Canon 200 mm USM F 2.8 With ZWO 7 nm Ha Filter - Is there a band pass shift issue - Forums

Topic starter

Feb 3, 2022 · Edited: Feb 3, 2022

I'm wondering if anyone has tried this combination from a light polluted back yard (Bortle 7). I took 2 hours of Ha data on both the Horse Head/M42 and the Rosette/Cone and was very underwhelmed with the results. Details that were clearly visible in similar exposures with my WO Z61 were just not there.

I had the camera attached to an ASI 6200 MM Pro and I was surprised because while back I got very good results with the same lens and an STL11000 and 12 nm Ha Filter.

Any insight much appreciated. I've read so many posts on band pass shift recently I just wondered if this could be the issue.

Many Thanks

John

Jérémie

Feb 3, 2022

I personally experienced some strong vignetage when I personally used narrowband filters with lenses with high apertures. And indeed, I read some tests that showed that most of the NB filters required light to arrive at an angle as much as possible parallel to the optical axis (and the larger aperture, the higher this angle on the borders of the image).
I tell you about that because you talk about band pass shift (and the vignetage is as far as I understand a consequence of that), but that being said, you talk about details that does not appear, wich is more related to the sharpness of your lense, isn’t it ? In this case, I think that most lenses have better sharpness when shutted bown a bit to f/4… except very good lenses.
Hope this helps a bit…

John Noble

Topic starter

Feb 3, 2022

Thanks Jeremie - there is vingetting but it corrects out easily with a flat frame and I don't think there is anything up with the lens as I've seen the details before with a different set up and these are good 200 mm lenses, focus was as good as one can get manually with a mask. So it was something else!

Thanks again

John

Jon Talbot

Feb 3, 2022

Thats an interesting result. The STL 11K is much less sensitive and has much lower QE than the 6200. I've done side by side tests with a scope and the difference is significant. I would expect the vignetting is the same since the size of the chip is the same. If it flatted out with the 11K it should do the same with the 6200.

I would expect the 12nm Ha filter to let quite a bit more light in vs the 7nm filter, which if using the same exposure time, the 12nm filter image would look like it has more signal.

I'm not to sure about the bandpass shift although I haven't used that lens and a 7nm Ha filter. I have a Canon 200mm f4 L lens with a Canon 6D and 12nm Ha clip in filter but I shoot at f5.6 and it seemed to work just fine. I just only get 1/4 resolution due to the pixel array.

Can you elaborate on the gain/offset settings you used on the 6200? I have gone to only imaging with the 6200 using Gain 100. Gain 0 is fine but I'd rather have the shorter exposure times and lower read noise with Gain 100.

Jon

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John Noble

Topic starter

Feb 3, 2022

Thanks for replying John - I use Gain 100 all the time with the ASI6200 no offset.

John

Jon Talbot

Feb 3, 2022

Hi John,

Ah, okay, gain 100 is it. The default offset should be set to 50. Not sure what acquisition software your using but an offset of 50 is the default for the camera. It's basically the bias level of 500. I suppose you could run it lower, say 20 which would give you a bias level of 200 (its a X10 offset). If yours is actually 0 you will have odd looking image. A friend with a 2600 somehow had offset 0 set in the Sky X and his images looked like they had virtually no signal. Once we changed it to the default of 50, life was good.

Not saying this is your issue but you mentioned you didn't use an offset. Maybe its set to the default but something to check in the fits header.

Jon

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