What the? Help please...Bizarre spots on flats

[Andy01]

Hi everyone,

Here's a weird new problem :screwy:
Check out these flats I took yesterday evening.
There appear to be strange black spots that move between frames and filters.

I use a QSI wsg-8 which has the OAG prism in front of the filters.
I used the same t-shirt I always use at dusk, yes it's clean!

I triple checked the image train for obvious dust or aliens, and removed and checked the flattener, LP filter and visually checked the objective and surface of the filters. Nothing obvious there.

Thinking maybe it's a light leak or flare I removed the OAG and replaced the cap - no change. I then wrapped all connecting image train elements in dark fabric - no change.

To try to i.d. the problem, I then varied both exposure times and filters.
In order l-r these are 1 sec Ha, .05 sec C, .05 sec SII, 5 sec Ha, 10 sec Ha, 30 sec Ha, 5 sec Cyan and 60 sec OIII.

I'm guessing it's a foreign body or dust BEHIND the filters or possibly on the CCD surface?

As it's a new camera, I'm reluctant to dismantle the camera & filter wheel to access the CCD.
Not sure how to open the shutter for a visual inspection of the CCD either.

Any ideas?

Darn shame as it was a rare clear spring night in Melbourne and I've now got these marks all over my subs from the night's imaging as well. :(

Cheers

Andy

[Merlin66]

Andy,
those bunnies are very sharp...to me that infers they are on the CCD chip or very very close to it.....

[RickS]

You can calculate the distance from the sensor fairly easily: http://www.wilmslowastro.com/software/formulae.htm#Dust

This will tell you where to look...

Cheers,
Rick.

[Andy01]

Thanks Merlin :)



Oh cool, cheers Rick - so looks like around 6mm in front - now to pull the specs and see how to safely access/clean in front of the CCD.

[ZeroID]

6mm, .. I'd say dew on the window in front of the sensor. Internal or external, ... dunno. They appear to move independently on longer exposures.

[MelD]

By any chance, are they alive? They certainly appear mobile. My pier and mount seem to be very attractive to tiny ants, despite the multiple applications of surface insecticides that I spread around. Just a thought.
Mel

[leon]

Tiny ants now that is possible, I have seen them that small one needs a magnifying glass to view them properly.

Leon

[alistairsam]

hi Andy,

can't you keep your camera on a desk, start a 2 min luminosity frame and look through from the scope end of the wheel, that would open the shutter and you should be able to see through to the CCD surface.

Does it disappear when your cooling is off?

Cheers
Alistair

[Andy01]

Sure was weird. I just recleaned the glass in front of the CCD chamber again.



Maybe? Not sure... Interesting theory :)



There have been some infestations of flying ants around, and the scope is set up near my pool- but they're the normal 1cm size. Not sure about microscopic ones?



Hey Alistair, yep did that - it's the only way I could figure out how to access the ccd chamber window. I spied a teeny tiny dust mote that i was able to remove but that was all. Havn't tried testing uncooled, are you thinking the fan is sucking something in?

Thanks for all your input everyone. After exhaustive cleaning it's a big improvement but I'm still very wary of something odd going on!

Cheers
Andy

[Merlin66]

One of my guys found condensation droplets forming on his CCD....

[Peter Ward]

I ran some image processing routines over your data and...yes, I've seen this sort of thing before.

You can get rid of the problem by purchasing a two axis joystick for the host PC....:D