Help me track down a light leak

[almcl]

This morning while sorting out a camera cooling issue with the ASI 2600, I tried an 'on-scope' dark.  Normally the darks for the dark library are taken with the camera off scope.

I was a bit shocked to see this 180sec 100 gain 30 offset image as a result.

I have had a peer down the focuser tube with camera removed and although there is a bit of light right at the edge of the field of view (from the cut-out section of the Moonlight focuser) if I shine a light close to it, there's nothing that appears anywhere the long edge of the camera sensor.

So could this be amp glow, I wonder?  The ASI2600 mc is supposed to be free of this, so if it's not, how to find the source? (And I know there are some dust issues as well, but think those can be sorted.)

[david_taurus83]

I could take darks on my old WO GT71 during the day. Never an issue. Now though, with my Altair Starwave scopes, the one time I tried it looked like there was light leak getting under the dew shield. It's appeared to get worse when I shined a torch on the shield.

[MaltaNewtF4]

I just posted this elsewhere on another thread, hope it helps you:

 

I spent 4 hours today really trying to make the inside of my scope dark in the daytime.  My observing location is quite light bright  With an f4 8 inch Newt Astrograph my darks taken no the scope have some reflected light gradients and spikes and my 240s subs mostly have weird spectral rings and gradients that just cannot come from sky conditions. 

OK, the cover doesn't totally block the light, but I'm not so worried about that, as I want light frames to be all light from the sky and I can do darks with that covered in material while parked or on the bench. 

But on the bench in the daytime, with the front covered, it's like a theme park in there, when the fireworks are going on! 

So I started tracing the issues at the mirror end, adding self adhesive velcro strip between the mirror frame and the mirror holder frame (a 1.5cm wide strip does the trick).  But the areas around the adjustment bolts let in light so they needed more velcro. gaffer tape, nothing would stop the little holes, and I wasn't willing to disassemble my nicely collimated F4!  I also used a smartphone camera light to shine at the mirror. 

Then I remembered the black bag that a motorbike helmet came in, and I found with it a smaller bag for the visor.  Putting both on the end, with them already elasticated and black, bingo! 240 second dark frame. no light.....from that end...l

I moved the covers from around the camera and focuser......and Oh No - more light!  I read up on focussers and how not to worry about the small amount of light they let in through the gap to allow the tube to go up and down.  But, the dark shots (now that he mirror end was sorted) showed me the culprit for the weird circular refractions...yep focuser leakage onto the secondary and onto the primary and back to the camera.....What to do?....Baffles Baffles...hmmm....oh flock it!? a special adhesive skirt around the focuser tube that could ride up and down with the focus? 

Er - queue brainwave!!!.  I found an old black sock, cut the foot part off, and yippee, the leg part of the sock covers the base of the focuser right up to the camera ring.  And this sock still had good elastic to hold it securely.   And another test....very very small amounts of light....but most acceptable for a budget solution! 

I really could see the gradients and rainbow patterns on my lights, and even after applying flats, darks etc....so I would suggest the loss of a sock and an old black bag, a good investment!  Astronomy is sooo romantic.