flatframes


	Last Modified: email dave@allmon.com


	My QuickCam only site


	This is a 50% size frame of the far focused white card flat fielded by the far unfocused white card. An additional artifact is seen here, that I have never seen before. Right of center is something which looks remarkably like the secondary obstruction. I don´t see this in close unfocused flats. The image is stretched considerably.


	After much rework on the SCT adapter, I managed to get the following flat. It is a median of 9 frames made with the same exposure time and at the same temperature as the image frames. The same dark frame was used to correct the image and the flat. The SCT adapter was painted inside with Krylon Ultra Flat Black.








	The rusult of flat fielding with this frame is an overcorrected image. The lower left corner has a mean value 113 ADUs above the center. There is also an undercorrected ring around the sweet spot.


	I reworked the SCT adapter again. This time I lined the inside with black velvet. I also baffled the front and back ends of the adapter with black posterboard. I cut a hole in the center of both baffles, sized such that about 1/8" of aperture is lost.


	A frame made with the new setup. It looks worse than before. The slope on the edges is steeper, but the circle of best illumination is larger. The moon is 97.3% full, and 45 degrees southwest.

	A cheesey sky flat.

	The image frame after flat fielding. The corners look good. The sky flat still had a few stars, so you can see some pits.


	NGC-891 30 x 60 seconds LX-200 @ f/3.3 Genesis KAF-0401 @ -20.5C Processed with CIP, PhotoImpact 4




	I picked up a piece of black velvet ribbon from the hobby shop, and RTV´d it around the inside of the NGF SCT adapter. The ribbon had to be cut to length, and width. This created all sorts of loose fuzzies. A better solution might be black felt or the sticky-backed paper velvet, which has the fuzzies glued on.



			The posterboard baffle installed on the front end of the adapter.


	The modification above does wonders for the flats, but it doesn´t quite do it all. I looked at the sky tube after seeing that Mike Fleenor had solved his f/6.3 flat fielding problems. The thing looks like black anodized aluminum. It glares through the surface coating.




	To get this image, I pointed the LX at the ceiling 24 inches above a 300 W. halogen lamp, stuck the nose of the Olympus up the tailpipe of the NGF-S, and snapped a couple of frames. I averaged them together. The gamma was adjusted to 3.5 to bring out the ring. I had already frosted the tube inside with Krylon UFB. The NGF mounts on a special back that replaces the visual back, and has a larger hole, and an NGF mounting ring. The ring in the image above is the inside surface of the mounting ring. It is about 2 inches ID, and as shiny as can be. I cut a baffle with about 2mm smaller ID than that and glued it inside the back, so that it acted to keep the reflections from hitting the surface. Everything got the standard coat of Krylon, and I assembled it and tried again.



			Same setup as before. The ring is gone.