Further advice regards flats

Quark (Trevor) · #1 19-01-2009, 02:54 PM

Hi All,

Getting ready for a night of piggyback imaging with my 450D.
Will be using my 17-85mm EFS lens.

Have been experimenting this afternoon with the camera and lens at 85mm taking flats using a light box.

Brought up the histogram display on the 450D and with the lens focused at infinity at 85mm and camera with W/B set at daylight or 5200 K, an exposure of 1/400 sec has my histogram peak just over 1/3 of the way across from the left toward the right of the graph.

How does that exposure sound for my flats, would that be adequate.

Regards
Trevor

gbeal · #2 19-01-2009, 03:03 PM

Sounds about right to me Trevor, although some of the DSLR aficionados will chime in if it isn't.
What does the finished flat look like, what sort of readout do you get when you run the cursor from the middle to the edges?
Oh, and I just re-read your post, you will need them at the exact focus point, and focal length that you capture at, so in my case I normally shoot them immediately after I finish imaging. Same with darks.
Gary

Octane (Humayun) · #3 19-01-2009, 03:16 PM

Trevor,

Try half-way to two-thirds of the histogram. This has always given me the best results.

So, if you had just finished imaging, and your light source was that bright, I'd be taking 1/160th to 1/200th exposures. Experiment and find which works best.

Regards,
Humayun

Quark (Trevor) · #4 19-01-2009, 03:39 PM

Hi Gary,
The flat looks ok.
I took one with IP camera control and used the display image after release & download. Zoomed the image and had a good look and it looks fine, it is a new camera and lens.

Not sure what you mean regarding a readout or what to run the cursor over.

Regards
Trevor

Quark (Trevor) · #5 19-01-2009, 03:45 PM

Quote:

Originally Posted by Octane

Trevor,

Try half-way to two-thirds of the histogram. This has always given me the best results.

So, if you had just finished imaging, and your light source was that bright, I'd be taking 1/160th to 1/200th exposures. Experiment and find which works best.

Regards,
Humayun

Thanks Humayun,
My light source is very bright. it is a proper light box I used to use years ago to check out my negatives whenever the Camera shop guy didn't process my film because he didn't think there was anything on it.

I just tried 1/200th and that takes me nearly two thirds of the way across the histogram. I will give this a try tonight.

Regards
Trevor

Octane (Humayun) · #6 19-01-2009, 06:07 PM

Hi Trevor,

No problem.

One more thing, make sure you're shooting at ISO-100, it's very easy to forget that you were imaging at ISO-400, or what have you, and then go and take flats at the same ISO.

Regards,
Humayun

Quark (Trevor) · #7 20-01-2009, 06:34 PM

Quote:

Originally Posted by Octane

Hi Trevor,

No problem.

One more thing, make sure you're shooting at ISO-100, it's very easy to forget that you were imaging at ISO-400, or what have you, and then go and take flats at the same ISO.

Regards,
Humayun

Hi Humayun,

Just wondering, if I take my lights, darks and bias frames at ISO 800 why should my flats be taken at ISO 100.

I read your message just before I went down to my observatory last night, so I took two sets of flats one at ISO 800 at 1/200 th sec and one set at ISO 100 at 1/30 th sec. Both exposure and ISO combinations got my histogram about 2/3 across the graph.

The image from last night that I have posted was calibrated with the ISO 800 flats and I am currently re processing with the ISO 100 flats for comparison.

I know nothing about this subject and just wondered at the reason for using ISO 100 for flats.

Regards
Trevor

Quark (Trevor) · #8 20-01-2009, 07:51 PM

Quote:

Originally Posted by Octane

Hi Trevor,

No problem.

One more thing, make sure you're shooting at ISO-100, it's very easy to forget that you were imaging at ISO-400, or what have you, and then go and take flats at the same ISO.

Regards,
Humayun

Hi Again Humayun,

Just finished re processing with the ISO 100 flats, looked no different until I zoomed both images and there was definitely less noise in the image that was calibrated with the ISO 100 flats, why is it so?

Regards
Trevor

Octane (Humayun) · #9 20-01-2009, 11:17 PM

Trevor,

The reason why you use ISO-100 is simply to gather information about anomalies in your imaging train, such as dust particles, vignetting and any other aberrations. They are not used to gather noise characteristics.

The dark frames that you take will remove the dark noise for you. The camera's sensor at ISO-800 is going to be much, much noisier than it is at ISO-100. This is why flats are taken at ISO-100.

To completely calibrate your image, you should also be taking dark frames for your flat frames, at ISO-100, because your flats will have noise in them, too. Think of your flats as a type of light frame.

I don't bother with bias/offset frames as I find they introduce noise into my images. When you think about it, sensor readout noise (which bias frames are supposed to counter) is already being captured in the dark frame.

I hope this helps.

Regards,
Humayun

Quark (Trevor) · #10 21-01-2009, 05:52 PM

Thanks very much for the explanation H, can I call you H.

It is invaluable for DSLR beginners such as me to be able to tap into the wealth of experience on this site and I really do appreciate your help.

What you say about the bias frames makes sense and yes I did take darks for the flats but I could not work out how to utilize them in Images Plus.

Thanks again
Regards
Trevor