Effect of dew on images

[bugeater (Marty)]

I had my setup out last night and set it taking shots of the Triffid nebula from about 3am. When I got up this morning I had a quick look at the subs on the back of the camera. The first couple were brighter and clearer - you could see the nebula and the background was brighter. Then over time they got darker and the nebula faded. Then the sun came up. Tracking was fine.

Is this the effect of dew? I had a heater on the secondary, but set quite low and there was dew on it at 7am. Alternatively I guess it could be the effect of the camera warming up (I'd set up before bed and left it out there turned off ready to go when the moon set). Hope my subs aren't all ruined.

[RB (Andrew)]

Yes, sounds like the effects of dew to me.

RB

[glend (Glen)]

It does sound like fogging. What sort of camera is this? First make sure that you eliminate mirror fogging as a cause. Primary mirrors can fog up as well as secondaries. Small blower fans can act as demisters for the primary/secondary if blowing across the face of the mirror, but don't use big fans as they can encourage tube currents. You may also have the front low pass filter (if a DSLR) fogging up. You could try wrapping a dew heater strap around the focuser to try and heat the camera adaptor to get some to transfer to the front low pass filter. I run a filter over the front of my DSLR coma corrector and adaptor and fill the space behind it with argon - this prevents condensation/fogging. There are front low pass filter heater options that get discussed in the Cold Finger thread but you need to disassemble the camera to get to it and have to know what your doing with electronics to install a nichrome wire heater around the filter.
Of course there is always the option of staying up and monitoring the capture and stopping the run if fogging starts.

[bugeater (Marty)]

My setup is a DSLR with a MPCC and a light pollution filter. When I looked inside at around 7:15am, the secondary was fogged, it looked like the primary may have had a bit and the guide camera was really bad. I didn't notice anything on the light pollution filter though.

I do have heaters on the guide camera and the secondary (newly installed last night), but I'd set it quite low with my homemade controller (a cheap PWM dimmer from Ebay). I'm being cautious because I don't want to set up tube currents. Probably should have left my homemade dew cap on though. Oh well, live and learn.