Your advice please - do I have a faulty filter

[seeker372011 (Narayan)]

Dear all

I purchased a 3 nm Antlia filter but suspect I have a bad copy; would appreciate any advice

I also purchased a 3 nm Optolong filter - and it works perfectly with the same telescope and camera ( Sharpstar Z4 and the ZWO ASI 533MM)

I have attached both flats- same telescope same camera ASIAIR auto flats - should be obvious I hope which is the one I am whingeing about- the first one is the Antlia Ha filter

Auto flats using ASIAIR but I had similar results when targeting 32000 ADU using NINA

Also have no problem using the same telescope and the Antlia triband filter- flat looks normal and the result after stacking is just fine ( obviously not the same camera but a 183 MM)

I wouldn’t worry about how the flat looks if it all calibrates out- but the stacked stretched image shows haze all around the centre of the image which cannot get rid of with background calibration or gradient exterminator or anything I try

I obviously reached out to Antlia and they responded saying this ( the Ha flat) is normal

Seems to me that’s not right but would appreciate if one of you knowledgeable people can please comment

Thanks in advance

PS Hate to say this but the OIII Optolong filter delivers tighter stars - don’t know if this pure selection effect - ie I had better seeing on the nights when I used the Optolong

All of which makes me wish I had bought the Optolong 3nm instead

[TrevorW]

Post this on Cloudy Nights you are more apt to get a quicker response, bigger forum with many knowledgable members

[ronson]

Hi Narayan,

Reading on the forums, my understanding is, it is expected to see weird patterns on narrowband flats, and as long as your lights calibrate fine you should be OK. If you are having troubles calibrating your Ha lights, try creating new master flat with different exposure and new flat darks.

Attached is how my Optolong 3nm SHO master flats looks like (ASI533MM Pro). Only STF applied. I recently purchased them, so haven't had a chance to test the calibration.

One of the reasons I chose Optolong, apart from being on sale making it significantly cheaper than Antlia, was reports that Antlia dismiss calls for suspected faulty filters saying it's all normal, while Optolong have in some cases been replacing suspected faulty filters. As Cuiv the lazy geek has showed in one of his videos, unless you have the equipment to test the filters if they pass the right wavelengths, it's a lottery what you get and almost impossible to prove something is wrong with the filter

Sorry, not much of a help, but if you suspect something is wrong, reach out to the retailer you bought the filter from and have a chat with them. They might be more understanding than Antlia.

PS: If you reach out to CN, please post the link here, should be interesting to see what people there share.

PS2: Found the CN link I read in the past with some similarly weird flats: https://www.cloudynights.com/topic/6...d-sii-filters/. Seems people suggest this is normal. Some suggest trying shorter exposure, different light source, and ensuring flats are taken with the same gain as the lights. Also, if unmounted, check the filters face the correct direction - clean reflective side to the sky/telescope.

[Paul Haese]

That certainly does not look normal at all. You should see any vignetting present and dust donuts but not that sort of gradients in a flats image with any filter.

If you bought the filter from an Australian retailer, simply use the words "not fit for purpose". The filter should be replaced or a refund given.

[Peter Ward]

Looks like spattering/inconsistent film thickness in one of the layers to me.
(some NB filters have dozens of layers...if just one is off specification then
you have to bin the production run)

This would also be evident through extended halos/scattering in brighter stars.

Based on what you have provided, I'd say it was a dud.