Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

Whats wrong with my Flats?

Keener_7

By Keener_7
in Getting Started With Imaging

Recommended Posts

Keener_7

Keener_7

Posted
Posted

Was trying to take some flats for the first time last night but didn't have much luck. I was using the white laptop screen method. When I would try and use Av mode on my cannon 1000D the camera would take 8sec exposures and the histogram  would be all the way to the left. Then I tried to use manual mode, when I manged to get the histogram about 1/3 of the way my flats all had lines running through them. 

post-24036-0-69069500-1379200704_thumb.j

I tried 2 different exposure times where the histogram would be in the correct location and always the the lines. Could it be the shutter moving across the image or is the exposure time somehow matching the refresh rate of my computer screen? So any ideas on what is causing this or how to correct it? 

Link to comment
Share on other sites
Keener_7

Keener_7

Posted
  • Author
Posted

when you take flats put the camera in AV mode and use the same iso as when you was imaging HTH

Dan

I tried using Av mode. For some reason when I would do this the camera would take an 8sec exposure and the the image would be completely blown out. Not sure what was happening there.

Link to comment
Share on other sites
Keener_7

Keener_7

Posted
  • Author
Posted

Yes, I had it very close to the scope held up against the dew shield. I just did a quick test point at the sky, ( its evening here and clear) and my flats seemed to come out fine! I also tried again with the laptop screen and got the same result. I really think the camera is catching the refresh rate of the laptop screen, looks like taking flats that way may not be an option for me.

Link to comment
Share on other sites
StuartJPP

StuartJPP

Posted
Posted

What I did with flats using a computer monitor was to set the camera's ISO to the lowest setting which increases the exposure time. This helps to eliminate shearing problems caused by display refresh. Used AV mode with +1 EV to get the histogram above 1/2 way mark.

Some monitors are not evenly illuminated though (invisible to the naked eye), so this may not work.

Link to comment
Share on other sites
ollypenrice

ollypenrice

Posted
Posted

Oh?!?!

I didn't know!!! I can use other ISO's for Flats??!! Great!!

I'm not sayng you can but I can't see why you shouldn't! I don't use DSLRs but as far as I can see the only thing you are trying to get is a brightness value per pixel. You do need to calibrate flats by subtracting a bias (or rather a master bias made of a couple of dozen average combined.) You could shoot dedicated darks for your flats but they will do nothing that a bias won't do and you only need one bias. Now, how do bias frames work on DSLRs? Do they vary with ISO setting? If so then the bias need to be taken at the same ISO as the flat. Someone will know.

Olly

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.