Thus for f10 this is
cfz = 10 * 10 * 2.2
cfz = 220 microns
so to get 10 steps within the critical zone we require
ss = 220 / 10
ss = 22
so we need a stepper that step size of around 22microns (in other words, each step will equate to moving the focuser 22microns)
Lets assume that you will connect the stepper direct to the shaft. This is good because that would minimize backlash. Using a gear/belt drive will make all the following figures wrong as we need need to factor in the ratio for the gear/belt system. Using a direct drive its always going to be 1:1 (assuming you do not connect to the 10:1 of the focuser - which is a pretty bad idea).
Lets assume you will select a stepper motor whose current is around 400mA at 12V. Less current = less heat and less power requirements.
What we do NOT know is how far your focuser moves in one full turn of the focuser knob. This is important and we cant go much further without this information. That distance is something you will need to measure (in millimeters) before continuing.
Lets assume that you did measure it and your focuser moves 20mm in one full revolution.
So lets proceed on that basic and in the following you can sustitute the real value instead of the 20mm I am using
Lets say we chose a NEMA stepper motor that runs at 12V, is rated at 400mA and has 200 steps per revolution.
Case 1: NEMA at 200 steps per revolution at FULL STEPS
So 1 full revolution is 200 steps and 1 full revolution moves 20mm, so there are
per step = 20/200 = 0.1mm or 100microns. This is not good enough because we need a per step size if 22microns
Even using HALF STEPS there would be 400 steps per revolution giving a step size of 50 microns, still too large.
CASE 2: NEMAPG5: 1028 Steps per Revolution at FULL STEPS
So 1 full revolution is 1028 steps and 1 full revolution moves 20mm, so there are
per step = 20/1028 = 0.019mm or 19microns. This is OK as we need a per step size of 22microns and what we have is 19. So this is good.
SO - if your focuser moved 20mm per revolution then a PG5 NEMA will be good to go.
So, what is important need to know information - how far your focuser moves in one revolution.
To do this see page 24 of PDF, measure it, then redo the calculations above using your value instead of the 20mm I assumed.
And purchasing the PG5 would be good because if you ever fit it to a faster ratio scope, you can still use it by reducing the stepper mode to 1/2, 1/4 or more.
Let me know your calcs if you want me to double check them or if you need additional help
Cheers
Robert
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I have scoured the internet for information regarding the distance that the focuser on my 9.25 SCT travels per revolution of the focuser knob and came up with this quote:
Focus tolerance:
On an f10 system, the “Critical Focus Zone” (CFZ) - to maintain a ¼ wave optical performance is 0.22mm (220 micron). The secondary in the SCT acts as an x5 magnifier, the allowable CFZ for the main mirror movement is therefore (220/5) 44 micron.
The standard focus knob on an SCT has a 40 tpi pitch thread (0.635mm travel per revolution) and a maximum travel distance of approx. 30mm.
To achieve the CFZ tolerance, the focuser must repeatably move the mirror to within 0.07 (44/635) of a revolution i.e. within 25 degrees (360 x 0.07).
Now if I put the figure of 0.635 in to the equation for CASE1 the result is 0.003175mm or 3.175microns.
For CASE2 the micron figure is even smaller, around 0.6.
Am I doing something wrong, because I cannot get anywhere near the required 22 microns.
Kind regards..,
Kirk
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Hi Kirk
SCT.... I guess most users will look to using a crayford focuser attached to the rear cell. There is a heap load of backlash in the standard celestron focuser knob so results will vary.
However, this is how.
Using an eyepiece and diagonal, focus as best you can on a star (bahtinov mask is preferred).
Once focused, turn the focuser knob one full turn (cannot remembr which way).
One you have turned the focuser knob one full turn, move the eyepice out till you get good focus. Now measure the distance between the eyepiece lip and the diagnonal lip. That will be the distance (measure using mm). If you cant get focus using the eyepiece (must mean the focuer knob needs to move the other way), then move the eypiece back down, and turn the focuser back through good focus, reverse and get good focus. You only want to focus in one direction to avoid backlash.
Let me know what it is.
I used have the figures but cannot find them as I moved away from SCT's a few years back.
Cheers
Robert
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Kirk, I also have a F10 9.25 SCT. The focus tolerance quote you have above is a little off for a 9.25 because the primary isn't the normal F2, it's said to be F2.3 (a few say F2.5). The secondary is then -F4.35. I had seen that the focuser lead screw was .75 mm thread pitch, but your source may be right that it's 40 tpi (0.635 mm thread).
Using F2.3 primary and 40 tpi thread I get a focus change distance per revolution of the focus knob of roughly 12mm = 0.635mm * (F10/F2.3)^2
I'd like to see what value you get from moving the eyepiece as Robert said above. I've had terrible weather lately so I haven't been able to test it myself.
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Hi
Yes, I entered your numbers into my own spreadsheet and they look accurate.
Based on those numbers, full stepping is the best option.
If going full stepping with PG27 then you can have choice of L293D Mini Driver Board, or L298N driver board or DRV8825 driver board to drive the stepper motor.
Regards
Robert
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Hi,
I am a little stuck understanding the CFZ of my scope to figure out which stepper motor I would need.
My scope is a CPC925 with f ratio of 10.
I calculate the CFZ as 220 as per the PDF equation but that is as far as I can get & have no idea which stepper motor is best suited.
Regards..,
Kirk
Hi Kirk
Welcome and glad to help.
