My darks are very light - what have I done wrong?

[Shanghai Joe]

For my last imaging session I used Smartcaps smart histogram to determine the optimum offset and gain for my new PlayerOne Ares-C Pro with an Optolong L-Enhance filter. It returned values of offset=600 and gain=380. The off the shelf values from PlayerONe are O=50 and G=125 so I ran the routine again and it came back with the same results. The telescope was pointing over a nearby streetlight although the light itself was hidden by a fence. The imaging session went well, the Fireworks galaxy was clearly visible with a 300s exposure. When it came to taking darks I used the same settings and the resulting images were quite light, almost like a flat frame in fact. Is this right? or have I messed up somewhere or misunderstood the Sharpcap results.

[admin]

Hi,

I think the most likely cause is that you have still got the 'display stretch' function in SharpCap enabled, which is brightening the image on screen (if you look at the saved files in another application then they would be dark. See https://docs.sharpcap.co.uk/4.1/#Display%20Stretch

If the saved images also look bright when viewed without any sort of stretching applied, then it would be worth checking the capturesettings.txt file saved alongside them - that contains all the camera settings used during capture, and maybe there is some setting that was not quite set as expected.

cheers,

Robin

[Shanghai Joe]

Thanks Robin. After several hours of inspection and testing it turns out that the answer was quite simple - light leakage into the filter housing. The PlayerOne filter housing has six tapped holes on both the front and rear faces so that the housing can be screwed directly matching holes on the camera on one side and to a PlayerOne OAG on the other. What they don't tell you is that these holes are drilled all the way through the front and rear faces. In my case I used a 0mm wide M48/M42 adaptor to fit the housing to the camera so the rear holes are effectively blocked. However the front holes were "open" and this was the source of the leakage. I managed to prove this by firstly taking flats with the imaging train covered (I hung a jacket over it ), and then by blocking the holes with blutack. PlayerOne provided screws to join the filter housing to the camera or OAG and I used these for a more permanent seal. It's not very elegant though, the screws are designed for countersunk holes and there is no countersink so the screwheads protrude slightly. There is no way that this method could be used on the camera side as the screwheads would foul on the front face of the camera. Incidentally, I've not tried it but I don't see how you could use these holes to connect to anything anyway, there appears to be no access for a screwdriver, certainly at the bottom of the housing.

The upshot is I can now take decent flats

[admin]

Hi,

glad to hear that you got it sorted in the end - I had a look at the images of the P1 filter drawer online and I can see the holes that you are talking about - an odd design choice!. I have had similar issues in the past with the T thread to CS thread converter plates that you get with some cameras - some have holes in them (useful to get them unscrewed if they get stuck), which can also let in light depending on the design of the camera front plate.

cheers,

Robin