Hi
I got a 2nd hand Orion extra narrowband imaging Ha filter and I am getting some really weird shapes of stars. See, for example, the attached crop photos for 120s and 30s exposures + single 120s uncropped exposure of M42 @ ISO1600. This is when focused as good as it gets on a brightest star using live-view on a laptop.
The same happens with two DSLRs models, Canon 450D and 1200D, both fully IR modded and not debayered (I am aware of the disadvantages).
- Stars change shape across the image.
- When slightly defocused the brightest star has a crescent moon shape and a complicated dual-lobe shape when slightly defocused on the other side of the focus.
- Visually the filter looks perfect.
- Combining the Ha filter with Orion luminance IR-cut filter gives the same result so the Ha filter is not letting through the FIR.
- Stars look round without any filters or even with a Baader SII filter which has a relatively similar wavelength.
- Field flattener does not seem to make a difference (the attached images are taken with the flattener).
Scope is ED80. The filter was sticky-taped to the front of the flattener - would tilting of the filter cause this? I know that sticky-tape is not the best but the filter looked very well placed and attached and also this was just a test, not a permanent solution.
Any ideas?
I saw the same effect with filter taken on/off multiple times over several nights.
Thanks in advance
Luka
edit: Corrected filter name, it is "extra narrowband" and not "ultra narrowband"