Ha, S2 filter flats have rings

k2k2e6

My ZWO Ha and S2 filters are producing flats which have some sort of ring. L, R, G, B are OK. O3 filter has some sort of pattern. But I am mostly interested in Ha and S2 flats having this bright ring.

The telescope stuff:

Televue 102, 0.8 focal reducer, ZWO OAG, ZWO filter wheel, ASI1600Mono, LED light panel for flats.

Here is the attached image - L flat and Ha flat stretched to same values using Pixinsight.

ha_lum.JPG

k2k2e6

Guys

Anyone has any ideas and workarounds for this problem?

wvreeven

I had a similar issue with my Ha filter and stopped using an artificial light source for taking the flats. I think the ring arises from excess IR radiation from the artificial light source. So instead I point the telescope at a white ceiling during day time. That solved it for me.

HTH, Wouter

k2k2e6

Thanks Wouter!

I have the problem with Ha lights as well - so it cannot be IR source from the LED flat frame.

katherine.tsai

You can also try calibrate it with flats.

k2k2e6

Thanks Katherine. It does not calibrate out using flats. I am using Pixinsight. I was hoping it would.

It seems like I need to buy better filters if someone cannot come up a with a solution for me.

There is extensive discussion on CloudyNights:

https://www.cloudynights.com/topic/605371-comparing-zwo-vs-astrodon-36mm-filters-a-controlled-test/

https://www.cloudynights.com/topic/600525-flats-issues-with-zwo-filters-and-un-correctable-ring-artifacts/

I wonder if these apply to my experience.

katherine.tsai

It maybe reflections, you can add the filter masks.

k2k2e6

I read more threads on Cloudy Nights.

New filters/filter wheels come with some sort of Filter Masks. When I purchased my set of filters and ZWO filter wheel, they did not.

Will it be possible for ZWO to ship me the filter masks? My filter wheel is 36mm filter wheel with 36mm filters.

k2k2e6

I went ahead and ordered filter masks from ZWO site. It seems like new filter wheels come with these filter masks - mine didn't. It will take 3-4 weeks to arrive. Didn't want to pay $20 for $15 filter masks.

I will report if filter masks solve the problem.

I saw another thread that ZWO has newer version of the Ha/S2 filters. But it will be too much for me to replace my first-gen ones - I cannot afford new ones. Hopefully, $15 fixes the problem.

katherine.tsai

Hope it can slove the problem, will wait for your message.:)