Wondering why moon's luminance changes in a short period

[blucenturion]

I made these over the course of 50 mins or so, and was struck by the differences in illumination; particularly puzzling is the dramatic drop near the end (top) of the session. All exposure times were equal throughout, as was shutter speed and ISO. At this point, all I can figure is some thin clouds intervened, perhaps? It did cloud up within a couple hours, so perhaps that's the answer. Thanks for looking.

[lensman57]

I made these over the course of 50 mins or so, and was struck by the differences in illumination; particularly puzzling is the dramatic drop near the end (top) of the session. All exposure times were equal throughout, as was shutter speed and ISO. At this point, all I can figure is some thin clouds intervened, perhaps? It did cloud up within a couple hours, so perhaps that's the answer. Thanks for looking.

Hi,

Did you take individual exposures and then lined them up or you allowed the Moon to drift across the FOV and then make the captures. If latter is the case then you have uneven field illumination in either the scope or the lens.

A.G

[Paul M]

Certainly looks like vignetting to me too.

Nicely composed image though!

[blucenturion]

These were stacked into Photoshop layers. Just enough frames were included to make a symmetrical arrangement.

[lensman57]

These were stacked into Photoshop layers. Just enough frames were included to make a symmetrical arrangement.

In this case something like fast moving clouds have obscured the view of the camera. Is it possible for you to equalise the histogram between the different frames?

A.G

[blucenturion]

Yeah, I could, but I'm not going to bother; these are pretty much a monthly exercise, clouds permitting. Clouds, I think, is it.

[Paul M]

So assuming that the images are laid out in sequence. First shot at lower left and last shot at upper right.

The gradual brightening of the early shots is obviously due to atmospheric extinguishing. It's even slightly red in colour.

But then considering the number of frames and how far apart they were taken the moon must have been heading back towards the westerly horizon and so back into atmospheric extinguishment.

The colour difference between the early and late frames will be down to changes in atmospheric conditions as the night went on.

I never absorbed the information about the time frame in the OP and just assumed the shots were a single frame with multiple exposures. 

It's still a nice composition though.

Me thinks bluecenturion should have a go at the analemma next

[nytecam]

I don't quite understand how the picture was produced bit IF you were to use a static camera and expose at 2min intervals this is the result I'd expect !  

Most lenses cause some vignetting especially with normal to wide-angle settings.  If, via an old film SLR with back open, you'd see that on the lens optical axis is the lens a perfect circle but viewed from the corners the lens is vignetted down to an oval shape giving less exposure in the corners.  In CCD imaging [and DSLR] this uneven illumination is corrected with a 'flat'  - well my two cents