|Parfocal Filters and Parfocal Systems
|People worry whether their filters are parfocal. But just having parfocal filters doesn't mean you don't need to refocus or determine offsets for
each filter because your entire system (e.g. OTA + flattener + filters) may well not be parfocal.
Below are some curves for the Tak TOA 130. First look at Figure 6 (first panel, upper left). Note that not only is the focuser position for each
wavelength uniform from 0 to 65mm off-axis, but the curves for each wavelength nearly superimpose on the curves for the other wavelengths. (I
get all those within a total range of about 0.02mm or +/- 0.01mm). This is a very well corrected scope.
Now look at the spot diagram Fig 8. The spot smears off axis which will affect your image.
But now look at Fig 14, the color curves with the 67 flattener. The curve for each wavelength is still pretty vertical, so you will be able to focus the
light passing through that filter at one focus position. But there is now a perceptible shift between the focus position for, say the 486nm light and
the 656 nm light.
So even if you have perfectly parfocal filters the optics themselves are now not parfocal for blue vs red light, and are not the same as for this
same scope without the reducer. Almost for sure every setup of OTA +/- reducer +/- flattener you use will be a little different from the others, so
you can't use the same set of filter offset data for every system.
Nor is it that critical that your filters themselves be perfectly parfocal (provided they are close enough you can get to focus with each which is
unlikely to be a problem). Since the entire system won't be parfocal you will have to refocus with each filter change or measure the (different)
offsets for each of your setups independently.