weird marks on images

[Andyk93]

Hi, I'm really stuck with my images at the minute. I am using a modded canon 1000d with baader mpcc C/C and skywatcher lp filter in a skywatcher 130pds. each image I take look pretty clean apart from a dust spot that gets sorted easily with flats. however when they are stacked in DSS and I start processing them in pixinsight. doughnut like objects start appearing in the image. I'm lost as to what causes this because they don't show on flats and I don't think they are on single frames. this never used to happen with a Nikon d3300 and skywatcher coma corrector. This isn't a fully processed image, its just as far as I need to go before they start showing up. thanks

[Peter Drew]

Dust bunnies on the sensor most likely, they can be microscopic.  

[Andyk93]

2 minutes ago, Peter Drew said:

Dust bunnies on the sensor most likely, they can be microscopic.  

They aren't showing up on the flats though? the flats are correcting for the dust that I see on them but those doughnuts don't show up at all

[Peter Drew]

Just quoting the common culprit, way out of my depth for further "advice"!  

[gnomus]

These are odd.  They do look a little like dust bunnies but if they were, you would expect the flats to take care of them.  I have found, however, that some software can 'read' (I don't know a better word) image files differently.  For example, if I stack in AstroArt, my images are flipped compared with what I get in PI.  Is it possible that your flats are being subtracted in the wrong orientation, I wonder.

I note that you stack in DSS and process in PI.  I wonder what would happen if you stacked in PI and cut DSS out altogether.  I would use the BPP script in PI and I would make sure to load in the raw subs and raw calibration files.  I wonder if you would end up with the doughnuts if you did that...

[Andyk93]

3 minutes ago, gnomus said:

These are odd.  They do look a little like dust bunnies but if they were, you would expect the flats to take care of them.  I have found, however, that some software can 'read' (I don't know a better word) image files differently.  For example, if I stack in AstroArt, my images are flipped compared with what I get in PI.  Is it possible that your flats are being subtracted in the wrong orientation, I wonder.

I note that you stack in DSS and process in PI.  I wonder what would happen if you stacked in PI and cut DSS out altogether.  I would use the BPP script in PI and I would make sure to load in the raw subs and raw calibration files.  I wonder if you would end up with the doughnuts if you did that...

Thanks, ill have to learn how to stack in PI and give it a go. I cant upload my master flat because when it uploads on here it puts a weird pattern on it. The flats do take care of the other dust bunnies so I assume they are working correctly.

[gnomus]

Under 'Scripts' you will find the Batch PreProcessing Script.  It is very easy to use.

[wimvb]

Dustbunnies is also my guess, but something is changing between the various subs, which creates a 3d effect.

If I were you, I would do the image calibration in PixInsight. Use the script, but check the box "calibrate only". Then examine each calibrated lightframe. You can use the blink script for this. Stack only those that do not show the bunnies.

If the bunnies look this weird in all calibrated frames, examine the uncalibrated light frames. You can use the Debayer script to turn you frames into colour images that are easier to view and examine.

The cause may also be a size difference between the dustbunnies in the flats and light frames. Whatever could have caused this, I don't know. But this can't be calibrated out.

(Consider this an exercise in PixInsight.  )

Hope this helps.

[cjdawson]

Could it be that the flat correction is overcompensating?

[Andyk93]

I think I have stacked them in PI now and the same doughnuts are showing up. I can probably rule out the stacking software, so I am thinking it's the flats. I use my canon 1000d in Av mode and the flats I used for this stack are 1/500s. probably way off the mark but if the exposure is too fast could I be catching the dust on the shutter which isn't there when I take lights?

[Davey-T]

A reflector should make dust into doughnuts a refractor should make them into spots.

Dave

[wimvb]

Try inspection of a calibrated and an uncalibrated light frame. Calibrate a light frame without flats. If the lightframe without flat calibration looks the same as the light frame with flat calibration, then clearly your flat correction is not working.

[Andyk93]

Thanks for the advice, I have inspected an uncalibrated light frame and there doesn't seem to be any dust where those doughnuts are. However when I stack the lights without calibration frames I'm still getting them. I'm guessing I just need to take better flats. I'm using a flats panel with paper between to dim the light down and then using aperture priority mode to take them, I don't think it's picking up all the dust so I'm getting light remainders

[iapa]

I have seen people sometimes suggest that rotating the OTA with repeat to the light source you are using for flats helps get better flats.

The intention, I believe, is to eliminate patterns from the light source, perhaps, patterns caused differences in intensity across the light panel, or the paper (watermarks for example - I am not suggesting you have used 'Basildon Bond' paper with its very conspicuous water mark :))