any ideas what went wrong here?

[Eeyarn]

i was imaging last night and decided to try flats for the first time, put a plain white sheet over the scope and pointed it at a light, read on Dss help page to set the camera to AV mode so i did but when i took the flats the exposure time was so fast the image came out completely black so set it myself at 20th of a second an they looked like i would expect flats to look like on the camera, but when i stacked them with the lights and darks i got this...

this is same data without flats 

it looks like the pattern of the fabric from the white sheet but would have thought it would be so out of focus it cant be. anyways will this just be because the exposure time was too long? like i said first time using flats so dont really know what im doing but any advice would be great.  

iain 

[ChrisLX200]

If you apply a histogram stretch to the falts what do you see?

ChrisH

[oldpink]

most likely is the fabric
I read somewhere in the imaging forum about this happening

[oldpink]

http://stargazerslounge.com/topic/208847-darksflats-bias/

have a read through this post

[ChrisLX200]

At the moment I'm using a simple illuminated sheet of white foamboard for flats, until I get around to making something a bit more sophisticated   Two battery powered diffused LED lamps provide the illumination (the red light is not on when taking the flats).

[malc-c]

Iain,

Looking at the two images I would say the flats have done their job.  They have taken out the vignetting to a degree giving the image a general all over level.

The info I've gathered is that often it's best to point the scope at an even  surface, and what might be happening is the tee shirt / sheet etc isn't being illuminated evenly.  I use a tee shirt doubled over and pulled tight over the scope, then when the sun is out, point the scope at a section of sky in the opposing direction, ie if the sun is due south then the scope is pointed north (almost like the default home position) and then with the camera set to AV reel off a batch via APT.  I was told that so long as the histogram peaks for RGB were in the middle of the histogram then they should be OK

[Eeyarn]

hi thanks for the replies had a read through that other thread oldpink, was helpful might try a different method like laptop screen or an illuminated white screen like yours Chris. 

[toxic]

i take it the photo's are in raw format.

i dont know if this helps but i get that all the time when i use raw files and my flats are taken from a part of the sky away from the sun and no cloth.

i dont seem to get it from jpegs i take flats and lights in jpeg then convert them to tiff then run them through dss then i dont get the odd texture but it still takes out the vignetting.

[Eeyarn]

hmm might give that a go toxic, yeah they're in raw, i just ran it through dss again but only used 3 of the flats to see if it made a difference and it did...came out a lot worse, which now that i think about it makes sense haha 

[Russe]

I use my iPad for flats - stick it right in front of the scope - camera to av mode / done.

[Pete Presland]

similar to Russe, white laptop screen, white shirt stretched over the end of the scope. your flats do seem to have worked though.

[dph1nm]

This looks like random noise being introduced by the flats to me. Hence it gets worse if you use fewer flats. If it was a pattern intrinsic to  the flats it wouldn't matter how many you used.

NigelM