Bright circle around centre of image

[mountainmadman]

Good evening,

hopefully a fairly straight forwards one.  In the attached image, you can see the nebula, but it is surrounded by a lighter blueish circular area that I seem to get on most of my images.

My questions for the community are:

1.  What is it and what causes it?

2.  What can I do to combat it?

I've posted on a couple of other forums to try and get a consensus too.

The image was made up of approx. 60 x 60 second lights, 10 flats, approx. 30 bias frames and 10 dark frames.  All stacked and processed in Pixinsight.  There is no filter involved and the image is taken with a Altair Hypercam 183c in RAW 12 using an Altair 80ED-R scope with 0.8 reducer.

Thanks for looking,

Tony

[RT65CB-SWL]

Hi Tony.

That is very, very good. I cannot fault it.

I am not into astro-photography, [yet], but if you mean the 'blue' in the centre of M27 (Dumbell Nebula), then it is the gas or dust being blown away. I have just looked a other images of the same and some are on par with yours. I do not understand when you are say: "...but it is surrounded by a lighter blueish circular area that I seem to get on most of my images." Would it be possible to share some of your other images, then more experienced SGL'ers may have a better idea? ...also if you can provide details as date, time etc, it may help, especially this image.

 

[michael8554]

Hi Tony

The blue halo could be so many things:

Is it on a Flat if you stretch it?

Is it there without the FR?

Does your camera have the optional optical flat window? Try without.

Try it with a cardboard hood on the OTA.

Do you get it with your 300p?

Think about why don't you get it on all your images.

etc etc .....

Michael

 

 

[mountainmadman]

8 hours ago, Philip R said:

Hi Tony.

That is very, very good. I cannot fault it.

I am not into astro-photography, [yet], but if you mean the 'blue' in the centre of M27 (Dumbell Nebula), then it is the gas or dust being blown away. I have just looked a other images of the same and some are on par with yours. I do not understand when you are say: "...but it is surrounded by a lighter blueish circular area that I seem to get on most of my images." Would it be possible to share some of your other images, then more experienced SGL'ers may have a better idea? ...also if you can provide details as date, time etc, it may help, especially this image.

 

Hi Philip,

thanks very much for your kind words.  I appreciate them.  The blueish circle I refer too is about 4 or 5 times as wide as the nebula itself, and if your screen brightness is too dim, it's hard to make out.  It is also visible on this image of C19, although a little more subtle.  Somehow, I managed to process most of it out to the detriment of the brightness of the nebula itself.  I used to get the same when using my unmodified DSLR and 150p reflector too, but I don't use that for imaging any more.  Cheers.  Tony

[mountainmadman]

8 hours ago, michael8554 said:

Hi Tony

The blue halo could be so many things:

Is it on a Flat if you stretch it?

Is it there without the FR?

Does your camera have the optional optical flat window? Try without.

Try it with a cardboard hood on the OTA.

Do you get it with your 300p?

Think about why don't you get it on all your images.

etc etc .....

Michael

 

 

Hi Michael,

thanks for your reply and questions.

1.  I've attached a screenshot of a stretched flat.  The feature is there although very hard to make out.  In fact, I don't think you will make it out here, but if I tilt my laptop screen a certain way, I can see it.

2.  I haven't tried omitting the FR from the set up. I will consider this next time I am out.

3.  I haven't see the optional optical flat window option with my camera.

4.  Next time I am out, I will try an additional hood on the end of the scope.

5.  My 300p is a manual dob, not really meant for imaging.  However, I also used to get the same thing when I used a 1100d DSLR with my 150p.  Almost identical.

Thanks

Tony

 

[RT65CB-SWL]

Hi again Tony.

I see it, now that I have my brightness set to the max.  My guess now is that it could be to be the focal reducer maybe causing an internal reflection. I cannot see it on your image of C19 though. Have you tried a brighter object... i.e. Moon, Venus, Jupiter or Saturn and if "Yes!" does it still occur?

Maybe @ollypenricecan offer us his thoughts and wisdom.

[Rico]

If you managed to get the same artefact with two different scopes and two different cameras, it almost implies that your imaging train is probably not at fault but possibly something else. I reckon that it probably has something to do with your flats. What light source are you using and was this the only common thing between the different setups?

A quick check would be to stack your images with out the flats and check if you still get the halo.

[mountainmadman]

Hi @Rico

I use either a white screen off my laptop, or, more recently an LED tracing tablet to create a series of flats. I've been taking the more recent ones with SharpCap and have been in touch with Robin from Sharpcap who was able to give me the right mean values I should be aiming for. Both methods (laptop and tracing tablet) have been via a white t shirt over the end of the scope. Pixinsight then generates a master flat which is then applied in due course.

