When photographing nebulas with narrowband filters, you want the weak signal in your image to be genuine and not from a dust or dark edge. The DayStar flat cap FC140 allows you to create flat frames for this purpose. You place the cap on the front of your telescope and take shots of a uniformly illuminated plane, which you calculate when stacking the images using your light frames.
You create flats per filter, so for each filter in your set as well. narrowband and LRGB filters. Before ordering, check which size fits the front of your tube or fog cap, so that the cap fits well and no stray light enters from the edge.
Make your flats preferably immediately after your shoot session, without changing your focus or the position of your camera. A narrow band filter reduces light, so expect longer exposure times for your flats than with a wide band filter.
Another measure in this series is the flat cap FC150.
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