Flats and DSLR

[dmahon]

OK, I've got the hang of taking (basic) pictures and I know how to take darks and bias frames. I haven't got the hang of processing (that's going to take a while), nor of using flats.

Can someone walk me through the process of taking flats with a DSLR (Canon 450D)?

I don't have an observatory - I set up at the beginning of the night and tear down at the end (inlcuding removing the camera from the scope), so I guess I would need to take flats at night before I pack away - i.e. there's no early evening/morning light flats for me.

If I need to buy an illuminated panel I will, but if there's anything cheap I can use in the meantime to get started, please let me know.

[MikeWilson]

I've been using tracing paper.

I put a sheet over the top of the OTA, then shine a LED torch (white) at it from a distance so that the tracing paper is more or less evenly illuminated. Then, with the camera in Av mode, I snap several pictures (about 10), turn the tracing paper around 90 degrees and snap another 10. Flats done.

I was in the process of making a light box but I think I'll abandon that plan now that my tracing paper method works just fine.

I had heard that a white t-shirt pulled over the scope works. Therefore for my first flats I used a pair of white boxer shorts (clean!) pulled over the end of the scope. That did remove the vignetting but it introduced a 'fibrous' look to my early M31 images, caused by the cotton in my pants. Tracing paper is much better...

[MikeWilson]

Alternatively you can take Twilight flats (during twilight point your scope at an evenly illuminated part of the sky away from the sun and snap ~20 images).

[swag72]

I asked a similar question about taking flats with a DSLR. One suggestion I liked was to put notebook onto your PC screen, and use that to take the pictures. Nice and white and even to boot.

I have never managed flats though - So really know nothing! I understand the principle, I understand about setting the camera into AV and just snapping the pics, but mine all turned out with some kind of colour cast and didn't look like any kind of flat I have seen on here. So maybe I just got it wrong, or perhaps they don't really look like flats until they've been processed.

I hope you don't mind the ramblings in your thread. I will be watching this with interest as I want to sort these out too!

[samtheeagle]

@Sara - My flats appear to be a light blue on the camera, but don't worry about what colour they are, as it's the illumination across the image that's important. I believe that the colour information is irrelevant.

@Mike - I love the tracing paper idea! Very clever indeed

@dmahon - I think the main points have already been covered, and from your initial post it seems that you realise that you'll need to take flats each imaging session, especially as you have to set up and break down every single time, so do I :S I'd love an obsy!

[jgs001]

I like the tracing paper idea... hmm... I use the laptop and Al's virtual lightbox (although notepad works just fine). I also do the complete setup and teardown every session, and my SLR gets a lot of use in between too, so can't leave it attached..

[Psychobilly]

Unused laminating pouches also work well as diffusers and are "dewproof"...

Billy...

[dmahon]

Well, something A4 sized from here (plus power supply) arrived today and I am hoping that some of these will arrive tomorrow.

To take flats I just set the DSLR to AV and take it for whatever length exposure is selected then? And I then take flat darks for the same period of time?

[MikeWilson]

Well, something A4 sized from here (plus power supply) arrived today and I am hoping that some of these will arrive tomorrow.

To take flats I just set the DSLR to AV and take it for whatever length exposure is selected then? And I then take flat darks for the same period of time?

Nope

Flats are there to compensate for the vignetting/dust/particles/optical aberrations in the optical path from the front of the telescope to the sensor.

Darks are there to compensate for noise. Noise is dependent on exposure length and temperature, so these need to be taken at the same length as your light images and at a similar temperature.

I used the fridge to get a library of dark images around 3-5 degrees celcius, but generally it's worth leaving your camera outside and getting darks at different temperatures.

This is one reason why folks like cooled CCD's, since you can tell the CCD what temperature you want to take the images at (usually -20 or such) and it's conveniently consistent to the point where you probably don't have to take darks at all (I'm sure someone can correct me on this if I'm wrong?).

[Glider]

I take the OTA off the mount and take my flats next day, being careful not to move anything on the OTA in between.

I point the OTA at the door of our white Freezer in the conservatory, put a few sheets of plain white printer paper over the end of the dew shield and hold in place with a sheet of glass. I've found it is important for the dew shield to be retracted our light gets in from the back.

I adjust the number of sheets of paper until a 1s exposure gives me 2/3 histogram.

Then I shoot 30 flats.

Seems to work with my Canon 1000d. They usually look blue'ish, like Samtheeagle's.

[dmahon]

Nope

Flats are there to compensate for the vignetting/dust/particles/optical aberrations in the optical path from the front of the telescope to the sensor.

Darks are there to compensate for noise. Noise is dependent on exposure length and temperature, so these need to be taken at the same length as your light images and at a similar temperature.

I used the fridge to get a library of dark images around 3-5 degrees celcius, but generally it's worth leaving your camera outside and getting darks at different temperatures.

This is one reason why folks like cooled CCD's, since you can tell the CCD what temperature you want to take the images at (usually -20 or such) and it's conveniently consistent to the point where you probably don't have to take darks at all (I'm sure someone can correct me on this if I'm wrong?).

I already take lights and darks. I have bias frames too (although it appears that they may not be necessary for DSLRs as someone appears to think that they are taken automatically for every image in this thread).

I need to take flats and dark flats (or flat darks, not sure which is the correct terminology).

I was led to understand that for darks, the temperature has to be pretty much spot on the same as it was for lights, and also that the temperature of the CMOS chip may not be directly related to the external temperature. Hence I have not created a library of dark images.

[jgs001]

How long are your flats ? Based on that same thread on the dSLR bias frame, I wouldn't worry about dark flats. I've not used them in a long time, I'm not convinced they did a lot.

[dmahon]

I adjust the number of sheets of paper until a 1s exposure gives me 2/3 histogram.

This is the bit I am not completely sure about (the actual taking of the flat frame).

What is a "2/3 histogram"? Why 1s exposures a opposed to other durations?

What do other people do when they take the flats in "AV mode" (is it just setting it the wheel to AV and taking an exposure of the duration suggested by the camera)?

[jgs001]

That's exactly it. set the mode dial to Av (aperture priority) and let the camera determine the correct shutter speed to use. It plonks the peak of the histogram in just the right place. I normally take one in Av mode, then change back to manual and set the shutter speed that Av determined. I have to do it this way, as I can't see the camera when holding a laptop over the objective...

[JB80]

Sorry to bump an old thread but I want to try this tonight possibly and want to see if I have this right but just using my dslr with no scope.

After taking my subs and darks, lets say 20 of them I can then open up notepads blank screen and switch the camera to Av and then take another 20 flats of the blank screen?

Is it that simple or am I missing something?

[Shibby]

Pibbles, yes it's (almost) that simple. You just need to make sure that the exposure is long enough that you don't catch the screen refreshing, causing uneven illumination. So you might need to put a couple of sheets of paper or a T-shirt over the end of the scope to darken things a bit.

Also, I don't use AV but adjust exposure manually until the histogram is roughly in the middle.

[JB80]

Thanks, I won't be using a scope though but I guess the principle is the same.

I'm going to give it a go next chance I get and some bias too because trying to process out the gradient I find very difficult to follow the few guides I've found.