Splodges on stacked image - something on the camera sensor?

[Delboy_Hog]

Hello folks,

I was wondering if you could help with diagnosing an issue that's cropped up when I've stacked some frames in DSS.  I've done no processing other than stacking, but some fairly awful splodges have appeared, particularly in the top half of the image - it looks like someone's sneezed on it!  If it helps, this image was taken with a Canon 1100d through an ED80 telescope.

I haven't used the camera for a little while, my other half has been using it for work, and I know she changes the lens quite regularly.  I'm guessing these splodges must be on the camera's sensor?  If so, is there anything I can do to fix this so it doesn't appear on future images?  I've been eyeing a new camera recently (expect another thread on here soon!) but had still anticipated using this one for widefields etc - please tell me there's something I can do?!

Many thanks for any help you can offer,

Derek

[MarkyD]

Looks like dust bunnies.  They crop up quite alot.  You can clean the sensor in the camera with a cleaning kit.  I use a rocket blower and carbon lenspen to clean mine but there are others like wet wipe kits about

http://www.visibledust.co.uk/index.php?route=common/home

 

Usefel info on cleaning sensors

https://www.parkcameras.com/digital-slr-camera-sensor-cleaning

[Magnus_e]

These are dust bunnies

They are virtually unavoidable, but quite easy to manage.

First off the spots are in your imaging train, not sensor. Dust on sensor is smaller, pitch black and looks to be in focus.

As your spots are dim and out of focus, they are somewhere on lens(es).

 

Fix 1. Take flats.

Fix 2. Dither between images.

 

So flats are images taken at a even light source. (Light box, Dawn/Dusk sky). You saturate them so they are about 2/3 to the left of the histogram and add them to your stacking routine. The flats will contain your dustbunnies and vignetting pattern. The stacking software will make the image brighter where there are dark spots, resulting in a flat image.

Dithering is usually done if you are using software to control your mount when imaging. After each subframe the telescope is moved randomly a few pixels, so that the signal from your dso moves in the field of view, but the dust spots are static. When the images are aligned in stacking software, then the dustspots will be (not aligned), making so they are not stacked.

 

A little quickly explaned, but taking flats is important

[Delboy_Hog]

OK thanks Mark, thanks Magnus - that's really helpful info.  I've been thinking about flats and dithering recently, so I guess it's a good time to get stuck into those, while I figure out where the spots are and how best to shift them!

[steppenwolf]

You can find out where in the light path the dust is by applying the following calculation:-

distance of dust from sensor = width of dust shadow in pixels x width of a single pixel in microns x focal ratio of telescope / 1000

[Thalestris24]

Hi

You can ignore the dust bunnies - just take flats and they will miraculously disappear. Take 30-40 flats and 30-40 bias frames. Do read the DSS documentation re-calibration frames and stacking.

Louise

[Delboy_Hog]

Thanks Steve.  Even with my ropey maths skills I stand more chance of figuring it out like that than by just shining bright lights onto various bits of glass!  I can see lots of dust, but had just concluded there was no way of know which was the offending surface!  Apparently I was wrong!

[Delboy_Hog]

Ah fabulous, thanks Louise, that is good news!  It's a relief to hear that nothing is seriously broken / malfunctioning!

[steppenwolf]

Quote

Even with my ropey maths skills I stand more chance of figuring it out like that than by just shining bright lights onto various bits of glass!

Funnily enough, I got out my older CCD camera a few days ago and was shocked to see dust bunnies so I checked out the LP filter I had inserted that day and it was perfectly clean. "It couldn't possibly be on the sensor glass, I am too careful to allow that to happen" I told myself but I ran the calculation above and it gave me a figure 7.02mm. I checked with Starlight Xpress (the camera manufacturer) to see where the front face of the sensor cover glass was - 7.0mm from the sensor apparently ...............

[Delboy_Hog]

Ha ha, oh dear!  Well I'm sure you are careful with it all Steve - for me, I still wouldn't be surprised if the Mrs had sneezed on the camera while the lens was off!  

...Though I will feel rather guilty if I've accused her of that and then the offending dust ends up being on the scope!

 

I'd just been revisiting your book Steve, to check up on my flat-frame technique, when your name popped up on here - Fair to say I am a big fan of your work, dusty sensors or no!

[steppenwolf]

43 minutes ago, Delboy_Hog said:

Fair to say I am a big fan of your work, dusty sensors or no!

Ohh the shame of it ...............