Problems with my Narrowband Flats

[CarlightExpress]

Hi All

I am having fierce problems with my Narrowband Flats, moreso with OIII and SII, I am using the Baader 36mm 7nm filters, my LRGB and HA flats all look fine

Luminance

Red

Green

Blue

HA

Now here's the two weird ones:

OIII

I've checked the filters are installed the correct way in the EFW, just cannot understand why they are an odd shape, it is really affecting my images!!!

Simon

[MiguelMJr]

I would try moving the filters to different filter position on the filter wheel and making new flats. If the problem remains on the same filter positions that may point to a filter wheel problem. If the problem follows the filters then the problem has something to do with your filters. There are other ways to accomplish the same process of elimination, feel free to do it another way, the important thing is to pin point what is causing the problem and then a solution should become obvious.

Well anyways, that is what I would try. Please keep us informd.

Miguel

[MiguelMJr]

To me, the Red looks a little bit off too.

[lensman57]

I would also check the seating of the filters in the wheel so they are not tilted.

A.G

[swag72]

As long as your NB filters are clean and dust free then have you tried using the same Ha flat for all filters? I have done this in the past and it worked well. I know that there are people that use the one set of luminance flats for all their RGB calibration as well - No reason why the same can't be done with narrowband.

[CarlightExpress]

I find that even with my images, there's the same level of lightness around the areas of the filter, just I would have expected a lot more in the way of uniformity with regards to the light, not "over to the right" with the OIII

[zakky2k]

Are your filters parfocal? If the SII and OIII are thicker could that contribute to the vignetting?

It also looked like they were significantly dimmer, which lead me to wondering what light source you're using when taking the flats, and how do you ensure even illumination?

Just thinking if the "problem" filters might be blocking that frequency causing the uneven illumination. If it's an artificial source try outside at dusk/dawn and vice versa... 7nm doesn't let much light through but clear sky would be enough while Its still bright enough or with longer exposure?

Word has it Santa's delivering my first OIII and SII filters at Christmas, so I'm keen to figure out hurdles like this!

[CarlightExpress]

For my flats I am using an EL Panel, the same panel for all the filters, as I use Sequence Generator Pro to calculate the correct times to acheive a specific ADU, I checked all the filters with a Micrometer and they are all the same thickness so I know it is not that, I have noticed on my Light frames also that there is a patch in the same area, but for some strange reason I cannot seem to elliminate it throguh flats which leads me to believe that my flats are not the right ADU (29000 for the ATIK 383L+, also tried 22000)

[zakky2k]

Hmm, did you try contacting the manufacturer with the sample flats to get their opinion, just in case it is a defect in the filters?

I would still try capturing the flats with a natural source to ensure the filters aren't interfering with the specific light emitted from your panel.

There is some similarity between the LRGB illumination/unevenness (i.e. top left corner). Did you try rotating your light box and observing the resulting flats? If any unevenness also rotates then there's an issue with the panel.

Again, taking some flats outside using the clear blue sky (with simple cloth filter if too bright)  would eliminate a lot of variables...

[carl20171]

In the light frames that patch in the same area, is that only with the NB filters or all?

[CarlightExpress]

The light patch only affects the SII and OIII filter, so on my narrowband images, there's a larger "Blue" section to the right of my images

[CarlightExpress]

I should have said, on my OIII imae there is a larger "Blue" section to the right of the image

[swag72]

I had an uneven gradient on my images - I tried everything to eliminate it and still it persisted - It was only in the OIII. 

I can't say for sure what it was - Never got to the bottom of it - Changed camera's (manufacturer) and the gradient is no longer there. I always believed that it was a camera issue, perhaps something to do with the sensor glass that the manufacturer uses, but that was never proved or disproved. 

[ollypenrice]

Like Sara, one flat fits all for me. I use L for everything and it works fine. If I divide a red flat by an L flat I get a flat result which confirms it. Absolute parfocality isn't required in the real world. After all, you don't shoot a new set of flats each time you tweak focus on a night's run (do you??  )

You might be running into difficulties with internal reflections. Some matt black internal finishes are, it turns out, quite reflective in some wavelengths. Paints using dyes are the culprits. High temperature barbecue or stove paints use pigments instead and are to be preferred. This paper tells all; https://www.sbig.com/about-us/blog/flat-fields-the-ugly-truth/

Olly

[lensman57]

Like Sara, one flat fits all for me. I use L for everything and it works fine. If I divide a red flat by an L flat I get a flat result which confirms it. Absolute parfocality isn't required in the real world. After all, you don't shoot a new set of flats each time you tweak focus on a night's run (do you??  )

You might be running into difficulties with internal reflections. Some matt black internal finishes are, it turns out, quite reflective in some wavelengths. Paints using dyes are the culprits. High temperature barbecue or stove paints use pigments instead and are to be preferred. This paper tells all; https://www.sbig.com/about-us/blog/flat-fields-the-ugly-truth/

Olly

On the rare occasion that I have used Ha , Oiii and Sii I too just used an Ha flat to calibrate the subs with the caveat that Pixinsight refused to load the flat for Ha with Oiii and Sii. Something to do with the fits header I believe. In the end I used the good old dumb DSS and it all worked fine. Of course the filters have to be spotless. I wonder if AstroArt 5 is as fussy as Pixinsight.

Regards,

A.G

[pmesquita]

Well, although I'm not as experienced as all of you I too have a problem with the OIII vignetting and the respective flats. and as some of you know, I'm working with the Luis Campos monoMod monochrome and TEC cooled 450d camera.

My 1st Bicolor image (HA + OIII) and my 1st Tricolor (HA + OIII + SII) images done a few days ago - M1 and the rosette - had issues with the vignetting shower on the OIII session. I've tried the flats underexposed, normal and ovewrexposed  at 1/10s, 1/20s and 1/40s (Histogram lump lower, at level and higher)  than the OIII lights lump. and on Pixinsight I was unable to remove the vignetting completely.

I've done over 40 sessions with the monochrome and RGB  cooled 450d cameras in HA and RGB and no vignetting issues. I did one session only with the SII and no vignetting. I did two sessions with the OIII...and vignetting.

Something has to be wrong!

cheers

Paulo

I talked to Luis Campos about my problem...and he complains of the same issue. I'm sure there is some issue with the Baader OIII.

btw, I use the Gerd Newman Aurora Flats panel and Luis, I believe, uses an entirely diferent flats gear.