Proper flats frames for 383L +

[geoshare]

Does anyone have a good idea of the proper exposure level for producing accurate flat frames on the 383L +? Does anyone know what the values of full pixel saturation is for this camera model?

The attached sample from my camera with Televue NP101IS focal reducer and red filter binned 2x2 read about 35,000 in Nebulosity. Applying these flats does not seem to be working well and I'm seeking input to improve the use of properly collected flat frames....

Sample was converted to jpg from the unprocessed raw file.

Thanks,
Robert

[niteman1946]

Hi Robert,

Doesn't seem to be a lot of activity on this forum.  Here is a response I got from the Cloudy Nights forum. 

http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Number=5398108&page=0&view=collapsed&sb=5&o=all&fpart=1&vc=1&PHPSESSID=

Hope this helps.

Mark

[joelshort]

Robert,
I noticed that  the bottom right corner of your flat is darker than the rest of the frame.  How long were your flat exposures?  I'm wondering if you've got a bit of shutter shadow in your flat?  I see something like this in my flats and resulting calibrated light images, and I will be experimenting with longer flat exposures.

[Kaidan]

I think you need to calibrate your light source to get around 40000 ADU in > 4 - 5s, depending the filter (it will take longer to go to 40000 with a Ha filter in instance but i wouldn't go under 4 or 5s minimum). If your flat is longer than 10s, it's better to do a flat dark as well.

[joelshort]

kaidan,
I'm curious, why do you recommend 40,000adu for the 383l for flats?  Most of what I have heard is to aim for about half the total well capacity of the ccd, in this case around 30,000-32,000.  Again, I'm just curious.
joel

[geoshare]

My exposure with the light box was .125 sec. So perhaps there is some shutter shadow. Sound like trying for a bit more exposure will help.
Thanks for the ideas..

Robert

[joelshort]

My exposure with the light box was .125 sec. So perhaps there is some shutter shadow. Sound like trying for a bit more exposure will help.
Thanks for the ideas..

Robert

Yep, 0.125 sec is way too short for flat with this or any camera with a shutter.  Let me know how the new flats work out with longer exposures; I'm trying to figure this one out too.

[Kaidan]

I do use 2/3 of the well capacity. Results are better than using only 1/2 of the range.

This is not really 383L related, just a "good practice" i guess.