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Lum Calibration Problem

Mikko Viljamaa
in General
I'm turning to you Adam as my last resource...

I have an odd problem, actually I have had this for a while already. All my other flats calibrate their corresponding filters just fine but for some reason the LUM master almost always end up horrible and I need to fix this later in the process. For a moment I thought this was a filter wheel issue and the wheel doesn't always return to the same position but then I thought that if this would be the case, I would have the same problem with all the filters. Next I thought that this is because I take so many LUM subs (200 - 250 pcs) compared to the others that this must be the reason. But my latest tests debunk also this theory. I've tried taking the Lum flats (I take 15 pcs for each filter) with different methods; sky, with and without shirts, short and long exposures, etc. and they all produce the same end results. So now I'm at the end of my wits here and not sure what to try next. Attached are Lum and Red flat and calibrated masters showing what I mean. Both flats look good and similar, yet the masters are totally different. 

 

Here's also ta link to Cloudy Nights where I have asked the question; https://www.cloudynights.com/topic/804722-lum-flat-problem/#entry11597494


Any thoughts and thanks,


Mikko

Comments

  • BlockHeadBlockHead
    I wonder why I am "last" ...lol.

    You have demonstrated the issue isn't in the process of acquiring the flat... that is a good step.

    So I would not so easily dismiss the mechanical change in the system (whether it is filter alignment or something shifting).

    The thing to do is acquire data...and then immediately without moving the telescope or filter wheel acquire the flats. At a minimum- you can test the filer wheel issue by acquiring some data- and NEVER moving the filter wheel (just do luminance). Then at the end, you take your flats... moving the telescope if you have to. If you *still* have the issue.. .it isn't the filter wheel and something if moving at the camera end.

    As I demonstrate in my YouTube video on this subject- a useful way to diagnose this issue is to acquire flats at different times and divide one flat by the other. *this* will show you if the flats are changing. You should get an almost perflectly uniform result if nothing changes.

    -the Blockhead
  • BlockHeadBlockHead
    You need to take flats EACH time if you have dust donuts. Flats that compensate for vignetting (field non-uniformity) will work for long periods of time because they do not have much spatial information. The addition of dust donuts changes the game dramatically. In addition, focus, in the presence of dust, is also very important. I read your CC post...I think you are being too permissive in what you think is acceptable in terms of characterizing your system/sensor. Be pickier... and you will have better results.

    -the Blockhead
  • Mikko Viljamaa
    Thanks Adam. I opened the camera yesterday for the first time long time, cleaned all the filters and used the new flat panel today to take new set of flats. If Sunday night is clear as it's supposed to, I'll try to take just Lum data and in the morning take matching flats. We'll see if that reveals anything. 

    But I cannot help thinking again, why the issue only with the Lum channel and shouldn't the dust donuts more around with other filters too?

    - Mikko
  • BlockHeadBlockHead
    The Lum is just easier to see issues... you all wavelengths of light going through... everything is there. 

    -the Blockhead
  • Mikko Viljamaa
    Adam here's a straight copy / paste from my respond in Cloudy Nights. Thanks once again for your help.

    Well I'll be darned... shocked.gif

     

    I guess a person should take flats more often than twice a year before he starts to complain that there's something wrong with his images. Although the flats themselves looked horrible and worse after I "cleaned" the filters the fresh Lum master itself looks flawless. The attached master is 60 pcs stacked 5 min subs all taken last night with corresponding Lum flats (25 pcs) taken this morning after the session. I have to keep experiencing now if I really need to take new flats after every session or would once a week / twice a month be for example enough. 

     

    I appreciate everyone's comments and advices, thanks!

     

     

    - Mikko

    masterLight_BIN-1_EXPOSURE-300.jpg
    1913 x 1278 - 201K
    masterFlat_BIN-1_FILTER-Lum_Mono.jpg
    1913 x 1278 - 139K
  • BlockHeadBlockHead
    That flat looks fine..and all that matters is where it calibrates the lights... 

    -the Blockhead
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