Flat frame puzzler...

dave summa

I just started using the L-extreme filter and already appreciate the benefits.  However, I've run into a problem with flats.  

I setup for flats using an LED sketch pad as the light source, per usual.  I capture using the ASIAir software and am using a ZWO 294MC Pro camera.  I adjust light level to achieve about 25000 ADU on the histogram at 3-5 sec exposure (due to the 294 quirks), then shoot my series of flats.  This routine has worked fine and my flats have never been a problem...until now.  I do not have a filter wheel, so I leave the filter in the imaging train for the flats and now, there is no data in the flats.  This is really strange, since I see each sub on the screen as it is captured which shows the usual dust motes and such.  The histogram is also presented for each sub, and that's steady around 25k.  However, when I process in PixInsight using a master dark flat, no bias (also normal procedure) and stack, i get no data in my flats and PI aborts any process using the master flat i generate.  When I examine the master flat, there is indeed no data in there.  It's a uniform image of basically no data.  

My first fix was to suspect the light source on the light pad.  I used the t-shirt and cloudy sky light source and same method.  I get the same good subs, same histogram... and same fail - no data in the master flat I create.

Any suggestions?  At this point, I am not sure if this is a PI processing problem or a data capture problem.  I am most puzzled that each sub APPEARS to be good/normal when I capture.  Each is displayed on my screen, but I am now 5 for 5 in creating a zero-data master flat.  It's got me puzzled, and stuck. The only difference in my normal flat method (which has worked perfectly) is the L-extreme filter, so I have to start there.  But if I remove it from the imaging train, I will no doubt rotate the whole image and the flat will no longer be useful for my lights.  Ack...

Appreciate any suggestions.

Imaging train: Meade 130 Apo triplet, ASI 294MC pro, Lextreme filter

Software: ASIAir capture, PixInsight 

BlockHead

Hi Dave,

The short part of the story- if your master flat has no information - the problem has something to do with the calibration of the flat. This means there is an over-subtraction. There are so many ways to do this!

1. There is something wrong with your master Flat Dark. What is the value recorded there?

2. Sometimes people accidentally load a real flat into the darks- and *this* is subtracted... of course giving your zero. 

3. Several other ways... (Optimization issues for example)

Without seeing the data... it is hard to say- but this isn't about the light source or PI... it is likely something more fundamental.  

By the way, about your light source. I am not a proponent of the LED sketch pad. You might think everything is OK now...but in the future you might disagree. Likely it will does not have the right spectrum- and you will have some subtle color gradients...that you might think are in the sky...blah blah blah. :)

If you can't follow the clues... make the data available to be looked at.

-the Blockhead