When I get home tonight, I'll use the same data without the master flat and see what the difference is. 

Thanks for your advice,

Tony

[ollypenrice]

I think what we're seeing here is an internal reflection. I would expect the removal of the reducer would change it or, quite likely, eliminate it. (I'm always going to argue that the reducer is a waste of time on this target anyway since it brings in no new photons from your 'area of interest,' which is M27. So this is a case of the dreaded 'F ratio myth' discussed at length on other threads.)

The trouble with internal reflections is that it's hard to know what to do about them. Often they just come down to bad luck: this bit of glass doesn't like being at that distance from this bit of glass.

I took a screen grab and looked at the separate colour channels in Photoshop. Not surprizingly the halo is far worse in blue than in the other colours. Blue is the hardest colour for lenses to control. (Well violet, really, but it is passed by the blue filter.) However, what I wasn't expecting, and can't explain, is that the halo is entirely absent in green but is quite strong in red. That's why it comes out magenta. This seems very odd to me. Could you try the 'proper' data you have in this way to see if you get the same result?

Since it's absent in green I wonder if the green filters on your chip are different in some way from the red and blue? Could there be a production problem with the Bayer matrix? If the halo were only in blue I'd not be surprized but I don't know how it gets into red without appearing in green.

Oh, just another thought. Cameras have to be filtered against 'light' which lies just outside the visible spectrum at each end, the red end and the violet end. That might be a clue: if you don't have such a filter in place, or if its bandpass is too wide, it might be letting near infra-red into the red and near UV into the blue, causing the problem in just these channels. The central green channel won't pass either. 

Olly

[Merlin66]

I use a achromat doublet in one of my spectroscopes.

The focal length is 200mm, and when you focus across the spectrum there's a 2mm difference between Violet/blue (CaK) and green and almost (1.8mm)  the same from red/NIR (Ha) to green.

Could this be the reason for the halos?

[mountainmadman]

18 hours ago, Rico said:

If you managed to get the same artefact with two different scopes and two different cameras, it almost implies that your imaging train is probably not at fault but possibly something else. I reckon that it probably has something to do with your flats. What light source are you using and was this the only common thing between the different setups?

A quick check would be to stack your images with out the flats and check if you still get the halo.

Hi,

thanks for your suggestion.  When I got home last night, I pulled out the data again, and re-ran the stacking process omitting the flats, but unfortunately, the same results came out the same.  It was definitely worth a try though.

Thanks

Tony

[mountainmadman]

Good morning,

firstly, thanks to everyone for you responses and suggestions.  I certainly have plenty to think about.  

So, to summarise what I'm going to try and what my next steps are, most of which are to be done the next time I have everything set up for another run...

Add a cardboard extension on the end of the scope to help prevent any additional light entering the scope.

Remove the reducer from the light train. - I'll need to buy an extension tube I expect, but I don't mind that whatsoever.

@ollypenrice I think I understand a majority of what you have told me, although I will need to work out isolating the channels to identify in which of them the anomaly occurs.  Nevertheless, I have attached the .tif if you want a closer look.  What has been on the shopping list for a little while though are a couple of filters.  I don't use any at the moment, but have already been told that a bog standard light pollution filter is almost mandatory.  From what I think I understand from your reply, are you suggesting trying some sort of IR/UV filter too?  This is where I start to get a little out of my comfort zone at the moment :-)

Given that the whole light train from scope to camera is all Altair based, I might drop them an email too to see if they have come across this as well.

Now all I need to get going is some time under the stars.  I will report back on my findings when I get the results.

Thanks once again everyone, I really appreciate your time,

Tony

M27integration_ABE.tif

[ollypenrice]

21 minutes ago, mountainmadman said:

Good morning,

firstly, thanks to everyone for you responses and suggestions.  I certainly have plenty to think about.  

So, to summarise what I'm going to try and what my next steps are, most of which are to be done the next time I have everything set up for another run...

Add a cardboard extension on the end of the scope to help prevent any additional light entering the scope.

Remove the reducer from the light train. - I'll need to buy an extension tube I expect, but I don't mind that whatsoever.

@ollypenrice I think I understand a majority of what you have told me, although I will need to work out isolating the channels to identify in which of them the anomaly occurs.  Nevertheless, I have attached the .tif if you want a closer look.  What has been on the shopping list for a little while though are a couple of filters.  I don't use any at the moment, but have already been told that a bog standard light pollution filter is almost mandatory.  From what I think I understand from your reply, are you suggesting trying some sort of IR/UV filter too?  This is where I start to get a little out of my comfort zone at the moment :-)

Given that the whole light train from scope to camera is all Altair based, I might drop them an email too to see if they have come across this as well.

Now all I need to get going is some time under the stars.  I will report back on my findings when I get the results.

