Dew/humidity related aberration or something else?

[ONIKKINEN]

The last 2 nights i was imaging from a very humid location by a lake and so i am assuming close to 100% humidity, the temperature also dropped by about 8 degrees in 4 hours coming to a stop at -2 just before sunrise. The scope was wet (and later frosty) within an hour of setting up but this is not the first time this has happened, just this time i noticed that stars on one side of the image were horrible, while the opposite side was much better, but still not great.

One of the problem images (calibrated and split green channel image) below. The top right corner is awful while the left side is not so awful.

An animation of a top right corner star from all the frames that night:

And then an animation of a left side star:

Ignoring the few frames that are out of focus these 2 look very different. The flaring out does not seem to happen on the left side of the image. I had the fan on at first as i always do to cool down the scope, but turned it off once i noticed the hairy flare on the right side. Doing this alleviated the issue a bit and the flaring retreated a bit, but came back after a few frames. Then turned the fan on and it again retreated, rinse and repeat for the entire night.

I looked down the scope with a bright light and noticed that the primary mirror was a little bit misty which is not unusual for a night this wet. Usually this just very slightly reduces the reflectance of the mirror and i may notice a slight drop in levels from the images i have taken. Not given it much thought before. I looked at the reflection of the secondary and i saw no obvious dew or frost there, and neither did i see any on the coma corrector/camera.

What dew related problem would this be then? Only affects one side of the image, leading me to think its something related to tube currents. But how the fan being turned on and more oddly, off, helps here i have no idea. Maybe the mirror fan heats up in use and so heats the back of the mirror very slightly? Then once the fan has ran for a while that heat gets overrun by the cooling humid air running over the mirror. Or maybe this is frost on the sensor itself? Could be, because it does not affect the entire image and the way i have understood it any smear on either the primary or the secondary would affect the entire image.

Thoughts?

[vlaiv]

I don't think it is dew, unless dew is perhaps on the filter. It has to form on element in converging beam and in such place so that some of the converging beams are not affected by it and others (hitting that part of sensor) are. Example of this would be something on the one edge of secondary if secondary is oversized and FOV in question is smaller. This is usually not the case and there is vignetting due to secondary on larger sensor on fast scopes, so if there is dew on secondary it will affect whole FOV.

It can be sort of tube currents / hot air that passes on one side of tube and not on the other. In general - those would affect whole field, but it can happen that disturb only part of converging beam and then it will cause issues only on one side of the frame.

Fact that fan introduced difference is supporting this idea.

[ONIKKINEN]

1 hour ago, vlaiv said:

I don't think it is dew, unless dew is perhaps on the filter. It has to form on element in converging beam and in such place so that some of the converging beams are not affected by it and others (hitting that part of sensor) are. Example of this would be something on the one edge of secondary if secondary is oversized and FOV in question is smaller. This is usually not the case and there is vignetting due to secondary on larger sensor on fast scopes, so if there is dew on secondary it will affect whole FOV.

It can be sort of tube currents / hot air that passes on one side of tube and not on the other. In general - those would affect whole field, but it can happen that disturb only part of converging beam and then it will cause issues only on one side of the frame.

Fact that fan introduced difference is supporting this idea.

Secondary is undersized by a fair bit at 63mm so anything on it should affect everything in the image.

Re:tube currents; i should mention that i was imaging towards south and quite low in the sky at 40-30 degrees. Later i switched to a zenith target (M51) and most of the fuzziness disappeared. Not all of it, but it appeares more uniform in the images and not skewed on one side like here. In that case if there were tube currents rising from the tube they would not rise from one side of the tube but just straight up.

Strange that this has not happened before as i always run the fan for about an hour after setting up. Sometimes not so long, sometimes longer, but never had issues caused by it. Maybe the bearings are gone and there is some extra heating from friction? I think i should test if it heats up in continued use 🤔.

[ONIKKINEN]

The fan stays at ambient for a while in use and peaks at +0.2c above ambient in about an hour. After turning the fan off the temperature shoots up to +1,5c above ambient so it does produce at least a little bit of heat. This is from the back of the fan by the way, so its possible the inside is warmer as it sits between the mirror and its cell.

So a theory: The fan removes heat from inside the tube for a while, helping with tube currents like it should, but then when it warms up it starts to create them. At this point i turn the fan off because i notice the stars have started to worsen and it helps for a short time. During this time the now warm fan slowly transfers its heat to the primary mirror and the tube which then restarts the tube currents. Rinse and repeat for a few hours. Maybe makes sense? Still dont quite understand why it was an issue on this particular night though and its not like the fan got really hot or anything.