how to take flat frames

[paul j]

Hi all.could anyone please advice me on the best and easiest way to take flat frames.I've heard of people putting a teashirt over there scope and shining a light at it? Then take photos that way .if so how long exposure and how many should I take? And do these have to be done straight after the darks? Also I've heard people taking there Camera of there scope and pointing it at a laptop white screen this obviously seems easy too but then I read about having take them at same temperature? Any help on what's best and the length of exposure setting would be greatly appreciated.I'm new to astrophotogrsphy slowly getting there but struggling on few things such as flat frames.

[swag72]

Hi Paul,

First of all, you don't say what camera you are using, DSLR or CCD so to give an indication of exposure time is nigh on impossible at this point in time!

You are looking at a couple of things with flats.

1) Ensure that the focus is the same as when you were imaging

2) Ensure that the camera orientation is the same

3) Ensure that the filter is the same

These make it ideal to take the flats at or around the same time as the image, incase any extra dust for example has got in between your imaging and flat taking time. I don't actually know how important it is to be at the same temperature as your imaging run, but I keep mine the same.

I used to use a laptop screen when I had a DSLR, with notebook open and held up to the telescope as close as I could to the dewshield to stop extra light getting in. Then I would take an exposure on AV, with the same ISO as well and just ensure that the exposure time got me a histogram that peaked at between 1/3 and 1/2 across the whole histogram.

With the CCD, I aim to exposure at approx 20-28k ADU. And don't forget that you will need seperate flats for each filter. Now I use a light panel balanced ontop of my telescope. I take about 25 flats for each session. That's my take on it, although I am fairly new to flats (for the second time) as I could never really process them properly!

Hope that helps.

[paul j]

Thanks for the help.sorry I completely forgot about the camera.I using a canon 1100d unmodded.I didn't realize you take the flats on av mode neither.that's for your help.

[paul j]

Meant thanks for your help .predictive text.

[Bizibilder]

A couple of points - you can take darks at anytime, just record the temperature at which they were taken and you can build a "darks library" over time. I take mine in the garage using a remote timer - just set it up and let 'em run. I use 2min and 5min subs, so I have taken darks for these at every 5°C from -5 to +20°C. I always image at ISO 800 so all mine are taken at this setting. Just remember to put the lens cap on the camera!! You will need at least 30 Darks at each setting. Saves you wasting good imaging time too!

Flats must be taken with exactly the same optical train as your lights. That means exactly!!! I have built a light box using white http://stargazerslounge.com/topic/81681-light-box-for-sw-200p/page__st__20__hl__+light%20+box%20+bizibilder#entry1571418 LED's and use that now but I used to reflect a battery lantern off a sheet of white melamine covered chipboard - crude but it worked! If you use the white laptop screen method you will need to hold the lappy over the end of the scope. You can move the scope to a convenient position but don't alter anything and certainly don't switch the camera off! Just flick the settings switch to Av and run off about 30 frames. For Solar work I now just point the scope at the sky and place a piece of 30% transmission white opal acrylic sheet over the end of the scope - works a treat!

[paul j]

Thanks bizibilder v helpful.that's great taking darks and noting temperatures down as u mentioned saves lot of missed time for viewing etc.I start a catalogue of darks.I've seen light boxes on ebay around 15 quid so if I was to either buy one or make one all I do is slew the scope to it with Camera attached same settings but on av shine a light on back of box.sounds simple enough :-)

[Russe]

Am I daft? If I'd use a laptop to provide me with white light - how would I control the camera???

I guess I need some sort of a white light panel.

[swag72]

It is tricky, but once you've got an idea of the exposure time needed - There will be a lot of up laptop, down laptop, check histogram, back to notepad, up laptop ......................... - you can rattle them off fairly quickly and only need to make an initial check as for each seperate time, the exposure time will be roughly the same.

[earth titan]

Am I daft? If I'd use a laptop to provide me with white light - how would I control the camera???

I guess I need some sort of a white light panel.

I'm currently using an old LCD picture frame for mine. It broke as a picture frame but still displays a blank screen when switched on which is very pale blue (think evening sky). I'm considering some electronic paper for future use, which is relatively inexpensive and comes in sheets similar to standard paper sizes.

Seems to do nice even flats.

Typed by me, using fumms...

[Russe]

Am considering to use my iPad. Anyone tried that?

[earth titan]

I have. Seems to work okay although I struggled to find a way to get the right colour screen.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[Russe]

How did you get a white screen?

[earth titan]

How did you get a white screen?

In the end I made a jpeg which was just a white background which I saved. I then emailed it to myself on the iPad, saved it as an image then called up the fullscreen image on the iPad :-)

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[jgs001]

I don't think the colour makes any difference as long as the light is even and constant. I use a laptop for flats, and set my SLR into Av mode (there's a decent amount of information in the DSS help for shooting the calibration frames).

[Breakintheclouds]

I recently made a light box for shooting flats. Can I just ask other people: do you worry about how short the exposures end up being? My lighbox is hardly blinding, but as I'm using a camera that's designed for picking up a few stray photons from billions of km away, it only takes 0.02 seconds to get a peak in the middle of the histogram. It kind of worries me that I'm using such a small exposure, although I can't put my finger on why. What do other feel?

[jgs001]

I've only had issues with exposure times that end up being just about in sync with the monitor refresh rates (camera lenses really) I don't see why you'd have a problem, as long as the exposure levels are correct.

[Tinker1947]

There are Flash Light Apps out there, i downloaded a free one, paying would do away with the add at the top....Shown on a tablet

[Russe]

Flash light app is the way to go!

By the by, is anyone of you using easy capture to take their flats? I watched astro shed videos and in one Dion explained how he got rid of the problem in a very strange picture. Anyway, the program he used sounded like "easycap", which I downloaded. But, it was a screen grabber - so I must have understood it wrong. Anyone here's got a download link?

[Russe]

I'm still struggling to take FLATS with my webcam. What program are you using?

[Freddie]

You should use the same one as you use to take your lights.

[ollypenrice]

Super short exposures don't matter unless your camera has a shutter, in which case the 'wipe' of the shutter will slide across the flat creating a gradient.

If you have a sealed filter wheel and clean filters then (heresy alert!!!) do you need flats for each filter? I no longer bother. My L filter does fine for all the others. Under torture I'll confess the name of another (APOD) imager who does likewise...

Olly

[Davey-T]

If you have a sealed filter wheel and clean filters then (heresy alert!!!) do you need flats for each filter? I no longer bother. My L filter does fine for all the others. Under torture I'll confess the name of another (APOD) imager who does likewise...

Olly

I have always done flats for each filter as I found it needed different exposure lengths or light intensity to achieve the same level of saturation, never really gave it much thought but will now try all subs with the L flat, probably won't make a lot of

difference to my attempts.

If it's good enough for you and ahem it's good enough for me

Dave

[Russe]

If you have an avi of flats, how can you process it?

[earth titan]

Super short exposures don't matter unless your camera has a shutter, in which case the 'wipe' of the shutter will slide across the flat creating a gradient.

If you have a sealed filter wheel and clean filters then (heresy alert!!!) do you need flats for each filter? I no longer bother. My L filter does fine for all the others. Under torture I'll confess the name of another (APOD) imager who does likewise...

Olly

Interesting.

Is there any restriction on the colour correction capability of the scope to make this work?

Typed by me on my fone, using fumms... Excuse eny speling errurs.