DSS and alignment, what do I do about this?

[5haan_A]

Hi guys,

I got round to processing some narrow band images for the first time and have encountered a little problems in DSS. Basically I had captured H O and S data on the lagoon nebula on separate nights. Silly old me I had not thought about framing too much when I was capturing the data. 

I have used a good Halpha light frame as a reference frame in DSS and have begun to stack the H O S light frames separately in three individual stacks. When I had stacked my O data this is what I saw.

 

Clearly my H reference reference frame is at odds with my O stacked data. How much of a problem is this? I anticipate that when I combine the channels in photoshop I am going to be with parts of the image without any data in say the blue channel if I mapped O to that. So what do I do about this, and whats the best method for trying to make sure that this doesn't happen again?

 

In hindsight I could have started off better when originally imaging with my scope, it does have the ability to rotate, but I haven't ever thought to use it, and I haven't given framing too much thought, because I always assumed that If you goto to the same point in the sky using software it would goto and everything would be hunky dory. 

 

Best,

[wimvb]

Not much of a problem, but you need to crop so that the final image doesn't have those black areas. In general, it's good practice to align one edge of your sensor with RA.

Take a 20 s exposure, and about 5 s in, slew ra at 1x sidereal speed. This will give you star trails. Rotate the camera and repeat until the trail lines up with the edge of your sensor.  If you do that every time you set up, you are sure that your images will have identical orientation.

[5haan_A]

19 minutes ago, wimvb said:

Not much of a problem, but you need to crop so that the final image doesn't have those black areas.

Ahh, I was hoping you wouldn't say that 😅

As you can see from this side by side comparison that sort of a crop would end up cutting out some really nice looking areas where the dust is. Still though if this is the only way to get something then I best take my medicine and try better next time. 

Best,

[5haan_A]

Actually to be fair the crop doesn't need to be so severe, I reckon I can keep the dusty parts and still cut out the black parts of the image. 

[wimvb]

You can also rotate the crop. Doesn't need to be horizontal landscape. Any orientation with the largest rectangle that will fit and exclude the black areas, will do. Any image processing software will allow you to do that.

[almcl]

+1 for Wimvb's point.

Here's an example using your screen grab:

[MarkAR]

What you can do in the future is to open a previous session sub then take a preview snap of the current set up. if they are rotated then tweak the camera rotation and take another snap until rotation is as close as possible.

[5haan_A]

3 hours ago, wimvb said:

You can also rotate the crop. Doesn't need to be horizontal landscape. Any orientation with the largest rectangle that will fit and exclude the black areas, will do.

Great idea. That's some good thinking outside the box 😅

Thanks, 

Edited July 19, 2020 by 5haan_A
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