Cfz in microns = focal_ratio * focal_ratio * 2.2;
Thus for f10 this is
cfz = 10 * 10 * 2.2
cfz = 220 microns
so to get 10 steps within the critical zone we require
ss = 220 / 10
ss = 22
so we need a stepper that step size of around 22microns (in other words, each step will equate to moving the focuser 22microns)
Lets assume that you will connect the stepper direct to the shaft. This is good because that would minimize backlash. Using a gear/belt drive will make all the following figures wrong as we need need to factor in the ratio for the gear/belt system. Using a direct drive its always going to be 1:1 (assuming you do not connect to the 10:1 of the focuser - which is a pretty bad idea).
Lets assume you will select a stepper motor whose current is around 400mA at 12V. Less current = less heat and less power requirements.
What we do NOT know is how far your focuser moves in one full turn of the focuser knob. This is important and we cant go much further without this information. That distance is something you will need to measure (in millimeters) before continuing.
Lets assume that you did measure it and your focuser moves 20mm in one full revolution.
So lets proceed on that basic and in the following you can sustitute the real value instead of the 20mm I am using
Lets say we chose a NEMA stepper motor that runs at 12V, is rated at 400mA and has 200 steps per revolution.
Case 1: NEMA at 200 steps per revolution at FULL STEPS
So 1 full revolution is 200 steps and 1 full revolution moves 20mm, so there are
per step = 20/200 = 0.1mm or 100microns. This is not good enough because we need a per step size if 22microns
Even using HALF STEPS there would be 400 steps per revolution giving a step size of 50 microns, still too large.
CASE 2: NEMAPG5: 1028 Steps per Revolution at FULL STEPS
So 1 full revolution is 1028 steps and 1 full revolution moves 20mm, so there are
per step = 20/1028 = 0.019mm or 19microns. This is OK as we need a per step size of 22microns and what we have is 19. So this is good.
SO - if your focuser moved 20mm per revolution then a PG5 NEMA will be good to go.
So, what is important need to know information - how far your focuser moves in one revolution.
To do this see page 24 of PDF, measure it, then redo the calculations above using your value instead of the 20mm I assumed.
And purchasing the PG5 would be good because if you ever fit it to a faster ratio scope, you can still use it by reducing the stepper mode to 1/2, 1/4 or more.
Let me know your calcs if you want me to double check them or if you need additional help
Cheers
Robert
Hi Robert,
thanks for your reply.
I have scoured the internet for information regarding the distance that the focuser on my 9.25 SCT travels per revolution of the focuser knob and came up with this quote:
Now if I put the figure of 0.635 in to the equation for CASE1 the result is 0.003175mm or 3.175microns.
For CASE2 the micron figure is even smaller, around 0.6.
Am I doing something wrong, because I cannot get anywhere near the required 22 microns.
Kind regards..,
Kirk
Hi Kirk
SCT.... I guess most users will look to using a crayford focuser attached to the rear cell. There is a heap load of backlash in the standard celestron focuser knob so results will vary.
However, this is how.
Using an eyepiece and diagonal, focus as best you can on a star (bahtinov mask is preferred).
Once focused, turn the focuser knob one full turn (cannot remembr which way).
One you have turned the focuser knob one full turn, move the eyepice out till you get good focus. Now measure the distance between the eyepiece lip and the diagnonal lip. That will be the distance (measure using mm). If you cant get focus using the eyepiece (must mean the focuer knob needs to move the other way), then move the eypiece back down, and turn the focuser back through good focus, reverse and get good focus. You only want to focus in one direction to avoid backlash.
Let me know what it is.
I used have the figures but cannot find them as I moved away from SCT's a few years back.
Cheers
Robert
Hi Robert,
fantastic, thank you so much for the help.
Regards..,
Kirk
Kirk, I also have a F10 9.25 SCT. The focus tolerance quote you have above is a little off for a 9.25 because the primary isn't the normal F2, it's said to be F2.3 (a few say F2.5). The secondary is then -F4.35. I had seen that the focuser lead screw was .75 mm thread pitch, but your source may be right that it's 40 tpi (0.635 mm thread).
Using F2.3 primary and 40 tpi thread I get a focus change distance per revolution of the focus knob of roughly 12mm = 0.635mm * (F10/F2.3)^2
I'd like to see what value you get from moving the eyepiece as Robert said above. I've had terrible weather lately so I haven't been able to test it myself.
Hi!
I'm not sure if my calculation are really correct - someone so kindly and can help me :(
regards Toni
Last edit: Rev.Antun 2017-09-04
Hi
I see the aperature is 203mm so I am guessing it is an SCT?
I would recommend the eyepiece method to confirm your numbers.
Regards
Robert
Hi!
Nope it is a 8"f5 Newton (8" = 203.2mm).
What do you mean with "eyepiece method" - how can i do confirm with ?
Sorry :( but i i've only been in Hobbyastronomy since 2013.
Hi
Yes, I entered your numbers into my own spreadsheet and they look accurate.
Based on those numbers, full stepping is the best option.
If going full stepping with PG27 then you can have choice of L293D Mini Driver Board, or L298N driver board or DRV8825 driver board to drive the stepper motor.
Regards
Robert
Hi!
Many many thanx for your help!
This is a realy great job you done - and a easy to made focuser!!!
Cheers Toni
Hi Toni
Looks great. Looking forward to more feedback once you have it mounted and on the telescope!
Cheers
Robert
Hi!
I will gladly make, but unfortunately still a few days need ;)
Cheers Toni