Overexposed Center on Narrowband Image

[seboman]

Hi all,

I am just getting into narrowband astrophotography and was hoping if someone could help me with a problem I ran into. Until now I took DSO images with an ASI533MC Pro and a William Optics ZenithStar 61/360 on a Skywatcher HEQ-5. All worked out pretty well, so I decided to jump into the deep water and get myself a QHY294M Pro with a QHY 7 position filter wheel, and a complete set of LRGB Ha Oiii Sii 36mm filters from Antlia. 

Yesterday, during my first imaging session I noticed that the LRGB filters work fine, but the narrowband filters (Ha and Sii in particular) show something that I would decribe as strong vignetting/overexposed center/washed out center. I made sure the filters are installed with the reflecting side towards the incoming light, but other than that I am not sure where to start troubleshooting. I attached a screenshot of a stretched single frame in N.I.N.A. with IC410 and IC405. I also tried to image the horsehead nebula and there barely anything is visible of the nebulosity. 

Has anyone see a similar behavior? Any suggestions or help is greatly appreciated. 

Many thanks!

[tooth_dr]

Could you take a flat frame to see if the problem is there.

[seboman]

I took a flat for each filter. Just as in the light exposures, the Ha and Sii flats show the same overexposed center. Interestingly, the Oiii filter flat does not show any sign of a circular pattern. 

I am not sure how to inperpret this test. Is the Oiii the wrong way around or the other two? Or is everything OK and the Oiii filter just behaves differently by default? 

[david_taurus83]

Try turning the 2 filters the other way round and see if the flats are different. Are they unmounted? They usually have an anti reflective coating on one side so its possible you have installed them backwards.

[wimvb]

Before you do anything else, calibrate your lights with darks and flats. I've read posts on CN, where this kind of flats worked perfectly fine. Make sure it's broke, before you try to fix it.

[seboman]

OK, so I took the light frames and calibrated them with the flats and darks, and the result looks awful. 😬 

Something is clearly wrong here. I could try turning the two filters around to see if that changes anything, but according to the instructions on the Antlia web site the more reflective side has to go towards the source of light. 

While doing the test I also noticed very prominent dark borders around the image - a wider one on the right side and a less wide at the bottom. I wonder where that comes from. They do not go fully away after stacking and there are even stars visible in that area. 

[wimvb]

Ok, something needs fixing.

What size filters do you have? If 31 mm unmounted, you may have reflections from the filters edge.

[seboman]

36mm unmounted

[wimvb]

3 minutes ago, seboman said:

36mm unmounted

Could also be edge reflections then. I have possible edge reflections in my rgb blue filter (31 mm optolong), and plan to make rings to cover the very edges of the filters.

[seboman]

Thanks for your help and tips. I will try adding a ring to cover the edges and see if that makes a difference. 

[tooth_dr]

12 hours ago, seboman said:

While doing the test I also noticed very prominent dark borders around the image - a wider one on the right side and a less wide at the bottom

Indeed these are quite visible in the Oiii flat. I get similar to these with my FF DSLR where light is reflecting on the edge of the mirror box.  I wonder is there something in the way that is causing reflections. 

[seboman]

Today, I got a set of 6nm Astronomik filters, Ha, Oiii, and Sii. All flats look just as I would expect them to look, no reflections, no overexposed center. Astronomiks have a 3mm black metal ring around 36mm unmounted filters. I guess my initial problem was caused by some reflections with the Antlia filters. These have a blackened edge, but maybe that is not enough. 

My Antlia LRGB filters show "normal" flats, but they all reflect the border of the sensor a little. I will try out filter masks eventually to see if I can stick to the Antlias or have to swap them out for Astronomiks. 

Edited December 25, 2020 by seboman

[seboman]

I managed to cut out black paper rings and place them as filter masks on top of the Antlia filters. The filter edges are now covered and the strange sensor reflections are gone. 😁

[Astropia NL]

I do have the 36mm unmounted version of them in a ZWO EFW 7*36mm without any problems. (Using a 294mm)

 

CS Bas