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Flats problem with long-focal length Mak imaging?

Aries Productions Astro Pixel Processor (APP) INDI Library Ekos Processing Calibration Deep Sky
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Leela.Astro.Imaging avatar
Leela.Astro.Imaging avatar
Hello,

I'm wondering if anyone has ever had a problem with flats calibration with long focal length Mak imaging.

I'm testing out a new setup  (details & example here: https://astrob.in/ij748l/0/) and for the life of me I cannot understand what is going on with the flats.



I'm using the exact same process as w a wide field refractor (calibrating in EKOS for about 23000 ADU, using APP to calibrate and stack) - with the wide-field this never happens.  Are long deep images more sensitive to autofocus change complicating flats calibration?  Or am I just being stupid and missing something obvious?

All help would be appreciated because this would make a nice galaxy imaging unit if I can get this sorted.

Thanks,

Vin
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andrea tasselli avatar
andrea tasselli avatar
Your master flat is wrong on many levels so you might want to revise how you take them. Nothing wrong with a long focus Mak in itself since I never ever had any issue with them and if anything a narrow FOV helps get it right.

I would, however, reconsider the wisdom of shooting for M101, a rather dim galaxy, at those pixel scales with a 5" reflector for just 3 hours of integration but to each his/her/their own.
Leela.Astro.Imaging avatar
Leela.Astro.Imaging avatar
Thanks Andrea.  Yes I hope it's just that I need to figure out a new flats process.  I'm using the same light panel as I do for my 4" f5 refractor and the same camera so I assumed it would work here.  But clearly it's not!

I just picked M101 as a handy target near the zenith (I have restricted FOV) and wanted an object with enough structure in the FOV to see if the image would work.  You're right about the dimness - that in itself was a learning process (can you believe I even tried a dual band filter on M101 at one stage, hilarious).  And for sure 3h is no where near enough data, but the aim for this test was more just to see if the guiding and the mount would handle the focal length, and whether the camera would vignette.

I think I will try sky flats next time we ever have clear skies again here.

Cheers - its good to know that its just user-error rather than something fundamental about the setup b/c that gives me hope for some deep targets!
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