Light leaks in imaging train

[Endolf]

Hi

I tried to get some flats and darks during the day time with my 8" newt and my ASI1600MM Pro cooled camera. I had assumed that being cooled, I could bring the temperature down to the same as when I'm shooting and be able to take calibration frames during the day. My first attempt was awful, the darks just were not. I remove the camera, filter wheel adapters and coma corrector assembly from the scope to try again and found them better, but still not looking like I expected a dark to look (I had glow in the corners for example). I waited till dark and tried again and got something that looked like I was expecting, stretched, it's a uniform noise/grey across the whole image.

As I wanted to take flats this way too, I decided I needed to sort out the light leaks. I realised that the gap round the primary mirror was one big source, so I've made a cap for the rear of the scope for when I'm shooting indoors. It should help reduce dust getting in to the scope when stored too. By wrapping an old pillow case around the focuser (covering from the scope body right up to the filter wheel) I've managed to get the light levels closer to that good dark image. But I've noticed another location where it leaks. I *think* it's in the camera body itself. Being a cooled camera it has cooling grills in the body, and I *think* light is getting in to the body this way, the bright spots are the 4 corners of the image. Covering these to get better darks would be a bad idea, as the cooling would stop working.

Has anyone else experienced this with cooled CMOS cameras?, is there a way to reduce this?, am I analysing my darks and flats too closely?, or am I really going to have to go back to shooting flats (with an ELP) and darks in darkness (given we have no astro dark at the moment).

I've attached a 300s dark.

Thanks.

 

Dark_300_secs_002.fits

[tomato]

I had a similar problem and traced the light leakage to the base of the filter wheel, where the cable connections are made.

 

[Endolf]

Good thought, I checked by doing a test dark at 300s, similar results, this was with the camera removed from everything and a cap screwed over the sensor, so it's coming in through the camera somewhere.

[Skipper Billy]

Cooling vents are a favourite for light leaks - take your cal frames indoors but put the rig in a completely dark room???

[ollypenrice]

I agree with Billy. It's best to do darks off the rig with the camera's window cover in place. Many have screw fit metal covers for this purpose. The tiniest light ingress will invalidate the darks.

I also do try to do flats in the dark, even on refractors which are sealed. When I can't I put a dark drape over the rig while shootng them. I think you'll struggle to get food reflector flats in the light.

Olly

[Endolf]

I might have to turn a cupboard in to a dark room....

Thanks.

[jimjam11]

My 200pds is terrible for light leaks as well. I resorted to curtain blackout material which works well when double thickness...

[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 on 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.

Edited May 30, 2021 by MaltaNewtF4
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