I have been reading through the documentation of the FPSESP on configuring
the the stepper motor. I have cameras connected so doing the way suggested is
not what I would prefer. However I do know the distance the mirror is moved
with on turn of the focuser knob.
The knob moves a threaded shaft that is 28 threads per inch which is 0.036"
per turn of the knob, Converting to mm 0.036" = 0.907 mm per turn. Dividing
that by 200 steps per rev we get 0.0045 mm. I still think mostly in inches, so I
find converting from mm to microns (which I believe is metres) confusing. So if
that is correct than that should be 4.5 microns per step.
Which according to your discussion in the manual should be more than adequate.
That is assuming that moving the mirror is the same thing for CFZ.
Is my reasoning correct?
Bob Parry
Chilliwack Observatory
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Hi Bob
4.5 microns per step - yes - this is step size.
CFZ is calculated from the focal ratio. The lower the focal ratio the smaller the step size needs to be.
You do not mention the f ratio of the telescope that you are using. And how you are connecting the motor to them.
I can provide a spreadsheet for you with the calculations for your setup. I need to know
the scope
the fratio
distance travelled by focuser/mirror in one turn - 0.907 mm
stepper motor- you have 200 steps per revolution.
Cheers
Rober
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The native Focal Ratio is F10. I have a Starizona 0,7 Reducer/corrector installed. My calculations
were based on F10. My Scope is a 14" LX200 GP. Stepper motor that I have is 200 teps/rev. Not
totally commited to this motor, they are not that expensive so if I have to change that is not a problem. I am planning on connecting directly to the knob without any gears or belts. Again that can change if necessary.
Bob Parry
Chilliwack Observatory
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
I think you may be missing something in the excel. I'm understanding that you compute the mirror movement and take it as the "focus displacement". However, the secondary mirror introduces a x5 multiplication in the focal length, which makes the distance traveled by the focal plane way larger than the distance traveled by the main mirror. I'm not sure, but I believe that the relation between travel distances is x26 (5^2+1). This is the same effect that produces SCT to change its focal length for different focus points (the outer the focal plane the larger the focal length). Using a Focal Reducer further complicates things as now the relation between mirror travel and focal plane travel gets another multiplication factor more, so while the CFZ is as computed in the spreadsheet, the "micrometers per step" is not, and should change for every configuration. This is also similar to when you change the distance from your camera to a Barlow, which also changes the multiplication factor of the Barlow.
I hope this helps
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
I have been reading through the documentation of the FPSESP on configuring
the the stepper motor. I have cameras connected so doing the way suggested is
not what I would prefer. However I do know the distance the mirror is moved
with on turn of the focuser knob.
The knob moves a threaded shaft that is 28 threads per inch which is 0.036"
per turn of the knob, Converting to mm 0.036" = 0.907 mm per turn. Dividing
that by 200 steps per rev we get 0.0045 mm. I still think mostly in inches, so I
find converting from mm to microns (which I believe is metres) confusing. So if
that is correct than that should be 4.5 microns per step.
Which according to your discussion in the manual should be more than adequate.
That is assuming that moving the mirror is the same thing for CFZ.
Is my reasoning correct?
Bob Parry
Chilliwack Observatory
Hi Bob
4.5 microns per step - yes - this is step size.
CFZ is calculated from the focal ratio. The lower the focal ratio the smaller the step size needs to be.
You do not mention the f ratio of the telescope that you are using. And how you are connecting the motor to them.
I can provide a spreadsheet for you with the calculations for your setup. I need to know
the scope
the fratio
distance travelled by focuser/mirror in one turn - 0.907 mm
stepper motor- you have 200 steps per revolution.
Cheers
Rober
The native Focal Ratio is F10. I have a Starizona 0,7 Reducer/corrector installed. My calculations
were based on F10. My Scope is a 14" LX200 GP. Stepper motor that I have is 200 teps/rev. Not
totally commited to this motor, they are not that expensive so if I have to change that is not a problem. I am planning on connecting directly to the knob without any gears or belts. Again that can change if necessary.
Bob Parry
Chilliwack Observatory
Bob
Looks good to go
Robert
I think you may be missing something in the excel. I'm understanding that you compute the mirror movement and take it as the "focus displacement". However, the secondary mirror introduces a x5 multiplication in the focal length, which makes the distance traveled by the focal plane way larger than the distance traveled by the main mirror. I'm not sure, but I believe that the relation between travel distances is x26 (5^2+1). This is the same effect that produces SCT to change its focal length for different focus points (the outer the focal plane the larger the focal length). Using a Focal Reducer further complicates things as now the relation between mirror travel and focal plane travel gets another multiplication factor more, so while the CFZ is as computed in the spreadsheet, the "micrometers per step" is not, and should change for every configuration. This is also similar to when you change the distance from your camera to a Barlow, which also changes the multiplication factor of the Barlow.
I hope this helps