Flats

[malc-c]

OK I'm all goggled out !

Steve set me some homework to produce some flats. Having googled the subject I'm even more confused as to the simple easy way of doing this. Some suggest the tee-shirt over the scope on a bright day, others suggest the same but on a uniform overcast day, whilst another suggested at Dawn or Dusk on a clear night. Then there's the point the scope at a monitor or laptop screen which is uniform white background, then others say make a lightbox !!

At the moment is it effectively 98% cloud cover, mostly white clouds. Could is simply stretch a white sheet or Tee-Shirt over the 200P and take some shots at the same ISO I use for my imaging (800ISO and 1600ISO). Do these shots have to be at the same exposure as my images or can they be shorter, and do I use manual or auto setting on the camera.....

[Ben Ritchie]

I always found dawn flats the easiest as they require no kit and are easy at the end of a night, but the tee-shirt method works fine. Flats can of course be taken at any time, as long as you haven't adjusted the position of the camera.

As for exposure time, you want to expose so that the pixels are at about half their maximum level - i.e. the flat is neither saturated nor under-exposed. Exact time doesn't much matter. Usually getting the right length takes a little trial and error, just keep taking exposures until the histogram is roughly centred.

[ollypenrice]

By far the easiest way is not on your list, but with an 8 inch it will be quite expensive. It's an EL panel. The Neumann ones are the best (Google Aurora Flatfield Foils). Cheaper ones come from Earlsmann in the UK.

No outdoor flats have ever worked well for me but it may be the intensity of the sun and sky clarity here. I always get gradients so use a foil in the dark. Or it might be incompetence which is far from unlikely!

Olly

[adamsp123]

If you are using the Canon just simply set the camera to the same ISO but use auto exposure - you should get the right light level - job done. Dunno about full sunny day I do mine at dusk/dawn but when the light is quite good or when it is cloudy - oh and with a clean white T shirt over the scope.

[malc-c]

Thanks for the tip.

[steppenwolf]

The white T shirt should work as will a couple of sheets of A4 white paper and a cloudy sky. Set the DSLR to Av mode and let it decide the exposure length for you. The ISO value is not critical and does not have to match the original 'LIGHTS' as it is simply a facsimile of the original light cone at the same focus position that you are trying to capture.

A laptop's blank white screen will also work although some of these do not give a uniform plane. I use an EL panel in the observatory and that seems to work well for me.

In any event, once you have captured your flats, combine them to make a master in MaximDL's 'Set Calibration' routine using a median combine with the 'apply boxcar filter' ticked.