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Resolution and autoguider question

New Member

First off, I'm finding your book very helpful.
My Son and I are new to the hobby and it has helped us quite a bit, especially with expectations, budgeting, and knowing what of our existing kit will/won't work.

On page 60/61 of the book you go into calculating the resolution of your Camera/Telescope combination.

The formula is simple enough, but on page 61 you go into how much your guider moves your mount when it is off by a pixel.
I'd like to be able to calculate and know this information for our own scope/camera and guidescope/autoguider combination.

Please forgive my mathematical ignorance, but I can't for the life of me figure out where you came up with your main camera being moved by 2.09 pixels for every 1 pixel that your guider is off. I want to just divide the resolution of the guide scope by the resolution of the main scope, but it just doesn't add up for me. (It's close, but not exact - even if I try using the specs of the Orion Mini Guider you mentioned.)

Any clarification light you could shine on us would be greatly appreciated.


Topic starter Posted : 22/07/2014 9:11 pm
Allan Hall
Member Admin

My main scope and guide scope are bolted together so when one moves, the other moves exactly the same amount.

The first thing you have to do is find the resolution of both of the scopes, and to do that we need to use the formula:

Resolution = (206.265 x PixelSize) / FocalLength

For my main scope that would be:

1.25 = (206.265 X 4.67) / 770
Or the main scope moves 1.25 ArcSec per pixel

The guide scope is:

2.68 = (206.265 X 5.2) / 400
Or the guide scope moves 2.68 ArcSec per pixel

Since they move exactly the same amount on the mount that means that when the the guide scope is off one pixel it moves the mount 2.68 ArcSec which moves the other telescope 2.68 ArcSec and that translates into 2.1 (2.09 in the book because I did not tend to round my numbers) pixels of motion (2.68 ArcSec of motion divided by 1.25 ArcSec per pixel of resolution is approximately 2.1 pixels).

This make more sense?


Posted : 23/07/2014 2:02 pm
New Member

Perfect Sense 🙂
1) I was WAAY overthinking this.
2) I was not rounding either, so I was coming up with 2.1 pixels, not paying attention to the fact that 2.1 is only .01 pixel away from the 2.09 in the book. - causing total panic 🙂

Just to make sure though:
I have the same Autoguider setup as you - Orion Awesome Autoguider kit w/ f/5 400mm focal length and Pixel size of 5.2, so 2.68 ArcSec per pixel.
I have an f/9 900mm focal length refractor and a Nikon D7100 which has a pixel size of 3.9 which gives me a resolution of .89 ArcSec per pixel there.
SO, by my math - for every pixel that my guidescope moves, my camera will move 3 pixels (2.68/.89= 3.01)

Does that sound right?
Thanks again!

Topic starter Posted : 24/07/2014 12:40 am
Allan Hall
Member Admin

Looks right.


Posted : 24/07/2014 10:04 pm

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