flats problem - advice

[martin_h]

I have a problem, I thought my flats were miss aligned with my light frames and leaving artifacts, so I devised a test last night, I shot 6 flats, rotated the filter wheel  1 full revolution and came back to the same filter(lum) and shot 6 more flats.

The first set of images I treated as true flats and made a master which I then used to calibrate the other six images, the theory being that if all was well and true alignment then I should end up with a perfect smooth final image.

Well this is the mess I'm left with( and the reason for the test in the first place)..

It seems that there is alignment because the small scale dist bunnies all disapear (good!) but what I'm left with is this large scale mess.................Master flat on the left, finished article is on the right.

Ideas please, because every time I image I end up with this.

 

[Robd]

Very odd. The large rings are just visible in the master flat. Is there any sign of them in either set of individual images? 

Rob 

[martin_h]

individual sub..............

 

[vlaiv]

Filter wheel is not accurate enough to position filters in the same exact location.

Whatever dust is there on camera or other surfaces - it gets calibrated out. Dust on filters does not calibrate out due to slight shift in position - this is why you get "beveled" dust shadows in calibrated flat.

You get beveled appearance because one side gets over correction (brighter than it should be) then proper correction then (proper gray value) then under correction (darker than it should be). You will notice that "bright" and "dark" side are always in same direction (shift of filter) and perpendicular to this there is thin section of shadow.

Some time ago Rodd had issue with ZWO filter wheel - same problem, and I did this "simulation" to show effect (in your image there is also central obstruction in dust shadow so shadow looks like doughnut):

He solved problem with ZWO filter wheel by using some sort of utility that "flashes" / "resets" something - if you have this filter wheel, lookup ZWO download section for this utility.

 

[Robd]

Large rings are again just visible, is this from set that created the master or second set? 

Vlaiv has just beaten me to it, the filter is slightly out of position on second set compared to first. 

The large rings are on the filter whereas the smaller corrected rings are on the sensor. The latter don't move and are therfore corrected. The former compound the problem since the aberrations are slightly offset. 

Very careful cleaning of filters may help. However if filter is contributing to the vignetting then incorrect application will still happen. 

As a OSC user I, fortunately, don't have this problem. 

Rob 

Edited May 3, 2019 by Robd
Wrong name

[martin_h]

Many thanks, unfortunately it is a Brightstar 7x2" which is no longer made.

[Laurin Dave]

Maybe try swapping out the usb cables..  bad cables have been known to cause this issue.  

[carastro]

Quote

Filter wheel is not accurate enough to position filters in the same exact location.

This was my first reaction.  So therefore the dust bunnies are not ending up in exactly the same place.  Must say I have never seen this before though so grabbing at straws.  

I have however seen the 3D dust bunny in Vlaiv's bottom left examples, but I got that years ago with a DSLR so no filters.  ??????????

Carole 

Edited May 2, 2019 by carastro

[steppenwolf]

1 hour ago, vlaiv said:

Whatever dust is there on camera or other surfaces - it gets calibrated out. Dust on filters does not calibrate out due to slight shift in position - this is why you get "beveled" dust shadows in calibrated flat.

You get beveled appearance because one side gets over correction (brighter than it should be) then proper correction then (proper gray value) then under correction (darker than it should be). You will notice that "bright" and "dark" side are always in same direction (shift of filter) and perpendicular to this there is thin section of shadow.

This is a really good explanation of what we are seeing here! ? Does this filter wheel only rotate in one direction or two (i.e. 'shortest route')?

[martin_h]

42 minutes ago, steppenwolf said:

This is a really good explanation of what we are seeing here! ? Does this filter wheel only rotate in one direction or two (i.e. 'shortest route')?

One direction only.

[martin_h]

So......... who wants to buy a slightly secondhand filter wheel? ?

[martin_h]

New filter wheel ordered - problem sorted

[ollypenrice]

Sometimes I wish I could just have a manual filterwheel...

Olly

[tooth_dr]

I'm the new recipient of this wheel.  I did find the motor was slipping so I adjusted it but it doesn't appear to have corrected it.  I may remove the position sensor and replace it to see if that helps.  I think I'll try my other EFW to see if it is completely correcting these faults too.

I shot 10 flats tonight, rotated the wheel 1 full turn anticlockwise and took another 10 flats, then rotated it 1 full turn clockwise and took another 10 flats.

 

[martin_h]

It was worth a shot, nothing ventured etc.

[tooth_dr]

5 hours ago, martin_h said:

It was worth a shot, nothing ventured etc.

I’ll give it another few goes. It’s a good time of year to try these things out 👍🏻

[martin_h]

If at first you don't succeed, a bigger hammer's what you need!