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20" x 180 @ ISO 800
Sylvania, Ohio 8-19-13 @ 00:21

Open clusters, like globulars, make for good targets when the Moon is full, or nearly so. I actually was guiding when taking these short subs, but I quickly learned that a bright moon and long exposures don't mix. 20" was about as far as I could push it without the entire frame being a brilliant white. Question: Might taking such over-saturated subs be a good way of acquiring flat frames? The question just occurred to me as I was entering this. Feedback on this topic is welcome.

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Comment by Jeffrey P Nunnari on August 20, 2013 at 7:29am

Thanks for the feedback, guys. It's one of the nice things about being a member of this site and learning from others who have knowledge of such matters.

Comment by Russ Ruggles on August 20, 2013 at 6:26am

First one needs to understand the purpose of flats. You don't want any stars or data in your flats. Just a picture of your optical train including dust, smudges (heaven forbid!!) and the uneven illumination across the FOV. Taking flats using the sky is the cheapest method. You could buy products such as a Flatman and take them at night as well.
The ISO generally isn't important as long as it matches the Bias frames. The duration is dependent on the brightness of the sky at the moment. Late evening or early morning the sky brightens or darken very quickly so you may need to make adjustments during the capture process. I usually go for 20 flats for a decent mater flat. Because of the quickly changing sky late and early in the day I will try to get my flats while the sun is still up. I point my scope to an area of the sky where the sun isn't. Take a few test shots and if I find the shot is over saturated I'll stretch a white tee-shirt over the end of the scope to block some of the light. I try to get about 50% on the histogram.
Keep in mind that for the flats to work you must not change anything - focus, camera position....nothing. And those flats are only good for that imaging session. The bias can be reused as well as darks as long as they match in time and temp.

Comment by Bob W on August 20, 2013 at 5:36am

Nice picture Jeff. Regarding the flats you want them to be a short time frame that way they can capture the dust and other stuff that is on the front of the scope. The long image you are talking about doesn't do that, it is just capturing to much light and flooding the sensor. What I have been doing is about 5 mins after sunset I set the camera to AV and point the scope straight up in the sky or you can do it in the morning.

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