As per the subject matter, here are my finds about light leaks in my large reflector, which I though I had tamed in the past. The primary end, normally "naked" to allow for cooling, had TWO caps on, one specifically (supposedly) design for the purpose of blocking light plus a double layer black shower cap (at the time I couldn't find anything in the right size so stop laughing!). The focuser, an old style Moonlight Crayford, had the proverbial sock wrapped around it and given time and exposure to the elements (the scopes are all permanently sited in the back-garden) has somehow worked rather loose around the edges (as it came to light, literally, later on) but, naively, I though, it would still provide the blocking function it was meant to give.

Now, my main source of damnable light pollution, isn't only the various and nefarious security lights of the industrial/retail estate to my south and east but the bloody security light of my neighbour which can trip off any moment at night and flood the location where the scope is sited with direct billion-lumen light. Hence, any, even minimal, fissure in the light-blocking "armour" can have significant consequences. Yet, in the face of all this I thought I was pretty covered. Inspect in daytime did not reveal, visually (!!!) any obvious fault. Yet I had light leaks, not of much consequence as I most shoot NB but yet there they were...

So, I decided to go and investigate further and deeper than previously done and come to the root of the issue and the only way to this is to take darks during daylight (something I always preached but rarely done). I did the experiment last year (when I installed the sock) I was pretty happy that a 30s dark did not reveal anything obviously amiss.

So I put the double thickness felt cap on top of the business end of the scope and started taking 3 min darks in full daylight and here are the results:

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Horror and despair! The sock didn't do its bloody job! But worse to come. I put a total black-out cap over the fort half of the scope so that the entire imaging system was in total blackness and here is what I got:

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While the "diamond ring" of the eclipsing focuser was gone there was still plenty of diffuse light where I expected none to be. Drastic action was thus required and I put a rubble refuse bag (very thick and very black) one the other end and slipping further up for a 20 cm length and fitting very tight. Finally I got the expected results:

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Lesson learned: Do not assume, TEST!
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