Darks/Flats & Bias

[willuk2010]

Although I have no decent pictures yet, I'm trying to get my understanding for thing like flats and darks ready..

I think I get darks, I need to take a number of dark frames after the lights are complete by putting the scope front cover on.. Doing this after the session is important due to temperature of the sensor etc.

In terms of flats, it sounds like they have to be done in advance or the next day.. My couple of questions are

1) Are flats temperature sensitive too? Do I have to have the flats at the same temperature sensor as the lights?

2) Are flats focus sensitive, i.e. do I have to have the image path, location, and focus exactly the same as the lights?

Some general questions re darks, flats & bias..

1) I think I have read somewhere that the darks contain the bias is that right?

2) for darks and flats do I have to have the dew shield on?

Many thanks!

Will

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[Tinker1947]

If you have DSS installed there is a Help tab and in there it runs through all you need to know and how to do it....this is a link to the help file......the bit you need is in the Introduction section in a grey box....

http://deepskystacker.free.fr/english/userguide.htm

[willuk2010]

Cheers tinker, I shall have a read!

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[Alien 13]

Flats have to be taken through the same optics and focus, what you are looking to capture is any optical defects in the scope and dust etc on the sensor  temperature is not important.

The funny thing is i took some yesterday while it was very light and had a look at them in PS after a quick stretch i found the image full of stars so a T shirt or something over the lens is recomended.

Alan

[willuk2010]

Thanks Alan, so in theory I could go out in the middle of the day, point the scope at the sky with at tshirt stretched over.. And set the cam to AV to get my flats? On the other hand I suppose I could take the scope indoors off the mount, after the session stretch the white tshirt over, and use an off camera flash pointed at the tshirt?

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[Alien 13]

Its better to do it outdoors you need an even gradient that is not too bright so auto exposure is ideal.

Alan

[Tinker1947]

The scope still needs to be at the same focus as when the images were taken.....i use a clear blue sky making sure the sun doesn't get any light on the Asda slice up pillow case i brought a large jubilee clip of E-bay to hold the white sheet in place so there were no wrinkles...

[willuk2010]

So, does the clear blue come out on the flat? Or does the aperture priority mode expose so it's white? Does it actually have to be white?

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[Tinker1947]

Its white light that's required thats why the white t-shirt, pillow case is used, the light has to be very even...

[willuk2010]

Okidoki, I'll give it a go! Thanks for your help!

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[frugal]

You need to make sure that you do not end up with the threads of the T-shirt showing up on the flats. I have not had a chance to try flats with a telescope, but with my 300mm camera lens I can clearly see a pattern fro the t-shirt, even when focused at infinity. Now it may well be that with the longer focal length of the telescope this is not a problem, but it is worth checking.

The only technique that has repeatably worked for me is to hold the end of the lens (well the lens hood really) against the laptop screen with a blank white screen showing. I needed to touch the lens hood to the screen to ensure that it was exactly at 90 degrees otherwise I got two corners darker than the others.

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[willuk2010]

Cheers Frugal,

I shall give it a go, so like just open notepad or something..... then move the scope to point to it ? or remove the scope from the mount and do the white laptop screen trick indoors?

Cheers,

Will 

[Russe]

I use an iPad White screen (there are apps for that "torch"). Put this right in front of the scope - put camera into AV mode and let APT do things for me.

[frugal]

Cheers Frugal,

I shall give it a go, so like just open notepad or something..... then move the scope to point to it ? or remove the scope from the mount and do the white laptop screen trick indoors?

Cheers,

Will 

I am still just using a telephoto lens on a camera, so taking it off of the tripod and holding it against the laptop screen is easy. With a scope I would probably hold the laptop screen up to the scope as the scope would probably be heavier (and more expensive) than the scope.

[Russe]

What I do as well is, I turn the iPad brightness right down. Why? Mhm, makes me FEEL better about my flats...

[dph1nm]

So, does the clear blue come out on the flat? Or does the aperture priority mode expose so it's white? Does it actually have to be white?

The colour isn't relevant, as I think you will find in most software packages each channel (R,G,B is processed separately. Al that matters is that you have a decent signal-to-noise in each channel. With daytime flats you have to be a little careful, as to avoid saturating the blue you end up with not a lot of red, so it is the signal-to-noise in the red channel which dictates how many flats you need to stack.

NigelM

[ollypenrice]

You need to make sure that you do not end up with the threads of the T-shirt showing up on the flats. I have not had a chance to try flats with a telescope, but with my 300mm camera lens I can clearly see a pattern fro the t-shirt, even when focused at infinity. Now it may well be that with the longer focal length of the telescope this is not a problem, but it is worth checking.

The only technique that has repeatably worked for me is to hold the end of the lens (well the lens hood really) against the laptop screen with a blank white screen showing. I needed to touch the lens hood to the screen to ensure that it was exactly at 90 degrees otherwise I got two corners darker than the others.

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Given that the large black form of the secondary mirror doesn't show, you really don't have to worry about the threads of a T shirt! They are so far out of focus as not to matter.

Doing flats in the daylight is well and good if you have no light leaks. But Newts, for instance, let in light from the bottom of the tube and that's a no-no. 

In terms of exposure, your flats should peak at about a third of the way from left to right on the histogram. Take a lot and calibrate them using a master bias as a dark. This is important or they'll over correct.

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2266922474&k=Sc3kgzc