Taking flats with a filter wheel

[mike1485]

I have been thinking about getting a filter wheel to automate my image capture routine but  one aspect concerns me. If I use just two filters over a night I can see that the flats for the first filter can be taken before the capture sequence starts and for the second filter can be taken at the end. But if three or more filters are used surely i would  need to intervene and take flats when the filter change takes place?  This would seem to rather negate the automation. If I leave it and take flats at the end doesn’t the motion of the filter wheel (potentially dislodging/moving dust specks) plus any slightly different alignment potentially cause an issue?  

[michael.h.f.wilkinson]

I have taken flats after a session with at least three filters and not seen dust being dislodged in my flats. Not ideal perhaps, but to date it has worked

[teoria_del_big_bang]

I have always taken my flats after a session for each filter used, sometimes after using 7 filters.

If you were to take in every possible instance of dust changing places then yes in theory you are correct about the precise way would be to take flats before every move of the FW and if you had an automated Flip Flat then you could do this as you could advance the Flip-Flat after every  filter use and take the flats but really in UK we get little enough clear dark skies so personally I would not waste my imaging time doing this even with an automated Flip-Flat.

Some people do not even take flats for all filters and use same flat for all.

I think the filter wheel moves pretty slowly and generally within a session very little dust actually moves.

Steve

[mike1485]

18 minutes ago, michael.h.f.wilkinson said:

I have taken flats after a session with at least three filters and not seen dust being dislodged in my flats. Not ideal perhaps, but to date it has worked

 

14 minutes ago, teoria_del_big_bang said:

I have always taken my flats after a session for each filter used, sometimes after using 7 filters.

If you were to take in every possible instance of dust changing places then yes in theory you are correct about the precise way would be to take flats before every move of the FW and if you had an automated Flip Flat then you could do this as you could advance the Flip-Flat after every  filter use and take the flats but really in UK we get little enough clear dark skies so personally I would not waste my imaging time doing this even with an automated Flip-Flat.

Some people do not even take flats for all filters and use same flat for all.

I think the filter wheel moves pretty slowly and generally within a session very little dust actually moves.

Steve

Thank you both. Experience from the field sounds reassuring. I guess I am over-thinking it. 

[teoria_del_big_bang]

21 minutes ago, mike1485 said:

 

Thank you both. Experience from the field sounds reassuring. I guess I am over-thinking it. 

That aspect of the hobby never stops 🙂 

Steve

[geeklee]

2 hours ago, mike1485 said:

This would seem to rather negate the automation. If I leave it and take flats at the end doesn’t the motion of the filter wheel (potentially dislodging/moving dust specks) plus any slightly different alignment potentially cause an issue?  

I'm always waiting to get caught out with this, but do something similar to @michael.h.f.wilkinson and @teoria_del_big_bang.  Either I take the flats the day after (with the scope and associated image train still attached and sitting somewhere) or I even take them in advance being careful at all times with the rig!  I've also let software create sky flats for filters in the dawn after a session - but I think that long a session only happens a few times a year in the UK!