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Yeah Paul, It's really strange I think.. Although the Deepsky instruments RC's and the RCOS have built in secondary heaters from the factory... So obviously the secondary dewing is not "uncommon" in this design.
I think its a case of the secondary being closer to the front of the scope, the secondary holder is exposed directly to the cold and it dews rapidly.. I remember the last couple of times I've mentioned it, both you and Trevor had mentioned that you'd had no problems with this, as such, last night I was half inclined to take a photo down the aperture showing the secondary completely covered in dew....
I'm making a 30cm long dew shield as I type this.. its made of thin plastic, I'll be putting a layer of felt around the rear of it so it fits snugly to the OTA, and using matte black paint or flocking material to stop it reflecting light.... and just to keep the overall look of the OTA, I'll be sticking some of that carbon fibre pattern contact around the outside of the shield.. so it will hopefully just look like an extension of the tube..
If your conditions are as damp as you say, I'm amazed that you operate an SCT by choice... My SCT was a dew nightmare here.. I loved the scope, but the dew was impossible to stop.. nothing I did stopped it. heaters, shield, hair drier every 2 or 3 minutes...
I hope a sheild stops ths issue.. a heater is not optimal. As you say, it could negate the sharpness of the optics for one, and a lesser issue, but an issue all the same, is trying to route its power cable in such a way that it does not impede the light path in any way....
The heater will be a last resort, and hopefully it wont come to that...
Mike:
I just opened the original stacked tiff file of M16 and did nothing but a power stretch and those squares around the stars are still evident... Perhaps its a stacking artefact? Maybe I'll try adaptive weighted median combine rather than sigma-clipping to see if that has any affect...
Cheers.
Alex.
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