Filter wheel focus

[Fieldsy]

Hi all I was just thinking if u set up a imaging session with a motorised filter wheel should u need to refocus for each filter ? Or should it be close enough  that it doesn’t  matter ?.and as I’m at least a week away from clear nights I must do something to ease my boredom 🌧🌧🌧🥺

 

[sloz1664]

I've found you need to re-focus per filter to get the best out of your data. Some filter manufacturers state they are par-focal, but in reality they are not that good. I use SGPro and sometimes MaximDL and they both provide suitable offset  parameters to achieve par-focal data.

Steve

 

[ollypenrice]

You'll probably need to refocus. Good quality filters are pretty much parfocal but your optics probably won't be. Even an excellent 'Apo' isn't totally apochromatic. Other factors come into play as well. Fast optics have steep light cones and so have a shallow depth of field, intolerant of imperfect focus. Likewise, fine pixel scales are intolerant, too. So there is no easy universal answer. 

On our widefield rig, working at a forgiving 3.5 arcsecs per pixel, I do my focus in Luminance and then scroll round the filters without a refocus. It works fine.

On the high res rig I focus 'per filter' but also time the shoot so that I catch red and green at low elevation and both blue and luminance at the highest elevation possible for the better seeing.

Olly

[Adam J]

7 hours ago, Fieldsy said:

Hi all I was just thinking if u set up a imaging session with a motorised filter wheel should u need to refocus for each filter ? Or should it be close enough  that it doesn’t  matter ?.and as I’m at least a week away from clear nights I must do something to ease my boredom 🌧🌧🌧🥺

 

Your using a 130pds reflector you won't need to refocus. The other comments above assume your using a refractor. Just focus on luminance and your good as all your filters are from baader. 

Edited June 11, 2019 by Adam J

[ollypenrice]

On 11/06/2019 at 20:06, Adam J said:

Your using a 130pds reflector you won't need to refocus. The other comments above assume your using a refractor. Just focus on luminance and your good as all your filters are from baader. 

Coma correctors use refracting lenses, however.

Olly

[MilwaukeeLion]

If imaging planets will need to focus each filter.  My red and blue filter are pretty close, slight adjustment but green needs bit more.

[Adam J]

12 hours ago, ollypenrice said:

Coma correctors use refracting lenses, however.

Olly

I see no change in focus at all with my TS max field coma corrector and Baader LRGB filters. You tend to get lateral chromatic aberration with cheap two element coma correctors but only noticeable on larger chips. However, I dont think that Fieldsy is using a coma corrector as his sensor is a imx178 thats very small.

[Fieldsy]

On 15/06/2019 at 09:42, Adam J said:

I see no change in focus at all with my TS max field coma corrector and Baader LRGB filters. You tend to get lateral chromatic aberration with cheap two element coma correctors but only noticeable on larger chips. However, I dont think that Fieldsy is using a coma corrector as his sensor is a imx178 thats very small.

🤨 it’s not the size it’s what u do with it 🤣