Setup gradually out of focus?

[jeffwjz]

Hello everyone, I was taking M51 this evening using my new Atik 314L+ Mono camera with a IR/UV block filter. I'm using a Skywatcher 150PDS. I took 10 images and each has an exposure time of 6mins with guiding. I adjust the focus using a Bahtinov mask prior to the session. By looking back the images in DeepSpaceStacker, I noticed that my images are gradually losing focus over the period of the imaging session.

Following is my first image and hovered to a star to check focus.

Then the next is my 5th image, it's already showing defocusing:

Next is the 10th image:

As seen on the images, looks to me they are gradually defocusing. I have never encountered such issues before when I was imaging with a DSLR. I'm very confused how this has happened.

I have a couple of ideas, but not sure if they are the case

1. The focuser is slipping. This seems possible to me as I didn't have the habit of locking the focuser after Bahtinov (bad practice!). However as I mentioned this behaviour was not shown when I was using a DSLR. But next time I will definitely lock the focuser screw after focusing.

2. The newtonian is not properly cooled down when the imaging started. I left the scope in yard for about 30mins before taking images, and the temperature over the evening dropped from 5degC to 0degC.  I'm not sure if focal point would change when scope is not completed cooled, or when temperature changes.

Can anyone provide any insights and suggestions about this issue? Could this be caused by some other reasons?

Thanks in advance. 

[symmetal]

The temperature dropping 5 degrees during the imaging session would cause a focus shift due mainly to the scope tube contracting with the falling temperature. It's normal to recheck focus several times during a session. Automated programs like SGP can be set to trigger an autofocus check every specified change in temperature (default 1 deg), and/or every time a certain period of time has elapsed.

It may not have been so noticeable on the DSLR due to the bayer mask making the actual pixel resolution lower on the DSLR compared to the mono camera. For the same size pixels the mono camera would produce a sharper image so focus errors are more easily seen.

Alan

[jeffwjz]

Thanks Alan for your explanation. This is very helpful!