Flats nightmare
Posted: Wed Mar 17, 2021 3:12 pm
Hi Robin, all,
I've been struggling for months with this issue, so after thorough testing I guess I'm ready to ask.
I'm currently using an SCT (C9) with a focal reducer and a ZWO ASI294MM Pro (all that follows also happens with my Altair 294c). An SCT with a focal reducer really has vignetting, so it may be uncovering an issue that also happens with optics that have a better field illumination.
I'm usually taking flats with Sharpcap, using a dimmable Lacerta 12V flatbox, and as the livestack goes I'm often seeing either undercorrection (residual vignetting) or, worse, overcorrection (inverse vignetting).
Here are some tests I ran lately:
- flatbox at maximum brightness
- 294MM set to unity gain (HCG = 120)
- set the exposure to get a decent histogram with no saturated pixels (either black or white) and ended up with 6ms or so
- captured various flats by either (a) leaving the "bias" unchecked, (b) checking the "bias" box, (c) separately capturing bias at the lowest possible exposure (around 40us), stacking them and using the obtained "master bias" as darks during the flat capture (effectively subtracting this master bias from each unitary flats).
I then applied those three different flats in order to see what would happen
Flat captured without bias Flat captured with Sharpcap's "capture bias" feature Flat captured by subtracting a "true" master bias The bias-subtracted flat is overcorrecting, the two other seem OK at this point.
I then lowered the flatbox brightness in order to get a non saturated histogram for 4-second exposures, still at unity gain, more in line with I do during EAA sessions, and tested the above flats
Flat captured without bias Flat captured with Sharpcap's "capture bias" feature Flat captured by subtracting a "true" master bias Now we have another issue!
-> The no-bias flat still undercompensates for vignetting.
-> The SC-captured "flat bias" undercalibrates slightly
-> The "master bias" flat seems to be compensating for vignetting, but both bias-corrected flats have this odd circular pattern.
-> The flat captured with Sharpcap's "capture bias" also has a rectangular zone at the top of the frame that is darker than the rest.
While I was there, I also tested "flat darks" by matching 4s darks with 4s flats. Same issue with those strange patterns.
Do you guys see where the issue could come from?
I've been struggling for months with this issue, so after thorough testing I guess I'm ready to ask.
I'm currently using an SCT (C9) with a focal reducer and a ZWO ASI294MM Pro (all that follows also happens with my Altair 294c). An SCT with a focal reducer really has vignetting, so it may be uncovering an issue that also happens with optics that have a better field illumination.
I'm usually taking flats with Sharpcap, using a dimmable Lacerta 12V flatbox, and as the livestack goes I'm often seeing either undercorrection (residual vignetting) or, worse, overcorrection (inverse vignetting).
Here are some tests I ran lately:
- flatbox at maximum brightness
- 294MM set to unity gain (HCG = 120)
- set the exposure to get a decent histogram with no saturated pixels (either black or white) and ended up with 6ms or so
- captured various flats by either (a) leaving the "bias" unchecked, (b) checking the "bias" box, (c) separately capturing bias at the lowest possible exposure (around 40us), stacking them and using the obtained "master bias" as darks during the flat capture (effectively subtracting this master bias from each unitary flats).
I then applied those three different flats in order to see what would happen
Flat captured without bias Flat captured with Sharpcap's "capture bias" feature Flat captured by subtracting a "true" master bias The bias-subtracted flat is overcorrecting, the two other seem OK at this point.
I then lowered the flatbox brightness in order to get a non saturated histogram for 4-second exposures, still at unity gain, more in line with I do during EAA sessions, and tested the above flats
Flat captured without bias Flat captured with Sharpcap's "capture bias" feature Flat captured by subtracting a "true" master bias Now we have another issue!
-> The no-bias flat still undercompensates for vignetting.
-> The SC-captured "flat bias" undercalibrates slightly
-> The "master bias" flat seems to be compensating for vignetting, but both bias-corrected flats have this odd circular pattern.
-> The flat captured with Sharpcap's "capture bias" also has a rectangular zone at the top of the frame that is darker than the rest.
While I was there, I also tested "flat darks" by matching 4s darks with 4s flats. Same issue with those strange patterns.
Do you guys see where the issue could come from?