Taking Flats with a DSLR

[Fay]

Blimey, Mark could give Stephen Hawking a run for his money!!!

Mark, I will say you have a lot of nerve to say, you may be wrong about this after all...............

[Ian]

Still trying to get head around the effect of a lens on an extended diffuse light source, regardless. I think an experiment is in order

Mark, I hope you're not wrong, I would then have to worry that your luck had run out too...   

[MarkS]

If I was wrong, at least it means I can now go out and buy myself an electroluminescnt sheet and do flats the easy way!

[Ian]

Or, there's a chance that you're right, but for the wrong reasons. This could run on until it challenges the legendary battery thread.

[mickw]

Aaah the battery thread................

Fond memories........................

And no maths 

[Rocket Pooch]

I just realised something as well and I do need to goto the garage get a book and then translate it, or scan it, and this will help explain it more.

But, the origional post from Carol said she could not see the dust etc.  Now I can only assume this is because the magic 1/3rd - 1/2 half well depth of the CCD has not been reached, but from my point of view how the hell do you measure it on a DSLR, I have never used one.

I show you what I do,

When I take a flat I'm aiming for 1/2 well depth for the flat, I take an image through the whole system and look at it in the image processing software, Carol look at the 1st image its similar to what you see i.e. not a lot.


Initial load by chrissuddell, on Flickr


Carol this is an image with an Auto Streach function showing similar to you flat.



Initial load streach by chrissuddell, on Flickr


You will see a dialog on the screen showing the stats on the image, this is how I find out if the flat is any good, cool eh!

How do you do this with IRIS, or DSS and a DSRL?

Chris

[Fay]

I always check the histogram in a processing program for DSLR

[Rocket Pooch]

Hi,

I just read DSS infomation about taking flats, how unscientific!!!!

I bet I could come up with a better way in err 30 second.

Chris

[Rocket Pooch]

I always check the histogram in a processing program for DSLR

Ok thats good, and on the back of the camera it should be unsaturated and half way up, is that what you do?

Chris

[Fay]

yes, I have read that it is now suggested that you go over the half way mark on the histogram. I only have a small view finder, so look via processing program, or Irfanview or Canon dedicated software

[The Thing]

I use the histogram in nebulosity or irfanview if I'm using canon utility.

[Carole]

Chris thanks for doing that, but I am not familiar with the statistics to be able to tell if it's half well depth.

I have been doing some experiments today which I have to analyse as yet.  It occured to me that the dome of my POD is white and shows the light through, so I have done a series of flats at different ISO settings and am going to compare the results or indeed see if it worked at all.  

Also you may recall in an early post I said that in some literature I had read it had said to do Flats at 100 ISO.  Well it seems that DSS is warning me that the ISO does not match the light frames which I do at 800, so would this affect the stacked image?

I also took the "flats" with the draw tube pulled right in to see if this shows I have focus sag.  So even if the flats are no good might confirm this at least (hopefully). 

Duncan

or irfanview if I'm using canon utility.

I use Canon Utility but what is Irfanview?

Carole

[MarkS]

A flat taken through the dome should work well (for a refractor).

Irfanview is an excellent program: http://www.irfanview.com/

Don't get too hung up on half well depth - just make sure no part of the flat is saturated.

I also took the "flats" with the draw tube pulled right in to see if this shows I have focus sag

Good idea.

Mark

[MarkS]

I'm now beginning to think I might be wrong about this after all.

Whilst it's true that the focal reducer can form an image of the objective lens (or of the primary mirror), this fact might be a complete red herring  

I've had another idea about this while cycling home last night!  I need to do a couple more practical experiments; revisit the discussion and come up with a new explanation ...

[Rocket Pooch]

I also took the "flats" with the draw tube pulled right in to see if this shows I have focus sag

Good idea.

Mark

Hi,

I guess this is a test, but when you actually take flats to be applied do not move the drawtube, it should be in the same place as when you are imaging.

Chris