Thanks once again everyone, I really appreciate your time,

Tony

M27integration_ABE.tif

Yes, I was thinking on my feet and wasn't very clear. It might be that you need a UV/IR cut filter to stop the light beyond both ends of the visible spectrum from reaching the chip. I gather an LP fiter will do this as well though I don't need one where I operate.

Certainly worth contacting Altair.

It will take a while to download the TIFF here but I'll have a look at it later.

Olly

[Rico]

If your seeing the effect in your flats, why not remove the reducer take some new flats and see if you've removed the halo. That way you don't have to wait around for a clear sky and risk ruining a new set of lights.

[ollypenrice]

Flats may not tell you all that much. If it's to do with the UV/IR cut, your panel might not emit much light outside the visible spectrum. Or it may have a spectrum dominated by the middle of the visible spectrum which, like green, is not causing a problem. Rather than peering at it and tilting the screen I'd suggest opening it in a programme which allows you to mouse over it with the pixel's ADU values visible. I'd do this in AstroArt, my stacking programme. I'm still struggling to download the TIFF. Bad internet day!

Olly

[ollypenrice]

The behaviour in each channel is the same for the TIFF as on the screen grab I did. Top to bottom we have red, green, blue.

The file is headed ABE. If this means you ran PI's ABE on it note the dark ring around the halo. This will probably be an additional artefact created by ABE. It will have over-darkened the area around the halo. It will often do this with galaxies, too. The advantage of DBE is that you can move the background markers further from bright sources to avoid this.

Olly

 

[Stub Mandrel]

If you get it with different cameras and different scopes , start by listing what is the SAME in both setups?

The focal reducer must be prime suspect, if used with both scopes.

[mountainmadman]

Wow! Once again my fellow Loungers have come flooding back with loads of help. I truly appreciate it very much.

@ollypenrice thanks for taking the time to explain your thoughts and looking at my images. I owe you some serious bandwidth! I will look further at the DBE feature in PI. I've got PI on trial at the moment so want to use it as much as possible before committing.

I need a bit of time now to test out all these suggestions, invest in a filter or too and wait for the clear skies.

Thanks once again!

[wimvb]

I'm not convinced that DBE is the best tool for removing this artefact. (I tried, but please prove me wrong.) The artefact is quite abrupt, and DBE is designed to remove smooth variations.

I tried the following blunt force attack:

• Isolated the artefact by extracting the blue channel (as per Olly's response).
• Removed the stars from this image and used the range mask tool in PixInsight to isolate the structure.
• Blurred the mask
• Applied the mask and used curvestransformation to lower the blue channel.

Here's the result

There's still a little unevenness in the background. This is much smoother and can be removed with DBE.

 

[ollypenrice]

5 hours ago, wimvb said:

I'm not convinced that DBE is the best tool for removing this artefact. (I tried, but please prove me wrong.) The artefact is quite abrupt, and DBE is designed to remove smooth variations.

I tried the following blunt force attack:

• Isolated the artefact by extracting the blue channel (as per Olly's response).
• Removed the stars from this image and used the range mask tool in PixInsight to isolate the structure.
• Blurred the mask
• Applied the mask and used curvestransformation to lower the blue channel.

Here's the result

There's still a little unevenness in the background. This is much smoother and can be removed with DBE.

 

Nicely done. It could also be done in Photoshop by various means but not without impacting on those precious faint outer tendrils from the main nebula, I don't think.

I guess the OP is primarily concerned to find the cause since the artefact is too severe for post processing fixes to provide long term satisfaction.

Olly

[wimvb]

2 hours ago, ollypenrice said:

Nicely done. It could also be done in Photoshop by various means but not without impacting on those precious faint outer tendrils from the main nebula, I don't think.

I guess the OP is primarily concerned to find the cause since the artefact is too severe for post processing fixes to provide long term satisfaction.

Olly

Yes, I think that your first thought, reflections, is a likely candidate. Since the artefacts occur independent of pointing location at the centre of the image, we can probably rule out an external light source. What baffles me is that according to @mountainmadman, he gets the same artefact with a completely different setup (dslr & reflector vs small cmos with refractor).

Tony, can you post an image with that artefact, taken with the reflector and dslr? Is the artefact the same size, or does it scale with image sensor/focal length? You also wrote that you get this in "most" of your images. Under what circumstances does this not appear? Can you rule out an external light source (led perhaps)?

[mountainmadman]

Hi Wim,

Most of the data I had from using my reflector and DSLR has been purged, because to be frank, it was rubbish.  I do have this though, which, although isn't exactly the same was similar.  Looking back on it now, it's not really the same artefact.  That'll be my inexperience I expect, and I fear that I might have lead people astray slightly with regards to the reflector/dslr combination.  Since I bought my refractor, I've packed away the small reflector and it's now up in the loft so don't use that for imaging any more.

