8 replies to this topic

[TodRiley]

I have been using the above filters and now are starting to doubt the above claim as to their parafocality.  I would like to hear from other users as to their experience with these filters. 

[ngc2218]

I have been using the above filters and now are starting to doubt the above claim as to their parafocality.  I would like to hear from other users as to their experience with these filters. 

I did measure different focus points for my Astrodons, so they are not identical in focus, but close. 

So, when I image, to keep things simple, I gather data using one filter for a few days then I move to another filter and so on. Basically I don't switch filters during a night 

[Tapio]

My impression too is that there's always focus steps between parfocal broadband filters.
But does this matter in images - don't know.
And you can always use focus offsets.

[Helyis]

Parafocality is not only a matter of the filters, it apply to the entire optical train.

Extrem example, if you are using an achromat (i'm sure you aren't, that's for the example), the red and blue filters will obviously not be parafocal, because the scope is not. This is still true to with apochromat, there will almost always have a bit of focus offset between wavelength, meaning that best focus for R will not be the same as for B.

For reflectors, I think reducers and correctors can introduce this effect as well

[Rasfahan]

My Astrodon LRGB are parfocal in a 10“ f/8 RC with a .8x reducer at mediocre seeing. Temperature fluctuations mean I need to refocus about once an hour anyway. Just get an EAF, refocus on each filter change (or use focus offsets) and be content.

[pyrasanth]

My take on par-focal filters is that with the same set of filters if your in focus with a selected filter a change to the next filter in the set will put you close enough to focus for auto focus to fine tune- I expect nothing more than that.

 

I image an hour through each filter so I think it would be unrealistic for the next filter to put the optics in focus without a focusing tweak.

[kathyastro]

I use Astrodon filters and I have found them, both broadband and narrowband, to be as parfocal as I can tell.  I don't apply filter offsets in the focusing routine, and I am happy with my results.  Other defocusing effects, such as poor seeing, are much larger than any difference in focus between filters.

[ngc2218]

My Astrodon LRGB are parfocal in a 10“ f/8 RC with a .8x reducer at mediocre seeing. Temperature fluctuations mean I need to refocus about once an hour anyway. Just get an EAF, refocus on each filter change (or use focus offsets) and be content.

I think it also depends on the focuser resolution. The Optec Leo I use has 80 nm (nano meter) per step. I would imagine if the change in focus is smaller than the step size, you might conclude the filters are parafocal, when in reality they are not. Does this make sense?

[Rasfahan]

I think it also depends on the focuser resolution. The Optec Leo I use has 80 nm (nano meter) per step. I would imagine if the change in focus is smaller than the step size, you might conclude the filters are parafocal, when in reality they are not. Does this make sense?

No, that would mean your step size is too coarse to hit focus reliably.