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Light leaks


woodblock
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I'm just getting used to my first proper camera the ZWO ASI1600mm PRO which is cooled. It seems that I can now make all my calibration frames in the daytime. I suspected that there was a problem with my dark frames. My scope is a SW 200PDS and when I make the dark frames I put the SW plastic cover in the end of the tube but I had an idea that it wasn't completely light proof. So I've made an extra cover which goes over the top of that and extends down the tube a couple of inches. It is better but still not quite right. I'm doing this in the daylight by the way. After some investigation I found that light is leaking in somewhere in the optical train between the camera and the eye piece tube. I use a SW coma corrector and there are a few spacers and adapters in the train but I was surprised that light was able to get in. I'm testing it by taking a picture using APT and then in the histogram it tells me the range of pixel values so I can see that the minimum value has dropped.

 

I should say that when I do this normally I have the lights out and all the blinds down.

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You could try taking the camera off the telescope and use the plastic cap that comes with the camera. That will be lightproof. When doing darks the optics of the scope are not involved.

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Posted (edited)

The PDS scopes tend to leak light through the focuser drawtube. This can be wrapped with some tinfoil when taking calibration frames. You also need to cover the front as you have mentioned, and also the rear (primary mirror end) as this leaks too. Most people recommend a black shower cap. As I don't have a black shower cap ( 🙂 ) I keep a cardboard disk permanently in place behind the primary, and cover it with a woolly hat. It works just as well.

Darks can (and possibly should) be done off-scope, but obviously flats can't. My biggest problem was with the light from the flat panel illuminating the focuser tube so even taking these at night is an issue. Also, any local/ambient light including LED indicator lights on your camera, focuser etc. can cause minor issues both with darks and flats.

The best way to isolate and remove any leaks is to set up APT or NINA in a loop (1 second or 2 seconds is fine) and shine a torch on the OTA. You will see streaks and circles in the test images.

Edited by Padraic M
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Thanks, I made a cover out of cardboard to go at the bottom of the tube a while ago. I figured that there was a problem at the top as well and made one for there but I didn't count on light getting in around the focuser draw tube and elements in train to the camera.

 

It reminds me that I should be more careful when taking images outside at night as well.  It's surprising how much light is around.

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I'd do your darks at night on a cloudy night when you can't image.  I used to own a 130PDS and do flats during the day time.  I think I reverted to doing them in the dark too for the same reason, and had to buy myself a flats panel.

 

Carole   

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Thanks Carol, when I got my new camera which had cooling I thought, great I can do the calibration frames the next day during the daytime but maybe it wasn't such a good idea.

Steve

 

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I've flocked and blocked my newts to reduce stray light. And I use an A3 drawing panel for my flats. With that setup I can do my darks in a draw. 

 

The DIY flat panel is an easy mod if you don't mind a bit of basic soldering and arduino code. Now NINA automates the Flats for me.. 

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I would suggest if you want to do them in the day time that you cover the primary end of the Newtonian with a black bag, and then cover the camera and filterwheel as well.  make sure you have a decent white cover (I use a pillow case several layers) over the aperture for flats), with dark obviously you need the lens cap on. I have done this and it does work, but it's more faff.

 

Darks you can make yourself a darks library and just re-use them. 

 

Carole  

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My Skywatcher newtonians both leak light. A tube of black silicon bathroom sealant solved the leaks around the focuser, flocking all but eliminated any remaining scatter that gets in around the mirror.

 

As my flats are fairly bright and short exposure almost no evidence remains of light leaks.

 

However if you shoot dark flats the leaks may continue to be a problem.

 

Ross

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