Is There Something Wrong With My Flats?

Nick Kohrn

Last night, I captured images of Castor and Pollux under a moonless sky. However, I have a bad glow around my subjects after processing. Below is a sample flat frame that I captured shortly after acquiring the lights, biases, and darks.

Below is the result after processing; I followed the steps outlined in the FastTrack course.

The data was captured as follows:

1. 30x60s lights (ISO 800, f/3.2)

2. 30x60s darks

3. 30x0s biases

4. 30 flats (using white t-shirt around lens, white image on iPad to illuminate, AV mode on Canon T3i to get centered histogram)

The equipment used was:

- Canon T3i

- Canon 50mm lens

- Sky-Watcher Star Adventurer

I don't understand why I continue to have a glow around my targets. Is there something obviously wrong with my flats that I may be overlooking? :/

BlockHead

Hi Nick,

1. Typically DSLR cameras do not fair well with "biases" ..you will want to create matched "darks" for your flats. Even if I am wrong (because this may very well end up being like a bias) you eliminate this as being a problem. Find the exposure time that gives you good signal. Getting the "half saturation" is NOT the critical thing that makes flats work... the most important is to make certain the flat does not have a pedestal left over after calibration and to make certain the sensor is evenly illuminated. 

2. T-shirts are not good diffusers. Based on your picture, I don't think this is the major problem. This looks like a pedestal problem.

3. Forget the t-shirt and panel. Just take pictures of the twilight sky. Dark twilight..perhaps 20-30 minutes after sunset. Use zenith.

The fast lens can cause a different problem ... but that is not evident here.

-the Blockhead

Nick Kohrn

Hey Adam,

Thank you for the advice. To make sure I understand correctly for my next outing, I should capture the following frames:

1. Flats (dark twilight sky, zenith)
2. Flat darks
3. Lights
4. Darks

Is that correct, based on your recommendation above?

Also, is this WBPP video the one that I should pay attention to with regard to your first point above?

BlockHead

Hi Nick,

Yes, I do think with a DSLR you will be fine with your 4pt plan above.

However, and this part I cannot say, if your flat exposures are really short (like a second or so), you might not need that dark frame from the Flat.  But with them in hand.. you can try with and without. When I calibrated my images using my Canon Ra ... I did not need darks for the flats..I just used the flats as is. 

So assemble that data..and lets see how it goes together. 

Do not let your camera do any kind of automatic dark subtraction (or noise correction) or anything. 

-the Blockhead