How to create DSLR or CCD Flat frames

[AstroTanja]

A while ago I imaged Corona Australis with my Canon 5D Mark III on a Orion 8" telescope. (All CCDs and modded Cameras were busy shooting at the time).

http://photographingspace.com/wp-content/uploads/2015/12/flats_featured.jpg

I was curious to see what the superb sensor would do on the fast Orion 8” Astrograph – I imaged 12x 300sec @ ISO800 of this target. Due to the full frame sensor I was concerned about vignetting. My husband, Cory Schmitz's method of shooting flats post-imaging worked well, and corrected the uneven illumination extremely effectively, some coma left in the left edge but tolerable.

Check out the blog he wrote for a step-by-step guide how to shoot effective flat frames! It really helped my data! (http://photographingspace.com/how-to-create-dslr-and-ccd-flat-frames-for-astrophotography/?utm_source=IceInSpace&utm_medium=post&utm_content=flatframe&utm_campaign=main)

Shooting flats seems difficult, but once you figure it out it’s easy and very reproducible. Flat frames fix vignetting and brightness imperfections and can really save your astro photos.

[billdan]

Thanks very much for those tips Tanja, I think I know why my flats have been over-correcting and leaving a gradient.

The article states that the goal should be to get the median value approx 50% of the maximum value.

Well a quick check on my flat subs reveal around 25,000 for median and 36,000 for maximum or approx 70%.

Next time I will be more careful and try for 50% and note the difference.

Thank you.

Bill

[LightningNZ]

It's a great method. I do the same thing but without the t-shirt (which would prevent me from scratching the screen but I have nothing appropriate to use at this stage).

Additionally I make sure to rotate the scope as well as move it around the screen (translate) to average out imperfection in the monitor. This does require a fairly big screen, but they're pretty cheap these days.

Because the screen is so bright, exposures are normally around 1/100th of a second, so I can easily collect 50 flats with this method very quickly.

Thanks for sharing,
Cam

[bojan]

I am using tracing paper (placed over aperture) and clear blue sky (I am guessing fogy shy would also work)... taking shots in monochrome.
This method never failed me in the past.

[BruceG]

Thanks Tanja. The work and advice you and your lesser half are gifting to the astro community is really appreciated. I have followed your work for a couple of years now and find your images so inspiring - (both astro and terrestrial). So from me a BIG thankyou.
regards,
Bruce.

[andyc]

Thanks Tanja! I shoot plain twilight flat just now, and sometimes have imperfect illumination, so I've bookmarked your tutorial and I'll give it a crack :thumbsup: