last night I wanted to take images of an area in Ursa Major; my plan was to take the flats in the morning I got up in the morning, to take flats; I thought I did everything okay; I used white T-shirt over the objective, minimal wrinkles, took my flats I stacked everything in WBPP; at first I was happy with how clean everything seemed to look, but when I did a boosted stretch I saw what I was hoping I wouldn't see: A BIG DUST MOTE THAT WAS POORLY CORRECTED, slightly faint, but BIGall the below examples are cropped into the far bottom right corner of the frame, because I really don't want to spoil the project; I could share the full master flat and a raw sub if it becomes necessary.here's an uncorrected single FIT file, 100s, Lum:
here's the Master Flat:
and here's a 2hr stack, calibrated:
I plan on imaging for another 2 nights, so I would appreciate tips, like, am I missing some setting I need to use in WBPP for flats? or, did the dust mote just move during the night? (it seems like the latter is a good explanation but, how would this specific one move so much when the scope is moving so slow?). This was the only dust mote that didn't get corrected okay at all. |
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Hi Oscar,
Any chance of image train/camera rotation between acquisition of light frames and taking the flats? Looking at the largest mote on the lower right, the slight over-correction on the left side and under-correction on the right side might suggest this. I think I can see a little under-correction on the right of that less bright smaller mote on the upper left.
Rob
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Or you may be using a ZWO or QHY filterwheel that turns in either direction.
Problem is these wheels don't stop in the same place, when turning in both directions.
Solution, if this is the case, set filter wheel to turn in only one direction in the ascom driver menu, or for QHY you must download new firmware from QHY that forces the wheel to only turn in one direction
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Rob Foster:
Hi Oscar,
Any chance of image train/camera rotation between acquisition of light frames and taking the flats? Looking at the largest mote on the lower right, the slight over-correction on the left side and under-correction on the right side might suggest this. I think I can see a little under-correction on the right of that less bright smaller mote on the upper left.
Rob true, I mean, my rotator was tight, but I guess it's possible I did put a thick heavy cover over my telescope very early in the morning, and when I came back a few hours later, and took it out, to take the flats, it's possible I took it out too fast and it pushed on the EFW slightly, but I don't know I put on the cover because of the fog that can come early in the morning; I guess I should not use it this next time, just to test thanks! Pete Bouras:
Or you may be using a ZWO or QHY filterwheel that turns in either direction.
Problem is these wheels don't stop in the same place, when turning in both directions.
Solution, if this is the case, set filter wheel to turn in only one direction in the ascom driver menu, or for QHY you must download new firmware from QHY that forces the wheel to only turn in one direction I'm using ZWO EFW, yes ASIair has this option for one way rotation I think; I'll turn it on this next time thx!
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When you turn on one way rotation, would be a good idea to make sure all the lights and flats were all shot with this setting.
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Pete Bouras:
When you turn on one way rotation, would be a good idea to make sure all the lights and flats were all shot with this setting. Will do. I have an update: I made sure to save a reference frame (one sub from last night) so that the ASIair can slew back to the exact framing like before, and when I tried slewing, it warned me that there was a full 1 degree difference in framing angle from the reference frame. so I think Rob might be onto something. I'm still gonna make sure I follow your tips too. thx again!
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None of those suggestions would really matter as the main culprit is up in the sky: the Moon!
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hi
train change position
the lights and the flats are in different optical position
focus change, filter change and back focus position change after lights or before flats
CS
Brian
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andrea tasselli:
None of those suggestions would really matter as the main culprit is up in the sky: the Moon! I mean, I understand the moon can cause messy gradients, but how would the moon be causing this mis-correction? Brian Diaz:
hi
train change position
the lights and the flats are in different optical position
focus change, filter change and back focus position change after lights or before flats
CS
Brian But I didn't really do any of those 3 you mention; I just imaged in Lum, and this is petzval, so no backfocus concerns; focus change - well yeah, but doesn't everyone do, like, hourly autofocuses? that was all I did. Rob's theory so far -seems- to be the right one. currently stacking another 2 hours of data taken with the new techniques, so I hope this data will be cleaner.
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Oscar:
andrea tasselli:
None of those suggestions would really matter as the main culprit is up in the sky: the Moon!
I mean, I understand the moon can cause messy gradients, but how would the moon be causing this mis-correction?
Brian Diaz:
hi
train change position
the lights and the flats are in different optical position
focus change, filter change and back focus position change after lights or before flats
CS
Brian
But I did none of those 3 you mention; I just imaged in Lum, and this is petzval, so no backfocus concerns; focus change - well yeah, but doesn't everyone do, like, hourly autofocuses? I thought minor changes in focus position wouldn't matter, that cannot be it, because everyone does autofocusing and they still seem to get clean data.
Rob's theory so far seems to be the right one.
currently stacking another 2 hours of data taken with the new techniques, so I hope this data will be cleaner. is just a rule( guide) , i mean general rule , not is for specific OTA or optical train let me tell you a history about 10 years on the field first rule , everything is fine and not until prove when we supestime some point ,( maybe we have to return and test again) the same problem couple years ago , after check everything i found the focus was slipping due a AF screw everytime i running AF the symmetric point was different , the last point after end the night was too long from the first one ( 5 hours before ) , but when you stacking all come together so my recommendation is check and test everything again small details make the different after that i add the af info( value ) in my lights , like the gain and temp , etc easy way to see any problem with the AF point CS Brian
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Oscar:
I mean, I understand the moon can cause messy gradients, but how would the moon be causing this mis-correction? *Plenty of examples there where any and all additional sky light sources causes different residual shading of dust motes (and also of sensor vignetting).
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From my experience, when Moon is over 70 percent and I’m shooting towards it (not opposite it) flats can’t correct my lights. No matter they are well exposed, long enough, etc. I think this is what Andrea is pointing out.
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thanks both! I didn't know that before; next broadband project I'll wait for the moon to go away I have sort of good news: my new 2hr addition of data seemed to clean up the image a bit here's the now 4hr stack, in the same section of the frame:  It's much fainter now, I guess the moon was more out of the way this time, also, perhaps, not touching the EFW also contributed to the cleanliness; I think I'm okay with this result. Now I need to focus on getting 1hr of R, G, B. Can't wait!
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