From: Milton Aupperle <milton@outcastsoft.com>

Date: November 19, 2009 9:44:08 AM MST

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] Re: proper darks and flats

Hi Willie;

On 19-Nov-09, at 1:25 AM, Willie Strickland wrote:

So what is your procedure for proper darks and flats?

Willie

By "proper darks", I mean they have the correct exposure time for the LRGB exposures. What I was doing before was to shoot flats for Luma and then do a Live dark frame subtraction. For R G B exposures I was using about 1.15 times for Red, about 0.85 times for Blue Green exposures and was using the same Luma dark frame for the dark subtraction.

I didn't think that being within +/- 10% exposure for the flats would make much of a difference, but it sure does when you stretch the image with log sqrt curves to bring up the faint areas. I would get weird color casts in different parts of the image which made balancing the background images tough, as shown for things like the veil nebula:

http://www.outcastsoft.com/AstroImages/NGC6960_20090722_MJA.jpg

The upper left corner is red and the bottom right corner is blue green - that's due to the difference in flats exposure times it isn't real.

Now I shoot my Luma Dark ahead of time ("Avg. Frame.." capture as a 16 bit tiff with a minimum of 4 frames) and then balance the Dark brightness up or down in Astro IIDC Image Processing for each R G B images to make synthetic flats of the correct exposure. Then I subtract the correct Dark for each LRGB movie and then run the resulting dark subtracted LRGB movies with the correct Flat for each color.

To balance my colors out properly and make the exposure the correct time to avoid ringed stars. I balanced them using a G2V (HIP 7918 Mag 4.9) star which I shot multiple times at different exposures and partially defocussed to spread it over more pixels for statistical purposes. I measured the total light captured for each LRGB image using Astro IIDC 's Photometry section in Image Processing module.

So to bring a G2v star to a "white" color with the Grasshopper EXHAD CCD camera and my LRGB Astrodon filters, I need to shoot at these exposure ratios:

Luma = 1.0 : Red = 1.75 Green = 1.0 Blue = 0.85

If I add the the Hutech IDAS LPR filter into the mix (which I'm doing now because I want to shoot longer than 9 minutes without having the sky background go above 25,000 ADU), I need to shoot with these exposures to compensate for it removing light from parts of spectrum.

Luma = 1.0 : Red = 1.5 Green = 1.0 Blue = 0.67

One thing I'm considering doing for Astro IIDC 5.0x is allowing you to save or load flats and darks up from a special binary file format before you begin image capture. This would also have a Applescript commands for it too so you could automate the process. I'd also like to add the ability for it to create a synthetic dark on the fly with the AppleScript command too so you can scale it up or down to match your exposure times. It would save both disk space and time, assuming your dark frames are consistent and your CCD exposure times are linear.

HTH..

Milton Aupperle

On Oct 13, 2009, at 5:52 PM, milton_aupperle wrote:

I was not collecting proper darks and flats images when I shot it

------------------------------------

Yahoo! Groups Links

<*> To visit your group on the web, go to:

http://groups.yahoo.com/group/Astro_IIDC/

<*> Your email settings:

Individual Email | Traditional

<*> To change settings online go to:

http://groups.yahoo..com/group/Astro_IIDC/join