Robin,
would it be a lot of hassle to add a "focus position" field to the internal manual filter wheel?
- I would envisage using this to store a ball-park focus position for a given filter, so that you've got a starting point for refocus efforts...
Cheers,
Carl
Suggested addition to internal "Manual" filter wheel
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Re: Suggested addition to internal "Manual" filter wheel
Hi Carl,
for consistency with other focusers (ASCOM), it would be easiest to add a *focus offset* to each filter rather than a focus position. That means that if you set a focus offset of 40 for red and 15 for blue, a move from red to blue will move the focuser by -25 and +25 if you go back from blue to red.
If the offsets are set correctly then the focus should stay 'good' during filter changes.
cheers,
Robin
for consistency with other focusers (ASCOM), it would be easiest to add a *focus offset* to each filter rather than a focus position. That means that if you set a focus offset of 40 for red and 15 for blue, a move from red to blue will move the focuser by -25 and +25 if you go back from blue to red.
If the offsets are set correctly then the focus should stay 'good' during filter changes.
cheers,
Robin
Re: Suggested addition to internal "Manual" filter wheel
Hmm, so designed for relative movement focus systems, but I suppose just as easily applied to an absolute case: is this always considered as an offset from the “current focus position”, or from some arbitrary, but pre-defined point, as your example would seem to imply? - trying to understand how this works during manual focuser moves that don’t involve a filter change, for example…
If this is an offset from some arbitrary point, is it constrained to a limited range by its data type (16-bit signed, at a guess)? - I had to find a focuser system that could cope with more than 64k steps from zero, otherwise, as was previously the case, I just had to define an arbitrary starting point for my “focus start position”, such that it was within the 16 bit range covered by most focuser drivers, rather than from the zero position of the scope.
Just trying to better understand how it works in practice. Thanks.
Carl
If this is an offset from some arbitrary point, is it constrained to a limited range by its data type (16-bit signed, at a guess)? - I had to find a focuser system that could cope with more than 64k steps from zero, otherwise, as was previously the case, I just had to define an arbitrary starting point for my “focus start position”, such that it was within the 16 bit range covered by most focuser drivers, rather than from the zero position of the scope.
Just trying to better understand how it works in practice. Thanks.
Carl
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Re: Suggested addition to internal "Manual" filter wheel
Hi Carl,
in theory the ASCOM filter focus offsets are 32 bit signed integers, so can hold large enough values to deal with any situation. Anyway, they only have to hold the offset from a default position, so 16 bit would be fine (Since the ASCOM driver for the wheel itself has to implement these offsets, it's possible that some wheels may not allow the full 32 bit range).
On the ASCOM focus position side of things, the specification says that focuser positions should be non-negative whole numbers, ranging from zero up to a maximum specified by the focuser ASCOM driver. Unfortunately some drivers allow for negative positions, as does some software. SharpCap sticks by the spec and only allows positive positions.
Now, for the focus offset values, the way I would use them is like this...
Find focus with no filter and record the value - say 12345
Now, without other changes, find focus with each filter in place - you will get a list of positions like
red : 12368
blue : 12324
green : 12340
Then you set the offset for each filter to the difference between the position for that filter and the 'no filter' reference of 12345 - ie
red : 23
blue : -21
green : -5
Set the offset for 'no filter/luminance' to zero.
Now everything is set to go. On another night you focus with no filter and the position is 12330 (different temperature maybe). When you switch to the red filter with the offsets set then SharpCap will move the focuser by +23 to 12355, which should be good focus for the red filter assuming that the *differences* in positions for the different filters are constant.
cheers,
Robin
in theory the ASCOM filter focus offsets are 32 bit signed integers, so can hold large enough values to deal with any situation. Anyway, they only have to hold the offset from a default position, so 16 bit would be fine (Since the ASCOM driver for the wheel itself has to implement these offsets, it's possible that some wheels may not allow the full 32 bit range).
On the ASCOM focus position side of things, the specification says that focuser positions should be non-negative whole numbers, ranging from zero up to a maximum specified by the focuser ASCOM driver. Unfortunately some drivers allow for negative positions, as does some software. SharpCap sticks by the spec and only allows positive positions.
Now, for the focus offset values, the way I would use them is like this...
Find focus with no filter and record the value - say 12345
Now, without other changes, find focus with each filter in place - you will get a list of positions like
red : 12368
blue : 12324
green : 12340
Then you set the offset for each filter to the difference between the position for that filter and the 'no filter' reference of 12345 - ie
red : 23
blue : -21
green : -5
Set the offset for 'no filter/luminance' to zero.
Now everything is set to go. On another night you focus with no filter and the position is 12330 (different temperature maybe). When you switch to the red filter with the offsets set then SharpCap will move the focuser by +23 to 12355, which should be good focus for the red filter assuming that the *differences* in positions for the different filters are constant.
cheers,
Robin