Mystery flat vignetting issue - any ideas?

[buzz]

I spent a cloudy week cleaning sensors and re-doing all my flats with combinations of WO FLT132/110 and WO Field Flattener IV and 68 versions.

I'm using a SX H18 (KAF8300) sensor with the StarlightXpress filter wheel, OAG and Baader LRGB Ha filters

I'm getting what appears to be corner vignetting on the Ha filter only - with the WO FF IV and with both scopes but it does not show up on the LRGB flats at all. Any ideas folks? The files are a Ha and Lum averaged flat (with the contrast turned all the way up)

This is a bit of a head scratcher - since all I did was revolve the filter wheel between sets of flats.

[Dave Smith]

Hi Chris, I see you're only 10 miles from me.

Is the Ha filter of the same make, i.e. does the filter holder look the same? Same diam, thickness etc. If the flats do their job, presumably it doesn't show on the calibrated lights, a bit of a puzzle none the less. Is the filter the same distance away from the sensor?

Dave

[buzz]

Hi Dave - yes - these are all identical Baader 2 inch filters, bought at the same time from IKI. I'm wondering if it is to do with narrow band filters in general and their angle of acceptance? The vignetting is minimal and should normalise out but it does nag at the back of the mind.

[martin_h]

What were you focused on? that filter edge looks quite sharp to me.

[buzz]

I'm focused on infinity, using a flat field panel resting on the lens hood. The thing that is the mystery is the difference between the Ha filter and the other four. Puzzling.

Regards Chris

[Mark72]

Chris,

May be your Ha flat is underexposed? What is exposure time compared with L filter?

Mark

[buzz]

It shouldn't be exposure. All my histograms have the bump roughly in the middle. To keep sub 10s exposures I use ND filter (Lee lighting acetates) over the flat field panel with the LRGB exposures but typically don't need them with Ha exposures.

I'm going to take out the filters and examine them more closely.

The Ha filter is quite reflective, could it be a reflection from the flat sensor window back and forth to the filter wheel?

[ollypenrice]

I think you might have hit it with the angle of acceptance theory. That angle will define a circle on the filter. However the WO is not particularly fast so I would have expected no issues there. I use a Baader Ha at F3.9 so that light cone has a steeper angle.

I suppose the other thing might be the ability of the flattener to handle the Ha wavelength around its outer part. Seems odd, though.

It's a remarkably hard edge to the vignetting.

Strange.

Olly