Processing with Flats... Quantities... Help please....

[AlexxxAA]

Hi everyone. Last night I took 60 minutes worth of data of M109 in 2 minute subs. I also took 13 darks of 2 minutes right after my lights in order to keep the same temperature on the camera (500D).

I've been trying to learn how to use flats but my images never come out evenly illuminated after processing. So last night I tried to take flats at dusk. I pointed my scope to the east/south-east and took 25 flats in that manner.

So my question is, how many flats should I use with the other data I have?

Any help would be greatly appreciated.

Thanks to everyone in advanced

PS

I've tried the white t-shirt on my telescope during the day... However, after processing images in CS5, half of the image becomes incredibly bright, while the other half loses it's data and becomes dark...

[jgs001]

Alex, I normally use about 15 flats. What setting do you use on your camera for the flat ?? If you set the camera into Av mode, and make sure that all exposure compensation is off, the metering should get the right exposure value for the flat. The T Shirt is to make sure that the light is as even as possible.

[AlexxxAA]

Hi John. Thanks for the quick response

Ive been told to just set the camera in Auto mode. What i did yesterday was point the scope to the East at about 34 degrees up(polar alignment for me). I set the camera to auto w/out the flash. It gave me ISO 800 which is the same for my regular subs, and 1/50 exposure. But back to the point (sorry i deviated), the camera is at full auto.

[swag72]

If you set the camera into Av mode, and make sure that all exposure compensation is off, the metering should get the right exposure value for the flat.

Interesting John - I over exposed by 2/3 stop least night and this morning for my flats. Does that mean that the flats are less effective?

Definately want the camera on AV,not auto, with all the same settings and focus as the images you are going to use the flats with. I did mine yesterday taking a picture of my laptop screen with a blank notebook page open so that it's all white.

[Psychobilly]

Sarah find out what works for you..

I tried a range of exposure compensation values and find out what worked the best with my setup... you can end up with flats that over or under correct... its a bit like the porridge with Goldilocks and the 3 bears...

[jgs001]

The key is to get the histogram of the flat on about the 75% bar or thereabouts on the preview screen. On some occasions I've had to dial in a little +ve exp comp, but normally 0 works fine. I suspect when I've had to use exp comp it's because of dew on the optics I didn't spot.

[AlexxxAA]

Oohhhhhhhh! Ok

That totally makes sense!

Ill try the white t-shirt again today in that manner... No imaging tonight since its going to be cloudy and raining

I might try the laptop as well

Thanks alot guys I'll try to process when I get home and post it here. Hopefully my other data is worthy as well :-/

[jgs001]

I agree though don't use full auto. For astro you need the creative modes on the camera.

[AlexxxAA]

Yea it totally makes sense to me now. I dont know why they suggested to me to do that in the telescope dealer i went to. I kind of had my doubts too, but i went with it since i didnt really know about flats.

Cant wait to process my data that i have

[jgs001]

One thing with flats, you must have everything in exactly the same position as when you captured the light frames. And without anything else changing, like getting more dust on the sensor etc.

[AlexxxAA]

Hey john, one more question about what you've just mentioned about having everything the same...

My lights were taken with the cls clip for my canon. Should I take the flats with that clip on my camera as well? Or should it be just the sensor by itself?

Thanks again for the help

[jgs001]

Everything should be identical and in exactly the same orientation. Ideally you should take the flats before taking anything apart. You may not be able to use the flats if you have to put the clip filter back in place.

[AlexxxAA]

oh i see... very good then. i will keep all of this in mind for my next night out. Thanks