Coma Corrector spacings and narrowband

[lukebl]

Hi all,

Just wondered if anyone knew if the Coma Corrector separation distance needs to be changed if using Narrowband filters. I'm using the Baader coma corrector with my 250mm f/4.7 Newtonian and SXVF-H9, and have the CC set at the recommended 55mm from the CCD sensor. However, I am getting stars which aren't perfectly round, and wondered if the Narrowband filters (O-iii and Ha) might be changing the focal length or something, thus requiring the spacing to be different?

[steppenwolf]

The use of filters in the light path will extend the effective focal length - but I'd be surprised if this would be enough to take you outside the operating envelope of the coma corrector.

[michael.h.f.wilkinson]

As a rule of thumb, each mm of glass (2 or 3 in many filters) adds 0.5mm of effective optical path (due to the fact that glass has an index of refraction of 1.5). The effective optical should not be more than 1 or 1.5mm off, in that case.

[lukebl]

Thanks, gents. I think I'll add a couple of millimetres and see if that makes any difference.