What's causing this? (Pictures attached)

[Diddykong]

So I'm using a Canon 1300D and I find that for the first hour or so my pictures are alot clearer than later on.  Could this be down to dew?  The exposures are around 3 minutes.

As you can see it's M101.  The first hour of exposures show the galaxy clearly, where as one taken around 2 hours after is alot darker and the galaxy has no detail.

 

EQ6R-Pro mount and Skywatcher 200PDS.

IMG_1374.CR2 IMG_1410.CR2

[Jonk]

My first guess is high cloud passing through.

Here are the images side by side, autostretched.

[Diddykong]

Thank you for the reply Jonk.

The only thing I'll say is that this has happened everytime I've imaged.  And when I go out 2 hours after the conditions are just as good normally from when I started.

[Jonk]

Do you see a gradual change over a few hours on different evenings, or is it random?

Sometimes, high cloud is not noticeable by eye but can play havoc with imaging.

Also, a DSLR image at this time of year is not going to be optimal, due to the sky darkness.

[Diddykong]

It is a gradual change yes.  So I normally start at around midnight then by half 1 onwards the exposures just become fainter as you can see.

[Jonk]

It could be dew, what are you using, looks like a reflector?

Check the secondary mirror the next time you notice it, in fact check both mirrors.

[Diddykong]

200PDS reflector.

 

The temp has been pretty mild was maybe around 10degrees last time though.

[Pompey Monkey]

15 hours ago, Diddykong said:

200PDS reflector.

 

The temp has been pretty mild was maybe around 10degrees last time though.

As long as the dew point temperature is higher than ambient, dew can form. It is even more likely on surfaces like, wait for it, metal and glass that radiate their heat directly into space (under clear skies). The temperature of these materials can be considerably cooler than the surrounding air.

This is why fans are often used on reflector primaries: passing the ambient air over the surface of the glass is actually a warming process.

So, it could still be dew.

[don4l]

I wonder if this might be the result of a change in sensor temperature.  This will seem a bit weird, but read on.

 

I tend to carry on imaging as my camera warms up at the end of a session. and I have noticed that the image appears to fade as the camera warms up.

It seems that the software that is displaying the image is autostretching when it puts the image on my screen.  I think that as the hot pixels get brighter, the rest of the image gets displayed as if it is darker.

If I manually stretch the image my target looks fine, but the hot pixels become very evident.  The background level also rises.

Just a thought...