Flat issues .. again

[Astroscot2]

New set up and I continue to have issues with flat frames.

Ive attached most recent image taken through Esprit 120, SXV694 camera and CLS filter.  140 mins, Darks, Flats, Bias, Dark flat calibrated.

AA5 used to gather frames and calibrate, Ive also tried DSS for calibration with same results.

Ive tried various calibration frame combinations .

lights - Darks / (flats / bias)

lights - darks / (flats / flat darks)

lights - bias / (flats)

Ive attached a copy of the light calibrated showing the dust bunny issue and also a copy of the flat frame both stretched.

Any thought?

 

[vlaiv]

It would help if you could tell what calibration method did you use for attached calibrated light frame.

I think I see couple of issues due to different things.

Could you post light calibrated by (light - dark) / (flat - dark flat) method - if above is not done like that (and I suspect it was not).

One thing that can make light / dark outlines on dust shadows is related to filter wheel repeatability. If filter wheel does not position filters at exactly the same place between lights and flats (and this implies that you took your lights, changed filters in between and took your flats later by "returning" to particular filter) - dust shadow is going to move a bit. Light will have over correction in place where there is dust shadow in flats but not in lights, under correction in places where there is shadow in lights and no shadow in flats - and normal calibration in between. If you've used inappropriate calibration method you can see over/under calibration in all parts but not that pronounced - this is why I asked about method of calibration.

There could be other possibility - it is not filter that does not return to proper position - it's the dust particles that move. This is highly unlikely but I suspect could hypothetically happen if filter wheel "snapped" to new position or something like that.

[coatesg]

I'd say this is the filter wheel not being in the same position for the flats as for the lights - the light and dark edges to the dust bunnies at the bottom of the frame is symptomatic. The only other thing (but seems very odd) would be if the light had been registered (thus shifting position) before reduction, but not sure why this would ever be the case...

[Astroscot2]

Hello and thanks for your replies.

The calibrated light frame is light - dark / flat - dark flat if I recall, I will check again tonight and confirm,   You are right in saying that I took my flats the following day, The filter wheel spins on start up so I also suspected the wheel was not placing the filter in the correct position, the only slight doubt I have with this theory is the small dust spots at the edge of the frame, these spots I suspect are on the chip its self and not the filter and should calibrate out ?

I will check and post new calibrated light frame as mentioned above, really want to get to the bottom of this, I'm hoping the issue is mechanical.

Thanks

Mark

[vlaiv]

38 minutes ago, Astroscot2 said:

Hello and thanks for your replies.

The calibrated light frame is light - dark / flat - dark flat if I recall, I will check again tonight and confirm,   You are right in saying that I took my flats the following day, The filter wheel spins on start up so I also suspected the wheel was not placing the filter in the correct position, the only slight doubt I have with this theory is the small dust spots at the edge of the frame, these spots I suspect are on the chip its self and not the filter and should calibrate out ?

I will check and post new calibrated light frame as mentioned above, really want to get to the bottom of this, I'm hoping the issue is mechanical.

Thanks

Mark

Yes, you are right, those two worry me as well. Filter position is not explanation for these two.

There are several smaller shadows - probably sensor cover window dust. Some of them calibrate out just fine (close to left edge), while other two have the same "signature" as larger ones.

This is why I mentioned "dust movement". I had this once - dust particle settled on filter between lights and flats, and I took my flats right after lights. After I finished shooting lights - I just parked scope to home and put flat box on it and proceed to take flats. Somehow one dust particle managed to find its way to filter. I setup every time (no obsy or pier, work from my back yard) and also work with filter drawer, so that is my usual routine - as soon as I finish lights, first thing I do is take flats for that filter.

[Astroscot2]

I have a permanent set up so would rather work from a library of flats than take new flats for every filter,  I may have to experiment with this by taking a set of flats straight after some lights then reset the wheel and take another set, compare both, may be the only way to rule out the filter not being placed correctly by the wheel.

Mark

[Star101]

I too have a permanent setup.

When I first setup my filters, I had similar issue. I have the Atik EFW3 with 2" filter holes. I use 1.25" filters in 1.25" to 2" adapters. The filters would rotate slightly when the wheel turned therefore moving the dust motes and making my flats unusable. My "fix" for this was to make sure the filter's were snug/tight in their adapters and put black tape across the adapters, when inserted in the filter wheel, to hold them in place to stop filter rotation. If that makes sense. So far so good

[Astroscot2]

Ive taken some additional lights last night via the red filter, I followed this up with a set of flats immediately after the lights and another set of flats after spinning the filters a few times, will process and confirm the outcome tonight.

Mark

[Astroscot2]

Well ive calibrated last nights short test image  Light - Dark / Flat-Bias and it seems to be acceptable. Ive re-done the original image with the same process but no luck. The images below are using flats taken straight after the lights and flats taken after the filter was spun a few times on the wheel, I don't think the issue is mechanical?

The original image was taken through a CLS filter, could there be an issue with the filter itself?

Mark

[ollypenrice]

If it's any help I'll give you my exact AA flats routine which I find reliable.

1) Load flats into the images section of the stacking page and a master bias into the darks for images. (Of course I'm lying to the program in doing so but it doesn't seem to mind!)

2) Set page 2 to average combine the flats without alignment, run it and save the output as Master Flat.

3) Reset the stacking page and load it up conventionally with master flat in the flats section, darks for flats empty since the master is calibrated, lights and darks for lights as per normal.

(Actually I no longer use darks myself but that has nothing to do with your issue.)

Olly

[Astroscot2]

Thanks Olly.

Im going to re-shoot the Lum and see what happens.

FYI , I still have the following link, that I refer to occasionally.   I do wonder if there is an issue wth the CLS filter, loose with in its cell? I dunno, Its causing problems.