Light rings/circles in image after automatic background extraction in Inpixsight

[morgandollar1]

I've started to learn about imaging deep sky objects a few months ago. I have a 10" Meade RCX400 scope on a permanent pier and wedge and it is still functioning except for the focus. I can collimate the scope manually and have added a Moonlight focuser so I never touch the scope focus. It seems that the focus and tracking are fine when I have a bright object or globular cluster to image. Inpixsight allows me to hide these rings of light for those but that doesn't work when its a dimmer galaxy or nebula. I use a Nikon 810a at prime focus in the Moonlight focuser, sometimes with or without a diagonal, dew shield on and my fittings are all 48mm. I shoot all the images in raw and have tried different iso settings and time but no difference.

My question is what are these rings/circles? What is creating them? I've been searching online for a discussion or set of similar pictures but haven't been able to find anything. I plan on doing a star test the next time the weather allows but not sure if the primary and secondary mirror separation, and resulting wavefront error, could cause this effect.

Any thoughts or suggestions would be appreciated.

[wxsatuser]

Do you calibrate your lights?

If not try taking some flats and see if they correct it.

[Tim]

Hello, and welcome to SGL

Sorry to hear you are having problems.  Is the issue evident in your light frames and your flat frames?

I have had a similar problem in the past when a tiny amount of light was able to enter the image. In my case it was a dim red light bouncing off my obsy floor and entering through the rear of my reflector telescope mirror. Blanking the rear solved it.

Is there any way stray light could be entering the setup somewhere? Perhaps through the sides of the Moonlite drawtube? Or anything bright or not blacked inside the tube or fittings? I am unfamiliar with the Meade RCX rear layout.

If you can, definitely ditch the diagonal when imaging, it is just an extra element which will reduce quality.

Let's hope someone can help you nail this as the pictures look to have real potential to be great.

Tim

[newbie alert]

If you using the diagonal then its not prime focus...do you use flats..is it evident before stretching?

[ollypenrice]

56 minutes ago, newbie alert said:

If you using the diagonal then its not prime focus...do you use flats..is it evident before stretching?

I'd still call it prime focus. The light cone is not refocused by the diagonal. Its just that the light path is including the diagonal. It is certainly a bad idea to have the diagonal in there, though, so I would replace it with an extension tube of the right length.

I read this post as being about ABE and ABE has one fairly consistent defect. It often picks up bright objects and reads them as part of the gradient it is trying to remove. As a result it will pull down the brightness in the region of the object, often creating a dark halo around it. You can check this by looking at the gradient map it offers you after you have run the algorithm. If you see any sructure at all reminscent of the object the gradient map is useless. It has read the object as background.

I would try using DBE instead. Don't be tempted to put in lots of markers. That is a sure fire way to pick up the object again. You'll read this advice from Rogelio Bernal Andrea and Warren Keller as well. What you are looking for is a very unstructured gradient map. Use subtraction for LP gradients but, maybe, division for vignetting.

Olly

[morgandollar1]

Olly,

  I tried the DBE and it did the same thing. To take the telescope out of the test I put the camera piggybacked on top of it. The M42 picture is from the same Nikon D810a with a Nikon 28-300mm lens with lens hood attached. Thinking that maybe its the camera I used a new Nikon D810 and it had the same light area although it was of the Rosette Nebula. What is all this extra light from? These are 60 second pictures. Driving me crazy!!!

[Stub Mandrel]

I've seen this before and it was caused by using flats that didn't match the imaging train I used.

[morgandollar1]

Neil,

  As these last two pictures were taken with the camera and regular lens, no scope, would I still need to use flats? What confuses me is the dark area in the middle. The vignette on the corners is understandable but the center eludes me. Don't think it can be little pollution or the center would look the same as the problem area. The two pictures from back in January were through the scope and had similar light issues. Are you able to process only lights and get a dark background in your images?

[morgandollar1]

'light pollution', sorry

[wimvb]

12 minutes ago, morgandollar1 said:

Neil,

  As these last two pictures were taken with the camera and regular lens, no scope, would I still need to use flats? What confuses me is the dark area in the middle. The vignette on the corners is understandable but the center eludes me. Don't think it can be little pollution or the center would look the same as the problem area. The two pictures from back in January were through the scope and had similar light issues. Are you able to process only lights and get a dark background in your images?

You definitely need flats to eliminate vignetting. While PixInsight DBE can correct a lot, it's not a magical wand. Correcting at the source is always better.

To get back to your original problem; I have seen this kind of rings before and I suspect it is related to the corrector plates. Repeated use of DBE can cure it.

[morgandollar1]

wimvb,

  I tried the DBE five times on the recent posted picture but no change to the light-fog-haze circle area. I'm not worried about the vignetting as I can always crop that out if needed. The question I have for yourself or others is if you take DSO images, register and star align, perform image integration, do ABE and then stretch with STF will the image have a nice dark background? This would mean that no flats, darks or bias were added to the process.

[wimvb]

No, it won't.

First of all, you need to do the full calibration process. There are just no short cuts in this hobby of ours. The first thing I do in PixInsight after image integration is to crop the image, so that edges are clean. The I apply dbe, until I'm satisfied with the results. If I later find that dbe could be improved, maybe because I can see faint dust better, or there is some gradient left, I go back to the very start, with the previously processed image as a reference. The second time, processing will be easier because PixInsight saves a full image history, so I know the settings of most tools.

Processing takes time. As I have mentioned before, I'm not the fastest dinosaur in the herd, and for me there's no need to rush. There are enough cloudy nights to keep me busy.

[wornish]

You say you are using a D810a. Are you closing the little shutter/cover over the rear of the eyepiece that could be a source of light entering the camera. 

[Stub Mandrel]

I think @wimvb has summed it up, really - yes you need flats