That thread about guiding got really confusing to me.
I think we can do a lot with numbers, and they are appropriate eventually. But most important is the overall concepts and their results.
The overall concept is that you need to magnify the image you are using to guide so that it is a lot bigger than the image that is going to the film.
You need to do this so that any error shows in the "guide" image before it shows in the "film" image. When a "guide" image is only as large as the "film" image, by the time you see it in the guidescope, it is already in the film.
So, most people say that for visual guiding, you need the image to be two to three times the power (and I think a lot more is even better) of the film image.
Now, this is where things got confusing in the "I"m new" thread.
To start with, I disagree that it is just focal length of the scopes that you think about. I think it is power of the respective images.
The power of the main scope at Prime Focus is the focal length of the scope divided by 50, roughly. (If you want to know why I said this, ask.) If you are using a Corrector or Barlow, or eyepiece projection, feel free to work out the power of the scope according to standard calculations.
The power of the guide scope is the focal length of the guide scope divided by the focal length of the eyepiece.
It does not matter (to me) how big the "box" on the reticle of the eyepiece appears. It is true that the size of the box may change if you change the focal length of the eyepiece. (This is because the reticle is at the focal plane, and before the magnification of the eyepiece--which is determined by its focal length. So, for any given 2 mm dimension to the reticle, it will be grown or shrunk dependent on how much magnification you put to it after that. ) This knowledge allows you to use terms like "one fourth of the box" or so to tell what is acceptable for drift. However, (again in my mind) you should not allow any drift that can be measured in "fraction" of the box. Hide a dim star right inside where two lines cross, and keep it right there, and push the buttons however necessary to keep it there if it looks like it moved. Don't even think of letting it go a quarter of the box.
And the bigger your magnification in the guide scope compared to the imaging scope, the quicker you will see the deviation.
There's more to say. (for instance an autoguider does not require such high power since it has better sensors and reaction times.)
In short--Just make sure you are guiding at the highest possible power, and especially highest compared to your imaging scope.
Alex
I think we can do a lot with numbers, and they are appropriate eventually. But most important is the overall concepts and their results.
The overall concept is that you need to magnify the image you are using to guide so that it is a lot bigger than the image that is going to the film.
You need to do this so that any error shows in the "guide" image before it shows in the "film" image. When a "guide" image is only as large as the "film" image, by the time you see it in the guidescope, it is already in the film.
So, most people say that for visual guiding, you need the image to be two to three times the power (and I think a lot more is even better) of the film image.
Now, this is where things got confusing in the "I"m new" thread.
To start with, I disagree that it is just focal length of the scopes that you think about. I think it is power of the respective images.
The power of the main scope at Prime Focus is the focal length of the scope divided by 50, roughly. (If you want to know why I said this, ask.) If you are using a Corrector or Barlow, or eyepiece projection, feel free to work out the power of the scope according to standard calculations.
The power of the guide scope is the focal length of the guide scope divided by the focal length of the eyepiece.
It does not matter (to me) how big the "box" on the reticle of the eyepiece appears. It is true that the size of the box may change if you change the focal length of the eyepiece. (This is because the reticle is at the focal plane, and before the magnification of the eyepiece--which is determined by its focal length. So, for any given 2 mm dimension to the reticle, it will be grown or shrunk dependent on how much magnification you put to it after that. ) This knowledge allows you to use terms like "one fourth of the box" or so to tell what is acceptable for drift. However, (again in my mind) you should not allow any drift that can be measured in "fraction" of the box. Hide a dim star right inside where two lines cross, and keep it right there, and push the buttons however necessary to keep it there if it looks like it moved. Don't even think of letting it go a quarter of the box.
And the bigger your magnification in the guide scope compared to the imaging scope, the quicker you will see the deviation.
There's more to say. (for instance an autoguider does not require such high power since it has better sensors and reaction times.)
In short--Just make sure you are guiding at the highest possible power, and especially highest compared to your imaging scope.
Alex
