Does my scope have light leaks?

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KathyNS Canada
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Does my scope have light leaks?

#1

Post by KathyNS »


Nothing serious. Just a question that I wasn't setting out to answer, but that I accidentally answered anyway.

I was noticing some hot pixels that weren't calibrating out. I figured that, after a couple of years since I last shot darks, I was probably due for some new ones. That's cool, I thought, I can run those in the daytime. I checked that the aperture cap was in place, set up a sequence in SGP to take darks and fired it up.

Oops, I thought, something is not right here, as I scanned the first frames. I cranked up the screen stretch in SGP as the frames came in, and this is what I saw:
120sec_2x2_None_frame1_19-1000px.jpg
As confirmation that this was due to a light leak, I can see the shadow of condensation on the camera window at the left side. Amp glow doesn't leave shadows, but light leaks do. The scope is capped, the lights are out, but daylight leaking under the dome skirt and into the ventilation openings around the mirror is getting to the camera. This is the view inside the dome:
Untitled.png
The reason I got this is because I accidentally forgot to specify in SGP that the frames were darks. If I had, it would have left the camera shutter closed. Taking them accidentally as lights, it opened the shutter.

A lesson in how dark it needs to be to take darks, and how to find a light leak if you suspect one.
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kt4hx United States of America
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Re: Does my scope have light leaks?

#2

Post by kt4hx »


This sort of harkens back to the idea of storage of a newtonian. Though one puts the end cap on at the front of the OTA they need to remember the back end as well, because it is not air tight (and thus light tight) and dust/insects can come in around the mirror. That is why I put shower caps on the the front end (even with the end cap in place) and on the back end of my solid tube dobs while stored in the garage.

While this is not exactly what you are getting at, I easily saw the correlation. :)
Alan

Scopes: Astro Sky 17.5 f/4.5 Dob || Apertura AD12 f/5 Dob || Zhumell Z10 f/4.9 Dob ||
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Re: Does my scope have light leaks?

#3

Post by OhNo »


My guess it is back end leaking light. Mine does the same on darks in the daylight! IF I shoot darks in the daylight, I cover the back and front with a coat of blanket....
Scopes: SkyWatcher 8" Quattro, Celestron C8, SkyWatcher ST120, Orion ST80, SharpStar 61EDPH II. SLT 130 Celestron
Mounts: CGEM, CG-4, EQ2, Alt Az, SLT
Cameras: ZWO ASI533MC Pro, ZWO ASI120MM, Canon 1100D
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Re: Does my scope have light leaks?

#4

Post by Star Dad »


And I don't about your scopes, but my focuser is a bit raised from the OTA and does not "seal" - light, air, etc can enter around it. Maybe I should put some heavy duty duct tape on it to seal it. mmmm another very small project to do.
"To be good is not enough when you dream of being great"

Orion 203mm/f4.9/1000mm, converted TASCO 114mm/f9/1000mm to steam punk, Meade 114mm/f9/1000, Coronado PST, Orion EQ-G, Ioptron Mini-Tower and iEQ30, Canon 70D, ASI120MM,ASI294MC, Ioptron SkyHunter
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Re: Does my scope have light leaks?

#5

Post by UlteriorModem »


My newtonian astrograph has more leaks than a sieve.

Main source is from behind the primary mirror believe it or not.
Tom

Current Equipment:
Mount: Celestron CGX-L
Scope: 130mm f7 APO
Cam: ASI071mc-pro
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