Problem with Flats

Simon - this is a snip of what I wrote at www.irishastronomy.org/index.php/kunena?...29&id=87780&start=25

My ROI was chosen by taking a very long dark (4 hours long!) and then examining the image and avoiding obviously regions containing lots of hot pixels. I narrowed it down to a region of 140 x 140 pixels - it just happened to very close to the center of the chip - though I don't think that mattered too much.
The practicalities were carried out by:
1. I got a cardboard box and lined it with black card
2. Imager went into the box
3. A lint-free cloth was placed tightly over the nosecone of the imager to prevent any paper particles falling onto the CCD window when the shutter was opened
4. Sheets of paper (I think about 15) were placed over the lint free cloth
5. I have a light-box - this was placed over the top of the cardboard box and swithced on. I let it "warm up" for about half an hour
5. Images were taken in my study with all other light sources off
Since my max exposure was about 80 or 90 seconds, it didn't take long to iterate through two exposures at each exposure duration (I started at about 0.1 sec). It took about an hour or so.
Images were taken at -20 deg c.

I took corresponding darks for flat calibration.
The ROI was cropped as well as the corresponding overscan region. The cropped areas and the overscan regions were then analysed and the data thrown into the spreadsaheet. Then I did the pixel-math with the duplicate ROI's - data again thrown into the spreadheet.

That's pretty much the process I wanted to write-up but haven't had the time yet...

So darks were taken at ambient, lights were taken cooled. I think I chose -20 because that was the temp I'd normally operate the imager at.

HTH
Dave

So this is what I am currently doing

1. 250 BIAS Frames at 0C and create a master BIAS
2. Take Multiple Darks at 0C at varying lengths, 1M, 2M, 5M, 10M and make a master dark for each exposure time
3. Pairs of Flat Frames at min exposure time at -25C (This is my usual imaging temp) doubling the exposure time until I reach Full Well
4. Back off the exposure time til I hit just below Full Well and take another pair of flats

That's the preliminary part, does that sound right?

Simon

I'm not sure about bias frames Simon - I didn't take them because the 1001e chip has an overscan region than can be used to subtract the offset. The question arose in the previous thread of 3 years ago but I don't think it was fully answered.

You can take darks at ambient temperature but I don't see the harm in cooling to zero.

Don't forget to take darks for your lights as well. The darks will be used for standard calibration. They are different to the darks you use for the DTC (i.e. those you take in step 2 of your process)

Also, the one dark that I took of 4 hours duration was to find the 'cleanest' part of the CCD (no hot pixels) to use as the Region of Interest (ROI). That was its only purpose. 4 hours was probably way too much. But try taking a dark of 1 hour and see what it looks like...

I'm going to go back to the folder of images I took and see what darks I took. I'll report back.
Dave

I took darks at the following durations (seconds)

0.1
0.2
0.4
1.0
2
5
10
20
60
120
300
600
900
1800
3600

You need to take two darks for each of those exposure times.

You could use the 3600 dark to find the best ROI - but use one of the two raw frames - don't combine them.

It looks like I set the CCD cooler to 15 degrees C

That will give you all the data you need for the Dark Transfer Curve (DTC).

I checked back over the light frames I took - for the 1001e. The longest usable exposure I took (using the lightbox) was 63 seconds. So I backed off and took lights from 50 secs to 63 secs at one second intervals. The data was then used to construct the PTC.

Hope that helps a bit more...

Dave

Lights being the flats in step 3?

250 BIAS Frames completed, I know you say it's not necessary but all the guides I have read on this explicitly call out to do BIAS Frames and typically 200+ of them

I only say they are not necessary *if* your chip has an overscan region and you have software that can include it in the image. If I recall, I used the Finger Lakes software to take the images then I used Maxim to analyse it. Though I've a suspicion I mat have managed to persuade Maxim to take full-frame-including-overscan images... The memory is a little hazy.

Now that you have the bias frames, I don't know what the process is for extracting the data from them... I'm not sure if they are stacked and then an average value is taken from the ROI - though an average of what, I'm not sure. But it could equally be simply taking a master bias from each light/dark frame - i.e. normal calibration procedures.

I've never actually taken bias frames - they are only required if you are going to use scaled darks.

Lights = flats - yes

One other point... When you choose your region of interest, you must use the same ROI for *all* your images

If you PM me your email address, I can send you on the spreadsheet with all my data in it. It'll maybe help you see the measurements and you can then delete my data and throw your own in...

Cheers
Dave