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Calibration steps by ASI Air plus


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I have been having a few problems with my tracking, I think I am getting the hang of things a bit better but trying to get to the bottom of some figures is a little confusing at times. These figures being the amount of time for calibration steps in the mount.

Before I drone on I found an article that covered it quite well:

Remember that the "Calibration steps" is in terms of the duration (milliseconds) of a calibration pulse at the Guide Rate. So, the same Calibration steps duration will move the mount by twice as much (in terms of arc seconds in the sky) if you use a Guide rate of 1x sidereal instead of 0.5x sidereal rate, for example.

1x sidereal rate moves at 15 arc-seconds per 1000 milliseconds of time. At 0.5x guide rate, the mount will move by 37.5 arc-seconds in 5000 milliseconds.

You want each calibration pulse to move a star by about 2 pixels on your sensor. PHD2 will try to move the star by 25 pixels during calibration; so the 2 pixels per calibration pulse will finish calibration in 12 or 13 steps.

You can use this tool to determine the pixel scale (arc-seconds per pixel) for your guide scope:

https://astronomy.tools/calculators/field_of_view/

Select Imaging Mode, and select any Messier object (M1 will do; the tool won't produce a result unless you have chosen an object first). Enter the focal length of your guide scope and the guide camera that you use.

Then look at the result for the "Resolution". It will be in arc-seconds per pixel.

From what I mentioned earlier, you want to scope to move about 2 pixels per calibration pulse. The plate scale will convert that to arc-seconds per calibration pulse.

For example, if the plate scale is 6" per pixel, and you want 2 pixels, then you want to move 12 arc-seconds per pulse.

If you use 0.5x sidereal rate as the guide rate, this 12 arc-seconds corresponds to 12/(15*0.5) = 1.6 seconds. So,for this example, you will want to use a Calibration Pulse of 1600 milliseconds.

Ok so I had to read it a couple of times before I got my head around it but here is my equation: Guide scope / camera combo gives Pixel arc second 6.45, sidereal rate being used 0.75 so it's 12.9/(15x0.75) = 1.146 or 1146 milliseconds.

Yet there are some articles giving what appears to be a rule of thumb type guide for a 30MM F4 guide of 8000-10000 

Have I missed something here as my calc is either rather low or the rule rule of thumb guide seems amazingly high.  (FWIW I prefer the scientific method)

 

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Posted (edited)

I've read it a lot of times and not sure of my calculations on it and completely baffled which isn't hard haha. 

I have my calibration step set to 2000ms. Max dec and RA at 500ms.

A guide rate of 0.9x seems to work for me and callibrates in 11 steps. 

My pixel scale is 2.2 for my 72ed and 294mc pro. 

Not sure if any of my nonsense there helps you Steve, as I've lost myself. 

Cheers 

Lee 

Edited by AstroNebulee
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7 minutes ago, AstroNebulee said:

I've read it a lot of times and not sure of my calculations on it and completely baffled which isn't hard haha. 

I have my calibration step set to 2000ms. Max dec and RA at 500ms.

A guide rate of 0.9x seems to work for me and callibrates in 11 steps. 

My pixel scale is 2.2 for my 72ed and 294mc pro. 

Not sure if any of my nonsense there helps you Steve, as I've lost myself. 

Cheers 

Lee 

you need your guide scope and guide cam pixel scale lee, half your problem right there, haha 😄

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Posted (edited)
19 minutes ago, bomberbaz said:

you need your guide scope and guide cam pixel scale lee, half your problem right there, haha 😄

That's my whole problem 😂😂

Check this out if I'm even half correct haha. 

Cheers from a complete baffoon. 

 

Screenshot_20220528_210943_com.microsoft.emmx.jpg

Screenshot_20220528_211151_com.microsoft.emmx.jpg

Edited by AstroNebulee
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3 minutes ago, AstroNebulee said:

That's my whole problem 😂😂

Check this out if I'm even half correct haha. 

Cheers from a complete baffoon. 

 

Screenshot_20220528_210943_com.microsoft.emmx.jpg

That gives us 2 pixels at 7.5/(15x0.9) = 0.555 or 555 milliseconds, even lower than mine, extraordinary.   

Although remember this is for calibration only and only done at the start of your session. Assuming you redo it at the start of every session, something a few videos I have watched seem to say should be done every time. 

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Posted (edited)
4 minutes ago, bomberbaz said:

That gives us 2 pixels at 7.5/(15x0.9) = 0.555 or 555 milliseconds, even lower than mine, extraordinary.   

Although remember this is for calibration only and only done at the start of your session. Assuming you redo it at the start of every session, something a few videos I have watched seem to say should be done every time. 

It does seem strange and thank you for working it out for me. 

I do clear the old callibration and do a fresh calibration at the start of every session and use the previous settings of 2000ms guide step and 500ms max RA and dec, with 0.9x guide rate. 

Edited by AstroNebulee
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38 minutes ago, AstroNebulee said:

With my asiair pro abs az gti last night and using the settings I have said above I had 9 calibration steps in both of the directions. 

Cheers 

Lee 

Sorry do you mean you set your callibration to 555 milliseconds? If so then 9 steps shows that is superb and it shows the maths behind it at least appears good.

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Posted (edited)
1 hour ago, bomberbaz said:

Sorry do you mean you set your callibration to 555 milliseconds? If so then 9 steps shows that is superb and it shows the maths behind it at least appears good.

Sorry Steve I meant. That my guide step calibration is set to get 2000 ms and max RA and dec set to 500ms.  I've added the calibration log from last night for you to look at . Think its 9 steps west and 7 north.  

Cheers

guide step.jpg

calibration.png

Edited by AstroNebulee
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