Lights getting progressively darker?

[fortytwo]

Hi all,

Been imaging for 5 hours with my Canon 600 SLR (un-modded) on my SW200 explorer and just went out to switch to taking the calibration frames and found something odd.

Reviewing the last half hour on the back of the camera the subs have been getting progressively darker. The settings are still correct ISO 1600 for 30 secs and the sky is clear.

There is no condensation on the mirror but the histogram is definately moving to the left and the spike is getting narrower.

Does anyone have any idea what might be going on?

Thankyou in advance,

Jeff

Edited March 28, 2022 by fortytwo

[ONIKKINEN]

Decreasing light pollution as the night progresses, or the target getting higher in the sky, probably both.

Some street lights are dimmed in the AM hours of the day, or even shut down. I notice that some patches of highway around here have only every second light on at the lowest traffic hours of the night. Some folks might also have their yard lights off for the night. Most businesses shut down lights at least partially at some point of the night and car traffic producing extra lights is at its minimum. All of these are small effects but it adds up and there can be a real noticeable difference in light pollution in the weird hours of the night.

But usually this isn't a shocking difference, but maybe a 10% reduction overall. Did you check your secondary mirror for dew? Or a coma corrector lens (probably the tube facing one) if you have one. A thin film of dew can be difficult to spot without shining a light directly on it so it might just seem like there was no dew but really there was. I sometimes got dew on my secondary before i started using a dew shield, which then made the problem go away even for the worst nights.

[fortytwo]

10 hours ago, ONIKKINEN said:

 Did you check your secondary mirror for dew? Or a coma corrector lens (probably the tube facing one) if you have one. A thin film of dew can be difficult to spot without shining a light directly on it so it might just seem like there was no dew but really there was. I sometimes got dew on my secondary before i started using a dew shield, which then made the problem go away even for the worst nights.

I think that was the reason ONIKKINEN, when I removed my camera the secondary had dew on it. I had looked down the OTA and the primary was clear and didn't occur about the secondary mirror.

I'm used to imaging with a refractor and a dew band so will have to investigate methods for preventing dew in the reflector.

I wish they would dim our lights at night, oh well 😂

Thankyou for your reply ONIKKINEN.

Cheers,
Jeff

Edited March 28, 2022 by fortytwo