A galaxy, and a question on technique

[DavidWright2010]

Last night I attempted my first 'real galaxy' shot (Andromeda doesn't count) This is 25 subs of 30 seconds each, a ZenithStar61 with the Fornax Lighttrack II tracker.



Taken from my back yard - not a very dark sky.

Anyway, my question is about stacking. I'm using Deep Sky Stacker, and when I previously processed images of M31 or the North America nebula, I had no complaints. On this latest effort, when I compare the stacked result (LHS) with a single exposure (RHS), the stacked stars are bloated and colorless. I'm using the standard Roger Clarke technique of subtracting the background in each color separately which has worked fine for me before. (300% enlargement here):



So, do you think this is a 'seeing' problem, a stacking problem, a stability problem, or something else?

Inquiring minds want to know. And I do, too.

David

 

[Tristimulus]

Most likely a focus problem.

Solution: Bathinov mask.

Turbulence can also cause trouble, at least from locations like mine.

 

[DavidWright2010]

Most likely a focus problem.

Solution: Bathinov mask.

Turbulence can also cause trouble, at least from locations like mine.

Thanks for your thoughts and suggestions. I have a bahtinov mask, and used it.

I think there were a number of issues, not the least being turbulence. Last night I was out with a K-70 and Rokinon 135 mm lens, It seems to me that the 135 mm version (left) shows better detail (such as it is) than the longer lens. Definitely the stars are smaller. Both images 100% crops.



I much prefer the K-70 to the K-1 for AP since it has smaller pixels and a fully articulating screen. That screen positioning is indispensable when the object of interest is at the zenith.

David

 

[Tristimulus]

At my locations steeply falling temperatures can be a problem. Focus is drifting.

So when taking out my lenses I have to refocus every five minutes or so for about the first half hour and thereafter I always check focus every between each series of exposures.

Have figured out which way focus drift so I can refocus ever so slightly before a session the worst nights when temperatures plummet. Then focus will drift in focus and then out of focus again, but most images will be in quite good focus.

During the most turbulent nights at my location stars are bloated even when using a measly 135mm focal lenght. Good focus helps but does not entirely save the situation.

Still fun to succeed occasionally! :-D