Fairy Rings?

[groberts]

For some unkown reason I seem to get fairy rings in some of my pictures, though not always, which are usually only apparent in the darker sky areas.

Below is an example of a highly stretched L sub  where the 'rings' show as a collection of slightly darker patches, not unlike mare on the Moon! 

In the subsequent part processed image the areas appear as the aformentioned Fairy Rings?

Any ideas on what these are and how to get rid of them? Thanks.

Graham

 

Edited March 20, 2020 by groberts

[MarkAR]

Almost looks like dew drops on the lens. Are they always in the same place in different images?

[groberts]

Definitely not dew drops, the object lens was bone dry on Monday evening.

As to position, I'm not sure, though it certainly has happended before.  I'll try and go back and take a look at this.

    

 

[newbie alert]

Dust on sensor or filters, take flats 

[groberts]

Thanks but already successfully been using flats, darks & bias calibration for some years to remove such issues.

I've now been back to look at the history of the problem which shows:

• No sign before 2018
• Only evident in LRGB subs i.e. does not apear to be present in narrowband wavelengths

At first I thought it was a stacking artifact but, of course, being present in the subs that can't be the case. 

If it's not in Ha OIII SII but it is in LRGB i.e. not the sensor which then would suggest the filters but it still has to be something common as it's the same pattern in each LRGB wavelength?  Frankly  I'm baffled! 

Edited March 21, 2020 by groberts

[carastro]

Hi Graham, 

I get things like this when I am doing broadband from home (Bortle 8), but I don't seem to get them with narrowband, and I don't seem to get them at a dark location with LRGB.

Flats don't seem to remove them all either.

I have just come to the conclusion that broadband imaging is really not suitable for LP locations, and that the background sky just seems to show up the dust more together with gradients, and combined it is simply too much for flats to cope with.  I am quite non technical but that's how it appears to me.

I wouldn't be doing broadband from home right now if it wasn't for the lockdown and there being no available narrowband targets in the sky.  

Does this make any sense? 

Carole 

[Sunshine]

I saw those on Interstellar, they're wormholes!

[groberts]

On 26/04/2020 at 16:05, carastro said:

Hi Graham, 

I get things like this when I am doing broadband from home (Bortle 8), but I don't seem to get them with narrowband, and I don't seem to get them at a dark location with LRGB.

Flats don't seem to remove them all either.

I have just come to the conclusion that broadband imaging is really not suitable for LP locations, and that the background sky just seems to show up the dust more together with gradients, and combined it is simply too much for flats to cope with.  I am quite non technical but that's how it appears to me.

I wouldn't be doing broadband from home right now if it wasn't for the lockdown and there being no available narrowband targets in the sky.  

Does this make any sense? 

Carole 

Thanks for your thoughts Carole.

I hear what you're saying but can't say that's my experience.  The so-called 'fairy rings' appear as light smudges always in the dark sky part of the image, almost entirely with narrowband and come out as fairy rings after fully calibrated processing, which I then deal with in PS.  Broadband seems much less affected, if at all  - FYI I'm 5/6 Bortle here.  The smudges + fairy rings can be seen in all narrowband wavelengths, in what I think are the same position.

Now we're running out of darkness for the summer I'm going to strip down the optical train (for the first time in 3-years!) to see if I can see anything.  I see that others have suggested changing the dessicant, which I'll also be doing for the first time.

Watch thsi space!

Graham

   

   

[Tom OD]

Well in the L frame that you stretched, they just look like Dust Bunnies.

If you are saying that the corrected frame with the rings is Flat subtracted, could it just be the dust has shifted a little between the flat you used and the recent L frames? Hence the ring as the dust  could be a slight bit offset now?

Tom

[groberts]

17 hours ago, Tom OD said:

Well in the L frame that you stretched, they just look like Dust Bunnies.

If you are saying that the corrected frame with the rings is Flat subtracted, could it just be the dust has shifted a little between the flat you used and the recent L frames? Hence the ring as the dust  could be a slight bit offset now?

Tom

I guess it's possible Tom.  As mentioned above I will use the summer months to take a closer look + refresh my calibration librbary.

Graham 

[Adam J]

those are caused but the alignment of the filter wheel changing between the light frams and the flat frames slightly moving the dust along with the filter.

[groberts]

So cleaning the filters should work?