Some of the other images that I have taken with the refractor set up, but haven't shown the halo effect have been using less frames and when I was first getting to grips with AltairCapture.

On the other hand, I'm dead impressed on the way that you have managed to almost totally remove the halo from my image using pixinsight.  I've got another set of data from that night when I was imaging the Pleiades so I will run it through PI and see if I can replicate your technique in there.  Thanks very much for explaining those steps.

@ollypenrice I haven't had chance to get outside again yet, though it was clear outside.  The fireworks cause my dog great distress so my priority has been with him.  That, and the fact that the moon phase is quite full at the moment.  The filters have gone on the Christmas list and I am yet to try removing the reducer from the light train.

I'm dying to get back out there again.  The forecast is mixed this week.  It was looking hopeful for tomorrow night at one point, but the forecast rain is not due to clear until gone midnight now, too late for me on a school night!  Later in the week is looking more possible though, and the moon will be out the way a little bit longer.

I will report back with my findings though.

Thanks once again everyone.  It's much appreciated,
Tony

 

[ollypenrice]

On 06/11/2017 at 20:00, mountainmadman said:

Hi Wim,

Most of the data I had from using my reflector and DSLR has been purged, because to be frank, it was rubbish.  I do have this though, which, although isn't exactly the same was similar.  Looking back on it now, it's not really the same artefact.  That'll be my inexperience I expect, and I fear that I might have lead people astray slightly with regards to the reflector/dslr combination.  Since I bought my refractor, I've packed away the small reflector and it's now up in the loft so don't use that for imaging any more.

Some of the other images that I have taken with the refractor set up, but haven't shown the halo effect have been using less frames and when I was first getting to grips with AltairCapture.

On the other hand, I'm dead impressed on the way that you have managed to almost totally remove the halo from my image using pixinsight.  I've got another set of data from that night when I was imaging the Pleiades so I will run it through PI and see if I can replicate your technique in there.  Thanks very much for explaining those steps.

@ollypenrice I haven't had chance to get outside again yet, though it was clear outside.  The fireworks cause my dog great distress so my priority has been with him.  That, and the fact that the moon phase is quite full at the moment.  The filters have gone on the Christmas list and I am yet to try removing the reducer from the light train.

I'm dying to get back out there again.  The forecast is mixed this week.  It was looking hopeful for tomorrow night at one point, but the forecast rain is not due to clear until gone midnight now, too late for me on a school night!  Later in the week is looking more possible though, and the moon will be out the way a little bit longer.

I will report back with my findings though.

Thanks once again everyone.  It's much appreciated,
Tony

 

This just looks like vignetting. I think it's entirely unconnected with the first artefact.

Olly

[Adam J]

Not totally convinced its a reflection if its always in the center of the FOV. Reflections normally associated with very bright objects. 

I suggest you go and image a bright star. The move it around to see it the 'reflection' moves with it. If not then its not a reflection, not in the classical sense in which you get reflections of bright stars etc at any rate.

If it is a reflection its a large one and the rule of thumb is that the bigger the reflection the greater the distance from the sensor at which it occurred. 

So given that its only appearing in red and Blue I would guess that its a lack of UV/IR block combined with poor anti reflection coatings on the primary optics. 

Its a color camera so it should be fitted with a UV/IR cut window. If not then something like this could result. 

As you say you have tried a DSLR I am going to go with it being the anti reflection coatings (or possibly lack of) on the primary lens. Contact Altair Astro and show them the image. 

[mountainmadman]

@ollypenrice @wimvb @Rico @Philip R

Good evening,

Well, I said I would report back with some results after some tampering.  From all of your feedback, the prime suspect, and easiest to amend to start off with was omitting the focal reducer from the light train.  So, I went through, every thing the same as I did before as best I could.  Same number of flats/bias/light/darks and used the same processing and stacking routines in PixInsight as I did before.  I used M27 as a target too, so I could get as good a comparison as possible.

I'm really pleased that in this instant, the halo effect has disappeared, and then image is fairly uniformed throughout.  While this is obviously a massive improvement on the first image, I also have data on another 3 targets from last night.  One set of which is for M45 which is another image that had issues with the phenomena at my last attempt too.  I doubt if I will have time tonight to go through the stacking and processing routine, but I will finish it off some time this week before declaring it a 100% success.

Even though in this case I have only omitted the FR from the light chain, I have on order a UV/IR filter too which I will introduce into the kit in an attempt to improve things further.

My thanks to everyone who has contributed and made suggestions to help me.  It's truly appreciated.  When I finish the M45 image, I will add that to this thread as a comparison too.

Thanks so much,
Tony