IN or OUT moves to focus with or without a filter

Voyager
1

Just purchased a new Askar FRA600 scope and trying to set everything up. I am using it with ZWO ASI2600MC cam and ZWO EAF focuser.
I run the first vcurve wizard and could not finish it due to errors. To make it work I ended up with the following HFD parameters: 8, 20, 35; MIN Flux: 200; MAX Flux: 1000. I built 5 vcurves for the best precision.

Now I am able to focus without any problem with dual band filter as well as without it. My focus position without a filter is 3877 and with filter it is 4123.

However, when I am imaging without a filter at 3877 position and switch to a filter, then I have to move the focuser out to 4100 so it could focus on a star (AcquireStar). And vice versa, if I am imaging with filter and want to image without it, then I have to move the focuser in to 3900. Without those preliminary moves IN or OUT I am getting HFD errors and the focuser cannot focus.

Is it a normal behavior? Previously with my Newtonian scope I didn’t have such an issue and I could focus straight with or without a filter. Am I doing something wrong?

Thank you!

2

It is pretty normal behaviour, though it can depend on your filters (How thick they are) the typical change of focus position is about one third of the thickness of the filter glass, so if your filter is 3mm thick it would be about 1mm, which might be enough to give you trouble.

Can you get a clear filter (Or maybe a Luminance) in the same thickness as your dual band? I did some narrowband imaging my with OSC camera as I bought equipment to switch to mono imaging (Filter wheel and filters for my 2600MC before I finally bought a 2600MM and swapped the filter wheel over to that camera) and I ended up putting a clear filter in the filter wheel of the same thickness to use when imaging for colour, so that OSC and narrowband were close enough to parfocal to avoid the problem you are having.

2 Likes
3

Thank you! Sounds like a great idea with a clear glass filter :grinning:.