimate the field flattener is several (like 3-4 inches) out, The filter is about 1 1/2", The protective glass is only about 1/4 to 1/2" out., The objective is way out there and in my mind not really in consideration (but will look at it).

Examples note two different targets, same splotch.

[UlteriorModem]

I found the formula D=pdf

Where D is distance p=width of individual pixels, d=diameter of defect in pixels, and f=focal length.

Now all I have to do is figure out how to measure the defect's pixel width.

[Juno16]

Hi Tom,
In Pixinsight, can’t you place your mouse cursor on the image at one side of the splotch and click the left mouse button. The pixel location should show. Then do the same at the other side of the splotch and calculate the difference? The difference would be the splotch size in px.
I can’t try it right now on my machine because I’m running
WBPP.

[UlteriorModem]

Yes indeed that worked here is what I came up with and it makes no sense!

Pixel size .00478mm, Defect diameter 296, focal length 910

PxDxF-D

.00478 * 296 * 910mm = 1,287,5 mm ?

That would put it beyond the end of the imaging train

So I am guessing it is some sort of smudge or other crud that is bigger than a dust speck.

Guess I will have to take it all apart for a close inspection.

[notFritzArgelander]

Yes indeed that worked here is what I came up with and it makes no sense!

Pixel size .00478mm, Defect diameter 296, focal length 910

PxDxF-D

.00478 * 296 * 910mm = 1,287,5 mm ?

That would put it beyond the end of the imaging train

So I am guessing it is some sort of smudge or other crud that is bigger than a dust speck.

Guess I will have to take it all apart for a close inspection.

Something is wrong with the formula or the way it's being used. The right hand side would then be in mm^2.
Are you sure it isn't something more like

(mm/pixel)(#pixels)(f ratio) = distance in mm? That would give ~10mm not the 1287.5 mm^2. It also makes more sense to me optically.

[Juno16]

Yes indeed that worked here is what I came up with and it makes no sense!

Pixel size .00478mm, Defect diameter 296, focal length 910

PxDxF-D

.00478 * 296 * 910mm = 1,287,5 mm ?

That would put it beyond the end of the imaging train

So I am guessing it is some sort of smudge or other crud that is bigger than a dust speck.

Guess I will have to take it all apart for a close inspection.

Something is wrong with the formula or the way it's being used. The right hand side would then be in mm^2.
Are you sure it isn't something more like

(mm/pixel)(#pixels)(f ratio) = distance in mm? That would give ~10mm not the 1287.5 mm^2. It also makes more sense to me optically.

Thanks nFA, that looks much better. Tom @UlteriorModem , you used the focal length in your calculation instead of the f-ratio.

[UlteriorModem]

Yea maths and me dont get along sometimes.

It is a little redundant at this point as I took the entire imaging train apart, inspected, cleaned, and re-assembled. Even cleaned the Objective inside and out. I never did see anything 'obvious' just the usual dust flecks. If the smudge is still there then it will be on the CCD face

Shot a new set of flats yesterday but have not processed them yet I did not see anything but then again you usually dont without some heavy handed stretching.