Filters

Filters and Filter Offsets

Filters used in an imaging train may not be parfocal - that is, they may not focus at the same position. You should only need to generate a set of Vcurves once in any color (even clear / no filter) and  then use the Vcurves to focus with any filter. If you are using  narrow band fillers then you will have to use bright stars with long exposure times to focus. An alternative is to measure the required filter offsets from a base filter (perhaps Clear) then focus with this filter and apply the required offsets. This can be accomplished with a simple script, MaxIm or form automated imaging software (ACP, CCDAutoPilot, CCDCommander, etc.). Here is my table:

[Focus Offset]
Slot=E,-775
Slot=C,0 <== base filter
Slot=U,51
Slot=B,69
Slot=V,12
Slot=R,8
Slot=I,-38

If imaging in C (0) and rotate to R (+8) I move the focuser +8 steps
I imaging in R and rotate to I (-38) I move the focuser 0-8 (back to C) -38 = -46 steps total

To measure the required offsets:

a)Pick a night of good seeing in which the temperature has stabilized - you do not want to perform this procedure when there is significant temperature change occurring since the focus positions will be a moving target

b)Open FocusMax and select the Features Tab

c)Enable Focus Convergence

d)Set Steps = 1 and Samples between 5 and 10

e)Create a table that lists all of the filters you will be using and add a columns for time, position, temperature and offset (steps)

f)Start with your base filter (say C) and perform an autofocus run

g)Record the time, position and temperature

h)Move to the next filter and repeat step g)

i)If the temperature has changed, cycle back to the base filter and repeat f) and g)

j)Set you base filter as 0 (zero) and calculate the difference in position for each filter relative to this filter.

Example:

C = 1500, R = 1508 => offset is +8 steps

k)If you experience a change in temperature, then use the position from the repeated base filter for the offset calculations

Papers / presentations:

Get Focused paper by Don Goldman and Barry Medal

New Critical Focus Zone by Dr. Winter