How to create DSLR or CCD Flat frames

[AstroTanja (Tanja)]

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-con...s_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!

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 (Bill)]

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 (Cam)]

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 (Bruce)]

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 (Andy)]

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