strange flats

Processing Calibration Deep Sky Artifacts
11 replies•360 views
José Carlos Rodríguez avatar
José Carlos Rodríguez avatar
Yesterday when I finished my lights, I did the corresponding FLATS. I do them automatically in NINA with the advanced scheduler.
But this time, I have obtained something very strange, a kind of donut.
Does anyone have any idea what happened? I did 50 FLATS and they are all the same.
The Lights are "normal"


Link the image in real size
Brian Puhl avatar
Brian Puhl avatar
I wouldnt have thought of this had another guy just posted about something similar, but could you have dewed up?
José Carlos Rodríguez avatar
José Carlos Rodríguez avatar
Brian Puhl:
I wouldnt have thought of this had another guy just posted about something similar, but could you have dewed up?
José Carlos Rodríguez avatar
José Carlos Rodríguez avatar
I don't think it's a dew problem. I have a heating tape on the tube.
If I look inside, I don't see any dew and the telescope is dry.
Lorenzo Dal Molin avatar
Lorenzo Dal Molin avatar
Could it be a focus changement?
Brian Puhl avatar
Brian Puhl avatar
Did you stack the data anyways?  Sometimes flats look overly aggressive after STF, it could be fine?
Deepan Vishal avatar
Deepan Vishal avatar
What camera are you using? 

Did you take the flats at the end of your imaging session?  Or at a different time? 

I once encountered dew and frost on the Camera AR window which produced similar pattern on the flats.
Well Written Concise
John Hayes avatar
John Hayes avatar
That’s the kind of pattern that frost (or dew) will create.  What temperature were you running the camera at and what was the ambient humidity?  What camera is it?  Is this a sky flat?   Is this a stretched or a linear image?  How long was the exposure?  What filter was used?
Well Written
Jonathan Paul avatar
Jonathan Paul avatar
Possibly dew on the sensor?
Scott Badger avatar
Scott Badger avatar
Possibly dew on the sensor?

That's what it looks like to me too. Do the lights show the same thing? Seems likely if you took the flats immediately after the lights.

Cheers,
Scott
Well Written
José Carlos Rodríguez avatar
José Carlos Rodríguez avatar
Well, in the end it seems that it is the humidity.
This afternoon, I did the FLATS again without touching anything, and they came out perfectly.
Without that strange "donut".
The strange thing about all this is that the Lights all came out perfect, and the flats, which I did immediately after, from the first to the last have this effect.
I use NINA, the advanced sequencer to take the flats after the lights, at the same temperature and focus. Everything remotely.
It is true that the humidity was very high (close to 95%), but as with the heating tape nothing strange appeared in the lights…
I guess the conditions changed, or maybe it was by covering the tube for the flat shots. I use a RB Focus eXcalibur. The camera is a HQY600 PH-M SBFL which is supposed to have an integrated anti-spray system.
The chamber temperature was -10ºC.

Thank you very much everyone for your answers. It's nice to see that when you ask for help, you find it.

Thank you so much
Helpful Respectful
Scott Badger avatar
Scott Badger avatar
Glad your back on track! Could be just bad timing as you said, but……odd. If it was the ota, not sensor, covering the tube should help I think. I'm in the northeast and dew has been an issue, but with a tape on the ota, plus a dew shield wrapped in foam (old camp sleeping pad), setting the sensor at 0 C, and cooling it slowly, I haven't had problems for awhile. Of course, now that I've said that out loud, I'm sure the hair dryer I keep handy in my observatory will be useful next time out……..

Cheers,
Scott
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